Nitrogen in boreal forest ecosystems - The influence of climate change, nitrogen fertilization, whole-tree harvest and stump harvest on the ...
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APPENDIX B
Nitrogen in boreal forest
ecosystems
- The influence of climate change, nitrogen fertilization,
whole-tree harvest and stump harvest on the nitrogen cycle
A report from Belyazid
Consulting & Communication
A report from Belyazid Consulting and Communication AB This report is an appendix to the report “Effects of climate change, nitrogen fertilization, whole-tree harvesting and stump harvesting on boreal forest ecosystems - A review of current knowledge and an evaluation of how these factors may influence the possibilities to reach the Swedish environmental objectives”. January 2013. Authors Ulrika Jönsson Belyazid & Salim Belyazid, Belyazid Consulting and Communication AB, Fersens väg 9, 211 42 Malmö, Sweden Cecilia Akselsson, Physical Geography and Ecosystem Sciences, Lund University, Sölvegatan 12, 223 62 Lund, Sweden Contact Ulrika Jönsson Belyazid, E-mail: ulrika@belyazid.com, Tel: +46 735 244462 Acknowledgements This work was funded by the research program “Climate Change and Environmental Objectives” (CLEO) hosted by IVL Swedish Environmental Research Institute and the strategic research area “Biodiversity and Ecosystem Services in a Changing Climate” (BECC) hosted by Lund University. The photos in the report are from dreamstime.com, freedigitalphotos.net and shutterstock.com. The photo on page 18 was kindly provided by Rasmus Kjoller.
SUMMARY
This report consists of a literature review about the effects of climate being fertilized decreased to reach its lowest points between 1995
change and forest management on nitrogen (N) cycling in boreal and 2005 (20 000 - 25 000 ha yr-1) and has then increased again
forest ecosystems, with specific emphasis on Swedish conditions (55 500 ha yr-1 in 2009).
and the terrestrial part of the N cycle. With regard to climate
change, the influence of temperature, moisture and increased levels N in plants
of carbon dioxide (CO2) on N cycling have been evaluated. With The plant N-uptake rate depends on the concentration of N in
regard to forest management practices, the effects of N fertiliza- the environment as well as on the demand of the plant, the latter
tion, whole-tree harvesting (WTH) and stump harvesting (SH) being at least partly determined by the plant growth rate. It is well-
have been evaluated. Results from empirical as well as modelling known that plants have several different mechanisms to enhance
studies are included in the review. the availability of nutrients in the rhizosphere. With regard to N,
mycorrhizal associations are one of the most important.
N input
N is mainly added to terrestrial ecosystems through biological N That N uptake increases with temperature, at least within the
fixation and atmospheric deposition. In production forests, it may temperature ranges that are observed in boreal forest ecosystems,
also be added in the form of N fertilizer. Previously, boreal forests have been demonstrated in both laboratory and field studies. The
have been considered to have a limited capacity for N fixation. impact of drought and excess water availability is less certain.
However, recent studies have shown substantial levels of N fixa- However, it seems likely to assume that drought results in an im-
tion by cyanobacteria colonizing pleurocarpus feather mosses in paired N uptake, since extended drought periods leads to reduced
northern boreal forests, and this process is now considered to be fine root biomass and may also influence the nutrient availability
the main source of N in these ecosystems. and transport in the soil. Increased levels of CO2 generally result
in an increased allocation of C to below-ground parts of trees,
Since microbial activity generally increases with temperature, and with subsequent increases in the growth of roots and mycorrhiza,
the efficiency of the N-fixing enzyme nitrogenase reaches its maxi- which should allow the trees to exploit a larger soil volume and
mum near 25°C, an increasing mean annual temperature is likely thus might be expected to promote N uptake. Indeed, more of-
to have a positive effect on N-fixation rates – something that also ten than not, an increase in N uptake seems to occur at elevated
has been demonstrated in a number of studies in northern Swe- CO2, provided there is an adequate supply of N in the soil. The
den. However, the high light sensitivity of feather mosses could increase is most probably a function of increased root growth,
mean that climate-induced increases in plant productivity could since most studies to date have shown little effect, or no effect
eventually limit biological N fixation because of greater shading. N at all, of elevated CO2 on soil N-mineralization rates. There are
fixation is also generally stimulated by increased moisture (nitro- basically no scientifically published studies on how WTH and SH
genase is very sensitive to oxygen) and by elevated CO2 (although may affect N uptake and acquisition of trees in the subsequent
the number of studies in boreal forests are limited). It is also well generation. The two studies that do exist are greenhouse studies.
established that N-fixation rates are directly influenced by soluble They show that N in brash and in decomposing roots can make
N concentrations. N additions to a late-succession forest in the small, but discernible, contributions to new tree growth after one
north of Sweden was found to have dramatic and lasting influence growing season.
on N-fixation rates and fertilization at a rate of 4.5 kg N ha-1 yr-1
basically eliminated any N fixation throughout the study period. Elevated temperatures are generally believed to increase the
Many other studies show similar results. The effects of WTH and concentrations of N in plant tissue, because mineralization and
SH on N fixation are basically unknown. diffusion of N are stimulated at high temperatures. However,
the response probably depend on the nature of the temperature
With regard to the deposition of N, human activities have led to increase. While a small and gradual increase in mean annual tem-
major increases in the global emissions of N to the atmosphere and, perature may give the above mentioned results, a period of very
subsequently, to increases in the deposition of biologically available high seasonal maximum is likely to have the opposite effect, not
N to the terrestrial biosphere. In Sweden, there is a clear gradient, least because of its interaction with drought. Drought generally
with wet deposition of inorganic N decreasing substantially from causes an impairment of the nutrient concentrations and contents
the south-west (≥ 11 kg ha-1 yr-1) to the north (≤2 kg ha-1 yr-1). In of plants. Modelling results suggest that this may be a consequence
addition, there may be substantial amounts of N reaching ecosys- of lower decomposition rates and, consequently, lower availability
tems by dry deposition. of N. It may also be a consequence of reduced C allocation to
below-ground, resulting in less C and energy available for uptake
Fertilization with N has been used in Sweden as a measure to and assimilation. The initial stimulation of photosynthesis under
increase tree growth since the 1960s. Around 2 million ha of elevated CO2 is often accompanied by a decrease in plant N con-
forest has been subjected to N fertilization at some occasion. centration, in particular foliar N and especially at low N supply.
After a peak in the 1970s (200 000 ha yr-1 fertilized), the area The decrease has been suggested to be a result of either decreased
N uptake per unit of biomass produced or preferential allocation result commonly found for WTH in nutrient budget calculations
of acquired N to root tissue. and when using dynamic modelling. In reality, however, signifi-
cant differences between WTH and CH are rarely discernible in
Fertilization experiments with N have generally shown increased the field, at least in the Scandinavian countries. Considering SH,
leaf N concentrations, although the increase may sometimes be keeping stumps were recently suggested to constitute an important
rather short-lived. Studies of WTH, on the other hand, usually mechanism of nutrient retention, potentially diminishing nutrient
show no effect on N concentrations of trees (in particular the long- leaching and maintaining site fertility and productivity.
term ones). The effects of SH on N concentrations of trees in
the subsequent generation have rarely been studied, but the two N losses
studies available showed lower concentrations of N in SH stands N is lost from ecosystems mainly through denitrification, leach-
as compared with conventionally harvested (CH) stands (despite ing and harvesting. However, N mineralisation, N fixation and
N having been added before planting in one of the two studies). nitrification also contribute to the losses of volatile N compounds.
Despite the theoretical temperature dependence of denitrification,
The quantity of N transferred from the trees to the soil by lit- the temperature dependence in the field is usually not pronounced.
terfall is primarily a function of tree biomass. Generally, increases This is probably due to the indirect effects of temperature (i.e. low
in temperature, moisture, CO2 and N are all regarded to result in temperature enhances soil oxygen consumption, inducing anoxic
increased production and thus litterfall. WTH and SH, on the conditions which promote denitrification) or the fact that other
other hand, implies a removal of potential litter, and thus N, from environmental controls exert a stronger influence on denitrification
the ecosystems. than does temperature. Denitrification is enhanced by anoxic con-
ditions and high soil water content thus usually result in enhanced
N in soils denitrification rates. Denitrification is also strongly dependent on C
Rates of decomposition, mineralization and nitrification generally availability and there exists a general correlation between total soil
increase with temperature, at least under favourable moisture con- organic matter content and denitrification potential. The increase
ditions and in ecosystems where temperature is a limiting factor. in C allocation below-ground as a consequence of elevated CO2 is
The effect of drought on mineralization rates in forest ecosystems thus expected to result in a stimulation of denitrification. However,
is still poorly understood and results are variable. Ultimately, how- conclusive field results are missing.
ever, reduced soil water availability limits the microbial activity
in soils, and it should thus also limit mineralization. With regard Most studies to date have demonstrated that N additions generally
to nitrification, it is generally inhibited at very low soil moisture lead to increased fluxes of gaseous N from the soil, although the
levels, increases with soil moisture to an optimum and then decline emissions are not necessarily high or correlated to the amount of
as the soil becomes water saturated. Freezing-thawing and drying- N added. To our knowledge, there are no studies on how WTH
rewetting cycles have both been demonstrated to occasionally result and SH influence denitrification.
in increased N mineralization and nitrification rates. However, the
increases, if observed, are usually rather small in relation to the There is still relatively little information available with regard to how
expected annual rates. Most studies to date have indicated little or various environmental factors affect the leaching of N. Primarily,
no effect of elevated CO2 on soil N-mineralization rates. The results leaching is limited by the availability of soluble N. Secondly, hydrol-
with regard to nitrification are inconclusive, with studies reporting ogy imposes a strong control on the transport of N in soil, with
increases, decreases and no change in response to elevated CO2. increasing precipitation being associated with more rapid leach-
ing. Although increased temperature generally results in enhanced
Gross N mineralization usually shows a strong negative correlation mineralization rates, this does not necessarily lead to additional N
with the substrate C/N ratio in long-term studies. The increase in losses, as N uptake by roots or immobilization by microbes might
mineralization in response to increased availability of N is probably occur. However, the potential risk of leaching increases. Studies
a result of increased plant productivity, but also of an increase in of N leaching from forests subjected to elevated CO2 are rare,
the activity and biomass of the soil microbial community. Possibly, and the results variable. Several fertilization studies have indicated
it may also be an effect of a change in the composition of the soil that leaching may be induced when the availability of N increases.
microbial community at high levels of N. N availability is also a However, leaching only occurs if the N input exceeds the capacity
major controlling factor of nitrification, and many studies have of the ecosystem to retain N, something that is generally the case
reported increased nitrification rates in response to N addition, only at sites with very large N inputs. Recent studies in several
especially in ecosystems experiencing elevated N inputs over pro- European countries have indicated that N leaching is one of the
longed periods of time. last things to occur in the succession of events related to increased
N availability in ecosystems.
Since WTH and SH result in reduced substrate availability (and
probably also influence the abiotic environment), it seems logical to Several studies have suggested that removal of logging residues
assume that they should influence the processes of decomposition, may reduce the accumulation of N in forest ecosystems and thus
mineralization and nitrification and, eventually, result in diminish- the potential leaching of N. Although there are some studies sup-
ing stores of N in forest soils. With regard to SH, the few studies porting this statement, the field-based evidence is relatively scarce
available do indeed report significantly reduced soil N pools and and there are studies (both field and modelling ones) which have
mineralizable N after SH as compared with CH. This is also a reported the opposite result, i.e. a decreased leaching after CH as
compared with WTH. The effects of SH on leaching is unclear, Conclusions
since the number of studies is very low. Although recent advances have resulted in a more comprehensive
understanding of how climate change and forest management
The amount of N lost through harvesting depends not only on practices affect the processes controlling the N dynamics of boreal
the amount of harvested biomass, but also on what fraction of the forest ecosystems, much information is still lacking. In particular,
tree that is harvested. Nutrient budget calculations have shown the long-term impact on soil N and N losses from ecosystems are
that WTH may result in up to two times higher N loss than CH. unclear. Improving our knowledge about soil N, endeavouring in
Even if the N losses are smaller when stumps are removed (since obtaining long-term data sets, and clarifying the couplings between
N concentrations are lower than in slash), they are not negligible the N and the C cycles are crucial if reliable predictions about the
since the biomass of stumps is substantial. future cycling of N in boreal forest ecosystems are to be made.
TABLE OF CONTENTS
SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1. BRIEF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2. NITROGEN CYCLING – AN OVERVIEW. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3. NITROGEN INPUTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1 N fixation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.1.1 Effects of temperature 12
3.1.2 Effects of moisture 12
3.1.3 Effects of CO2 13
3.1.4 Effects of N 13
3.1.5 Effects of WTH 13
3.1.6 Effects of SH 13
3.2 Atmospheric deposition of N . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.2.1 Throughfall 15
3.3 N fertilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4. NITROGEN IN PLANTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1 N acquisition and uptake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.1.1 Effects of temperature 17
4.1.2 Effects of moisture 17
4.1.3 Effects of CO2 18
4.1.4 Effects of N 19
4.1.5 Effects of WTH 19
4.1.6 Effects of SH 19
4.2 N content of trees, allocation, retranslocation and storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.2.1 Effects of temperature 19
4.2.2 Effects of moisture 20
4.2.3 Effects of CO2 20
4.2.4 Effects of N 20
4.2.5 Effects of WTH 22
4.2.6 Effects of SH 22
4.3 Plant growth and above-ground production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.4 Root growth and mycorrhiza production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.5 Litterfall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.5.1 Effects of temperature 22
4.5.2 Effects of moisture 22
4.5.3 Effects of CO2 22
4.5.4 Effects of N 23
4.5.5 Effects of WTH 23
4.5.6 Effects of SH 23
5. NITROGEN IN SOILS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1 Decomposition, mineralization and immobilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
5.1.1 Effects of temperature 25
5.1.2 Effects of moisture 25
5.1.3 Effects of CO2 26
5.1.4 Effects of N 26
5.1.5 Effects of WTH 27
5.1.6 Effects of SH 29
5.2 Nitrification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.2.1 Effects of temperature 29
5.2.2 Effects of moisture 30
5.2.3 Effects of CO2 30
5.2.4 Effects of N 30
5.2.5 Effects of WTH 30
5.2.6 Effects of SH 30
6. NITROGEN LOSSES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.1 Denitrification and volatilization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
6.1.1 Effects of temperature 32
6.1.2 Effects of moisture 32
6.1.3 Effects of CO2 32
6.1.4 Effects of N 33
6.1.5 Effects of WTH 33
6.1.6 Effects of SH 33
6.2 Leaching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
6.2.1 Effects of temperature 33
6.2.2 Effects of moisture 34
6.2.3 Effects of CO2 34
6.2.4 Effects of N 34
6.2.5 Effects of WTH 35
6.2.6 Effects of SH 36
6.3 Tree harvest. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
7. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
8. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 381. BRIEF
This appendix consists of a literature review about the effects of levels of carbon dioxide (CO2) on various processes in the N
climate change and forest management on nitrogen cycling in cycle are evaluated.
boreal forest ecosystems, with specific emphasis on Swedish condi-
tions. The literature review is an appendix to the report “Effects of The focus of the literature review is on the terrestrial part of the N
climate change, nitrogen fertilization, whole-tree harvesting and cycle. The processes included are: N fixation, N acquisition and
stump harvesting on boreal forest ecosystems - A review of current uptake of plants, plant growth, litterfall, decomposition, minerali-
knowledge and an evaluation of how these factors may influence zation, immobilization, nitrification, denitrification and leaching.
the possibilities to reach the Swedish environmental objectives”
published by Belyazid Consulting & Communication AB in Janu- We are aware of the difficulty in drawing coherence from a large
ary 2013. number of studies, which to varying degrees, are site-, age- and
species-specific. However, we have tried to include as much infor-
The aims of this review are: mation as possible within the limited time available. Foremost,
• To identify what processes are controlling the N dynamics in we have collected information from scientifically published ar-
boreal and temperate forest ecosystems. ticles concerning boreal forest ecosystems in Scandinavia. When
• To investigate how these processes are influenced by climate the information available was scarce, we extended the literature
change and some forest management practices that are likely search to other types of forest ecosystems and other regions of the
to become more common in Sweden in the future, namely world. Both empirical and modelled data have been included in
nitrogen (N) fertilization, whole-tree harvesting (WTH) and the review. For SH, very little scientifically published material is
stump harvesting (SH). With regard to climate change, the available. Consequently, the conclusions with regard to the effects
influence of temperature and moisture as well as increased of SH on forest ecosystems are uncertain.
910
2. NITROGEN CYCLING
– AN OVERVIEW
The overview is mainly based on the description provided by before proceeding along either grazing or detritus trophic chains
Brady & Weil (1999). (decomposition) to end up as soil N.
As it moves through the N cycle, an atom of N may appear in The soil is the major sink for N in most ecosystems, and Kauppi et
many different chemical forms, each with its own properties, al. (1995), Tietema (1998) and Nadelhoffer et al. (1999) showed
behaviours, and consequences for the ecosystem. The principal that between 70 and 90% of the N added to forest ecosystems in
pools and forms of N, and the processes by which they interact 15
N experiments ended up in the soil compartment. Initially, the
in the cycle, are briefly described below. For more detailed in- main sink is the forest floor, but later, most of the 15N is recovered
formation about the major pathways and processes governing N from the lower mineral soil (Emmett et al., 1995). The great bulk
in boreal forest ecosystems, see sections three to six. (95 to 99%) of the soil N is in organic compounds (Brady & Weil,
1999). Organic N may be stored in soils for many centuries, or
In the atmosphere, N exists mainly as dinitrogen gas (N2), with even millennia (Kimmins, 1997). Sooner or later, however, much
traces of nitric oxide (NO), nitrogen dioxide (NO2), nitrous of it is converted to NH4+ by the process of mineralization.
oxide (N2O), and ammonia (NH3). N in the form of N2 is and
has always been abundant, making up about 78% of the world’s The NH4+ ions are subject to five possible fates in the N cycle
atmosphere. N2 is, however, unavailable to plants. It has to be (Brady & Weil, 1999). They may be: 1) removed by plant up-
fixed, i.e. pulled from the air and bonded to hydrogen or oxygen take, 2) immobilized (i.e. converted into organic form again)
to form inorganic compounds, mainly ammonium (NH4+) and by microorganisms, 3) fixed in the interlayers of certain clays,
nitrate (NO3-), before the plants can use it. This conversion is
4) transformed into NH3 and lost to the atmosphere by vola-
performed naturally either by lightning or by certain micro-
tilization or 5) oxidized to nitrite (NO2-) and subsequently to
organisms (including some species of bacteria, a number of ac-
NO3- by the microbial process of nitrification. Nitrogen in the
tinomycetes and cyanobacteria). Globally, enormous amounts
NO3- form is highly mobile in the soil and in the environment
of N are fixed each year – terrestrial ecosystems alone fix an esti-
and NO3- released by nitrification is thus quickly: 1) taken up by
mated 139 million Mg N per year, while lightning fixes around
plants, 2) immobilized by soil microbes or 3) lost by leaching in
8 million Mg N per year (Brady & Weil, 1999).
drainage water. It may also be 4) volatilized to the atmosphere
by denitrification, a process in which NO 3- is reduced to NO,
Besides this “natural” fixation, human activities during the past
N2O, N2 or NH3.
century have resulted in substantial increases in the global N
fixation, effectively doubling the annual transfer of N from the
atmospheric pool to biologically available forms (Vitousek et The rates of the various steps included in the below-ground part
al., 1997). The major sources of this enhanced supply include of the N cycle depend mainly on temperature and soil conditions.
industrial processes that produce N fertilizers, the combustion At each step, plants or microbes can take up soluble N, or N
of fossil fuels, and the cultivation of soybeans, peas, and other can be leached from the system, reducing the substrate available
crops that host symbiotic N-fixing bacteria. As a consequence, for the next N transformation. Therefore, the supply rate of dif-
substantial amounts of N nowadays also enter the ecosystem as ferent forms of “available” N to plants and microbes follow the
atmospheric deposition (NH3, NH4+ or NO3-) or, in production sequence: dissolved organic N ≥ NH4+ ≥ NO3- (Lambers et al.,
forests, in the form of N fertilizer. Although biological fixation 1998). However, although the N supply rate always follows the
is the most important process for the introduction of combined same sequence in soils, the quantities and relative concentrations
N into natural environments on a global scale (Son, 2001), of these soluble forms of N varies substantially.
deposition and fertilization may be of greater significance locally.
That the recycling of N from plants to soil and back to plants is a
Once N has entered a terrestrial ecosystem, it undergoes a com- key component regulating the N availability in soil and hence the
plex series of alterations. Ammonia gas may be absorbed directly rate of forest growth is evident from the fact that approximately
by plant foliage (Hutchinson et al., 1972), whereas NO3- and 93% of the N that is available for plant growth in temperate for-
NH4+ ions enter the ecosystems as dry fallout or in precipitation, ests originates from this type of recycling, while only 7% comes
and is taken up either by leaves or roots of trees. The ions are from the atmosphere (Lambers et al., 1998). In arctic tundras,
then converted to organic N, in a process known as assimilation, the number is even higher reaching 96% (Lambers et al., 1998).
113. NITROGEN INPUTS
N is mainly added to terrestrial ecosystems through biological in boreal forests, showed that warming enhanced N-fixation rates of
N fixation and atmospheric deposition. However, in production the two feather mosses investigated – Pleurozium schreberi and Hylo-
forests, N may also be added to the ecosystem in the form of N comium splendens, with the former responding more strongly to the
fertilizer. Among these three input forms, biological fixation is the temperature increase than the latter. However, for both species, the
most important process for the introduction of combined N into highest warming treatment (30.3°C) eventually caused N fixation to
natural environments on a global scale, although deposition and decline. Light strongly interacted with warming treatments, having
fertilization may be of greater significance locally (Son, 2001). a positive effect at low or intermediate temperatures but damaging
effects at high temperatures. That temperature has a positive effect
on N-fixation rates and that the temperature sensitivity may differ
3.1 N fixation between species was also shown in a field experiment by Markham
(2009), examining N-fixation rates in wet lowland and dry upland
Previously, boreal forests have been considered to have an extremely boreal forests in central Canada.
limited capacity for N fixation, with fixation rates estimated at only
0.5 kg N ha-1 yr-1 (DeLuca et al., 2002). However, recent studies Based on their results, Gundale et al. (2012) concluded that the
have shown substantial levels of N fixation by cyanobacteria colo- maximum predicted increases in mean annual temperatures (i.e.
nizing pleurocarpus feather mosses (DeLuca et al., 2002), and N 5°C; IPPC, 2007) during the next century is likely to directly in-
fixation is now considered as the main source of ecosystem N in crease N-fixation rates by the two feather moss species investigated,
northern boreal forest ecosystems (DeLuca et al., 2008). The fixa- meaning that increased biological N inputs may satisfy the higher
tion rates in feather moss carpets may, however, vary greatly, both N requirements of a more productive vascular plant community
spatially and in terms of time (DeLuca et al., 2002) as well as with in warmer climates. However, Gundale et al. (2012) cautions that
stage of secondary succession (Zackrisson et al., 2004). For early- N fixation by the two mosses are also sensitive to light availability,
succession sites (25-80 years since fire), Zachrisson et al. (2004) which could mean that climate-induced increases in plant produc-
reported N fixation rates below 0.5 kg ha-1 yr-1, while mid-succesion tivity could limit biological N fixation because of greater shading.
sites (100-200 years since fire) had fixation rates between 0.4 and Furthermore, exposure to extreme temperature events substantially
1.6 kg ha-1 yr-1 and late-succession sites (>200 years since fire) had damaged the N-fixation capacity of the feather-moss species, sug-
fixation rates between 1.0 and 2.0 kg ha-1 yr-1. Recently, N fixation gesting that increases in the frequency and intensity of extreme
rates of up to 4 kg N ha-1 yr-1 in boreal forests were reported by temperature events in the future could reduce biological N inputs
Gundale et al. (2010; 2011). Despite the importance of biological in some boreal environments.
N fixation in boreal ecosystems, very little is known about how
this process may respond to climate change (Gundale et al., 2012).
3.1.2 Effects of moisture
In addition to symbiotic fixation of atmospheric N, asymbiotic The N-fixing enzyme, nitrogenase, is very sensitive to oxygen. Gen-
fixation may also contribute to the input of N in forest ecosystems erally, N fixation is thus stimulated by reduced oxygen levels and
(Son, 2001). Asymbiotic fixation is known to occur in decaying low redox potentials (Paul & Clark, 1996; Son, 2001). Soil moisture
wood, in the forest floor, in the rhizosphere, by bacteria associated levels are thus linked to N fixation through its influence on oxygen
with mycorrhizal fungi and even in the stemwood of living trees supply, but also through its influence on transportation of mineral
(Kimmins, 1997). Although the rates are quite low (generally less nutrients, many of which are essential for N fixation to occur (P,
than 3 kg ha-1 yr-1 and mostly less than 1 kg ha-1 yr-1), this addition Mo, Fe, Co, Mg, P, W and Ca) (Son, 2001; Vitousek et al., 2002).
has been suggested to play an important role in maintaining N eco-
system budgets over long time periods (Kimmins, 1997; Son, 2001). Recent studies on the influence of variations in moisture and rain-
fall intensity and frequency in boreal forests in northern Sweden
have verified the strong dependency of N fixation on soil water
3.1.1 Effects of temperature availability. Gundale et al. (2009) found that prolonged drought
In general, microbial activity increases with temperature (Brady & (>45 days) resulted in a diminished N-fixation capacity by the
Weil, 1999). According to Gundale et al. (2012), it is also known feathermoss-cyanobacteria association investigated, whereas per-
that the efficiency of the N-fixing enzyme nitrogenase reaches its sistent moisture resulted in an increase in the N-fixation capacity.
maximum near 25°C. An increasing mean annual temperature is The sensitivity to moisture was larger in feather-mosses collected
thus likely to have a direct positive influence on N-fixation rates. from old forests than from young forests. Gundale et al. (2009)
However, it is also possible that warmer temperatures may enhance thus concluded that moisture availability may be an important
evaporative water loss, which have been shown to affect feather moss regulator of N-fixation rates in some boreal forest environments.
N-fixation rates negatively (Gundale et al., 2009; Jackson et al., Their results were supported by those of Jackson et al. (2011), who
2011). A recent study by Gundale et al. (2012), investigating the showed that N fixation of boreal feather mosses under greenhouse
interactive effects of temperature and light on biological N fixation conditions was strongly influenced by both rainfall amount and
12frequency, as well as their interaction (an increase in the frequency tion treatment (3 kg N ha-1 yr-1) resulting in a nearly 50% reduction
of rain had greater effect when the amounts were higher). Jackson in N fixation per unit mass relative to control. At high inputs (50
et al. (2011) also found that feather mosses had a buffering effect on kg N ha-1 yr-1) biological N fixation was nearly completely absent.
the decomposer subsystem under moisture limitation, promoting The combined effect of decreased N fixation and the sequestration
vascular litter decomposition rates, concentrations of litter nutrients of N into feather moss tissue accounted for more than half of the
and active soil microbial biomass and reducing N release into soil N input (56,7%) at the lowest simulated N-deposition rate. At the
solution under low soil moisture conditions. highest rate, the feather moss tissue was no longer acting as a sink
for N. Rather, a net release of N from bryophyte tissue occurred.
Several studies performed in non-boreal systems have also demon-
Gundale et al. (2011) suggest that this “bryophyte effect” is partly
strated correlations between moisture and N fixation in bryophytes
responsible for why soil inorganic N availability and acquisition
or lichens (Turetsky, 2003; Zielke et al., 2005).
by woody plants often remain unchanged at low N addition rates.
Vascular plants cannot acquire anthropogenic N inputs until the
3.1.3 Effects of CO2 bryophyte layer is N saturated.
According to a review on long-term responses of forest ecosystems
to elevated CO2 (Johnson, 2006), elevated CO2 has been found
3.1.5 Effects of WTH
to stimulate symbiotic N fixation in trees in several studies (see for
example Norby, 1987; Arnone & Gordon, 1990). Arnone & Gor- We are not aware of any studies investigating the impact of WTH
don (1990) found evidence for the presence of a positive feedback on N-fixation rates. For effects of WTH on bryophytes and lichens,
loop between N fixation and photosynthesis in nodulated Alnus see Appendix D.
rubra plants growing at elevated CO2. The plants had greater whole-
plant photosynthesis, nitrogenase activity, leaf area, N content, and
nodule and plant dry mass relative to nodulated plants grown at 3.1.6 Effects of SH
ambient CO2 and non-nodulated plants grown at both CO2 levels. We are not aware of any studies investigating the impact of SH
The relative amount of dry mass allocated below-ground decreased on N-fixation rates. For effects of SH on bryophytes and lichens,
for all seedlings over time and the amount allocated above-ground see Appendix D.
increased. The proportion of dry mass allocated below-ground was
consistently greater in non-nodulated plants. However, a stimula-
tion of symbiotic N fixation does not occur in every case (Arnone,
1999).
With regard to non-symbiotic fixation, there are very few stud-
ies (Johnson, 2006), but Verburg et al. (2004) found no effect of
elevated CO2 on non-symbiotic fixation of N.
3.1.4 Effects of N
It is well established that N-fixation rates in cyanobacteria are
directly influenced by soluble N concentrations (Paul & Clark,
1996) and N additions to a late-succession forest in the north
of Sweden was found to have dramatic and lasting influence on
N-fixation rates (Zackrisson et al., 2004; DeLuca et al., 2007). N
fertilization, at a rate of 4.5 kg ha-1 yr-1, basically eliminated any N
fixation throughout a three-year period. DeLuca et al. (2008) also
reported a close relationship between increasing N in throughfall
and decreasing N fixation, suggesting an ecosystem-level feedback
in which N bioavailability in early successional yields increased N
deposition on the moss carpets via canopy throughfall and litter,
which down-regulates N fixation. Other studies have also shown
an inverse relationship between rates of biological N fixation and
N availability (see for example Gundale et al., 2011).
Recently, Gundale et al. (2011) demonstrated that bryophytes actu-
ally attenuate anthropogenic N inputs in boreal forests by limiting
the availability of anthropogenic N to vascular plants by directly
offsetting N capital as a result of down-regulation of N fixation and
through sequestration of N capital into their biomass. This effect is
most pronounced at low rates of N input, with the lowest N addi-
13Figure 1. The total deposition of inorganic N (NH4+ and NO3-) to coniferous forests in Sweden as modelled with the MATCH model. In order to reduce the effect of extreme years, the average values for 2009, 2010 and 2011 have been used. For more information, see http://www.smhi.se/klimatdata/miljo/atmosfarskemi. 14
3.2 Atmospheric deposition of N average 20% of the total N deposition. The organic N deposition
varied between 0.2 and 4.0 kg ha-1 yr-1 with a median value of
Human activities have led to major increases in the global emis- 1.1 kg ha-1 yr-1. The highest deposition values were found in the
sions of N to the atmosphere, and, consequently, to an increase southern parts of the country, but there was no clear gradient
in the deposition of biologically available N to the terrestrial from the south to the north of the country.
biosphere, particularly in Europe and North America. On a global
basis, it is estimated that in 1860, 34 Tg N yr-1 was emitted as
NOx and NH3 and then deposited to the Earth’s surface as NOy 3.2.1 Throughfall
and NHx. In 1995, this value had increased to 100 Tg N yr -1, In two ecosystem manipulation studies, covering twelve sites with
and by 2050, it is projected to be 200 Tg N yr -1 (Galloway et varying atmospheric N inputs, Tietema & Beier (1995) found
al., 2008). As a comparison, N deposition to ecosystems in the a strong correlation between N input by precipitation and the
absence of human influence is generally around 0,5 kg N ha -1 throughfall of both NO3- and NH4+. In general, their sites, domi-
yr-1 or less (Galloway et al., 2008). According to Galloway et al. nated by coniferous forests in central and western Europe, had
(2008), there are now large areas of the world where the average higher concentrations of NH4+ and NO3- in throughfall than in
N-deposition rates exceed 10 kg N ha-1 yr-1. By 2050, some regions bulk precipitation, indicating the importance of dry deposition
may reach N deposition levels of 50 kg N ha-1 yr-1 (Galloway et as well as exchange processes in the canopy for the amount of N
al., 2008). reaching the soil. Furthermore, they found that NO3--dominated
sites had higher N concentrations than NH4+-dominated sites,
Measuring N deposition on more than 200 sites in 23 countries despite the same input by throughfall. According to Tietema &
in the end of the 1990’s, De Vries et al. (2003) showed that the Beier (1995), this implies a higher NH4+ uptake in the canopy in
amount of inorganic N deposited in Europe ranged from 1.4 to the NO3--dominated sites (with generally low N input), compared
42 kg ha-1 yr-1. Approximately 55% of the plots got an external with the NH4+-dominated sites (with generally high N inputs).
input of N exceeding 14 kg ha-1 yr-1 (De Vries et al., 2003). The Also De Vries et al. (2003) found higher NH4+/NO3- ratios in
measurements by De Vries et al. (2003) confirmed that there is bulk deposition (1.5) than in throughfall (1.14), indicating a
still a clear gradient in the deposition of N compounds, start- preferential uptake of NH4+ in the canopy of trees.
ing in northern Scandinavia (boreal forests), increasing towards
southern Scandinavia (boreal-temperate forests) and reaching its In contrast to the forest sites investigated in Tietema & Beier
peak in western, central and eastern Europe. The gradient within (1995), forests in southern and central Sweden have generally
Sweden has been confirmed by for example Hallgren-Larsson et shown a considerable canopy uptake of N, resulting in substan-
al. (1995) and Pihl-Karlsson et al. (2011). In the latter study, the tially lower concentrations of N in throughfall compared with
open-field deposition of inorganic N decreased from ≥11 kg ha-1 wet deposition in adjacent fields (Westling et al., 1995). In areas
yr-1 in the south-west to ≤2 kg ha-1 yr-1 in the north, reflecting the with high deposition of N, like coastal areas in the south-west
prevailing wind direction carrying pollutants from the continent of Sweden, the dry deposition may be substantial though and N
to the south-western part of Sweden (note that dry deposition is in the throughfall may then exceed that in the wet deposition
not included in these measurements). (Westling et al., 1995). Balsberg-Påhlsson & Bergkvist (1995)
found, for example, that the supply of N was increased by a
The relative contribution of NH4+ and NO3- to European N depo- factor of 3.4 in a spruce forest front (1 m from the forest edge)
sition varied largely over plots, but in general, there was a weak in comparison to the bulk deposition in the open field. The flux
but significant correlation between both N species (De Vries et al., of N then decreased exponentially with distance from the forest
2003). In the boreal and boreal-temperate region, the deposition edge. Twenty-five meters into the forest, where the edge effect
of NH4+ was generally almost two-fold that of NO3-, with values disappeared, N in the throughfall was 1.5 times higher than that
ranging from 156-250 molc ha-1 yr-1 for NH4+, and 84-172 molc of the bulk deposition. This effect was apparent for both NH4+-N
ha-1 yr-1 for NO3-. Most of the variation in deposition could be and NO3--N. An adjacent beech forest gave similar results with
explained by region, although the deposition of NH4+ was also
strongly influenced by the amount of precipitation and altitude
(De Vries et al., 2003).
Several studies have shown that organic N deposition may con-
tribute substantially to total N deposition (Cornell et al., 1995;
2003; Cape et al., 2001; Neff et al., 2002; Ham & Tamiya, 2006).
According to Neff et al. (2002), approximately 23% of the N
deposited in Europe constitutes organic N. Most likely, this pro-
portion has not changed much during the 20th century (Cape et
al., 2001). The contribution of organic N to total N deposition
in Sweden seems to be similar to the values estimated for Eu-
rope in general. Measurements on 24 open field sites within the
SWETHRO network (Pihl-Karlsson et al., 2011) during October
2011 to September 2012 showed that organic N constituted on
15regard to the throughfall. However, in this stand, the edge effect
was not pronounced (Balsberg-Påhlsson & Bergkvist, 1995).
The filtering effect of tree canopies is generally obvious in all forest
types, although different species may have different filter capacity,
with coniferous forests usually being more effective in capturing
atmospheric particles than deciduous forests (Balsberg-Påhlsson
& Bergkvist, 1995). Bergkvist & Folkesson (1995) found that the
deposition of NH4+-N and NO3--N was 1.5 to 3 times higher in a
spruce stand than in two deciduous forest stands (with deposited
amounts to the two deciduous stands, consisting of one beech and
one birch stand, being quite similar). The factors determining the
filter capacity of canopies was suggested by Balsberg-Påhlsson &
Bergkvist (1995) to be total leaf area (particularly different when
deciduous trees are defoliated during winter) and leaf surface char-
acteristics together with the general structure of the trees.
3.3 N fertilization
Fertilization with N has been used as a measure to increase tree
growth since the mid 1960’s. In Sweden, around 2 million ha (or
10% of the productive forest land) has been subjected to N ferti-
lization on some occasion (Nohrstedt, 2001). When at its highest
peak, in the late 1970s, nearly 200 000 ha were fertilized annu-
ally. In the end of the 90’s, this number had decreased to 30 000
ha (Nohrstedt, 2001). Between approximately 1995 and 2005,
the area that was fertilized varied between 20 000 and 25 000 ha
(Swedish Forest Agency, 2007). In 2009, this number had increased
to 55 500 ha (Swedish Forest Agency, 2010).
Both urea, NH4NO3 and NH4NO3 amended with dolomite have
been used as N fertilizers in Sweden. Nowadays, the recommenda-
tion from the Swedish Forest Agency is to apply maximum 200 kg
N ha-1 of NH4NO3 amended with dolomite, with an intermediate
interval of at least 8 years (Swedish Forest Agency, 2007). To avoid
negative effects, the total amount that should be added during a
rotation period varies depending on the geographical location.
The recommendations for the different parts of the country can
be seen in Figure 2.
Previously, both experimental and commercial rates of fertilizer
addition have often exceeded the recommended amounts, and
application rates of several hundred kilos of N per hectare have
been reported in Sweden as well as in other countries (Aber et al.,
1989; Andersson et al., 1995; 1998; Tamm et al., 1999; Jacobson
& Pettersson, 2001). In many of these studies, fertilizer was applied
annually or with only a few years interval.
Figure 2. The Swedish Forest Agency’s recommendations for
N fertilization in Sweden. In the southernmost part of the The rates of atmospheric deposition of N are generally low com-
country (dark green and middle green), no fertilization is
pared with the rates of fertilizer application. Furthermore, ferti-
allowed (an exception is if whole-tree harvesting is applied in
the middle green area). In the middle and the northern part lizer applications are often one or a few-time applications, whereas
of the country (light green and yellow areas), fertilization atmospheric deposition is chronic. In this review, N deposition
is allowed but should not exceed 300 kg (light green) and and N fertilization are both considered as input processes, only
450 kg (yellow area) of N per hectare and forest generation. with different rates. We assume that the resulting effects of N on
For a more detailed description, see Swedish Forest Agency ecosystem processes depend on the concentrations and forms of
(2007). N rather than on the source of input.
164. NITROGEN IN PLANTS
4.1 N acquisition and uptake clear-cutting or intensive selective felling), soil temperatures in the
uppermost soil layer may approach the optimum for net N-uptake
Approximately 1.5 % of the dry weight of plants is accounted for (Rennenberg et al., 2009).
by atoms of N (Kimmins, 1997). Plants adsorb N in three distinct
forms: NO3-, NH4+ and amino acids. The rate of N uptake depends That soil temperatures affect NO3- and NH4+ uptake of trees has
on both the concentration in the environment and the demand of also been observed in the field (Geßler et al., 1998). For spruce
the plant, the latter being at least partly determined by the growth and beech growing in temperate forests, a seasonal pattern with
rate of the plant (Lambers et al., 1998). The majority of terrestrial maximum uptake rates of NH4+ in mid-summer and strongly re-
plants absorb N primarily via their roots from the soil (Lambers stricted uptake rates in spring and autumn when soil temperatures
et al., 1998). However, leaves are also capable of N uptake (Sutton were below approximately 12°C was found (Geßler et al., 1998).
et al., 1995).
According to Rennenberg et al. (2009), N uptake from the soil
It is well-known that plants have several different mechanisms to also depends on transpiration rate. It thus remains to be elucidated
enhance the availability of nutrients in the rhizosphere. With regard whether higher soil temperatures in spring increase early season
to N, mycorrhizal associations are regarded as one of the most N uptake of roots since transpiration is either low (evergreens), or
important (Smith & Read, 2008). Roots of boreal forest trees are not present (deciduous).
usually heavily colonized by ectotrophic fungi. In Norway spruce,
for example, more than 90% of the root apical zones are usually Temperature also seems to influence the proportions of NH4+
enclosed by a fungal sheath (Marschner, 2003), and microcosm and NO3- taken up by trees (Rennenberg et al., 2009). At lower
studies of EM fungi have shown that 20 to 30 % of the C assimi- soil temperatures (up to 10°C), NO3- uptake is minimal, whereas
lated by the host plant may be consumed by the fungal partner NH4+ uptake already reaches >50% of its temperature-dependent
(reviewed in Söderström, 2002). maximum in spruce and beech. With increasing soil temperature
(>10°C), the preference for NH4+ is generally maintained but the
relative proportion of NO3- increases (Geßler et al., 1998).
4.1.1 Effects of temperature
According to Lambers et al. (1998), low temperature directly re-
duces N uptake by plants, as expected for any physiological process 4.1.2 Effects of moisture
that is dependent on respiratory energy. Rennenberg et al. (2009) Reduced soil water availability and extended drought periods are
emphasized the temperature sensitivity of the high-affinity trans- known to reduce fine-root biomass (Cudlin et al., 2007; Rennen-
port system (HATS) prevalently used in forest ecosystems with low berg et al., 2009), thus decreasing the nutrient absorbing surface of
soil solution concentrations of N. For NO3--uptake, a consistent root systems. Both drought and excess water availability may also
increase of the HATS activity was observed when root temperature impair the mineral nutrition of trees by influencing: 1) the nutrient
increased in a range from approximately 5 to 25°C. The strong availability in the soil and 2) the physiology of the uptake systems
response of HATS to root and soil temperature may, according to of the mycorrhizal tree roots (Kreuzwieser & Geßler (2010). Ac-
Rennenberg et al. (2009), be explained by the enzymatic nature of cording to Rennenberg et al. (2009), not much is known about
the HATS (as proposed by Glass et al., 1990). Several other stud- the effect of increased drought on the physiology of N transport.
ies also suggest that nutrient uptake by trees increase with rising Most studies to date allow the characterization of the N status of
temperatures, at least until a certain treshold value (Bassirirad, the tree, but do not provide information as to whether changes in
2000). For spruce and beech, Geßler et al. (1998) found a maxi- the N status are due to changes in soil water availability or root
mum uptake of NH4+ at 20°C and 25°C, respectively, followed by N-uptake capacity, or both.
a decrease at higher temperatures. Maximum uptake for NO3- was
found to be at 25°C. According to Lukac et al. (2010), such high Applying 15N labelling, Geßler et al. (2005) assessed NO3- and
temperatures are not likely to be reached under field conditions, NH4+ uptake kinetics of non-excised intact mycorrhizal roots of
at least not in a closed canopy forest. Geßler et al. (1998; 2005) adult beech trees on two aspects of a site that differed in soil water
observed soil temperatures ranging from 10 to 16°C in the up- availability. At the drier aspect, the maximum rate of NO3- up-
per soil layers of temperate beech and spruce forests during the take (and thus uptake capacity) was reduced by more than 50%
growing season. At temperatures of 10 and 15°C, net uptake of as compared with the wetter aspect during most of the growing
NO3- by spruce and beech amounted to around 16% and 11% of season. As differences in NO3- availability, rooting patterns and
maximum uptake at 25°C. At 10°C, net uptake of NH4+ reached in the affinity of the transport system were ruled out as respon-
73% and 31% of maximum uptake for spruce and beech (Geßler et sible for the difference, a long-term effect on NO3- transport of
al., 1998). However, in open canopies (such as after storm events, continued water depletion was assumed. Whether differences in
17NO3- transporter abundance/activity, the expression of different between trees subjected to ambient and elevated CO2 (Finzi et al.,
NO3- transporters and/or mycorrhizal colonization are responsible 2007). Johnson (2006), reviewing the implications for long-term
for the drought-driven reduction remains to be clarified (Ren- responses of forests to elevated CO2, concluded that, more often
nenberg et al., 2009). than not, N uptake is increased with elevated CO2 and the increase
is a function of increased root growth, since most studies to date
Shi et al. (2002) observed large changes in the species composition have shown little effect, or none at all, of elevated CO2 on soil
of fungal partners in beech mycorrhiza as a consequence of drought N-mineralization rates (see section 5.1.3).
and also an accumulation of stress-related metabolites in the myce-
lia, indicating that mycorrhizal fungi may exert a strong influence However, the relationship between elevated CO2 and N uptake
on the drought reaction of plants. The drought-sensitivity of N is not completely straightforward (see for example review by
acquisition in beech was also emphasized by Geßler et al. (2004) Bassirirad, 2000). Calfapietra et al. (2007), investigating the ef-
and Geßler et al. (2002) found that net uptake of mycorrhizal fect of elevated CO2 on a short-rotation poplar plantation, found
roots of beech was closely related to the transpiration rate of trees. no change in cumulative N uptake by trees over the rotation as
a consequence of elevated CO2, despite a substantial increase in
biomass production. This resulted in a significant increase in NUE
4.1.3 Effects of CO2 and a decrease in the concentration of N in most tissues. Several
Increased levels of CO2 generally result in an increased allocation examples of increases in NUE are also given in the review by Stitt
of C to below-ground, with subsequent increases in the growth & Krapp (1999). The authors suggested that the decreased N con-
of roots and mycorrhiza (see Appendix A, section 4.3.3 ), which centrations observed under elevated CO2 indicate that N uptake
should allow the trees to exploit a larger soil volume and thus might and assimilation often fail to keep pace with photosynthesis and
be expected to promote NO3- and NH4+ uptake (Stitt & Krapp, growth under elevated CO2. However, they emphasized that there
1999). In several of the FACE experiments, the increase in biomass is no hinderance for elevated CO2 to lead to increased N up-
production was indeed accompanied by an increase in N uptake by take, provided there is an adequate supply of N outside the plant.
trees, resulting in a negligible variation in N-use efficiency (NUE) Bassirirad (2000), on the other hand, suggested that differences in
18the experimental protocols as well as differences in species-specific With regard to the influence of WTH on N content in litter and
responses were the major determinants behind the variable results. forest soils, see sections 4.5.5 and 5.1.5.
Alberton et al. (2007) found that mycorrhizal growth (extraradi-
cal hyphal length) under elevated CO2 was negatively correlated 4.1.6 Effects of SH
with shoot N-content and total plant N-uptake. According to Using a similar method as above (stable isotope technique; Weath-
Lukac et al. (2010), one explanation for this seemingly contra- erall et al., 2006a), Weatherall et al. (2006b) traced nutrient release
dictory observation of N uptake may be the observed increase from decomposing roots and the subsequent uptake into newly
in fungal abundance and proliferation and the increase in fungi/ planted Sitka spruce seedlings. They found that decomposing roots
bacteria ratio in soils under elevated CO2 (Treseder, 2004; Hu et contributed up to 3-10% of the N subsequently taken up by new
al., 2006; Parrent et al., 2006; Carney et al., 2007). Fungi have trees after one single growth season. The percentage contribution
higher C/N ratios than soil bacteria, thus using smaller amounts from decomposing roots to new tree growth was markedly higher
of N to produce equivalent amounts of biomass. Furthermore, in a nutrient-poor soil than in a more nutrient-rich soil.
they translocate C and N within the fungal mycelium (Boberg
et al., 2010), something that according to Lukac et al. (2010) With regard to the influence of SH on N content in litter and
may explain the low mineralization rates and, hence, the lower N forest soils, see sections 4.5.6 and 5.1.6.
availability in fungal dominated ecosystems. Lukac et al. (2010)
concluded that the positive effects of elevated CO2 on infection
rates of mycorrhiza (Hu et al., 2006), mycorrhizal activity (Lukac 4.2 N content of trees, allocation,
et al., 2009) and turnover (Godbold et al., 2006) may enhance tree retranslocation and storage
nutrition in the future, but only if mycorrhizal fungi proliferate at
the expense of bacteria or other functional types. According to Lukac et al. (2010), total N uptake and N allocation
of trees are two of the main factors controlling foliar N concen-
In one of the few studies examining the impact of elevated CO2 on tration. In theory, trees optimize their N allocation for attaining
organic N uptake, no effect of elevated CO2 was reported (Hof- maximum growth, thus allocating available N to organs with great-
mockel et al., 2007). est benefit for net growth. It is well-established that trees store
N, and that the remobilization of stored resources can provide
an appreciable proportion of the annual N requirement used for
4.1.4 Effects of N above-ground growth each year (Millard et al., 2007). Deciduous
While increasing N supply usually enhances both shoot and root plantation trees of oak and larch in Wisconsin, USA, were found
growth, shoot growth is generally more stimulated than the root to have retranslocation values for N of around 80%, supplying on
growth, leading to a fall in root-shoot dry weight ratio with sub- average 77% of the above-ground annual N requirement (Son &
sequent implications for nutrient uptake in the long term (see Gower, 1991). Evergreen plantation trees in the same study retrans-
appendix A, section 4.3.4). However, the effectiveness of N inputs located between 13 and 55% of their N from senescing foliage,
seems to be limited by immobilization and other mechanisms. For supplying on average 27% of the above-ground annual N require-
example, when labelled N was added to soil and litter in a forest ment (Son & Gower, 1991). For Scots pine in Scandinavia, values
over seven years, only a small fraction became available for tree have been found to be even higher, approaching 70 and 80% in
growth (Nadelhoffer et al., 2004). some studies (Helmisaari, 1992a; Nieminen & Helmisaari, 1996),
thus supplying between 30 and 50%s of the N required for annual
Atmospheric deposition of N to the surface of leaves may reduce biomass production (Helmisaari, 1992b). Norway spruce seem to
N uptake from the soil. The flux of N from soil into the roots has have lower retranslocation efficiencies than pine (Bothwell et al.,
been shown to be down-regulated to an extent that equals the N 2001), with values found in the literature ranging from 13 to 36%
influx into the leaves (Rennenberg & Geßler, 1999). (Son & Gower, 1991; Bothwell et al., 2001). The information with
regard to the effects of climate change and forest management on
storage and retranslocation of N in trees is generally sparse. Most
4.1.5 Effects of WTH of the information below thus refers to the effects of climate change
There is basically no scientifically published data on how removal of and forest management on N concentrations in trees.
slash may affect N uptake and acquisition of trees in the subsequent
generation. One exception is the study by Weatherall et al. (2006a).
By labelling growing Sitka spruce (Picea sitchensis (Bong.) Carr.) 4.2.1 Effects of temperature
seedlings with 15N, then harvesting the above-ground biomass According to a review by Pendall et al. (2004), investigating the
and placing it on soil in a pot containing newly planted seedlings, responses of below-ground processes to elevated temperatures and
they showed that N in brash can make a small, but discernible, CO2, elevated temperatures are generally reported to increase the
direct contribution to new tree growth one growing season after root N-concentration, probably because mineralization and diffu-
the application of the brash. The authors emphasized, however, sion of N are stimulated at high temperatures. In coherence with
that the contribution is likely to vary considerably depending on the results of Pendall et al. (2004), Tingey et al. (2003) found
site conditions, and that field studies are necessary before conclu- increased leaf N concentrations in Douglas fir as a consequence
sions can be drawn about the effects of slash removal on N uptake. of elevated temperature. However, Lukac et al. (2010) suggested
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