Conditions Under Which Nitrogen Can Limit Steady-State Net Primary Production in a General Class of Ecosystem Models

Human activity is drastically altering global nitrogen (N) availability. The extent to which ecosystems absorb additional N—and with it, additional CO₂—depends on whether net primary production (NPP) is N-limited, so it is important to understand conditions under which N can limit NPP. Here I use a general dynamical model to show that N limitation at steady-state—such as in old-growth forests—depends on the balance of biotically controllable versus uncontrollable N inputs and losses. Steady-state N limitation is only possible when uncontrollable inputs (for example, atmospheric deposition) exceed controllable losses (for example, leaching of plant-available soil N), which is the same as when uncontrollable losses (for example, leaching of plant-unavailable soil N) exceed controllable inputs (biological N fixation). These basic results are robust to many model details, such as the number of plant-unavailable soil N pools and the number and type of N fixers. Empirical data from old-growth tropical (Hawai’i) and temperate (Oregon, Washington, Chile) forests support the model insights. Practically, this means that any N fixer—symbiotic or not—could overcome ecosystem N limitation, so understanding N limitation requires understanding controls on all N fixers. Further, comparing losses of plant-available N to abiotic inputs could offer a rapid diagnosis of whether ecosystems can be N-limited, although the applicability of this result is constrained to ecosystems with a steady-state N cycle such as old-growth forests largely devoid of disturbance.     

Organic matter and nitrogen accumulation and nitrogen fixation during early ecosystem development in Hawaii

We used a chronosequence comprised of 10 y, 52 y and 142 yold `a'a lava flows on Mauna Loa, Hawaii, to determine theaccumulation of organic matter and nitrogen and rates of nitrogenfixation through time. The mass of organic matter (live and deadbiomass and soil organic matter) on the 1984, 1942 and 1852 lavaflows was 0.6, 2.2 and 7.6 kg m^{– 2}, respectively, while total N was 4.8, 10.9 and 85.7 g m^{– 2}.We estimated the total rates of nitrogen fixation for thethree different aged ecosystems using an acetylene reduction assaycalibrated with ¹⁵N incubations. While mean rates of total N fixation remained largely constant across the three sites – between2.0 and 3.1 kg ha^{– 1} y^{– 1} – the most important sources of N fixation changed. On the 10 y flow, the most important fixer was the pioneering cyanolichen, Stereocaulon vulcani. After 52 years ofecosystem development, the most important N fixer was a cyanoalga,while after 142 years, the predominant N fixers were heterotrophicbacteria associated with leaf litter, twigs and detritus. The totalamount of N accumulated after 52 years of ecosystem development wasequivalent to cumulative inputs through biological N fixation. After 142 years, however, cumulative inputs from N fixation couldonly account for between 27–59% of the total nitrogen accrued.We used fertilizer additions of all essential nutrients otherthan N to test whether the availability of lithophilic nutrientsregulated rates of N fixation in early ecosystem development. Ratesof nitrogen fixation by the lichen, S. vulcani, approximately doubled when fertilized on the 1984 and 1942 flows. Rates of N-fixation by heterotrophic nitrogen fixing bacteria on leaf litter ofMetrosideros polymorpha also increased significantly when fertilized with lithophilic nutrients. These findings suggest that weathering rates of lava in part regulate rates of nitrogen fixation in these young ecosystems.     

Nitrogen limitation on land and in the sea: How can it occur?

The widespread occurrence of nitrogen limitation to net primary production in terrestrial and marine ecosystems is something of a puzzle; it would seem that nitrogen fixers should have a substantial competitive advantage wherever nitrogen is limiting, and that their activity in turn should reverse limitation. Nevertheless, there is substantial evidence that nitrogen limits net primary production much of the time in most terrestrial biomes and many marine ecosystems. We examine both how the biogeochemistry of the nitrogen cycle could cause limitation to develop, and how nitrogen limitation could persist as a consequence of processes that prevent or reduce nitrogen fixation. Biogeochemical mechansism that favor nitrogen limitation include:

the substantial mobility of nitrogen across ecosystem boundaries, which favors nitogen limitation in the “source” ecosystem — especially where denitrification is important in sediments and soils, or in terrestrial ecosystems where fire is frequent;

differences in the biochemistry of nitrogen as opposed to phosphorus (with detrital N mostly carbon-bonded and detrital P mostly ester-bonded), which favor the development of nitrogen limitation where decomposition is slow, and allow the development of a positive feedback from nitrogen limitation to producers, to reduced decomposition of their detritus, and on to reduced nitrogen availability; and

other more specialized, but perhaps no less important, processes.

A number of mechanisms could keep nitrogen fixation from reversing nitrogen limitation. These include:

energetic constraints on the colonization or activity of nitrogen fixers;

limitation of nitrogen fixers or fixation by another nutrient (phosphorus, molybdenum, or iron) — which would then represent the ultimate factor limiting net primary production;

other physical and ecological mechanisms.

The possible importance of these and other processes is discussed for a wide range of terrestrial, freshwater, and marine ecosystems.     

Nitrogen Transfer from Four Nitrogen-Fixer Associations to Plants and Soils

Nitrogen (N) fixation is the main source of ‘new’ N for N-limited ecosystems like subarctic and arctic tundra. This crucial ecosystem function is performed by a wide range of N₂ fixer (diazotroph) associations that could differ fundamentally in their timing and amount of N release to the soil. To assess the importance of different associative N₂ fixers for ecosystem N cycling, we tracked ¹⁵N-N₂ into four N₂-fixer associations (with a legume, lichen, free-living, moss) and into soil, microbial biomass and non-diazotroph-associated plants 3 days and 5 weeks after in situ labelling. In addition, we tracked ¹³C from ¹³CO₂ labelling to assess if N and C fixation are linked. Three days after labelling, half of the fixed ¹⁵N was recovered in the legume soils, indicating a fast release of fixed N₂. Within 5 weeks, the free-living N₂ fixers released two-thirds of the fixed ¹⁵N into the soil, whereas the lichen and moss retained the fixed ¹⁵N. Carbon and N₂ fixation were linked in the lichen shortly after labelling, in free-living N₂ fixers 5 weeks after labelling, and in the moss at both sampling times. The four investigated N₂-fixer associations released fixed N₂ at different rates into the soil, and non-diazotroph-associated plants have no access to ‘new’ N within several weeks after N₂ fixation. Although legumes and free-living N₂ fixers are immediate sources of ‘new’ N for N-limited tundra ecosystems, lichens and especially mosses, do not contribute to increase the N pool via N₂ fixation in the short term.     

Interactive effects of N:P ratios and light on nitrogen‐fixer abundance

Nitrogen‐fixers can contribute significant amounts of nitrogen (N) and impact ecosystem functioning in diverse aquatic and terrestrial ecosystems. What determines N‐fixer abundance still remains poorly understood. Here we experimentally investigate major environmental controls on the abundance of N‐fixers: nitrogen to phosphorus (N:P) ratio and light. We grew a N‐fixer, cyanobacterium Anabaena flos‐aquae, in a multispecies community of freshwater phytoplankton in replicated factorial design treatments with two N:P ratios and two light levels. We show that low N:P ratios promote the dominance of the N‐fixer in the community, but only under high light. Under low light, N:P ratio did not have a significant effect on the abundance of the N‐fixer. N fixation occurred at low N:P only and increased with increasing light. In contrast, the density of non N‐fixing cyanobacteria did not depend on N:P ratios. Green algae dominated under high N:P and high light only, exhibiting the opposite pattern of dominance to N‐fixers. These results are consistent with patterns observed in nature and help explain the N‐fixer distribution along the environmental gradients of nutrients and light.     

Nitrogen fixation does not axiomatically lead to phosphorus limitation in aquatic ecosystems

A long‐standing debate in ecology deals with the role of nitrogen and phosphorus in management and restoration of aquatic ecosystems. It has been argued that nutrient reduction strategies to combat blooms of phytoplankton or floating plants should solely focus on phosphorus (P). The underlying argument is that reducing nitrogen (N) inputs is ineffective because N₂‐fixing species will compensate for N deficits, thus perpetuating P limitation of primary production. A mechanistic understanding of this principle is, however, incomplete. Here, we use resource competition theory, a complex dynamic ecosystem model and a 32‐year field data set on eutrophic, floating‐plant dominated ecosystems to show that the growth of non‐N₂‐fixing species can become N limited under high P and low N inputs, even in the presence of N₂ fixing species. N₂‐fixers typically require higher P concentrations than non‐N₂‐fixers to persist. Hence, the N₂ fixers cannot deplete the P concentration enough for the non‐N₂‐fixing community to become P limited because they would be outcompeted. These findings provide a testable mechanistic basis for the need to consider the reduction of both N and P inputs to most effectively restore nutrient over‐enriched aquatic ecosystems.     

Nitrogen fixation and the mass balances of carbon and nitrogen in ecosystems

Ecosystems with high rates of nitrogen fixation often have high loss rates through leaching or possibly denitrification. However, there is no formal theoretical context to examine why this should be the case nor of how nitrogen accumulates in such open systems. Here, we propose a simple model coupling nitrogen inputs and losses to carbon inputs and losses. The nitrogen balance of this model system depends on plant (nitrogen fixer) growth rate, its carrying capacity, N fixed/C fixed, residence time of nitrogen and carbon in biomass, litter decay rate, litter N/C, and fractional loss rate of mineralized nitrogen. The model predicts the requirements for equilibrium in a nitrogen-fixing system, and the conditions on nitrogen fixation and losses in order for the system to accumulate nitrogen and carbon. In particular, the accumulation of nitrogen and carbon in a nitrogen-fixing system depend on an interaction between residence time in vegetation and litter decay rate in soil. To reflect a possible increased uptake of soil nitrogen and decreased respiratory cost of symbiotic nitrogen fixers, the model was then modified so that fixation rate decreased and growth rate increased as nitrogen capital accumulated. These modifications had only small effects on carbon and nitrogen accumulation. This suggests that switching from uptake of atmospheric nitrogen to mineral soil nitrogen as nitrogen capital accumulates simply results in a trade-off between energetic limitations and soil nitrogen limitations to carbon and nitrogen accumulation. Experimental tests of the model are suggested.     

The presence of nitrogen fixing legumes in terrestrial communities: Evolutionary vs ecological considerations

Nitrogen is often a limiting factor to net primary productivity (NPP) and other processes in terrestrial ecosystems. In most temperate freshwater ecosystems, when nitrogen becomes limiting to NPP, populations of N-fixing cyanobacteria experience a competitive advantage, and begin to grow and fix nitrogen until the next most limiting resource is encountered; typically phosphorus or light. Why is it that N-fixing plants do not generally function to overcome N limitation in terrestrial ecosystems in the same way that cyanobacteria function in aquatic ecosystems? To address this question in a particular ecosystem, one must first know whether the flora includes a potential set of nitrogen fixers. I suggest that the presence or absence of N-fixing plant symbioses is foremost an evolutionary consideration, determined to a large extent by constraints on the geographical radiation of woody members of the family Fabaceae. Ecological factors such as competition, nutrient deficiencies, grazing and fire are useful to explain the success of N-fixing plants only when considered against the geographical distribution of potential N-fixers.     

DIAZOTROPHIC GROWTH OF THE MARINE CYANOBACTERIUM TRICHODESMIUM IMS101 IN CONTINUOUS CULTURE: EFFECTS OF GROWTH RATE ON N2‐FIXATION RATE,BIOMASS, AND C:N:P STOICHIOMETRY1

Trichodesmium N₂ fixation has been studied for decades in situ and, recently, in controlled laboratory conditions; yet N₂‐fixation rate estimates still vary widely. This variance has made it difficult to accurately estimate the input of new nitrogen (N) by Trichodesmium to the oligotrophic gyres of the world ocean. Field and culture studies demonstrate that trace metal limitation, phosphate availability, the preferential uptake of combined N, light intensity, and temperature may all affect N₂ fixation, but the interactions between growth rate and N₂ fixation have not been well characterized in this marine diazotroph. To determine the effects of growth rate on N₂ fixation, we established phosphorus (P)–limited continuous cultures of Trichodesmium, which we maintained at nine steady‐state growth rates ranging from 0.27 to 0.67 d^?1. As growth rate increased, biomass (measured as particulate N) decreased, and N₂‐fixation rate increased linearly. The carbon to nitrogen ratio (C:N) varied from 5.5 to 6.2, with a mean of 5.8 ± 0.2 (mean ± SD, N = 9), and decreased significantly with growth rate. The N:P ratio varied from 23.4 to 45.9, with a mean of 30.5 ± 6.6 (mean ± SD, N = 9), and remained relatively constant over the range of growth rates studied. Relative constancy of C:N:P ratios suggests a tight coupling between the uptake of these three macronutrients and steady‐state growth across the range of growth rates. Our work demonstrates that growth rate must be considered when planning studies of the effects of environmental factors on N₂ fixation and when modeling the impact of Trichodesmium as a source of new N to oligotrophic regions of the ocean.     

Contrasting Climatic Controls on the Estimated Productivity of Global Terrestrial Biomes

Net primary productivity (NPP) represents the greatest annual carbon flux from the atmosphere to the biosphere, is an important component of seasonal fluctuations in atmospheric CO₂ concentrations, and is the most critical biotic component of the global carbon cycle. NPP measures products of major economic and social importance, such as crop yield and forest production. Given that global NPP can not be measured directly, model simulations must provide understanding of its global spatial and temporal dynamics. In this study, we used the biogeochemical model BIOME-BGC to simulate global terrestrial NPP and assessed relative importance of climatic controls (temperature, water availability, and radiation) in limiting NPP in the array of climatic combinations found globally. The degree of limitation on NPP by climatic controls was defined by using an empirical membership function. Results showed that temperature or water availability limited NPP over larger land areas (31% and 52%, respectively) than did radiation limitation (5%). Climatic controls appeared to be important in limiting productivity in most vegetation biomes, except for evergreen broadleaf forests. Nevertheless, there were areas of the globe (12%) where none of the climatic factors appeared to limit NPP. Our research has suggested that other environmental controls, such as nutrient availability or biological constraints, should then be considered. The wide distribution of NPP between zero and the upper boundary values in the correlation plots indicated that multivariate environmental balances, not single limiting factors, controlled biospheric productivity. Received 27 August 1997; accepted 19 November 1997.     

Improving nitrogen-fixing systems and integrating them into sustainable rice farming

This paper summarizes recent achievements in exploiting new biological nitrogen fixation (BNF) systems in rice fields, improving their management, and integrating them into rice farming systems. The inoculation of cyanobacteria has been long recommended, but its effect is erratic and unpredictable. Azolla has a long history of use as a green manure, but a number of biological constraints limited its use in tropical Asia. To overcome these constraints, the Azolla-Anabaena system as well as the growing methods were improved. Hybrids between A. microphylla and A. filiculoides (male) produced higher annual biomass than either parent. When Anabaena from high temperature-tolerant A. microphylla was transferred to Anabaena-free A. filiculoides, A. filiculoides became tolerant of high temperature. Azolla can have multiple purposes in addition to being a N source. An integrated Azolla-fish-rice system developed in Fujian, China, could increase farmers' income, reduce expenses, and increase ecological stability. A study using Azolla labeled with ¹⁵N showed the reduction of N losses by fish uptake of N. The Azolla mat could also reduce losses of urea N by lowering floodwater-pH and storing a part of applied N in Azolla. Agronomically useful aquatic legumes have been explored within Sesbania and Aeschynomene. S. rostrata can accumulate more than 100kg N ha^-1 in 45 d. Its N₂ fixation by stem nodules is more tolerant of mineral N than that by root nodules, but the flowering of S. rostrata is sensitive to photoperiod. Aquatic legumes can be used in rainfed rice fields as N scavengers and N₂ fixers. The general principle of integrated uses of BNF in rice-farming systems is shown.     

Modeling Productivity in Mangrove Forests as Impacted by Effective Soil Water Availability and Its Sensitivity to Climate Change Using Biome-BGC

Ecosystem dynamics and the responses to climate change in mangrove forests are poorly understood. We applied the biogeochemical process model Biome-BGC to simulate the dynamics of net primary productivity (NPP) and leaf area index (LAI) under the present and future climate conditions in mangrove forests in Shenzhen, Zhanjiang, and Qiongshan across the southern coast of China, and in three monocultural mangrove stands of two native species, Avicennia marina and Kandelia obovata, and one exotic species, Sonneratia apetala, in Shenzhen. The soil hydrological process of the model was modified by incorporating a soil water (SW) stress index to account for the impact of the effective SW availability in the coastal wetland. Our modified Biome-BGC well predicted the dynamics of NPP and LAI in the mangrove forests at the study sites. We found that the six mangrove systems differed in sensitivity to variations in the effective SW availability. At the ecosystem level, however, soil salinity alone could not entirely explain the limitation of the effective SW availability on the productivity of mangrove forests. Increasing atmospheric CO₂ concentration differentially affected growth of different mangrove species but only had a small impact on NPP (<7%); whereas a doubling of atmospheric CO₂ concentration associated with a 2°C temperature rise would increase NPP by 14–19% across the three geographically separate mangrove forests and by 12% to as much as 68% across the three monocultural mangrove stands. Our simulation analysis indicates that temperature change is more important than increasing CO₂ concentration in affecting productivity of mangroves at the ecosystem level, and that different mangrove species differ in sensitivity to increases in temperature and CO₂ concentration.     

Do top-down and bottom-up controls interact to exclude nitrogen-fixing cyanobacteria from the plankton of estuaries? An exploration with a simulation model

Explaining the nearly ubiquitous absence of nitrogen fixation by planktonic organisms in strongly nitrogen-limited estuaries presents a major challenge to aquatic ecologists. In freshwater lakes of moderate productivity, nitrogen limitation is seldom maintained for long since heterocystic, nitrogen-fixing cyanobacteria bloom, fix nitrogen, and alleviate the nitrogen limitation. In marked contrast to lakes, this behavior occurs in only a few estuaries worldwide. Primary production is limited by nitrogen in most temperate estuaries, yet no measurable planktonic nitrogen fixation occurs. In this paper, we present the hypothesis that the absence of planktonic nitrogen fixers from most estuaries is due to an interaction of bottom-up and top-down controls. The availability of Mo, a trace metal required for nitrogen fixation, is lower in estuaries than in freshwater lakes. This is not an absolute physiological constraint against the occurrence of nitrogen-fixing organisms, but the lower Mo availability may slow the growth rate of these organisms. The slower growth rate makes nitrogen-fixing cyanobacteria in estuaries more sensitive to mortality from grazing by zooplankton and benthic organisms.We use a simple, mechanistically based simulation model to explore this hypothesis. The model correctly predicts the timing of the formation of heterocystic, cyanobacterial blooms in freshwater lakes and the magnitude of the rate of nitrogen fixation. The model also correctly predicts that high zooplankton biomasses in freshwaters can partially suppress blooms of nitrogen-fixing cyanobacteria, even in strongly nitrogen-limited lakes. Further, the model indicates that a relatively small and environmentally realistic decrease in Mo availability, such as that which may occur in seawater compared to freshwaters due to sulfate inhibition of Mo assimilation, can suppress blooms of heterocystic cyanobacteria and prevent planktonic nitrogen fixation. For example, the model predicts that at a zooplankton biomass of 0.2 mg l^–1, cyanobacteria will bloom and fix nitrogen in lakes but not in estuaries of full-strength seawater salinity because of the lower Mo availability. Thus, the model provides strong support for our hypothesis that bottom-up and top-down controls may interact to cause the absence of planktonic nitrogen fixation in most estuaries. The model also provides a basis for further exploration of this hypothesis in individual estuarine systems and correctly predicts that planktonic nitrogen fixation can occur in low salinity estuaries, such as the Baltic Sea, where Mo availability is greater than in higher salinity estuaries.     

Different nitrogen saturation thresholds for above-, below-, and total net primary productivity in a temperate steppe

Identifying the thresholds for the positive responses of total net primary productivity (NPP) to nitrogen (N) enrichment is an essential prerequisite for predicting the benefits of N deposition on ecosystem carbon sequestration. However, the responses of below-ground NPP (BNPP) to N enrichment are unknown in many ecosystems, which limits our ability to understand the carbon cycling under the scenario of increasing N availability. We examined the changes in above-ground NPP (ANPP), BNPP, and NPP of a temperate meadow steppe across a wide-ranging N addition gradient (0, 2, 5, 10, 20, and 50 g N m⁻² year⁻¹) during 5 years. Both ANPP and NPP increased nonlinearly with N addition rates. The N saturation threshold for ANPP (T_A) and NPP (T_N) was at the rate of 13.11 and 6.70 g N m⁻² year⁻¹, respectively. BNPP decreased with increasing N addition when N addition rates ˃5 g N m⁻² year⁻¹, resulting in much lower T_N than T_A. Soil N enrichment played a key role in driving the negative impacts of high N addition rates on BNPP, and consequently on the earlier occurrence of N saturation threshold for NPP. Our results highlight the negative effects of soil N enrichment on NPP in natural grasslands super-saturated with N. Furthermore, by considering ANPP and BNPP simultaneously, our results indicate that previous findings from above-ground might have over-estimated the positive effects of N deposition on primary productivity.     

Comparing global models of terrestrial net primary productivity (NPP): the importance of water availability

Given that neither absolute measures nor direct model validations of global terrestrial net primary productivity (NPP) are feasible, intercomparison of global NPP models provides an effective tool to check model consistency. For this study, we tested the assumption that water availability is the primary limiting factor of NPP in global terrestrial biospheric models. We compared a water balance coefficient (WBC), calculated as the difference of mean annual precipitation and potential evapotranspiration to NPP for each grid cell (0.5° × 0.5° longitude/latitude) in each of 14 models. We also evaluated different approaches used for introducing water budget limitations on NPP: (1) direct physiological control on evapotranspiration through canopy conductance; (2) climatological computation of constraints from supply/demand for ecosystem productivity; and (3) water limitation inferred from satellite data alone. Plots of NPP vs. WBC showed comparable patterns for the models using the same method for water balance limitation on NPP. While correlation plots revealed similar patterns for most global models, other environmental controls on NPP introduced substantial variability.     

Nitrogen to phosphorus ratios of tree species in response to elevated carbon dioxide and nitrogen addition in subtropical forests

Increased atmospheric carbon dioxide (CO₂) concentrations and nitrogen (N) deposition induced by human activities have greatly influenced the stoichiometry of N and phosphorus (P). We used model forest ecosystems in open‐top chambers to study the effects of elevated CO₂ (ca. 700 μmol mol^?1) alone and together with N addition (100 kg N ha^?1 yr^?1) on N to P (N : P) ratios in leaves, stems and roots of five tree species, including four non‐N₂ fixers and one N₂ fixer, in subtropical China from 2006 to 2009. Elevated CO₂ decreased or had no effects on N : P ratios in plant tissues of tree species. N addition, especially under elevated CO₂, lowered N : P ratios in the N₂ fixer, and this effect was significant in the stems and the roots. However, only one species of the non‐N₂ fixers showed significantly lower N : P ratios under N addition in 2009, and the others were not affected by N addition. The reductions of N : P ratios in response to elevated CO₂ and N addition were mainly associated with the increases in P concentrations. Our results imply that elevated CO₂ and N addition could facilitate tree species to mitigate P limitation by more strongly influencing P dynamics than N in the subtropical forests.     

Interactions of ecosystem processes with spatial heterogeneity in the puzzle of nitrogen limitation

We examined the potential effects of spatial heterogeneity and its development on the distribution, abundance, and functioning of nitrogen fixing and non-fixing components of a model ecosystem. CAECO, a spatially explicit model individual based approach, simulated the interactions between nitrogen fluxes and plant species community dynamics. Self-organized spatial patterns of nitrogen concentrations and plant occupancy were observed as the system approached an apparently meta-stable state. Nitrogen limitation was tested using chronic and gradient nitrogen amendments to the landscape. The dynamic arrangement of ecosystem components was sufficient to maintain indefinite nitrogen limitation at a local scale. However, landscape scale productivity was not similarly increased with nitrogen amendments. Landscape productivity was independent of nitrogen additions while fixers were present in the ecosystem. The probability of fixer loss from the system responded non-linearly to increasing nitrogen addition. The results of these model experiments suggest local and landscape constraints of primary productivity may be fundamentally distinct.     

Net Primary Production and Canopy Nitrogen in a Temperate Forest Landscape: An Analysis Using Imaging Spectroscopy, Modeling and Field Data

Understanding spatial patterns of net primary production (NPP) is central to the study of terrestrial ecosystems, but efforts are frequently hampered by a lack of spatial information regarding factors such as nitrogen availability and site history. Here, we examined the degree to which canopy nitrogen can serve as an indicator of patterns of NPP at the Bartlett Experimental Forest in New Hampshire by linking canopy nitrogen estimates from two high spectral resolution remote sensing instruments with field measurements and an ecosystem model. Predicted NPP across the study area ranged from less than 700 g m⁻² year⁻¹ to greater than 1300 g m⁻² year⁻¹ with a mean of 951 g m⁻² year⁻¹. Spatial patterns corresponded with elevation, species composition and historical forest management, all of which were reflected in patterns of canopy nitrogen. The relationship between production and elevation was nonlinear, with an increase from low- to mid-elevation deciduous stands, followed by a decline in upper-elevation areas dominated by evergreens. This pattern was also evident in field measurements and mirrored an elevational trend in foliar N concentrations. The increase in production from low-to mid-elevation deciduous stands runs counter to the generally accepted pattern for the northeastern U.S. region, and suggests an importance of moisture limitations in lower-elevation forests. Field measurements of foliar N, wood production and leaf litterfall were also used to evaluate sources of error in model estimates and to determine how predictions are affected by different methods of acquiring foliar N input data. The accuracy of predictions generated from remotely sensed foliar N approached that of predictions driven by field-measured foliar N. Predictions based on the more common approach of using aggregated foliar N for individual cover types showed reasonable agreement in terms of the overall mean, but were in poor agreement on a plot-by-plot basis. Collectively, these results suggest that variation in foliar N exerts an important control on landscape-level spatial patterns and can serve as an integrator of other underlying factors that influence forest growth rates.     

Ecosystem nitrogen fixation throughout the snow‐free period in subarctic tundra: effects of willow and birch litter addition and warming

Nitrogen (N) fixation in moss‐associated cyanobacteria is one of the main sources of available N for N‐limited ecosystems such as subarctic tundra. Yet, N₂ fixation in mosses is strongly influenced by soil moisture and temperature. Thus, temporal scaling up of low‐frequency in situ measurements to several weeks, months or even the entire growing season without taking into account changes in abiotic conditions cannot capture the variation in moss‐associated N₂ fixation. We therefore aimed to estimate moss‐associated N₂ fixation throughout the snow‐free period in subarctic tundra in field experiments simulating climate change: willow (Salix myrsinifolia) and birch (Betula pubescens spp. tortuosa) litter addition, and warming. To achieve this, we established relationships between measured in situ N₂ fixation rates and soil moisture and soil temperature and used high‐resolution measurements of soil moisture and soil temperature (hourly from May to October) to model N₂ fixation. The modelled N₂ fixation rates were highest in the warmed (2.8 ± 0.3 kg N ha^?1) and birch litter addition plots (2.8 ± 0.2 kg N ha^?1), and lowest in the plots receiving willow litter (1.6 ± 0.2 kg N ha^?1). The control plots had intermediate rates (2.2 ± 0.2 kg N ha^?1). Further, N₂ fixation was highest during the summer in the warmed plots, but was lowest in the litter addition plots during the same period. The temperature and moisture dependence of N₂ fixation was different between the climate change treatments, indicating a shift in the N₂ fixer community. Our findings, using a combined empirical and modelling approach, suggest that a longer snow‐free period and increased temperatures in a future climate will likely lead to higher N₂ fixation rates in mosses. Yet, the consequences of increased litter fall on moss‐associated N₂ fixation due to shrub expansion in the Arctic will depend on the shrub species’ litter traits.     

Global patterns of nitrogen limitation: confronting two global biogeochemical models with observations

Projections of future changes in land carbon (C) storage using biogeochemical models depend on accurately modeling the interactions between the C and nitrogen (N) cycles. Here, we present a framework for analyzing N limitation in global biogeochemical models to explore how C‐N interactions of current models compare to field observations, identify the processes causing model divergence, and identify future observation and experiment needs. We used a set of N‐fertilization simulations from two global biogeochemical models (CLM‐CN and O‐CN) that use different approaches to modeling C‐N interactions. On the global scale, net primary productivity (NPP) in the CLM‐CN model was substantially more responsive to N fertilization than in the O‐CN model. The most striking difference between the two models occurred for humid tropical forests, where the CLM‐CN simulated a 62% increase in NPP at high N addition levels (30 g N m^?2 yr^?1), while the O‐CN predicted a 2% decrease in NPP due to N fertilization increasing plant respiration more than photosynthesis. Across 35 temperate and boreal forest sites with field N‐fertilization experiments, we show that the CLM‐CN simulated a 46% increase in aboveground NPP in response to N, which exceeded the observed increase of 25%. In contrast, the O‐CN only simulated a 6% increase in aboveground NPP at the N‐fertilization sites. Despite the small response of NPP to N fertilization, the O‐CN model accurately simulated ecosystem retention of N and the fate of added N to vegetation when compared to empirical ¹⁵N tracer application studies. In contrast, the CLM‐CN predicted lower total ecosystem N retention and partitioned more losses to volatilization than estimated from observed N budgets of small catchments. These results point to the need for model improvements in both models in order to enhance the accuracy with which global C‐N cycle feedbacks are simulated.