that the forcing due to increasing N deposition and frequency of ecosystem disturbance over the past 50–100 years may have resulted in non-steady-state N cycles in much of the world (34, 35 and 36).

Equilibration of Nitrogen and Water Limitations

In ref. 24 we argued that spatial patterns of biophysical and nitrogen limitation are correlated because carbon and nitrogen fluxes are both strongly influenced by water and energy availability. This mechanism of equilibration is evident in Century because the model simulates the inputs and losses of N, rather than being calibrated to observed ecosystem N stocks (25). The equations in Century governing nitrogen fluxes include biophysical and soil biogeochemical processes. Atmospheric inputs of N are directly linked to precipitation (wet deposition). Biological nitrogen fixation is influenced by soil N and C availability and is assumed to be correlated with annual evapotranspiration (ET). The correlation is based on information indicating high rates of N fixation in humid tropical and temperate rain forests, and generally lower rates in mesic and arid systems, although the biogeography of nitrogen fixation is poorly known (23). It is noteworthy that, globally, patterns of N inputs through all processes are poorly known, and given their importance, require much more study (37). N inputs, as expected (summing biological and atmospheric processes) are strongly correlated with annual ET (Table 1).

Losses of nitrogen are controlled by soil moisture and water flux. Leaching losses of NO3 and dissolved organic N (DON) are directly controlled by the product of water flux and NO3/DON concentrations (28). Losses of N trace gases are linked to the rate of mineralization of NH4 and NO3 from organic matter, a rate that increases as temperature and soil moisture increase (28, 38). The proportional as well as absolute losses of gaseous N from inorganic N also increase with increasing soil moisture (30). Century simulates several pathways of N trace gas losses: the summed losses of N2, N2O, and NO from soil nitrification and denitrification are likewise highly correlated with ET (Table 1). This arises because of the strong first-order kinetic regulation of trace gas emissions with respect to soil inorganic N turnover.

A key index of soil inorganic N turnover, N mineralization, is likewise strongly correlated with ET (Table 1). As noted in ref. 24, the correlation of N mineralization and ET, though strong, varies among ecosystem types, as is evident for other processes (see Fig. 2). Trace gas losses show similar patterns (data not shown), indicating ecosystem type-specific relationships between biophysical controls and N trace gas emissions, a factor not widely recognized (24). Spatial patterns of nitrate N leaching (data not shown) show strong dependence on ecosystem type, with many systems showing no or low losses; here we computed correlations for systems with non-zero leaching losses. Leaching losses are less directly related to ET, perhaps because ET is a poor predictor of available water below the rooting zone. Nitrate leaching is, however, strongly correlated with precipitation minus ET (P-E), which is related to the amount of water available for movement below the rooting zone (Table 1). Organic N leaching only occurs in a small fraction of grid cells (˜10%) and generally at low rates. It is poorly correlated with either ET or P-E (Table 1). The low leaching losses of N from many of the world’s ecosystems in this simulation of a preindustrial biosphere are consistent with Hedin et al. (35), who suggested that undisturbed ecosystems may have very low losses compared with the bulk of extant ecosystems. The results indicate significant correlation between key fluxes in the nitrogen budget and biophysical controls, although ecosystem-specific processes such as organic N leaching add some variability to patterns of equilibration.

Table 1. Correlation structure emerging from key linkages between mechanisms shown in Fig. 1, as implemented in the simulation described in Model and Methods

 

NMIN

NPP

NINPUT

NGAS

NO3

DON

NMIN

0.90

 

0.54

ET

0.67

0.71

0.96

0.71

0.33

0.00

P-E

 

0.74

0.05

NINPUT

 

0.74

NMIN, nitrogen mineralization; NPP, net primary productivity; NINPUTs, nitrogen inputs; NGAS, trace gas losses of N; NO3, nitrate leaching; DON, organic nitrogen leaching; ET, evapotranspiration; P-E, precipitation minus ET. All correlations shown are significant at P < 0.05 (except for ET vs. DON).

In Century the potential for carbon fixation increases as evapotranspiration increases via an equation that constrains primary production based on moisture available for transpiration (28). This equation integrates precipitation, energy, and soil hydrological constraints over the water flux in evapotranspiration. ET is linked to both precipitation, soil properties and radiation, as radiant energy is the driving force for ET. Thus, ET, which together with soil hydrological properties, controls the partitioning of soil moisture into runoff and fluxes back to the atmosphere or to depths below the rooting zone. Primary production also requires nitrogen to form organic matter meeting critical C/N ratios for wood, foliage, and roots. On an annual time scale most plant-available N is derived from nitrogen mineralization, which arises from organic matter turnover (decomposition); rates of N mineralization range from 0.2 to 30 g·m2·yr-1, greatly exceeding inputs in most cases. N inputs range from 0.5 to 1.5 g·m2·yr-1. Whereas N availability can vary substantially from year to year, the natural nitrogen budget changes on centennial time scales, as inputs and losses are small fractions of soil N stocks, which typically exceed 500 g·m2.

As a consequence of the tight coupling of the water/energy fluxes and nitrogen budget in Century, strong correlations between ET, nitrogen availability, and net primary productivity appear in global Century simulations (see Fig. 2). The correlations arise because water and energy fluxes controls both carbon and nitrogen fluxes (Fig. 1). These fluxes of carbon and nitrogen are mutually interdependent through the dual requirements of nitrogen in the formation of organic matter and of the role of organic matter decomposition in nitrogen mineralization. As water flux increases, N flux increases (inputs and losses), and likewise, the potential for carbon fixation increases. As carbon fixation increases, the amount of the N flux that can be captured in organic matter increases. As more nitrogen is captured in organic matter, its subsequent turnover also contributes to plant available N, allowing more plant productivity. Thus, water/energy and nutrient limitation of plant primary productivity and ecosystem carbon storage tend to “equilibrate” in near-steady-state ecosystems, as illustrated by the spatial patterns of correlation in Fig. 2.

The relationships of NPP and N availability with ET are modulated by other factors that influence turnover times. The relationships between NPP, ET, and N are modified by ecosystem type-specific factors that control resource use efficiencies. Effectively these are the carbon-to-nutrient stoichiometry of plants and microorganisms, and water use efficiency (or organic matter produced per unit water transpired) (Fig. 2). Ecosystems with wider C/N ratios in plant tissue have higher NPP per unit N mineralization (higher nitrogen use efficiencies). Systems with lower C/N ratios in leaf and root tissues have higher rates of N cycling per unit ET. C/N ratios reflect both plasticity in foliar and root composition, and more significantly, changes in allocation between high and low-N tissues (wood vs. leaves or roots). Although large-scale patterns arise from system-level interactions of the biogeochemical and hydrological cycles, substantial variation is induced by



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