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Phosphorus (P) limits productivity in many ecosystems and has the potential to constrain the global carbon sink. The magnitude of these effects depends on how climate change and rising CO2 affect P cycling. Some effects are well established. First, P limitation often constrains CO2 fertilization, and rising CO2 often exacerbates P limitation. Second, P limitation and P constraints to CO2 fertilization are more common in warmer and wetter sites. Models that couple P cycling to vegetation generally capture these outcomes. However, due largely to differences between short-term and long-term dynamics, the patterns observed across climatic gradients do not necessarily indicate how climate change over years to decades will modify P limitation. These annual-to-decadal effects are not well understood. Furthermore, even for the well-understood patterns, much remains to be learned about the quantitative details, mechanisms, and drivers of variability. The interface between empirical and modeling work is particularly ripe for development. [ABSTRACT FROM AUTHOR]

We derive a simple prediction about euphotic zone N : P stoichiometry from a large class of models that use saturating nutrient uptake functions to characterize N and P acquisition by phytoplankton. The prediction is: At an ecological steady state, the ratio of phytoplankton N : P to inorganic N : P in the euphotic zone equals the ratio of phytoplankton maximum uptake rates of N and P. We estimate this predicted ratio using nutrient uptake parameters measured in laboratory growth experiments and compare the predicted ratio to empirical observations from long-term sampling in the Atlantic and Pacific oceans. The model predictions for the ratio of phytoplankton N : P to inorganic N : P are at odds with the majority of data from extensive long-term sampling in the Atlantic and the Pacific oceans. This discrepancy calls into question the scope of applicability of ecosystem models that explicitly describe phytoplankton growth as a function of N and P availability. We discuss efforts to resolve this discrepancy, including the need for performing more comprehensive N and P uptake experiments and by reexamining models of nutrient uptake. doi: 10.4319/lo.2010.55.3.0997

Biological nitrogen fixation (BNF) is the major nitrogen (N) input in many terrestrial ecosystems, yet we know little about the mechanisms and feedbacks that control this process in natural ecosystems. We here examine BNF in four taxonomically and ecologically different groups over the course of forest ecosystem development. At nine sites along the Franz Josef soil chronosequence (South Westland, New Zealand) that range in age from 7 to 120000 yr old, we quantified BNF from the symbiotic plant Coriaria arborea, cyanolichens (primarily Pseudocyphellaria spp.), bryophytes (many species), and heterotrophic bacteria in leaf litter. We specifically examined whether these groups could act as 'nitrostats' at the ecosystem level, turning BNF on when N is scarce (early in primary succession) and off when N is plentiful (later in succession and retrogression). Coriaria was abundant and actively fixing (~11 kg N x [ha.sup.-1] x [yr.sup.-1]) in the youngest and most N-poor site (7 yr old), consistent with nitrostat dynamics. Coriaria maintained high BNF rates independent of soil N availability, however, until it was excluded from the community after a single generation. We infer that Coriaria is an obligate N fixer and that the nitrostat feedback is mechanistically governed by species replacement at the community level, rather than down-regulation of BNF at the physiological scale. Biological nitrogen fixation inputs from lichens (means of 0-2 kg N x [ha.sup.-1] x [yr.sup.-1], bryophytes (0.7-10 kg N x [ha.sup.-1] x [yr.sup.-1]), and litter (1-2 kg N x [ha.sup.-1] x [yr.sup.-1]) were driven primarily by changes in density, which peaked at intermediate-aged sites (and increased with soil N availability) for both lichens and bryophytes, and grew monotonically with soil age (but did not change with soil N) for litter. This non-nitrostatic link between soil N availability and lichen/bryophyte BNF likely stems from increased tree biomass in more fertile sites, which increases epiphytic moisture conditions and habitable surface area. This apparent positive feedback could produce N-rich conditions. Key words: bryophyte; Coriaria arborea; ecosystem development; feedback; Franz Josef New Zealand; leaf litter; lichen; nitrostat; primary succession; Pseudocyphellaria.

Symbiotic nitrogen (N) fixing trees are absent from old-growth temperate and boreal ecosystems, even though many of these are N-limited. To explore mechanisms that could select against N fixation in N-limited, old-growth ecosystems, we developed a simple resource-based evolutionary model of N fixation. When there are no costs of N fixation, increasing amounts of N fixation will be selected for until N no longer limits production. However, tradeoffs between N fixation and plant mortality or turnover, plant uptake of available soil N, or N use efficiency (NUE) can select against N fixation in N-limited ecosystems and can thereby maintain N limitation indefinitely (provided that there are losses of plant-unavailable N). Three key traits influence the threshold that determines how large these tradeoffs must be to select against N fixation. A low NUE, high mortality (or turnover) rate and low losses of plant-unavailable N all increase the likelihood that N fixation will be selected against, and a preliminary examination of published data on these parameters shows that these mechanisms, particularly the tradeoff with NUE, are quite feasible in some systems. Although these results are promising, a better characterization of these parameters in multiple ecosystems is necessary to determine whether these mechanisms explain the lack of symbiotic N fixers--and thus the maintenance of N limitation--in old-growth forests. evolutionary ecology | model