Nutrient availability limits future productivity and carbon storage in terrestrial ecosystems
Uncertainty surrounding the continued strength of the terrestrial carbon (C) sink presents a significant challenge for projecting future climate-C cycle feedbacks. Most current Earth System Models (ESMs) indicate that terrestrial C storage will increase with climate warming, reflecting CO2 fertilization effects on net primary production (NPP) that outpace climate-driven ecosystem C losses via respiration. However, the extent to which nutrient availability could constrain rates of NPP is poorly understood. Few models from the 5th phase of the Coupled Model Intercomparison Project (CMIP5) currently represent terrestrial C-N biogeochemistry, but those that do show that N strongly constrains CO2fertilization effects, NPP and C storage. Yet, despite the fact that phosphorus (P) limitation is widespread throughout the terrestrial biosphere and expected to increase in the future, the potential effects of P limitation on future climate-C cycle feedbacks have not been thoroughly explored. Given these shortcomings, and in the absence of a robust empirical database on global nutrient limitation, we applied a theoretical framework based on observations of plant C allocation and stoichiometry to calculate the additional NPP that could be met with new nutrient inputs to identify potential biases in climate-C cycle feedbacks simulated by CMIP5 models over the next century.
The nutrient demand needed to fuel projected increases in NPP greatly exceeds new inputs, implying that unrealistically high rates of nutrient recycling are necessary to realize the CMIP5 model projected increases in terrestrial NPP and C storage. Compared with "no nutrient constraint" scenario, we estimate that N limitation and NP co-limitation will reduce mean global NPP 19% and 25%, respectively. While new N inputs and accelerated mineralization could be sufficient to meet plant N demand with increasing atmospheric CO2, similar outcomes seem less likely for P. Although P availability could substantially limit future productivity over much of the globe, uncertainty in the availability and dynamics of soil P across the land surface present significant challenges in projecting potential changes in plant P acquisition in global change scenarios. Overall, estimates of terrestrial C storage are highly variable, but our analysis indicates that the effects of nutrient limitation on NPP could result in net terrestrial C losses, and turn the land surface into a net source of CO2 to the atmosphere by 2100. Our findings underscore the need to consider potential effects of nutrient limitation in models used to predict global climate and C cycling responses to environmental change.