Nutrient Limitations Lead to a Reduced Magnitude of Disequilibrium in the Global Terrestrial Carbon Cycle

The terrestrial carbon (C) cycle is shifting to a state of dynamic disequilibrium under a rapid global climate change. However, the magnitude of such disequilibrium is inherently hard to measure directly. Abundant studies have revealed that the availability of nutrients, particularly nitrogen (N) and phosphorus (P), constrains ecosystem productivity and carbon stocks across the globe. However, whether and how nutrient limitation affects the disequilibrium magnitude of the terrestrial C cycle (X-p) has never been evaluated. Here, we developed an approach by combining a process-based numerical model and an analytical framework to evaluate the role of nutrient limitation on X-p. We found that nutrient limitation did have significant impacts on the X-p. Over the modeled period of 1901-2013, absolute change in X-p was 497.6 PgC under the C-only run, while it decreased to 155.6 and 124.3 PgC under N and NP limitations, respectively. To understand the underlying reasons, we further disaggregated the changes of X-p into changes in steady-state C storage and transit C storage with the former being decomposed into a productivity-driven change, an ecosystem-C-residence-time-driven (tau(E)-driven) change, and a change induced by productivity-tau(E) interactions. We found that nutrient constrained the increase in X-p primarily by dampening the productivity-driven changes in the steady-state C storage. Reductions in the productivity-driven term under N and NP limitations accounted for 94.7% and 94.9%, respectively, of the reductions in the steady-state C storage. These results indicate that nutrient limitations have profound impacts on future climate-biosphere feedback by reducing the disequilibrium magnitude of the terrestrial C cycle.