The logic of carbon and nitrogen interactions in terrestrial ecosystems

Simple ecosystem models of carbon (C) and nitrogen (N) interactions between plants and soil show that differences in N-use efficiency cause convergence of plant growth in ecosystems with a closed N cycle because rapid growth associated with high N-use efficiency results in litter with a high C:N ratio and a low N mineralization rate, whereas slow growth associated with low N-use efficiency leads to a low C:N ratio and a high N mineralization rate. This plant-induced negative feedback on production contrasts with the positive feedback that had previously been hypothesized. Our model explains the causes and results of several important ecological patterns. First, all ecosystems with a fixed N pool will show a small increase in C storage (especially in soils) in response to elevated CO2 despite constraints by litter-quality feedbacks to N mineralization rate. Second, the decreased N-use efficiency and plateauing of primary production in forest ecosystems with a high N supply reflect saturation of photosynthetic rate with high plant N pools. Finally, the addition of inorganic N to ecosystems induces a quick increase in productivity and N supply. However, these increases disappear if N additions are not sustained. These findings suggest that those global changes that alter N input to or output from ecosystems are likely to have larger long-term impact on biomass, productivity, and C storage of ecosystems with a tightly closed N cycle than would changes in plant N-use efficiency.