Long-term increases in soil carbon due to ecosystem fertilization by atmospheric nitrogen deposition demonstrated by regional-scale modelling and observations.

Fertilization of nitrogen (N)-limited ecosystems by anthropogenic atmospheric nitrogen deposition (N) may promote CO removal from the atmosphere, thereby buffering human effects on global radiative forcing. We used the biogeochemical ecosystem model N14CP, which considers interactions among C (carbon), N and P (phosphorus), driven by a new reconstruction of historical N, to assess the responses of soil organic carbon (SOC) stocks in British semi-natural landscapes to anthropogenic change. We calculate that increased net primary production due to N has enhanced detrital inputs of C to soils, causing an average increase of 1.2 kgCm (c. 10%) in soil SOC over the period 1750-2010. The simulation results are consistent with observed changes in topsoil SOC concentration in the late 20 Century, derived from sample-resample measurements at nearly 2000 field sites. More than half (57%) of the additional topsoil SOC is predicted to have a short turnover time (c. 20 years), and will therefore be sensitive to future changes in N. The results are the first to validate model predictions of N effects against observations of SOC at a regional field scale. They demonstrate the importance of long-term macronutrient interactions and the transitory nature of soil responses in the terrestrial C cycle.