Modeling the effects of nitrogen deposition on carbon budget in two temperate forests

The NOx input terrestrial ecosystems are increasing significantly induced by human activities, yet the understanding of the responses of carbon cycle to nitrogen deposition is still poor. The northern temperate forest ecosystems have seen the greatest changes in nitrogen inputs from atmosphere. It is necessary for us to understand how the carbon cycle would change under the nitrogen addition in temperate forests, as an important carbon sink. In this study, we present a biogeochemical process model, CEVSA2, and use this model to elucidate the key processes that may strongly influence the carbon budget response to anthropogenic nitrogen addition. The CEVSA2 model has included the effect of nitrogen on photosynthesis, carbon allocation, soil organic matter decomposition, etc. Our simulations show nitrogen addition stimulates the photosynthesis, net carbon sequestration, carbon accumulation in vegetation and soil, by contrary, the low level of nitrogen addition decreases the heterotrophical and total respiration. The long-term chronic nitrogen addition experiments show that the low and high level nitrogen addition would reduce the carbon sequestration and accumulation. The model failed to simulate the effect of nitrogen addition on plant mortality, the de-coupling of nitrogen and photosynthesis when nitrogen saturates. In addition, the responses of soil respiration to nitrogen deposition involve so many complex biochemical processes; however, we have little knowledge about them. Sequentially, there is large uncertainty of model simulation on the effect of nitrogen deposition on soil respiration. With increasing rates of anthropogenic nitrogen deposition, there is a strong need to understand links between nitrogen inputs and carbon cycle.