Effects of nitrogen deposition on carbon sequestration in Chinese fir forest ecosystems.

Nitrogen deposition and its ecological effects on forest ecosystems have received global attention. We used the ecosystem model FORECAST to assess the effects of nitrogen deposition on carbon sequestration in Chinese fir planted forests in SE China. This topic is important as China is intensifying its reforestation efforts to increase forest carbon sequestration for combating climate change impacts, using Chinese fir as the most important plantation species. A series of scenarios including seven N deposition levels (1, 5, 10, 20, 30, 40 and 50kg ha(-1)y(-1)), three management regime (rotation lengths of 15, 30 and 50 years) and two site qualities (nutrient poor and fertile sites) were defined for the simulations. Our results showed that N deposition increased carbon sequestration in Chinese fir forests, but the efficiency of the increasing effect is reduced as N deposition levels increase. When N deposition levels exceeded 20-30kg ha(-1)y(-1), the incremental effects of N deposition on forest C pools were marginal. This suggests that N deposition levels above 20-30kg ha(-1)y(-1) could lead to N saturation in Chinese fir forest soils. Any additional amounts of N input from deposition would likely be leached out. Total above-ground C was more sensitive to N deposition than to rotation length and site quality. It was also estimated that the contributions of N deposition to C sequestration in all Chinese fir forests in South-East China are 7.4×10(6)MgCy(-1) under the current N deposition levels (5 to 10kg ha(-1)y(-1)) and could reach up to 16×10(6)MgCy(-1) if N deposition continues increasing and reaches levels of 7.5 to 15kg N ha(-1)y(-1).