Sinks for Inorganic Nitrogen Deposition in Forest Ecosystems with Low and High Nitrogen Deposition in China

We added the stable isotope 15 N in the form of ( 15 NH4) 2 SO 4 and K 15 NO 3 to forest ecosystems in eastern China under two different N deposition levels to study the fate of the different forms of deposited N. Prior to the addition of the 15 N tracers, the natural 15 N abundance ranging from −3.4‰ to +10.9‰ in the forest under heavy N deposition at Dinghushan (DHS), and from −3.92‰ to +7.25‰ in the forest under light N deposition at Daxinganling (DXAL). Four months after the tracer application, the total 15 N recovery from the major ecosystem compartments ranged from 55.3% to 90.5%. The total 15 N recoveries were similar under the ( 15 NH4) 2 SO 4 tracer treatment in both two forest ecosystems, whereas the total 15 N recovery was significantly lower in the subtropical forest ecosystem at DHS than in the boreal forest ecosystem at DXAL under the K 15 NO 3 tracer treatment. The 15 N assimilated into the tree biomass represented only 8.8% to 33.7% of the 15 N added to the forest ecosystems. In both of the tracer application treatments, more 15 N was recovered from the tree biomass in the subtropical forest ecosystem at DHS than the boreal forest ecosystem at DXAL. The amount of 15 N assimilated into tree biomass was greater under the K 15 NO 3 tracer treatment than that of the ( 15 NH4) 2 SO 4 treatment in both forest ecosystems. This study suggests that, although less N was immobilized in the forest ecosystems under more intensive N deposition conditions, forest ecosystems in China strongly retain N deposition, even in areas under heavy N deposition intensity or in ecosystems undergoing spring freezing and thawing melts. Compared to ammonium deposition, deposited nitrate is released from the forest ecosystem more easily. However, nitrate deposition could be retained mostly in the plant N pool, which might lead to more C sequestration in these ecosystems.