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

摘要

We added the stable isotope ¹⁵N in the form of (¹⁵NH4)2SO4 and K¹⁵NO3 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 ¹⁵N tracers, the natural ¹⁵N 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 ¹⁵N recovery from the major ecosystem compartments ranged from 55.3% to 90.5%. The total ¹⁵N recoveries were similar under the (¹⁵NH4)2SO4 tracer treatment in both two forest ecosystems, whereas the total ¹⁵N recovery was significantly lower in the subtropical forest ecosystem at DHS than in the boreal forest ecosystem at DXAL under the K¹⁵NO3 tracer treatment. The ¹⁵N assimilated into the tree biomass represented only 8.8% to 33.7% of the ¹⁵N added to the forest ecosystems. In both of the tracer application treatments, more ¹⁵N was recovered from the tree biomass in the subtropical forest ecosystem at DHS than the boreal forest ecosystem at DXAL. The amount of ¹⁵N assimilated into tree biomass was greater under the K¹⁵NO3 tracer treatment than that of the (¹⁵NH4)2SO4 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.