Atmospheric nitrogen (N) deposition, an important component in the global N cycle, has increased sharply in recent decades in China. Here, we constructed national-scale inorganic N wet deposition (Ndep) patterns in China based on data from 280 observational sites and analysed the effects of anthropogenic sources and precipitation on Ndep. Our results showed that the mean Ndep over China increased approximately 25%, from 11.11 kg ha−1 a−1 in the 1990s to 13.87 in the 2000s. Ndep was highest over southern China and exhibited a decreasing gradient from southern to western and northern China. The decadal difference in Ndep between the 1990s and 2000s was primarily caused by increases in energy consumption and N fertiliser use. Our findings conformed that anthropogenic activities were the main reason for the Ndep increase and provide a scientific background for studies on ecological effects of N deposition in China.

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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.