[Meta-analysis on the Effects of Nitrogen Addition on Soil Organic Carbon Content in Terrestrial Ecosystems]

Several studies have demonstrated that the increased deposition of nitrogen(N) has significantly affected the content of soil organic carbon(SOC); however, the change significantly varies in different regions. In this study, Meta-analysis, Meta-regression, and linear regression were performed to systematically evaluate the effects of climate, soil properties, and field design factors on the responses of SOC to N addition based on 408 data points from 49 field experiments in China. The results revealed that the response of SOC to N addition was significantly positively correlated with the mean annual temperature(MAT) and mean annual precipitation(MAP) of the sample sites(P<0.05). In the regions with lower MAT(<3℃) or MAP(<500 mm), SOC significantly decreased after N addition. In the areas with higher MAT(>3℃) or MAP(>500 mm); however, SOC significantly increased. For soil properties, SOC significantly accumulated after N addition in the plots with a higher soil C:N ratio(>15) or acidic soil(pH<6.5) but less changed in the plots with a lower C:N ratio(≤ 15) or higher pH(≥ 6.5). For ecotype, after N addition, SOC decreased significantly in the grassland ecosystem(-5.34%) but less changed in the wetland ecosystem. SOC accumulated the most after N addition in the forest ecosystem(10.52%), particularly in the broad-leaved forest ecosystem(13.10%). Further analysis showed that the soil C:N ratio was the most important factor. For type of N application, the addition of ammonium nitrate or urea increased the SOC content significantly, but the effect of nitrate was not significant. In summary, when accurately evaluating, predicting, and analyzing the effects of N addition on SOC content, the effects of climatic characteristics and soil properties of sample sites and field design factors should be comprehensively considered.