Different responses of soil organic carbon fractions to additions of nitrogen

Carbon-nitrogen (C-N) interactions in terrestrial ecosystems regulate climate-C cycle feedbacks. How additions of N affect soil C sequestration and then regulate climate change, however, are not fully understood. Previous studies have assessed effects of N on bulk soil organic C (SOC) but have not yet carefully examined its effects on different SOC fractions, which determine how fast the N-regulated C cycle feeds back to climate. Here we synthesized data from 36 studies with 296 observations by a meta-analysis to evaluate the responses of SOC fractions to N additions. We hypothesized that additions of N might increase both labile and non-labile C fractions. The SOC that was separated by density fractionation increased by 18.3% for free light fractions and by 3.0% for heavy density fractions without change in the occluded light fraction under N addition compared with the control. The SOC that was separated by aggregate fractionation increased by 4.4% for macroaggregate-associated fractions and by 6.5% for aggregate mineral-associated fractions without change in the microaggregate-associated fraction under N additions. When bulk SOC was separated by chemical permanganate oxidation, the oxidizable fraction increased by 10.4%, whereas the unoxidizable fraction increased by 4.5%. The changes in different soil C fractions were related to the mechanisms of SOC stabilization. Because of the significant increases in non-labile SOC fractions, our findings suggest that on a global scale, increased additions of N might promote SOC accumulation and slow down climate change in the long term. Our results will be useful in model development for better prediction of SOC sequestration under additions of N.