[Effects of Nitrogen Addition on Soil Organic Carbon and Its Fractions in Karst Farmland and Forest Ecosystems of China Based on Meta-analysis].

In recent decades, with the intensification of human activities, atmospheric nitrogen （N） deposition has been increasing. N deposition affects carbon （C） cycling in terrestrial ecosystems, especially in fragile karst ecosystems. Karst ecosystems are considered to be an important C pool. To evaluate the impact of N deposition on soil organic C （SOC） and its fractions in karst ecosystems of China, we collected and collated 14 English literature published through the end of March 2023, yielding a total of 460 sets of experimental data. The meta-analysis examined the effect of N addition levels [low N: ≤50 kg·（hm²·a）^-1, medium N: 50-100 kg·（hm²·a）^-1, and high N: >100 kg·（hm²·a）^-1, in terms of N] on SOC and its fractions [particular organic C （POC）, readily oxidized organic C （ROC）, microbial biomass C （MBC）, and dissolved organic C （DOC）]. The results showed that N addition levels significantly affected the responses of farmland and forest soil SOC and their active fractions to N addition. Specifically, low and high N additions significantly increased SOC concentration in farmland ecosystems, whereas medium N addition significantly increased SOC concentration in forest ecosystems. In addition, soil active C fraction concentrations increased under high N addition in farmland ecosystems and under low and medium N addition in forest ecosystems. Without considering the level of N addition, N addition significantly enhanced soil organic matter （SOM） mineralization in both farmland and forest ecosystems and increased the SOC concentration in farmland ecosystems but not forest ecosystems. The responses of different active C fractions to N addition were diverse. In farmland ecosystems, the POC and ROC concentrations increased, but DOC did not change with N addition. In forest ecosystems, the DOC and POC concentrations increased, but there was no significant effect on MBC. Moreover, the response ratios （RR） of SOC and its fractions in different ecosystems to N addition were influenced by different environmental factors. In farmland ecosystems, the response ratio of SOC was related to the annual average temperature and soil pH. The response ratio of DOC was affected by the annual average temperature, mean annual precipitation, and N addition rate. The POC response ratio was related to the N addition rate. In forest ecosystems, the effects of N addition on the SOC response ratio were significantly altered by the annual average temperature, mean annual precipitation, and soil pH. However, the response ratios of DOC, POC, and MBC were not affected by the annual average temperature, mean annual precipitation, soil pH, and N addition rate. Consequently, these findings indicate that N addition could enhance soil SOC concentration and promote soil C sequestration in farmland and forest ecosystems in karst regions, but this effect relies on the level of N addition. This provides a scientific basis for predicting the soil C sink function in karst ecosystems under climate change scenarios.