Effects of nitrogen addition on the concentration and composition of soil-based dissolved organic matter in subtropical Pinus taiwanensis forests

Purpose Dissolved organic matter (DOM) plays an important role in soil C and N cycling. However, the driving mechanisms of DOM concentration and composition under the background of N deposition remains ill-defined. Through experimental N addition, we explored the influence and contribution of microbial community characteristics on DOM concentration and composition. Materials and methods Field experiments with different N-addition levels were set up in the Daiyunshan Nature Reserve, Fujian, China. Using water extraction and DAX-8 resin separation and phospholipid fatty acids as well as 96-well microplate methods, we mainly probed into the response of DOM concentration and composition, microbial community structure, and extracellular enzyme activity at two soil depths (0–10 and 10–20 cm) to N addition, separately. Furthermore, structural equation models (SEMs) were constructed to explore the effects of microbial community characteristics on the DOM concentration and composition. Results and discussion The dissolved organic carbon and hydrophilic matter (HIM) decreased significantly with N addition. Moreover, low N addition significantly increased the abundance of gram-negative bacteria and microbial biomass nitrogen, and the activities of urease and cellobiohydrolase in soil. SEMs revealed that the contribution rates of microbial community characteristics to dissolved organic matter (DOM) concentration and HIM were 65% and 60%, respectively. This study focused on the microbial regulation paths for the DOM concentration and composition, including mechanical decomposition, extracellular enzyme secretion, and absorption assimilation. Conclusion Short-term N addition stimulated microbial communities to regulate soil DOM concentration and composition via three pathways in the subtropical Pinus taiwanensis forest. Under short-term N addition, the absorption and utilization of unstable hydrophilic DOM by microbial communities is the main regulation process, which eventually leads to soil C loss in the form of water-soluble organic C.