Interactions between fine root-derived dissolved organic matter and K-strategy-dominated soil microbes regulate soil CO2 emissions in a Pinus tabulaeformis plantation under N deposition

The mechanisms by which belowground plant deposits influence soil organic carbon dynamics under increasing nitrogen (N) deposition remain unclear. In this study, ingrowth cores with different mesh sizes (1 µm, 45 µm and 1 mm) were used to investigate the effects of mycelium and fine root deposits on soil dissolved organic matter (DOM) and CO₂ emissions under N addition. Results indicated that mycelium did not significantly alter DOM composition or microbial community, whereas several labile (including amino sugars and carbohydrates) and recalcitrant DOM (including lignin and tannin) were enriched in the fine root and coarse root treatments, respectively. The fungal community shifted towards a K-strategy in the presence of mycelium and roots compared to the control treatment (1 µm). N addition increased the abundance of recalcitrant DOM molecules, particular in fine root treatments. Root deposit inputs increased DOM transformation and the complexity of the DOM-microbe network. The associations between microbes and labile carbon were enhanced in the mycelium and fine root treatments. The relationships between oligotrophic Basidiomycota and recalcitrant carbon were strengthened in the coarse root treatment. CO₂ emissions in mycelium treatments were inhibited by N addition, primarily due to a decrease in mycorrhizal colonization. Root deposit inputs and DOM-microbe interactions dominated the CO₂ emissions in the forest soil under N addition. Our findings confirm the essential role of fine root deposits, in regulating soil CO₂ emissions by influencing DOM characteristics under N deposition.