Soil environmental changes drive root decomposition under acid and nitrogen deposition in Chinese fir plantation, China

The decomposition of underground roots is a crucial component of soil carbon turnover and nutrient input processes. However, our understanding of how root chemistry and soil environments interact during root decomposition remains limited, and differences in the decomposition processes among different root orders are still unclear. Therefore, this study was conducted using Chinese fir (Cunninghamia lanceolata) as the experimental subject. We combine root chemistry-only change, soil environment-only change, and in-situ experiments to investigate the effects of root chemistry and soil environments during root decomposition, as well as the differences in decomposition processes among different root orders under the context of acid rain and nitrogen deposition. Our findings indicate that the soil environment has a long-term and more significant influence on root decomposition. In the SHAP analysis of relative contributions, soil environmental factors explained 61.2% of the variation in low-order roots and 55.9% of the variation in high-order roots. The root decomposition process primarily shifts from being dominated by root chemistry in the short term to being governed by the soil environment in the long term. Different root orders show similar changing trends but differ in their timing of change: for low-order, the dominant factor shifts to the soil environment after 6 months, whereas for high-order, this shift occurs after 10 months. The influence of the soil environment is mediated through soil nutrient. For instance, high-order roots are primarily driven by the soil carbon-nitrogen ratio, with a direct effect of 0.38, while low-order roots are dominated by available potassium, with a direct effect of 0.57. Interestingly, under the context of acid rain and nitrogen deposition, soil environment and root chemistry showed a synergistically enhancing trend in inhibiting the decomposition of low-order roots, whereas for high-order roots, the two factors exhibited an opposite trend of mutually weakening inhibition. In summary, this study highlights the long-term importance of the soil environment in root decomposition and the varying responses of different root orders, contributing to a deeper understanding of root decomposition and soil carbon turnover processes in forest ecosystems.