Efficiency and driving factors for the retention of exogenous inorganic nitrogen in different forest soils

Forest soils play a vital role in purifying exogenous nitrogen ions and mitigating eutrophication in downstream water bodies. However, the heterogeneity of nitrogen ion removal efficiency in different forest soils and the underlying driving mechanisms remain not fully known. In our research, we selected soils from four typical forest stands, namely, Pinus massoniana-Quercus variabilis mixed forest (MF), Quercus variabilis forest (QF), Pinus massoniana forest (PF), and Citrus reticulata forest (CF), surrounding China's second largest reservoir in the Yangtze River Basin. Using laboratory simulation experiments, we observed variations in concentration of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3--N) in soil leachate after addition of exogenous NH4+-N and NO3--N solutions, along with the corresponding physical, chemical, and microbial factors. The results showed that on Day 21, all soils exhibited comparable removal rates of approximately 93% for NH4+-N and 70% for NO3--N. On Day 4, 6-15, PF and QF demonstrated significantly higher removal efficiencies for NH4+-N and NO3--N, respectively. Combining direct with indirect effects from Partial Least Squares Path Modeling (PLS-PM), NH4+-N removal efficiency was promoted by total organic carbon (TOC), soil NO3--N and NO2--N, while it was inhibited by soil water content (SWC), urease activity, soil NH4+-N, and ammonia-oxidizing archaea (AOA). NO3--N removal efficiency was facilitated by SWC, TOC, and urease, while it was restrained by soil NO3--N and ammonia-oxidizing bacteria (AOB). Notably, TOC was the only factor that had a comprehensive promoting effect on both NH4+-N and NO3--N removal efficiency. Our study indicates that soil in coniferous forest excels in NH4+-N removal while broadleaf forest in NO3--N, and the spatial layout of water source forests should consider water replenishment time, the form of nitrogen ion pollutants, and the nitrogen purification features of different forest types when the prime objective is to purity the water. Additionally, enhancing soil organic carbon could be an effective forest management approach to improve soil nitrogen ion purification and mitigate agricultural and nitrogen deposition pollution.