[Soil aggregate stability and soil nutrient contents in Robinia pseudoacacia plantation with different stand...

Stand density is a key factor influencing forest structure and function. Its regulatory effects on soil structure and nutrient cycling are directly related to forest productivity and ecosystem functions. To investigate the effects of stand density of Robinia pseudoacacia plantation on soil stability and nutrient content, we selected stands with five density gradients (800-1100, 1100-1400, 1400-1700, 1700-2000, 2000-2300 plants·hm^-2) in the Cai-jiachuan watershed of Ji County, Shanxi Province. The composition of soil aggregates and nutrient characteristics in the topsoil (0-10 cm) and subsurface layers (10-20 cm) were determined. The results showed that soil aggregate stability decreased and nutrient content declined as stand density increased from 800-1100 to 2000-2300 plants·hm^-2. In the topsoil layer, the proportion of macroaggregates, mean weight diameter (MWD), and geometry mean diameter (GMD) decreased by 2.3%, 33.0%, and 19.4%, respectively. In the subsurface layer, they decreased by 10.9%, 25.3%, and 24.2%, respectively. The fractal dimension (D) showed no change. Total nitrogen (TN) and organic carbon (SOC) contents generally decreased with increasing stand density, but no significant trend was observed in the total phosphorus (TP) content. The aggregates with 0.25-1 mm size fraction had the highest contents of TN, TP, and SOC. The MWD, GMD, TN, TP, and SOC content in the topsoil were significantly higher than that in the subsurface soil. The interaction between stand density and soil layer had a significant negative effect on GMD, and a highly significant positive effect on D, collectively explaining 60.1% of the variation in soil aggregate stability. TN content was significantly positively correlated with GMD and negatively correlated with D. The stand density of 800-1100 plants·hm^-2 could effectively maintain soil structural stability and carbon and nitrogen sequestration.