Effects of stand density on soil enzyme activities and microbial metabolic limitation in differently aged Robinia pseudoacacia Linnaeus plantations

Soil extracellular enzyme activity and microbial metabolism limitation, critical for forest ecosystem functions, are strongly influenced by stand density, yet their responses to stand density in differently aged plantations remain poorly understood. We investigated these dynamics in Robinia pseudoacacia Linnaeus plantations across three age classes (young, middle-aged, and near-mature) with three densities (low density, < 1000 trees ha⁻¹; medium density, 1000–1500 trees ha⁻¹; high density, >1500 trees ha⁻¹) in the loess hilly-gully region of northern Shaanxi Province, China. Increasing stand density elevated carbon (C)-acquiring enzyme activities and the enzyme C: nitrogen (N) ratio in young stands, but decreased them in middle-aged and near-mature stands. Young stands exhibited suppressed N- acquiring enzyme activities under denser stands. Conversely, in middle-aged stands, increasing stand density elevated N-acquiring enzyme activities and enzyme N: phosphorus (P) ratio. In near-mature stands, increasing stand density significantly elevated P-acquiring enzyme activities but decreased enzyme N:P ratio. Microbial metabolism was primarily N-limited across all stands. Hierarchical partitioning analysis showed that soil properties explained most of the variation in soil microbial metabolic C limitation (45.71 %) and N limitation (79.09 %). Partial least squares path model showed that soil organic C had the highest total negative effect on microbial C limitation (−0.47) and microbial N limitation (−0.64). Our findings suggested that increasing stand density decreased microbial C and N limitations in young and near-mature stands but intensified microbial C and N limitation in middle-aged stands. Soil organic C was the main factor affecting microbial metabolic limitation, with enhanced soil organic C levels mitigating microbial metabolic C limitation and exacerbating microbial metabolic N limitation. Therefore, there is a need to implement age-based stand density management strategies for plantations to decrease microbial metabolic limitation and hence improve soil nutrient availability.