Terracing influences soil microbial assembly in citrus orchards: stochastic processes dominate community dynamics in a karst sloping land

Terracing is a key soil conservation practice in karst citrus orchards, yet its long-term effects on rhizosphere microbial community assembly remain poorly understood, especially the relative influence of deterministic (e.g., environmental filtering) versus stochastic processes (e.g., dispersal limitation). We investigated rhizosphere soil microbial communities along a terrace chronosequence (0–12 years) in the Lijiang River Basin using MiSeq sequencing and metabolomics, with null model analysis employed to assess community assembly processes. Terrace age did not significantly affect microbial α-diversity, but was associated with subtle changes in community composition: Proteobacteria, a copiotrophic group, decreased slightly, while Chloroflexi, an oligotrophic group, increased modestly. These shifts suggest a weak trend toward lower soil nutrient availability rather than a clear successional reorganization. Microbial diversity and structure were significantly correlated with soil stoichiometric ratios and available phosphorus (p < 0.05). Terracing also affected microbial network complexity and potential function. Potential functional profiling and metabolome data revealed that L-glutamine, a key nitrogen source, was negatively correlated with potential catabolic nitrate reduction (p < 0.05). This relationship was most pronounced at the Y5 phase (peak diversity/network complexity), suggesting accelerated L-glutamine utilization tightly coupled with enhanced potential for dissimilatory nitrate reduction to maximize nitrogen-use efficiency during the successional climax. Notably, stochastic processes explained over 96% of the microbial assembly. Bacterial communities were primarily driven by homogenizing dispersal, while fungal communities followed undominated processes. The prominence of stochasticity in our results complements current understanding of agricultural microbiome assembly, particularly emphasizing its vital role in fragile karst environments. We propose that optimizing terrace rotation intervals (e.g., every 5-year) could be a practical strategy to enhance nitrogen-cycling efficiency and support sustainable nutrient management in karst citrus cultivation.