Chemical Fertility Gains Mask Physical Degradation and Microbial Stress in a Tropical Oxisol

Soil quality in tropical agroecosystems depends on the integration of physical structure, chemical fertility, and biological efficiency, which is frequently disrupted by intensive land use. We quantified soil quality trade-offs under four land uses---no-tillage cropping, no-tillage intercropped with Urochloa ruziziensis, long-term pasture, and native forest---in a clayey Oxisol in southern Brazil. Physical, chemical, and biological indicators were evaluated at 0--10 and 10--20 cm depths. Intensive agricultural systems improved chemical fertility but caused pronounced physical degradation, including increased clay dispersion, soil compaction, and reduced macroporosity, along with lower microbial biomass and higher metabolic quotients, indicating microbial stress. Native forest maintained greater structural stability, buffering capacity, and microbial efficiency. Intercropping partially mitigated subsoil carbon losses and microbial stress but did not restore soil physical integrity, highlighting the limitations of fertility-based soil quality assessments.