Interactions between defoliation severity and canopy height drive root-derived organic matter inputs in tropical grasslands

Quantifying soil organic matter (SOM) and root decomposition rates is key to elucidating nutrient cycling and pasture sustainability. We evaluated the effects of defoliation severity (18, 33, and 46% height reduction) and pre-grazing heights (20 and 30 cm) on SOM and root decomposition of Mulato II brachiariagrass pastures. The two-year (2019 - 2020) experiment followed a randomized complete block design with six treatments and three replications. The study was carried out at ESALQ/USP (Piracicaba, SP, Brazil). Root decomposition was assessed using litter bags incubated for 256 days. A significant height × severity interaction was observed for SOM (P < 0.10) and decomposition rates (P < 0.01). Managing at 20 cm with 18% severity (high grazing frequency) resulted in the lowest SOM (21.03 g kg⁻¹). This was corroborated by C decomposition dynamics, which followed a quadratic broken-line model (R² = 0.99) with a significant join point at 109.8 days. Initially, about 29% of C was rapidly degraded. Subsequently, the C decomposition rate stabilized at a constant linear loss of 0.077 g kg⁻¹ DM day⁻¹, reaching a final C level of 60.01 g kg⁻¹ at 256 days. Shorter grass canopy and greater grazing frequency stimulated faster root turnover and C disappearance, accelerating microbial consumption and reducing SOM accumulation compared to taller canopies. These findings indicate that frequently defoliation management (20 cm/18%) accelerates carbon cycling, leading to lower SOM in tropical pastures due to increased decomposition rates.