Canopy height and defoliation severity drive root decomposition and C:N stabilization in tropical pastures

Understanding grazing management impacts on root dynamics is essential for soil organic matter (SOM) stabilization and nutrient cycling in tropical grasslands. We evaluated four grazing strategies on root dry biomass (RDB) and decomposition quality (C:N ratio) of Mulato II brachiariagrass under rotational stocking. The experimental design was a randomized complete block with three replications. Strategies combined two target average canopy heights (20 and 30 cm) with two defoliation severities: 1) lenient (33% reduction) averaging 20 cm (L20) and 30 cm (L30); and 2) moderate (47% reduction) averaging 20 cm (M20) and 30 cm (M30). RDB was assessed, and decomposition was monitored via C:N ratio changes in initial (0 days) and residual root mass at 273 and 512 days of soil incubation. No significant grazing effects were observed for RDB (P>0.05), averaging 184.8 g/m². Initial root quality (0 days) was uniform (C:N ~54.8). However, management influenced decomposition over time. At 273 days, roots under 20 cm height (L20, M20) showed lower C:N (39.5) than 30 cm strategies (45.1; P<0.05). By 512 days, a height × severity interaction emerged (P<0.02). L30 maintained greater C:N (28.2), while M30 resulted in the lowest ratio (23.3), nearing stabilization. Results demonstrate that while grazing strategies do not immediately alter RDB, canopy height and defoliation severity drive long-term decomposition. Lower heights and moderate defoliation accelerate C:N reduction, potentially increasing nitrogen mineralization and carbon cycling in tropical pastures.