Root Decomposition of Grazed Signalgrass in Response to Stocking and Nitrogen Fertilization Rates

Abstract

Belowground root biomass can respond differently to grazing management affecting grass sward quality and longevity. Linking grazing management with belowground responses is scarce in warm‐climate grassland literature. The objective of this research was to investigate the effect of different stocking rates (SRs) and N fertilization on root chemical composition and decomposition in signalgrass pastures [Urochloa decumbens (Stapf.) R. D. Webster]. The experiment consisted of a factorial arrangement of three SRs (2.0, 3.9, and 5.8 animal units [AU] ha−1; 1 AU = 450 kg live weight) and three N fertilization rates (0, 150, and 300 kg N ha−1 yr−1). Response variables included root remaining biomass and N, root N and lignin concentrations, root C/N ratio, and root lignin/N ratio. Response variables were not affected by SR and N fertilization, except for remaining root biomass. All response variables were affected by incubation time. Root decomposition fitted a single exponential decay model. Remaining biomass was 30% of initial biomass (k = 0.0022 g g−1 d−1) after 512 d. Remaining N was 31% of initial N (k = 0.0007 g g−1 d−1) at the end of the incubation. Remaining root biomass was reduced with increasing N levels for 3.9 AU ha−1, but not for 2.0 and 5.8 AU ha−1. Root decomposition released ∼65 kg N ha−1 during 512 d trough mineralization. Grassland ecosystems have important C stock in their belowground plant biomass, which is relevant for the global C cycle. Therefore, practices that maintain or increase belowground C stocks should be preferred.