Litter Decomposition of Signalgrass Grazed with Different Stocking Rates and Nitrogen Fertilizer Levels
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Litter decomposition is an important pathway of nutrient return on grazed pastures and it may be affected by management practices. The objective of this study was to evaluate the effects of stocking rate (2, 3.9, and 5.8 AU ha1; 1 AU = 450 kg animal live weight) and N fertilization level (0, 150, and 300 kg N ha1 yr1) on signalgrass (Brachiaria decumbens Stapf.) litter decomposition. Signalgrass litter was placed in nylon bags and incubated for 0, 4, 8, 16, 32, 64, 128, and 256 d during 2009 and 2010. Litter decomposition rates differed between years and among incubation times. The decomposition rate was less in 2009 than in 2010 (3.46 and 4.20 mg g1d1, respectively). Stocking rate had no effect on litter decomposition rate. Decomposition rates were greater for fertilization with 300 vs. 0 kg N ha1 yr1 (4.47 vs. 3.29 mg g1 d1, respectively), which resulted in less N remaining in fertilized signalgrass litter. Concentrations of N and lignin as well as lignin/N ratio fit linear plateau models, increasing 7, 151, and 25 g kg1, respectively, during the 256d incubation period. The effect of year on residual N and C/N ratios highlighted the need for longterm trials that measure the peaks of nutrient release and availability to plants. Data are needed to guide nutrient management decisions in tropical pastures. Pasture N fertilization may shift the balance between litter immobilization/mineralization, altering N dynamics in the litter pool.
