Winter cover crop integration on soil nitrogen loss from a subtropical warm-season pasture field and its potential mitigation

Bermudagrass is a summer forage crop with high nitrogen (N) demand. Introducing winter cover crops may sustain bermudagrass yields with less fertilizer, reducing environmental N losses. Field trials and lab incubation were carried out to evaluate the effects of white clover (Trifolium repens L.) and ryegrass (Lolium multiflorum) as winter cover crops on soil N losses through runoff and nitrous oxide (N₂O) emissions in a subtropical bermudagrass (Cynodon dactylon L.) pasture field. The 2-year field experiment included five treatments: (1) no winter cover and N fertilization as a control (CLT), (2) white clover without N fertilization (WC) (where WC represents white clover), (3) white clover mixed with ryegrass without N fertilization (WCR), (4) white clover with half-rate N at 112 kg N ha⁻¹ year⁻¹ (WC112N), and (5) no cover crop with full-rate N 224 kg N ha⁻¹ year⁻¹ (224N). Results showed that without N fertilization, WC increased bermudagrass biomass by 38% compared to CLT, while WCR had a similar bermudagrass biomass yield to CLT. WC112N produced comparable bermudagrass biomass as 224N as well as reduced NH₄⁺ and NO₃⁻ runoff loss by 30%–35% and 11%–24%, respectively, compared than 224N, due to decreased runoff volume. There was no difference in N₂O emissions between 224N and WC112N. Laboratory incubation of white clover residue-amended soil showed that nitrification inhibitors dicyandiamide and 3,4-dimethylpyrazole phosphate lowered N₂O emission significantly, with a maximum reduction of 77%–91%, while urease inhibitor N-(n-butyl) thiophosphoric triamide had no effect. Overall, clover incorporation during the winter season helps in developing a low-N-input pasture production system, and nitrification inhibitors could be applied to mitigate associated N₂O losses from clover residue decomposition.