Tailoring nitrogen input based on waterlogging duration improves root growth and biomass translocation in cotton

Nitrogen fertilization serves as an effective approach to alleviating abiotic stresses, including drought, waterlogging, and heat stresses. Nevertheless, the interactive impacts of nitrogen application rates and waterlogging durations on biomass accumulation, translocation, and root growth remain largely unknown. To address this knowledge gap, a two-year (2018 and 2019) field experiment was conducted to investigate the impacts of varied waterlogging durations (0, 5, 10, and 15 days, denoted as W0, W5, W10, and W15) at the early flowering stage and nitrogen application rates (0, 240, and 360 kg N ha⁻¹, denoted as N0, N1, and N2) on cotton biomass accumulation and translocation, soil nitrogen status, root traits, and lint yield. The results showed that waterlogging stress significantly delayed cotton phenology by 1–6 days, inhibited plant biomass accumulation and translocation, reduced soil nitrate nitrogen content, suppressed root growth and nitrate reductase activity, and ultimately decreased lint yield by 37–83 %, with the reduction proportional to waterlogging durations. Nitrogen input effectively alleviated waterlogging stress on cotton growth, and increased lint yield by 139–306 %. Compared to N1, N2 more effectively mitigated the adverse effects of W10 waterlogging by increasing root length density in deeper soil layers (40–100 cm), enhancing overall biomass production, and facilitating dry matter translocation from vegetative parts to reproductive structures. In contrast under W15 conditions, no significant differences were found between N1 and N2 in root traits, biomass translocation, or lint yield, indicating the 15-day waterlogging stress at early flowering caused irreversible damage to cotton growth. Consequently, tailoring nitrogen application rates according to waterlogging intensity is of great significance for improving nitrogen use efficiency and alleviating waterlogging stress.