Optimizing Water and Nitrogen Strategies to Improve Forage Oat Yield and Quality on the Tibetan Plateau Using APSIM

Abstract

There is a great need for improving oat forage production to increase forage supply and protect grassland ecosystems on the Tibetan Plateau. We conducted two field experiments and modeling work to investigate the responses of oat (Avena sativa L.) forage yield and N uptake to water and N applications, and to optimize the water and N scheduling under rainfed and irrigated conditions. The experiments were conducted in 2017 and 2018 at Jintai farm in the northeast of the Tibetan Plateau. Two N-applying rates of 120 and 60 kg ha−1 were tested in 2017, and four irrigation treatments (no irrigation—NI, irrigated 50 mm at flowering—I1, irrigated 50 mm at tillering and jointing—I2, and irrigated 50 mm at tillering, jointing, and flowering—I3) were applied under every N rate in 2018. The Agricultural Production System Simulator (APSIM) was calibrated and validated for the local oat variety. Under rainfed conditions in both years, oat yields under high and low N were 7.98–8.52 and 5.09–6.53 t ha−1, respectively; the high N rate significantly increased forage yield and N uptake compared to low N conditions by 22.2–67.4% (p < 0.01) and 42.0–162.0% (p < 0.01), respectively. In 2018, irrigation increased oat forage yield by 29.8–96.6% (p < 0.01) and increased N uptake by 19.6–50.5% (p > 0.05); N rates had no significant effect on forage yield (p > 0.05), but significantly increased N uptake by 42.6–64.7% (p < 0.01). I2 was superior to I3 in terms of increasing water use efficiency (WUE) while maintaining high forage yield and N uptake. APSIM-oat was calibrated with data under both rainfed and irrigated conditions and was confirmed to have good accuracy and lower normalized root mean square errors (NRMSEs) for phonology dates, forage yield, soil water storage, and N uptake. Scenario analysis was performed with 30-year historical weather data; five N rates were designed for rainfed conditions, and 25 scenarios comprising five N rates and five irrigation levels were designed for irrigated conditions. Simulations showed that the N rate of 90 kg ha−1 resulted in the best performance for oat under rainfed conditions. Under irrigated conditions, irrigation promoted oat nitrogen uptake. Thus, overall an N rate of 120 kg ha−1 in combination with irrigation of 120 mm applied during the vegetative growth period performed the best. This optimized strategy may provide guidance on water and N management of oat forage production in the Tibetan Plateau and similar alpine regions worldwide. The promoted strategy increases yields while reducing water and nitrogen resource wastes, thus decreasing the environmental pollution from agriculture and responding to the sustainable development of farmland ecosystems.