Fall nitrogen application increases seed yield, forage yield and nitrogen use efficiency more than spring nitrogen application in Leymus chinensis, a perennial grass

Efforts to maximize seed or forage yield with nitrogen (N) fertilizer are common in cropping systems. However, only limited attention has been paid to investigate N fertilizer application in different seasons, especially for perennial grasses, whose seed production requires two or more growing seasons. We conducted a two-year field experiment to evaluate the impacts of N application timing (fall in previous year and spring in current year) and N application rate (0, 28 and 56 kg N ha(-1) yr(-1)) on seed yield, forage production and N use efficiency for a semi-arid, perennial grass (Leymus chinensis). As compared to the spring N application, we found that fall N application at the inflorescence stage in the previous year significantly increased seed yield, forage yield, seed agronomic N use efficiency (s-aNUE), forage agronomic N use efficiency (f-aNUE), seed physiological N use efficiency (s-PE) and forage physiological N use efficiency (f-PE). Seed yield and forage yield were both higher with high N-rate treatment as compared to low N-rate in both years. Nitrogen rate effects on N use efficiency indices varied with year and N-timing. We conclude that N application in the fall of the previous year matches N demand for increases in seed yield and forage yield and this is an optimum choice to achieve greater seed yield and forage yield with high N use efficiency. Our research implies that N application in the fall increases inflorescence number and fall tillers by stimulating inflorescence and/or fall tillers numbers. Future research should focus on determining for each physiological stage that control the seed and/or forage yield components, the limiting resource. Then targeting the limiting resource at each physiological stage can increase yield, and, for perennial crops, the key stages regulating seed and/or forage yield components can be in different years, as we found for Leymus chinensis.