Dynamics of Productivity and Nitrogen Contribution in Mixed Legume/Grass Grasslands in Rain-fed Semi-arid Areas in Northwest China

Mixed legume/grass grasslands are the most significant type of artificial grassland in rain-fed semi-arid regions. Understanding the contributions of legumes and grasses to grassland productivity, as well as the nitrogen-sharing mechanisms between them, is crucial to maintaining the sustainability, stability, and high yield of mixed grasslands. In this study, four commonly used cultivated species were selected: smooth bromegrass (Bromus inermis Leyss.), orchardgrass (Dactylis glomerata L.), sainfoin (Onobrychis viciifolia Scop.), and red clover (Trifolium pratense L.). Combinations of two and three species of legumes and grasses were established, with monoculture serving as the control. The results revealed that in all the monocultures and mixed grasslands comprising two or three species, the average dry matter yield (DMY) of mowed grasslands in 2017 was significantly higher than in 2018, while the average DMY of grazed summer regrowth in 2018 surpassed that of 2016 and 2017. Over the period from 2016 to 2018, smooth bromegrass and sainfoin gradually dominated the mixed grasslands, while orchardgrass and red clover exhibited a declining abundance. Over time, the ratio and amount of nitrogen (N) fixation in legumes significantly increased in both the monoculture and mixed grasslands. Similarly, the amount of nitrogen (N) received by grasses also increased significantly in mixed grasslands. However, the proportion of nitrogen fixed by legumes remained below 10% in 2016, 20% in 2017, and 30% in 2018. In contrast, nitrogen transfer from legumes to smooth bromegrass was less than 10%, while in orchardgrass, it was even lower, at less than 2%. The interannual variability in dry matter yield (DMY) and nitrogen contribution in the mixed grasslands of rain-fed semi-arid areas is primarily influenced by forage adaptability and average annual precipitation. Increasing the proportion of grazed forage relative to hay in annual forage consumption should be considered, as more extensive grazing can reduce damage from field rodents and provide higher forage quality at lower costs and energy consumption. To maintain grassland productivity, targeted grazing should be carefully planned and implemented.