Seed mixture of oats and common vetch on fertilizer and water-use reduction in a semi-arid alpine region

The overuse of synthetic fertilizers damages terrestrial and aquatic ecosystems on the Tibetan plateau. We conducted this study to investigate the possibility of reducing fertilizer use by applying cereal-legume mixture planting in forage crops cultivation. A field experiment was conducted in 2017 and 2018 at Jintai farm in the northeast of the Tibetan plateau. The experiment was a randomized complete block design with four treatments: sole oat with nitrogen application rate of 120 kg ha⁻¹, sole oat with a lower nitrogen rate of 60 kg ha⁻¹, and oat and common vetch mixtures applied with those two nitrogen levels. Results showed that forage biomass production of oat-common vetch mixture was comparable or higher than that of corresponding sole oat under different nitrogen levels. Nitrogen uptake of the mixture was 19.6–81.7% higher than that of sole oat under high nitrogen fertilization and was 107.3–144.6% higher under low nitrogen. Biological nitrogen fixation of common vetch in mixture stands ranged from 11.1 to 59.1 kg ha⁻¹. Nitrogen use efficiency in the mixture was lower than that in sole oat under both nitrogen levels. However, the partial factor productivity of applied nitrogen (PFPN) in the mixture was higher than that in sole oat. Mixture with low nitrogen application attained the highest PFPN of 102.8 and 150.6 kg kg⁻¹ in 2017 and 2018 respectively. The mixture system depleted much more soil water storage than sole oat in both years. Water use efficiency based on biomass production in the sole and mixture systems were comparable. However, that based on crude protein production was far greater in the mixture system, and the advantage of the mixture under low nitrogen level was more significant compared to that under high nitrogen level. Therefore, we conclude that practicing cereal-legume mixture planting in forage production could improve nitrogen and water use efficiencies, maintain forage production and improve forage quality. We suggest applying the mixture planting system and reducing the amount of nitrogen application in forage production in the semi-arid farming area on the Tibetan Plateau and similar alpine regions worldwide.