Abstract:

To ascertain the appropriate N fertilizer rate for forage sorghum （Sorghum bicolor） production in dry regions of the Longdong Loess Plateau， a two-year field experiment was conducted at the Qingyang National Field Station of Grassland Agroecosystem during 2019 and 2020 to determine the effects of different N fertilizer application rates on dry matter accumulation， allocation， water consumption， and water use efficiency of forage sorghum cultivar ‘F10’. Five N application rates： 0， 80， 160， 240， and 320 kg·ha^-1 （denoted N₀， N₈₀， N₁₆₀， N₂₄₀， and N₃₂₀， respectively）， were used in this study， and each treatment was replicated three times using a randomized complete block design. In both seasons， dry matter accumulation （DMA） of forage sorghum at the jointing， heading， flowering， and filling stages showed an increasing trend with successive increments in N application rate. At the harvest stage in 2019 and 2020， the DMA of forage sorghum increased initially and then decreased with further increase in N application rate， and DMA peaked at N₁₆₀ with values of 22.3 t·ha^-1 in 2019 and 18.0 t·ha^-1 in 2020. As the growing season progressed， dry matter proportion （DMP） of leaves in forage sorghum gradually decreased， while DMP of stems initially increased and then decreased. At the harvest stages， the DMP of stems was the highest with average values of 70.8% in 2019 and 73.8% in 2020. In 2019， compared to N₀， the N fertilization treatments significantly increased DMP of ears and decreased DMP of stems， however， no significant difference was observed in 2020. In 2019， there were no significant differences in water consumption （WC） and water consumption rate （WCR） of forage sorghum among the different treatments. In 2020， the WC of forage sorghum in the treatments varied from 483.4 to 505.8 mm， and the WCR varied from 3.1 to 3.3 mm·d^-1. Compared to N₀， the WC in 2020 was significantly increased by 4.6% in N₈₀ and by 3.9% in N₃₂₀， respectively， and the WCR in 2020 was significantly increased by 6.5% in both N₈₀ and N₃₂₀. The water use efficiency （WUE） of forage sorghum at N₁₆₀ was the highest with values of 42.9 kg·ha^-1·mm^-1 in 2019 and 36.4 kg·ha^-1·mm^-1 in 2020. A regression analysis indicated that the maximum DMA at the harvest stage （19.2 t·ha^-1） was obtained at the N application rate of 166.7 kg·ha^-1 （close to 160.0 kg·ha^-1）， and the maximum WUE （37.8 kg·ha^-1·mm^-1） was achieved at the N application rate of 150.0 kg·ha^-1. Based on combined consideration of DMA and WUE， 150-160 kg·ha^-1 can be recommended as a suitable N application range for forage sorghum production in dry regions of the Longdong Loess Plateau.

Key words: forage sorghum, nitrogen application rate, dry matter accumulation, water consumption, water use efficiency