Effects of water and nitrogen gradients on growth and water use efficiency of forage sweet sorghum

Objectives This study investigates the effects of different irrigation levels and nitrogen (N) application rates on the growth, water consumption, and water use efficiency of forage sweet sorghum (Sorghum bicolor L. Moench) to provide theoretical and technological support for the efficient cultivation of forage sweet sorghum.

Methods A pot experiment was conducted under greenhouse conditions using the sweet sorghum cultivar ‘Hunnigreen’ as test material. The three irrigation rates employed were maintaining soil moisture content at 30%～50% (I1), 50%～70% (I2), and 70%～90% (I3) of the field capacity. The three N application rates were 0 kg/hm² (N0), 200 kg/hm² (N1), and 400 kg/hm² (N2), giving a total of 9 treatments. The soil moisture and plant water consumption were automatically monitored by a soil moisture sensor and auto-weighed lysimeters, respectively. The plant height and stem diameter were investigated 30, 40, 50, 65, and 85 days after emergence. The fresh and dry weights of plant were investigated 85 days after the emergence of forage sweet sorghum.

Results Compared with I1, I2 and I3 increased plant height by 20.0% and 34.5% and stem diameter by 18.0% and 36.6%, respectively. N application rate had no significant effect (P>0.05) on plant height and stem diameter. Increasing irrigation (P<0.05) promoted the fresh weight and dry weight of stems, leaves, and whole plants, while increasing N application rate only affected the fresh leaf weight and stem dry weight. Compared with I1, I2 and I3 increased the whole plant fresh weight by 158.1% and 290.1% and dry weight by 84.8% and 139.3%, respectively. Compared with N0, N1 and N2 increased the fresh leaf weight by 7.6% and 4.6% and decreased dry stem weight by 8.5% and 10.4%. The stem-leaf ratio was <1 under I1, >1 under I2 and I3, and higher under I3 than I2. Under I1 and I3, the stem-leaf ratio decreased with an increase in the N application rate. Increasing irrigation (P<0.05) increased the individual plant water consumption and decreased the water use efficiency. Compared with I1, I2 and I3 increased the cumulative water consumption by 101.3% and 178.9% and decreased the water use efficiency by 8.0% and 13.9%, respectively. N application rate had no significant effect (P<0.05) on cumulative water consumption and dry matter water use efficiency.

Conclusions The growth, water consumption, and dry matter water use efficiency of forage sweet sorghum were highly affected by irrigation level and slightly impacted by nitrogen application rate. The increase in irrigation level promoted the growth of plant height and stem diameter, the accumulation of dry matter and the dry matter allocation to stem, and increased the water consumption, but decreased water use efficiency. Therefore, moderate irrigation and low N fertilizer application rate are promising for efficient forage sweet sorghum production and water use efficiency.