The regulating effects of water and fertilizer coupling on the growth of the continuous cropping forage sorghum ‘Jintian No. 1’ in oasis irrigation area

Based on the water and fertilizer demand (irrigation amount: 7800 t·ha⁻¹; fertilization: 600 kg·ha⁻¹) and cultivation mode (plant spacing × line spacing: 23 cm × 40 cm; planting method: one plant per hole) of forage sorghum‘Jintian No. 1’ (BJ0601) for the first planting in the desert oasis irrigation area of the Hexi Corridor, the effects of continuous cropping on the growth adaptability (growth and development, fresh weight of stem, and seed yield and quality), soluble sugar content of the stems, and soil fertility (organic carbon, mineral nutrients) of ‘BJ0601’ were studied. The first planting and rotation were used as the control. The results showed that compared with the first planting, no significant difference was found in the stem fresh weight of ‘BJ0601’ under rotation, but the seed yield (925.1 kg·ha⁻¹) was significantly lower than that of the first planting (2029.5 kg·ha⁻¹). The growth adaptability of ‘BJ0601’ was significantly inhibited after 3 years of continuous cropping; compared with the first planting, the number of seedlings, seedling rate and plant height of ‘BJ0601’ in continuous cropping decreased by 11.5%, 11.4%, and 43.7%, respectively. The stem fresh weight (57.8 t·ha⁻¹) and seed yield (677.2 kg·ha⁻¹) under continuous cropping decreased by 26.4% and 71.5%, respectively. No significant difference was found in the soluble sugar content of the stems among the treatments. The soil fertility of ‘BJ0601’ significantly decreased after 3 years of continuous cropping. Compared with the first planting and rotation, the soil organic carbon content in the rhizosphere soil (10－20 cm depth) of ‘BJ0601’ decreased by 34.4% and 36.7% after 3 years of continuous cropping, respectively; the contents of alkaline N, available P, and available K in the rhizosphere soil significantly decreased by 71.9% and 43.9%, 85.7% and 38.8%, and 30.9% and 16.9%, respectively (the changes in the nutrients in the other soil tillage layers were the same as those in the 10－20 cm soil layers). However, under both high irrigation and fertilizer amounts (irrigation: 9000 t·ha⁻¹ and fertilization: 825 kg·ha⁻¹), the stem fresh weight (74.9 t·ha⁻¹) and seed yield (962.6 kg·ha⁻¹) of ‘BJ0601’ under continuous cropping for 3 years increased by 29.6% and 66.3%, respectively, compared with those under continuous cropping for 3 years under the optimal water and fertilizer treatments. The fresh weight and soluble sugar content of the stems were not significantly different from those of first planting and under rotation. The above findings showed that continuous cropping significantly affects the growth adaptability and soil fertility of ‘BJ0601’, but high irrigation and fertilizer amounts together under crop rotation can reduce the negative effects of continuous cropping on the growth of ‘BJ0601’ and slow the decline in soil fertility. In the desert oasis areas, the irrigation and fertilization methods, tillage measures, and cultivation management must be comprehensively improved to mitigate the adverse effects of continuous cropping on ‘BJ0601’ growth and soil fertility, so as to ensure high productivity of ‘BJ0601’ and the efficient utilization of water and fertilizer resources.