Effects of ridge mulching biochar on soil moisture and fertilizer, quality and yield of alfalfa in ridge-furrow rainwater harvesting

Ridge-furrow rainwater harvesting (RFRH) is an effective method of collecting rainwater to alleviate drought and control soil erosion in the hilly areas of the semi-arid Loess Plateau in China. The lack of durable covering materials for environmental protection is the key factor limiting the development of RFRH. To determine a suitable ridge-mulching material for RFRH, a field trial with a randomized split-plot design was carried out to determine how the ridges compacted with soil mixed with two types of biochar (rice straw biochar and cow dung biochar) affect the runoff, soil moisture, soil nutrients, soil quality, fodder yield, and water use efficiency (WUE) of alfalfa (Medicago sativa). Three application rates (no biochar application, 0 kg·ha⁻¹; single rate of application, 3 × 10⁴ kg·ha⁻¹; and double rate of application rate, 6 × 10⁴ kg·ha⁻¹) were used, and the ridges compacted with soil without biochar was used as a control. The treatments were RSB (rice straw biochar) and CDB (cow dung biochar) as the main plot, and SR (ridges compacted with soil), SBR (ridges compacted with soil mixed with a single biochar application), and DBR (ridges compacted with soil mixed with double biochar application) as split-plot treatments. The results showed that, compared with CDB, RSB increased the runoff coefficient, soil moisture, fodder yield, and WUE of alfalfa. Similarly, the fodder yield and WUE of alfalfa in RSB increased by 3.34% and 0.82 kg·(ha·mm)⁻¹, respectively. Furthermore, the runoff coefficient and soil water storage capacity decreased, while the soil nutrients, pH, nutrition content, and relative feed value (RFV) of alfalfa increased with the increase in biochar application rates. Compared with SR, the increases in the fodder yield of alfalfa in SBR and DBR were 3.96%～11.05% and 3.33%～6.43%, respectively, and the increases in the WUE of alfalfa of the SBR and DBR were 0.96～1.88 and 0.98～1.38 kg·(ha·mm)⁻¹, respectively. Rice straw biochar applied at a rate of 3 × 10⁴ kg·ha⁻¹ was found to be the most suitable form of biochar for fodder yield and WUE in RFRH.