Runoff and nutrient losses in alfalfa (Medicago sativa L) production with tied-ridge-furrow rainwater harvesting on sloping land

Ridge-furrow rainwater harvesting (RFRH) is known to be effective in controlling water loss and soil erosion, and increasing soil moisture and crop yield in semiarid regions. However, it can cause waterlogging, ridge overtopping, and harvest failure if not properly designed. A four consecutive-year field trial was carried out to assess the impacts of various slope gradients and tillage practices on soil moisture, runoff, sediment yield, associated soil nutrient losses, fodder yield, and water use efficiency (WUE) of alfalfa in the Loess Plateau, China. The trial adopted a split-plot design, taking slope gradient (5° and 10°) as main plot treatment and tillage practice (traditional tillage, open-ridging, and tied-ridging) as split-plot treatment, from 2015 to 2018. There were greater variations in runoff, sediment yield, and associated soil nutrient losses than in fodder yield and WUE. Tied-ridging and open-ridging resulted in decreased runoff and reduced sediment transport and associated soil nutrient losses, and increased soil moisture, fodder yield, and WUE. Runoff, sediment transport, and associated soil nutrient losses inclined with increasing slope; while fodder yield and the WUE declined. For slopes of 5° and 10°, the average decrease in runoff, sediment yield, total nitrogen, total phosphorus, and organic matter losses for open-ridging were 47.7–56.2%, 91.0–92.7%, 90.4–93.1%, 90.1–92.2%, and 88.9–90.2%, respectively, while these for tied-ridging were 62.4–68.4%, 94.5–96.4%, 93.6–95.5%, 93.9–95.9%, and 93.0–94.9%, over 4 years as compared with traditional tillage. The mean annual runoff, sediment, total nitrogen, total phosphorus, and organic matter losses for slopes of 10° were 1.31, 1.96, 1.87, 1.99, and 1.64 times greater than the corresponding values for slopes of 5°, respectively. The increase of fodder yield and WUE for open-ridging and tied-ridging was 40.1–45.6% and 16.0–17.5%, and 3.66–5.99 and 1.35–2.50 kg ha−1 mm−1, respectively, for slopes of 5° and 10°. The average fodder yield and WUE for slopes of 5° were 1.19 and 1.23 times greater than corresponding values for slopes of 10°, respectively. The disparities in fodder yield and WUE of alfalfa between slopes of 5° and 10° were not clear in dry years, but were evident in wet years. Tied-ridging was recommended for water and soil conservation, while open-ridging was recommended for increase of fodder yield and WUE of alfalfa. The implementation of RFRH in crop production should be evaluated by the use of a model in accordance with climatic conditions, soil type, and plant species.