Strategies for reducing the fertilizer application rate in the ridge and furrow rainfall harvesting system in semiarid regions

The ridge and furrow rainwater harvesting (RFRH) system is a promising water-saving planting technique for dryland farming, but we lack a full understanding of the effects of different fertilizer rates (N:P) on plant nutrient uptake and nutrient use efficiency (NuUE) in foxtail millet using this planting method, as well as the available nutrient residues in the soil. We conducted field studies (Loess Plateau, China) comparing RFRH planting (R) and traditional flat planting (T) at four different fertilizer rates to determine suitable fertilizer application rates for R during 2013–2015. Compared with T, R improved the soil moisture and the utilization of rainwater and fertilizer, thereby enhancing the grain yield, water use efficiency (WUE), grain nutrient uptake, and NUE in a dry year, but with no improvements in a rainy year. The grain yield and WUE exhibited parabolic increasing trends as the fertilizer application rate increased over three years, but no significant increase was found when the fertilizer rate exceeded 189:96 kg N:P ha(−1) under R, which significantly reduced the NuUE and might waste nutrients. Therefore, we recommend R combined with 189:96 kg N:P ha(−1) as a promising planting strategy for foxtail millet in semiarid areas.