Optimizing the ridge–furrow ratio and nitrogen application rate can increase the grain yield and water use efficiency of rain-fed spring maize in the Loess Plateau region of China

Providing a suitable water environment and nitrogen (N) supply are essential for improving the crop productivity, especially in rain-fed areas affected by drought and nutrient deficiencies. The ridge–furrow mulching system (RFMS) is an efficient planting pattern in the rain-fed area of the Loess Plateau in China. Changing the ridge–furrow ratio in RFMS often modifies the hydrothermal environments. Few previous studies have investigated the combined effect of changing the ridge–furrow ratio and N application rate. Thus, we conducted a field experiment from 2020 to 2021 in the Loess Plateau region of China with two planting patterns (flat planting without film mulching and RFMS with three ridge–furrow ratios of 40:70 cm, 55:55 cm, and 70:40 cm) and three nitrogen application rates (N1, 180 kg ha–¹; N2, 240 kg ha–¹; and N3, 300 kg ha–¹). We analyzed the effects of different N application rates and planting patterns on the soil hydrothermal environment, growth and development, grain yield, and water use efficiency (WUE) of spring maize. The results showed that the soil hydrothermal environment improved as the ridge–furrow ratio increased, but increasing the N application rate risked aggravating the soil water consumption. Compared with FP, the warming effect of RFPM accelerated senescence and shortened growth period (1–4 days) of spring maize under N1, and the negative effect could be effectively alleviated with the increase of N application rate. Increasing the ridge–furrow ratio and N application rate increased the photosynthetic capacity, leaf area, and accumulated dry matter. Compared with FP, RF40–70, RF55–55, and RF70–40 significantly increased the grain yield by 12.96%, 20.28%, and 28.98%, and significantly increased WUE by 16.01%, 22.35%, and 31.83%, respectively. Grain yield and WUE also increased with the increase of N application rate. Regression analysis further showed that, under RFMS at 70:40 cm, the highest spring maize grain yield and WUE were achieved with N application rates of 274.14 kg ha–¹ and 268.56 kg ha–¹, respectively. Therefore, we suggested that RFMS at 70:40 cm and N application at 268.56–274.14 kg ha–¹ are most suitable for rain-fed spring maize production in the Loess Plateau region.