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Appropriate water and nitrogen fertilizer management is crucial for achieving sustainable development in the apple industry. However, the soil nitrogen supply capacity and residual nitrate characteristics under drip fertigation remain poorly understood. Therefore, this study aims to (1) investigate the coupling effects of irrigation and nitrogen levels on the concentrations and distribution characteristics of soil soluble nitrogen fractions and residual soil nitrate; (2) explore the relationships between Christiansen uniformity coefficient (CU) of soil nitrate and apple yield, water productivity (WPc) and nitrogen use efficiency (nitrogen agronomic efficiency, AEn). The field experiment was conducted in a drip-fertigated apple orchard starting in 2017, which included two irrigation levels, 85% (W1) and 100% (W2) of field capacity (FC), and four nitrogen rates, 0 (N1), 120 (N2), 240 (N3) and 360 (N4) kg ha–1. The results showed that nitrogen inputs significantly increased soil soluble inorganic nitrogen and soluble organic nitrogen (SON) contents. However, the SON content significantly decreased when 360 kg N ha–1 was applied, particularly at W1. The maximum nitrate accumulation in the 0–3 m soil layer at N4 reached 2540.97 kg ha–1 in 2021, but the maximum value decreased to 1251.62 kg ha–1 in 2022 due to extreme rainfall. The average CU of residual nitrate across irrigation levels in the 0–3 m soil layer was highest at N3, with values of 0.68 and 0.57 in 2021 and 2022, respectively. High irrigation level increased the diffusion distances of nitrate both vertically and horizontally, resulting in a significant effect on CU. Apple yield and WPc first increased and then decreased with increasing nitrogen rates, while AEn consistently decreased, but they were all greater with a higher CU. The recommended combination of irrigation and nitrogen application was 85% FC and 240 kg N ha–1, which can achieve the dual goals of enhancing soil nitrogen supply capacity and minimizing environmental risks while improving orchard productivity.

Appropriate water and nitrogen fertilizer management is crucial for achieving sustainable development in the apple industry. However, the soil nitrogen supply capacity and residual nitrate characteristics under drip fertigation remain poorly understood. Therefore, this study aims to (1) investigate the coupling effects of irrigation and nitrogen levels on the concentrations and distribution characteristics of soil soluble nitrogen fractions and residual soil nitrate; (2) explore the relationships between Christiansen uniformity coefficient (CU) of soil nitrate and apple yield, water productivity (WP c) and nitrogen use efficiency (nitrogen agronomic efficiency, AE n). The field experiment was conducted in a drip-fertigated apple orchard starting in 2017, which included two irrigation levels, 85% (W1) and 100% (W2) of field capacity (FC), and four nitrogen rates, 0 (N1), 120 (N2), 240 (N3) and 360 (N4) kg ha–1. The results showed that nitrogen inputs significantly increased soil soluble inorganic nitrogen and soluble organic nitrogen (SON) contents. However, the SON content significantly decreased when 360 kg N ha–1 was applied, particularly at W1. The maximum nitrate accumulation in the 0–3 m soil layer at N4 reached 2540.97 kg ha–1 in 2021, but the maximum value decreased to 1251.62 kg ha–1 in 2022 due to extreme rainfall. The average CU of residual nitrate across irrigation levels in the 0–3 m soil layer was highest at N3, with values of 0.68 and 0.57 in 2021 and 2022, respectively. High irrigation level increased the diffusion distances of nitrate both vertically and horizontally, resulting in a significant effect on CU. Apple yield and WP c first increased and then decreased with increasing nitrogen rates, while AE n consistently decreased, but they were all greater with a higher CU. The recommended combination of irrigation and nitrogen application was 85% FC and 240 kg N ha–1, which can achieve the dual goals of enhancing soil nitrogen supply capacity and minimizing environmental risks while improving orchard productivity. • High nitrogen rate increases soil soluble inorganic nitrogen content, but decreased soluble organic nitrogen content. • Irrigation, nitrogen and interactions significantly affect residual soil nitrate distribution and accumulation. • Soil nitrate distribution uniformity shows a positive correlation with apple yield, water and nitrogen use efficiency. • 85% field capacity and 240 kg N ha−1 is the optimal irrigation and nitrogen management. [ABSTRACT FROM AUTHOR]

Optimizing fertilization strategies to improve water utilization efficiency and obtain higher crop yields and low nitrate leaching is essential for balancing food security and environmental risks in the rain-fed area of the Loess Plateau. The eight consecutive years (2014–2022) field experiment followed a split-plot design, and the experimental factors were nitrogen application rate (N0, N75, N150, N225, and N300) and manure application rate (M0 and M1). In general, (1) the relationships between nitrogen application rate and grain yield (GY), crop N uptake and water utilization efficiency (WUE) followed a quadratic curve. Compared with chemical fertilizer alone (NPK), chemical fertilizer plus manure (MNPK) significantly increased the annual average grain yield by 28.5%, 18.3%, 8.9%, 6.9% and 4.2% at the N application rate of N0, N75, N150, N225 and N300. (2) The nitrate accumulation (SNA) in the same soil layer increased with the nitrogen application rate. The residue nitrate will increase dramatically when the N application rate exceeds 150 kg ha-1. The peaks of SNA gradually moved to the deeper layer with increasing years of fertilization. The total soil nitrate accumulation (TSNA) of NPK was significantly higher than that of MNPK in dry and normal years, but the opposite was true in wet years. The total soil water storage (TSWS) decreased and then increased with the N application rate, and the annual average TSWS of the MNPK treatments was 3.9% less than that of the NPK treatments. (3) The addition of manure significantly increased the annual average moisture storage rate (MSR) during the fallow season by 4.6%. The GY in dry and normal years was more dependent on MSR than that in wet years. In terms of the long-term effects, the MSR contributes more to GY than growing season precipitation (GSP), and the high MSR caused by manure application reduced TSNA and TSWS by promoting crop growth. The results showed that M1N150 could obtain a high grain yield while maintaining low nitrate residue. Taken together, this study emphasized the importance of optimizing nitrogen fertilizer management and cattle manure application in combination with precipitation types to ensure food production and reduce negative environmental effects in the rain-fed area of the Loess Plateau in China. [Display omitted] • The impact of the long-term addition of manure on nitrate residue was investigated. • Evaluated coupling effects of water and fertilizer using the structure equation model. • The addition of manure reduced nitrate residues in dry and normal years. • The addition of manure will greatly promote the residual nitrate in wet years. • Manure application can regulate nitrate residue by increasing moisture storage rate. [ABSTRACT FROM AUTHOR]