Effects of different rates of nitrogen fertilizer on apple yield, fruit quality, and dynamics of soil moisture and nitrate in soil of rainfed apple orchards on the Loess Plateau, China

Abstract

Nitrogen (N) and water deficiency jointly limit apple production in semiarid apple-producing regions worldwide. A 4-year field experiment (2017–2020) was conducted on China’s Loess Plateau to systematically identify the responses of apple yields, fruit quality, and the dynamics of soil water and NO3−-N in 6-m soil profiles to different N (i.e., urea) application rates of 0, 300, 450, 600, 750, and 900 kg N ha−1. The results showed that apple yield averaged 30.02 t ha−1, with remarkable interannual fluctuations, although its differences between N rates were not statistically significant during 2017–2020. Similar responses were recorded for fruit quality indices, except that N-fertilization significantly increased total soluble solids compared with the non-N treatment. Interestingly, soil moisture differed considerably between N rates within the 100–300 cm depth, showing increasing soil moisture with increasing N rates, while an opposite trend was observed at 500–600 cm. The calculated minimum N rates achieving the balanced and maximum soil water storage within the 6-m profile (SWS6 m) were 244 and 323 kg ha1, respectively. Distinctive responses of NO3−-N to N rates were observed at 100–300 cm depth, whereas equivalent NO3−-N concentrations between N rates were recorded across the 520–600 cm depth. The minimum N rates achieving the balanced and maximum residual soil NO3−-N within the 6-m profile (RSN6m) were 412 and 509 kg N ha1, respectively. Notably, SWS significantly linearly increased as a function of RSN across the 6-m soil profile, implying the capacity of N fertilizer to facilitate soil moisture. Collectively, N fertilization slightly affected apple yield and fruit quality but significantly altered soil moisture and NO3−-N within the 6-m soil profile. An N application rate of 412 kg ha1 was recommended to support apple production and maintain balanced SWS6 m and RSN6 m for rainfed apple orchards on China's Loess Plateau.