Optimizing irrigation and nitrogen fertilizer management to improve apple yield, quality, water productivity and nitrogen use efficiency: A global meta-analysis

Inappropriate strategies in water use and nitrogen (N) fertilizer application in apple production can decrease apple yield, water productivity (WP), nitrogen use efficiency (NUE), and quality, leading to severe environmental pollution. However, there is a lack of systematic analysis on the effects of water use and N fertilizer inputs on apple yield, quality, WP, and NUE at the global scale. Therefore, we conducted a comprehensive meta-analysis with 1834 observations from 72 published studies across 18 countries to assess the effects of water use and N inputs on apple yield, quality, WP, and NUE. We aim to quantify the potential for water-saving and N-saving for global apple production. Our results showed that the global average apple yield, organic acids (OA), total soluble solids (TSS), vitamin C (VC), firmness (FN), WP, and NUE were 34.7 t ha−1, 0.51 %, 13.9 %, 4.7 mg 100 g−1, 7.4 kg cm−2, 6.8 kg m−3, and 214.5 kg kg−1, respectively. Reducing above-optimal water input to optimal water input may increase apple yield by 19.4 %, WP by 36.1 %, OA by 12.5 %, TSS by 8.4 %, VC by 17.5 % and FN by 9.8 %, respectively. Reducing above-optimal N inputs to optimal N inputs may increase apple yield by 8.9 %, NUE by 48.1 %, OA by 10.7 %, TSS by 6.1 %, VC by 13.3 % and FN by 5.2 %, respectively. The overall global mean water-saving and N-saving potentials for apple production were 20.4 % (136 mm less water irrigated) and 37 % (203 kg ha−1 less N fertilizer used), respectively. Our findings provide guidance for water use and N fertilizer management of apple production and can thus help to improve/ maintain yield and quality for greatly saving water and N fertilizer.