Enhancing soil water, carbon, and nitrogen by partially substituting chemical fertilizer with organic fertilizer integrated with a rainwater collection system in rainfed orchards

Rainfed apple orchards, particularly in loess hilly regions of China, face significant challenges due to water scarcity and declining soil fertility, necessitating their restoration for sustainable apple production. This study investigated the impact of partially substituting chemical fertilizers with organic fertilizers integrated with a rainwater collection system on restoring soil water storage (SWS), soil organic carbon (SOC) stocks, and total nitrogen (TN) stocks and their relationship with apple production. Three orchard management practices were evaluated: (1) OSCR: partially substituting chemical fertilizer with organic fertilizer (60% N from organic fertilizer) integrated with a rainwater collection system; (2) CR: chemical fertilizer integrated with a rainwater collection system; and (3) CK: chemical fertilizer alone without a rainwater collection system. Averaged across five years, the OSCR treatment (457.83 mm) substantially enhanced SWS from 0 to 280 cm profile by 19.12% compared to the CK (384.34 mm). OSCR system outperformed CR treatment in terms of the 0–280 cm SWS compensation index (SWSCI). The OCSR and CR treatments had higher SWSCI values in 2016 (dry year) than the other years, indicating more effective SWS restoration in dry years. Average across five years, OSCR treatment had 55.26% and higher SOC stock (214.1 Mg ha^–1) and 45.91% higher TN stock (5.72 Mg ha⁻¹) from 0 to 280 cm than the CK; and the OSCR treatment positively restored SOC and TN stocks, while the CR treatment decreased SOC stock and restored TN stock from 0 to 200 cm profile. Compared to 2016, OSCR, CR and CK treatments increased SOC stock by 36.84%, −5.78% and −6.98%, and TN stock by 36.31%, 37.88 and −3.36% in 2020, respectively. Average across five years, the OSCR treatment had the highest mean apple yield (24.61 Mg ha⁻¹) and crop water production (CWP, 48.64 kg ha⁻¹ mm⁻¹), 64.62% and 66.69% higher than the CK treatment across five years; no significant differences in fruit size or shape index occurred between treatments. The difference in apple yield is unfavorably connected with SWSCI and favorably connected with SOC and TN stock restoration. Overall, the OSCR system successfully restored SWS, SOC, and TN stocks, increasing apple yield and CWP, making it a promising management strategy for rainfed apple orchards.