Divergent responses of soil physicochemical properties in 6-m profiles to long-term overfertilization in rainfed apple orchards on China's Loess Plateau

Overfertilization is prevalent in rainfed apple orchards on China's Loess Plateau; yet, its impacts on soil physicochemical properties in deep soil profiles are poorly documented. To address this, we sampled sixty 6-m soil profiles from cropland, 8-year, 17-year, and 25-year apple orchards to determine soil water content (SWC), pH, soil organic carbon (SOC), NO3--N, NH4+-N, Olsen-P (AP), available potassium (AK), and electrical conductivity (EC). Divergent distribution patterns of the studied properties for various land-use types were observed in the 6m profiles. The variability was low for SWC and pH, moderate for NH4+-N, AP, and AK, and high for SOC, NO3--N and EC. Notably, a 6-m soil profile might be robust for exploring the vertical dynamics of SOC, NO3--N, NH4+-N, AP, and EC rather than SWC, pH and AK. Averaged across the 6-m profiles, 8-year orchards had equivalent values for all the selected variables, while the 25-year orchards exhibited significantly decreased soil water storage by 165 mm and depth-averaged pH by 0.50, but increased NO3--N by 4530 kg ha-1, AP by 235 kg ha-1, AK by 2435 kg ha-1, and depth-averaged EC by 460 mu s cm- 1, when compared with cropland. Factor analysis illustrated that pH and EC were jointly driven by NO3--N, AP, and AK, although NO3--N was of paramount importance for regulating pH and EC. A partial least squares path model further revealed that stand age rather than fertilization negatively affected SWC, and fertilization significantly influenced NO3--N, AP, and AK, the direct effects of which on pH were depth-dependent, highlighting the importance of deep profiles for elucidating the evolution of soil quality. Collectively, rainfed apple orchards exposed to repeated severe overfertilization were linked to aggravated soil degradation in terms of desiccation, acidification, N-P-K surplus and salinization. Hence, farmers' rampant overuse of fertilizers must be urgently controlled in order to develop sustainable apple production.

Automated summary

Overfertilization is common in rainfed apple orchards on China's Loess Plateau, but its impacts on soil physicochemical properties in deep soil profiles are poorly understood. This study found that different land-use types showed divergent distribution patterns in soil properties, with low variability for SWC and pH, moderate variability for NH4+-N, AP, and AK, and high variability for SOC, NO3--N, and EC. The results also showed that fertilization influenced soil water content, NO3--N, AP, and AK, and NO3--N played a crucial role in regulating pH and EC. Soil profiles were useful for studying the evolution of soil quality.