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), NO 3 –-N, NH 4 +-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 6-m profiles. The variability was low for SWC and pH, moderate for NH 4 +-N, AP, and AK, and high for SOC, NO 3 –-N and EC. Notably, a 6-m soil profile might be robust for exploring the vertical dynamics of SOC, NO 3 –-N, NH 4 +-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 NO 3 –-N by 4530 kg ha−1, AP by 235 kg ha−1, AK by 2435 kg ha−1, and depth-averaged EC by 460 μs cm−1, when compared with cropland. Factor analysis illustrated that pH and EC were jointly driven by NO 3 –-N, AP, and AK, although NO 3 –-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 NO 3 –-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. • Soil chemical properties in the 6-m profile are explored in rainfed apple orchards. • 6-m soil profile is insufficient for evaluating the dynamics of pH and available K. • pH is jointly driven by NO 3 –, Olsen-P, and available K, though NO 3 – is paramount. • Stand age rather than fertilization is responsible for soil desiccation. • Overfertilization causes significant acidification, N-P-K surplus and salinization. [ABSTRACT FROM AUTHOR]