Conversion of Farmland to Apple Orchards Modifies Water–Carbon–Nitrogen Trade-Offs in Deep Loess Deposits

Abstract

Conversion of shallow- to deep-rooted plants greatly alters the water–carbon–nitrogen coupling relationship in the deep soil, but has not been fully explored because of the difficulty and one-variable-at-a-time method in sample collection. This study aims to understand the effects of conversion of farmland (F) to apple orchards on deep soil water (SW)–carbon–nitrogen coupling in the loess covered region. We collected soil samples and determined SW, soil organic carbon (SOC), and soil total nitrogen (STN) contents and other soil indicators (pH, ammonium nitrate nitrogen, and soil texture) within 0–20 m under F and apple orchards with trees of different stand ages (years of 5, 10, 15, 20 and 24; abbreviated as A5, A10, A15, A20, and A24, respectively) in China's Loess Plateau. We used the partial least squares structural equation to ascertain the influence of vegetation and edaphic factors on the water–carbon–nitrogen coupling relationship. The mean SW contents in old orchards (A15, A20, and A24) decreased by 14%–22% compared to those of F, A5, and A10. The contents of SOC showed insignificant differences between F and apple orchards (p > 0.05). While nitrate nitrogen (NO₃⁻N) content in F was significantly lower than that in apple orchards (p < 0.01), no significant difference was identified in STN. Under F and apple orchards, the relative benefits of water storage were higher than those of carbon and nitrogen in 0–5 m. However, within 5–20 m, the water-dominated benefits under F shifted to nitrogen- and carbon-dominated benefits under orchards due to large water deficit caused by the planation of apple trees. Furthermore, the fertilization-induced changes in soil properties dominated the water–carbon–nitrogen coupling processes within 0–5 m, while root density and soil texture had significant impacts on the coupling processes below 5 m. This indicates that the deep soil carbon and nitrogen fixation is at the expense of water consumption. This study provides valuable information for agricultural land management in arid regions with degraded soil quality and deep unsaturated zones.