Multivariate Controls of Water–Carbon Coupling Relationship Under Various Land Use Types in the Thick Loess Deposits

ABSTRACT Prior studies have conducted extensive investigations of the water–carbon coupling relationship for aboveground vegetation and shallow soils, but the characteristics and multivariate controls in deep soils have not been fully explored. This has important implications for better understanding the water and carbon cycles of ecosystems. In this study, we attempted to examine the water–carbon coupling relationship in 18 m profiles under farmland (F), grassland (G), willow (SP), and poplar (P) in China's Loess Plateau. Specially, the multivariate controls of the water–carbon coupling relationship at diverse depths and scales were explored via the wavelet analysis. Individually, the conversion from F to G, SP, and P decreased the soil water storage within the 0–15 m by 553 (22% of F), 557 (22%), and 943 mm (38%). Land use change had little impact on soil organic or inorganic carbon. Even so, the poplar, compared with the willow, resulted in higher vertical variations in deep water and carbon. Furthermore, land use conversion increased the coherence between soil water and carbon in the whole layer. In comparison to F, which was dominated by water retention, G, SP, and P exhibited dominant carbon sequestration. The poplar reduced the root‐mean‐square deviation between soil water and organic carbon from 0.29 (F) to 0.13 in the 6‐ to 15‐m layer and between soil water and inorganic carbon from 0.32 (F) to 0.20 in the 0‐ to 2‐m layer. Sand content and electrical conductivity both played negative roles in maintaining water and carbon in the 6‐ to 15‐m and 0‐ to 15‐m layers at different scales, respectively. This study endeavors to present a novel viewpoint on land use management, with the potential to enhance ecosystem services in water‐limited and land‐degradation regions.