Spatial distribution of soil water and salt in a slightly salinized farmland

It is important to study the mechanisms associated with the spatial distribution of soil water and salt to control soil salinization and promote the sustainable development of farmland. In this study, six plots in gully farmland in the loess hilly region with different spatial locations were selected to determine the spatial distributions of soil water and salt and their correlation using the multifractal method. A grid method (15 m  15 m, 3,600 m2) was applied in the 0–20 and 20–40 cm soil layers where each sampling site was located at the center point coordinates. The results showed that the spatial variability of the soil water and salt were 1.41 and 1.73 times higher, respectively, in the upstream farmland than the downstream farmland. The uneven runoff and sediment distributions from gullies in the upstream farmland increased the spatial variability of the soil water and salt. In addition, the vulnerability of upstream farmland to waterlogging caused further in their spatial variability due to narrow landform features. Analysis using the joint multifractal method showed that the spatial variability of the soil water and salt was strongly correlated (P < 0.05) because of the coupling between soil water and salt. In addition, the spatial variability of the soil water and salt was strongly correlated in the 0−20 and 20−40 cm layers because of the spatial autocorrelations of the soil properties (P < 0.05), thereby indicating that the spatial distributions of soil water and salt in the whole soil layer could be represented by those in the 0−20 cm layer. Thus, we recommend using the 0−20 soil layer to sample the distributions of the soil water and salt. Our results provide a theoretical basis for studying the interactive mechanisms associated with the distributions of soil water and salt, and for optimizing the sampling method in the study area.