Spatial Distribution of Soil Moisture and Fine Roots of Apple Trees under Water Storage Pit Irrigation

This study was conducted to determine the spatial distribution of soil moisture and fine roots (white, diameter $<2\mathrm{mm}$) growth under water storage pit (WSP) irrigation of apple trees in an experimental apple orchard. Three irrigation treatments have been used since 2009—${\mathrm{WSP}}_{60}$ (water storage pit irrigation, pit depth 60 cm), ${\mathrm{WSP}}_{40}$ (water storage pit irrigation, pit depth 40 cm), and CK (border irrigation)—in controlled trials. In the considered soil profile of a 0–2 m depth, the soil moisture was mostly distributed in the middle-deep soil layer (40–150 cm) of the rhizosphere of apple trees, yet this was lower at a depth of 0–40 cm under WSP (${\mathrm{WSP}}_{60}$ or ${\mathrm{WSP}}_{40}$) irrigation. For CK, the soil moisture was mainly concentrated within a depth of 0–60 cm of the rhizosphere of apple trees. Compared with CK, the fine root length density (FRLD) in the surface soil (0–40 cm) was reduced by 7.52–7.56% under WSP (${\mathrm{WSP}}_{60}$ and ${\mathrm{WSP}}_{40}$) irrigation, whereas in the middle-deep soil layer (40–110 cm), it increased by 10.32–11.70%. The total amount of FRLD in a 0–2 m soil profile under WSP irrigation was 1.19–1.10 times that under CK. The FRLD increased, and the location of the highest concentration of fine roots deepened under WSP (${\mathrm{WSP}}_{60}$ and ${\mathrm{WSP}}_{40}$) irrigation. Thus, this method benefited roots by allowing them to absorb water in a wider area and improving drought resistance. The results obtained provide a theoretical basis for application of the WSP irrigation method.