Effects of planting apple trees on distribution of soil cementing materials in Weibei apple orchards

The aim of this study was to explore the effects of planting apple trees on soil cementation substances, to determine the relationship between soil aggregate stability and soil cementation substances, to reveal the factors affecting soil aggregate state and to build theoretical basis for the scientific management of apple orchard in Weibei Plateau. Four replicates of 10-year-old and 20-year-old orchards and four farmlands (wheat-corn rotation, CK) were selected for the experiment in Binxian County, Shaanxi Province. Soil samples within two-thirds of the radius of apple tree canopy were collected. Four replicates of undisturbed core samples were taken by driving volumetric rings (100 cm³) into the 0-10 cm, 10-20 cm, 20-30 cm, 30-40 cm and 40-50 cm soil layers to determine soil aggregates. Another four replicates were collected from 10 cm and 20 cm intervals respectively over a depth of 0-60 cm and 60-100 cm in each plot to determine the soil contents of clay, organic matter (SOM) and calcium carbonate (CaCO₃) contents. The results showed that soil clay content increased with increasing depth in the 0-100 cm soil layer. The increase was in the order of farmland > 10-year-old orchard > 20-year-old orchard for the 0-40 cm soil layer and reverse order was noted for the 40-100 cm soil layer. Planting apple trees increased SOM content in the 0-100 cm soil layer of the orchard. However, SOM content decreased with increasing planting age of orchard. The content of soil CaCO₃ was in the order of 10-year-old orchard > farmland > 20-year-old orchard for the 0-40 cm soil layer, 20-year-old orchard > 10-year-old orchard > farmland for the 40-100 cm soil layer, and 10-year-old orchard > farmland > 20-year-old orchard for the 0-100 cm soil layer. Pearson correlation analysis showed that the quantity and mean weight diameter (MWD) of > 0.25 mm soil aggregates were closely link to soil clay, SOM and CaCO₃ contents. Macro aggregates (> 0.25 mm) and MWD of mechanically stable aggregates were mainly affected by CaCO₃ and SOM contents. However, the quantity and stability of water stable aggregates were mainly affected by soil clay and CaCO₃ contents. Overall, planting apple trees changed the evolution processes of soil clay, SOM and CaCO₃. Soil clay and CaCO₃ migrated to deep soil with increasing age of orchard. The total storage of SOM in orchard was higher than that in farmland, but decreased with increasing planting age. It was concluded that planting apple trees enhanced soil residual viscosity and calcification, which affected aggregation of surface soil and compaction and hardness of bottom soil.