Effects of planting year and density on deep soil desiccation of apple orchards in Weibei dryland

In order to explore the effects of planting year and density on deep soil desiccation of apple orchards in Weibei dryland, soil moisture deep to 15 m belowground were measured twice for 5 apple orchards with different planting year and density in 2008 and 2015, respectively, and cropland was used as control.The results showed that: 1) the soil desiccation index (SDI) measured in 2008 for 5, 10, 15, 20 and 25 a were 6.46%, 32.68%, 37.65%, 63.33% and 62.81%, while SDI for the same orchard 7 years later were 35.98%, 59.65%, 42.21%, 75.06% and 70.09%, respectively, indicated that the longer the planting year was, the higher the SDI was.In addition, the degrees of soil desiccation were quickly from 5 to 12 a and 10 to 17 a, the increasing proportion per year were 4.22% and 3.85% averagely, but slowly from 20 to 27 a with an increasing proportion of 1.7%.The soil desiccation of 5 a apple orchard was slight and the soil moisture was close to cropland, and after planting 15 a, the soil desiccation spread to 11 m belowground (paleosol layer), and to 17 a, the buffer function of paleosol layer was lost and if the rainfall in a year cannot meet the water consumption of apple trees, the quality and yields would be limited by and linearly dependent on annual rainfall.2) Thinning could slow the soil desiccation process.The orchards with a planting density of 4 m×6 m (15 a, 25 a) had significantly lower rates of soil desiccation during 7 years, with an average of 0.65% and 0.61% per year, respectively.The soil desiccation index of 22 a orchard was 42.21%, which was 17.44% and 32.85% less compared to orchards of 17 a and 27 a, respectively.In addition, the soil moisture of 2-5 m layer with thinning treatment was higher than the non-thinning treatment, indicating orchards with lower density could compensate soil water in deep layer by rainfall in a wet year.However, in a dry year, even for the orchard with lower density, the water consumption of trees would be inevitable to be compensated by deep soil water, which was more common in old orchard, and thinning can only delay the formation of soil dry layer.Therefore, this study suggested corresponding measures should be taken around 12 a after planting, before the soil desiccation expanded to the paleosol layer.