种植年限及密度对渭北旱塬苹果园深层土壤干燥化的影响

为探究种植年限及密度对苹果园土壤干燥化变化的影响,以渭北旱塬洛川县不同种植年限和密度的果园为研究对象,采用烘干法测土样,2008年及2015年分别2次测得不同果园15 m深土层含水量,以自然条件下农田为对照,分析其干燥化变化规律。结果表明:1)2008年测定5、10、15、20、25 a果园的土壤干燥化指数分别为6.46%、32.68%、37.65%、63.33%和62.81%,2015年测定当年果园经过7 a后的12、17、22、27、32 a果园的土壤干燥化指数分别为35.98%、59.65%、42.21%、75.06%和70.09%,随着树龄增大,土壤干燥化指数整体增大,干燥化程度加剧;5 a生苹果幼园土壤干燥化轻微,程度接近自然条件下农田;树龄15 a后的果园土壤干燥化波及深度达11 m左右的古土壤蓄水层,至17 a后渭北旱塬古土壤层的缓冲作用已经丧失,果园产量及果实品质受限于当年降雨。2)研究中株行距密度从3 m×4 m变为4 m×6 m的22、25和32 a果园,干燥化进程得到控制,干燥化速率大幅下降,降低果园密度可以缓解因苹果树树龄增长造成的果园干燥化加剧,间伐措施后的果园降雨有所盈余,可以改善土壤干燥化现状;但在亏水年,间伐后的果园在充分利用降雨时,还需深层土壤水分的弥补,在树龄大的果园尤为常见,单靠间伐措施只能延迟土壤干层的形成。因此,要控制和减缓土壤干燥化的发展,应在干燥化影响波及古土壤层前采取措施,也即种植12 a左右,果园处于中度干燥化程度时采取相应对策最佳。

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.