黄土高原不同林龄苹果树根系吸水策略对降水的响应

在黄土高原陕西省长武塬区选取品种和管理手段均相同的3种林龄果园(尚未结果的5年幼龄果园、已结果的8年初果园和13年壮果园)苹果树,采用空间换时间的试验设计,分别于2015年7月12日和8月19日对0—500cm深度土壤及对应取样处的苹果树枝条取样,测定土样和枝条样中水分的稳定氢氧同位素,并利用贝叶斯模型量化降水前后不同土层对苹果林耗水的贡献。结果表明:(1)不同林龄苹果树降雨前后的主要水分来源深度不同。干旱时,13年壮龄果树的主要吸水深度比5年和8年果树深;而生长旺季,雨季降水只能补充未挂果的5年幼龄果园土壤水分消耗,即使降水量很大,也无法满足已经开始挂果的8年和13年果园土壤水分消耗。(2)在干旱期,5年和8年果树50%以上的水分来自表层0—100cm土壤,而13年果树50%的水分来自100—300cm土层。而降水后,5年和8年果树的主要水分来源变为100—300cm土层,贡献值在40%左右;13年果园的主要水分贡献层为0—100cm土层,贡献了近50%的水分。(3)3种林龄果树根系对300—500cm土层土壤水分的吸收对降雨的响应非常弱,降雨前后贡献率始终保持在30%。

Responses of Water Uptake Pattern of Apple Trees with Different Stand Ages to Precipitation on the Loess Plateau

Using the space-for-time substitution experimental design, three apple orchards with different ages but the same species and management (i.e. 5-year old young orchard has not yet borne fruit, 8-year old orchard has borne fruit for two years, and 13-year old mature orchard, respectively) were selected on the Loess Plateau in Changwu County, Shaanxi Province. On July 12th and August 19th, 2015, soil samples were taken at 0.2 m intervals from the surface to 5 m depth of a profile, and the corresponding apple tree branches were taken at each site. The stable oxygen and hydrogen water isotopes of the soil and branch samples were measured and the Bayesian mixing model was used to quantify the contribution of soil layers from different depths to apple water uptake and their responses to precipitation. Results showed that:(1) The depth of the primary water source for apple trees before and after precipitation differed with stand ages. During the dry season, the main water absorbing depth of 13-year old trees was deeper than that of 5- and 8-year old trees. At the peak growing season, rainfall in the rainy season could only increase the soil water storage in 5-year old young orchards that has not borne fruits, thus young orchards were free of deep soil water deficit. For the 8- and 13-year old mature apple orchards that has borne fruits for several years, the deep soil water could not be replenished even under the large precipitation. (2) For 5- and 8-year-old apple trees, the contribution of 0-100 cm shallow soil layer to the plant transpiration was more than 50% during the drought period. However, 50% of tree water came from 100-300 cm depth for the 13-year-old apple orchard. After precipitation, the main water source of trees was 100-300 cm for the 5- and 8-year-old orchards, and the resulting contribution was about 40%. For the 13-year-old apple orchard, the main water contribution layer was 0-100 cm soil layer, with nearly 50% of the total contribution. (3) The contribution of soil water in 300-500 cm layer showed very weak response to precipitation for all the three orchards, maintained around 30% consistently.