苹果园改种粮食作物渭北旱塬深层土壤水氮变化特征

黄土高原退耕还林(草)工程实施20年来，长期苹果种植导致了普遍的土壤干层和大量的硝态氮累积，严重制约了农业和区域经济可持续发展。因此，明确不同树龄苹果园改种粮食作物后对深层土壤干层恢复(土壤水分变化)、土壤硝态氮累积与运移的影响，对于黄土高原土壤质量改善和农业可持续发展具有重要意义。以渭北旱塬为研究区，选取10、15、20、30 a树龄的苹果园以及对应树龄苹果园改种为2、5 a和6 a粮食作物为研究对象，通过对比分析各样地0—10 m剖面的土壤含水量、土壤储水量和硝态氮含量的差异，基于空间换时间的方法定量评估苹果园改种为粮食作物后对于深层土壤水氮的影响。结果表明：(1)不同林龄苹果园改种粮食作物后土壤水分迅速恢复，在2年之内均可恢复到7.0 m左右深度。(2)改种后土壤储水量对于改种后土壤硝态氮累积量的直接影响最显著，不同林龄苹果园改种粮食作物后，土壤剖面中硝态氮随着土壤水分的恢复发生了不同程度的淋失。改种前苹果园种植年限对于改种后土壤硝态氮累积量起决定性作用，改种前林龄越长，改种后硝态氮累积量越大、淋失深度越浅。(3)土壤累积硝态氮的淋失滞后于土壤水分的向下运动。可见，不同林龄苹果园...

Variation characteristics of the deep soil water and nitrogen in orchards in Weibei dry plateau after replanting food crops

Since the implementation of the Loess Plateau Returning Forestry (Grass) Project in late 1990s, long-term apple planting has led to the universal soil dry layer and a large amount of nitrate nitrogen accumulation, which has seriously restricted the sustainable development of agricultural and regional economy. Therefore, clarifying the effects of apple orchards of different ages converted to food crops on the recovery of deep soil dry layer (soil moisture change), soil nitrate nitrogen accumulation and transport is crucial to the improvement of soil quality and sustainable agricultural development on the Loess Plateau. Weibei dry plateau was used as the study area, and apple orchards of 10, 15, 20 and 30 years old and apple orchards of the same age converted to food crops of 2, 5 and 6 years old were selected for the study. A comparative analysis of the differences in soil water content, soil water storage and nitrate-N content between 0-10 m profiles of each site was used to quantitatively assess the effect of conversion of apple orchards to food crops on deep soil water and N based on a space-for-time approach. The results showed that (1) soil moisture recovered rapidly after conversion to food crops from apple orchards of different stand ages, reaching a depth of around 7.0 m within 2 years. (2) The direct effect of soil water storage after conversion on the accumulation of soil nitrate after conversion was the most important. The conversion of apple orchards of different ages to food crops resulted in varying degrees of nitrate-nitrogen leaching from the soil profile with the recovery of soil moisture. The age of the apple orchard before conversion played a dominant role in the accumulation of nitrate nitrogen in the soil after conversion. The older the stand before conversion, the greater the accumulation of nitrate nitrogen and the shallower the depth of leaching after conversion. (3) Soil leaching of the accumulated nitrate nitrogen lagged behind the downward movement of soil moisture. Above all, soil moisture recovered rapidly after the conversion from different age apple orchards to food crops, which accelerated the leaching of nitrate-N, while the downward movement of soil moisture and nitrate-N did not coincide. A combination of soil moisture restoration and loss of soil quality due to rapid leaching of nitrate nitrogen needs to be considered when carrying out conversions.This study has far-reaching implications for rational optimisation of land use structure and improvement of soil quality.