黄土区退果还耕对黑垆土硝态氮积累和迁移的影响

Effect of land use change from orchard to farmland on the nitrate accumulation in caliche soils in the Loess Plateau

在黄土高原沟壑区王东沟小流域,针对塬面、梁地和坡地三种地形,分别选取了盛果期果园、老果园和退果还耕地等,研究了退果还耕条件下,三种地貌类型土壤剖面中水分含量变化及NO_3^--N、NH_4^+-N积累和迁移规律。结果表明,无论塬面、梁地或者坡地上,果园土壤水分含量显著降低(10%～14%);果园退耕后,土壤水分开始缓慢恢复。盛果期,塬面NO_3^--N峰值主要处于100—200 cm之间,退果还耕后,100 cm以上土层中NO_3^--N含量降低,100cm以下NO_3^--N积累量在增加,并且峰值不断向下移动。盛果期果园NO_3^--N积累量为631～3032 kg/hm^2;退果还耕地、老果园NO_3^--N积累量都显著高于盛果期果园。盛果期40%以上的NO_3^--N积累在100—200 cm土层,但退果还耕地上50%左右集中分布在200—300 cm土层。土地利用与管理方式的变化对NH_4^+-N分布特征的影响并不明显。

A young orchard, old orchard, and cropland converted from orchard were selected on table land, ridge land and slope land at the watershed of Wangdonggou of gully region in the Loess Plateau. Soil moisture content, nitrate and ammonium contents in 0-400 cm （in ridge land and slope land） or 0-600 cm （in tableland） depth were measured. The results showed that soil moisture content significantly decreased in table land, ridge land and slope land, while it began to be resumed in cropland converted from orchard. During the high yielding stage of orchard, for table land, the peak value of nitrate concentration were mainly located between 100-200 cm in young orchard, but once converted to cropland, NO3--N content in upper 100 cm layer decreased and NO3--N accumulation under 100 cm increased, furthermore, the peak value of NO3--N had the trend of moving downward. The accumulated NO3--N ranged from 631 to 3032 kg/ha in the young orchard. The nitrate accumulation in cropland converted from orchard and old orchard was significantly higher than that in young orchard. For young orchard, 40% of the nitrate accumulated between 100 and 200 cm, but for farmland converted from orchard, 50% of nitrate accumulated between 200 and 300 cm. Land use change had a little effect on ammonium in soil profiles.