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

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.