Soil nutrients of different land-use types and topographic positions in the water-wind erosion crisscross region of China's Loess Plateau

The Grain-for-Green program was implemented in China in 1999 to reduce soil erosion, increase biodiversity, conserve natural resources and improve the natural environment by modifying patterns of land use. The located field experiment about the influence of different land-use types (LUTs) on nutrient contents and changes in the soil profile or the degree of influence over time was conducted for many years in Liudaogou watershed. The contents of soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), available P (AP) and two inorganic N forms, ammonium-N (AN) and nitrate-N (NN), were determined in 0–200 cm soil profiles in five terraced and three sloped LUTs. The terraced LUTs were bare land (BAT), farmland (FAT), abandoned land (ABT), forage land (FOT, with alfalfa) and shrubland (SHT, with the peashrub Caragana korshinskii), and the sloped LUTs were farmland (FAS), abandoned land (ABS) and forage land (FOS). All of these LUTs were converted from traditional farmland except for FAT and FAS and they are carefully maintained for the last 50 years. SOM and TN contents were significantly higher after conversion from farmland to shrubland or grassland (15.6–34.9%) and decreased (51.5–77.1% for SOM in 0–40 cm, 49.7–72.1% for TN in 0–60 cm) with depth for all LUTs. TP and AN contents throughout the profiles were not significantly affected by land use, but NN was leached and accumulated in farmland only. AP content was highest throughout the profile in ABS when TP and AP contents were higher in sloped land than terraced land. SOM, TN and AN contents did not differ significantly for the same land-use patterns in the two topographies. SOM and TN contents in the surface soil increased with the length of recovery. Our observations demonstrated that the Grain-for-Green program has improved the nutrient contents and fertility of the soil and has increased SOM and TN stocks over time and that these effects were not limited to the topsoil. Abandoned land may be the optimal choice for nutrient sequestration in terraced and sloped land. Shrubland and artificial grassland are also recommended for increasing soil nutrient content in the area.