Effects of cultivation and agricultural abandonment on soil carbon, nitrogen and phosphorus in a meadow steppe in eastern Inner Mongolia

Grassland conversion into cropland and further into abandoned land may result in substantial changes when such conversion neglects the historical context of the land use pattern in question in most cases. The objectives of this study were to examine the effects of land use shifts on soil organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) in a meadow steppe in eastern Inner Mongolia. Four sites within 50 km were selected as replicates, and within each of the four sites, the plant and soil properties were investigated separately for three land use types, grassland, cropland and abandoned land from cropland, to reveal the effect of land use changes on soil nutrient contents and stocks. We found that cropland had the highest canopy biomass, whereas grassland maintained the highest root biomass. Significant decreases in OC and TN contents occurred mainly in the 0-20 cm soil layer with the conversion of grassland into cropland, whereas TP contents significantly increased in the 10-30 cm soil profile. Abandonment obviously increased the soil OC and TN contents in the 0-20 cm soil layer compared with cropland, although the levels of native grassland were not reached. The C:N, C:P and N:P ratios were highest in the entire soil profile for the grassland followed by the abandoned land and then the cropland. Changes in nutrient stocks with land use shifts showed similar trends as those in the relevant nutrient contents, but the change ratios of stocks among different land use types could be significantly underestimated due to the cultivation-compaction effect. Thus, we concluded that the abandonment and maintenance of native grasslands should be adopted to protect and maintain the soil organic matter and nitrogen in the meadow steppe of Inner Mongolia.