Changes in some soil properties induced by re-conversion of cropland into grassland in the semiarid steppe zone of Inner Mongolia, China

"Grain for Green Program" (GGP), i.e., re-conversion of cropland into forest or grassland, initiated by Chinese government has a profound impact on mitigating environmental degradation. The objectives of this study were to assess the changes of some soil properties during the processes of re-conversion from cropland to grassland over time in the semiarid steppe region of north China. Two sites with different ages of re-conversion were selected for measurements of organic matter (SOM), total nitrogen (TN) and phosphorus (TP), bulk density (BD) and grain size distribution. Saturated hydraulic conductivity was determined by the constant hydraulic head method and unsaturated hydraulic conductivity by disc infiltrometer at tensions of 30, 60 and 150 mm. Soil water content was measured using the gravimetric method. Wetting front depths in the soil after rainfall were also recorded at the study sites. Natural grasslands had higher belowground biomass than re-converted grasslands. Re-converted grasslands had lower SOM and TN at depths of 0-20 cm and higher saturated hydraulic conductivity at depths of 0-10 cm than natural grassland. The natural grassland soils had higher soil water contents in the surface soil (0-20 cm) and lower soil water contents at deeper depths than re-converted grassland soils. Soil aggregate stability reached the natural steppe level 12 years after re-conversion. The recovery of soil properties after GGP appeared to be slow, and these properties did not return to natural grassland status before cultivation after 12 years of re-conversion.