The effect on soil nutrients resulting from land use transformations in a freeze-thaw agricultural ecosystem

There is limited information on the effect of land use changes on spatial variation of soil nutrients in freeze-thaw area, which is one of the key points to achieving a sustainable agricultural ecosystem. With the application of GIS technology on a freeze-thaw agricultural system of Sanjiang Plain, Northeast China, a region that has been undergoing land use transformation of wetlands, dry and rice paddy lands, this study aimed to clarify the role of these land use changes in the spatial variation of soil nutrients in freeze-thaw farming system. In the spring thawing period of 2011, soil pH, organic carbon (OC), total nitrogen (TN), total phosphorus (TP), available nitrogen (AN), available phosphorus (AP) and total sulfur (S) in surface soils (0–20 cm) and subsoils (20–40 cm) were measured. All nutritional contents attributed to the surface soils were about twice those in the subsoils, but nutrients in surface soils showed a lower correlation than that of subsoils due to more disturbances of cultivation. From the spatial analysis perspective, the spatial heterogeneities of surface soil nutrients degraded greatly under the influence of long-term farming practices. The farming history here was shown to be an important factor for managing soil fertility contents in surface soils. Compared to existing wetlands, lower OC and TN contents were all found in soils of the transformed drylands, wetlands or drylands transformed paddy lands, and continuous drylands. The soil OC loss of continuous drylands reached 47.2% from existing wetlands, which was more than lower latitude areas of China. Soil phosphorus contents, especially AP, were low in soils of paddy lands and were influenced by the high variation of soil moisture and temperature during thawing. In general, with the rapid expansive reclamation and intensive tillage, the spatial variation patterns of soil nutrients in this freeze-thaw farming area have been significantly influenced by land use transformation. These findings can provide information for the increased responses in agricultural management and studies conducted in similar regions.