Spatial and temporal changes of soil properties and soil fertility evaluation in a large grain-production area of subtropical plain, China

Understanding the spatial and temporal characterization of soil properties, along with soil management practices, helps in analyzing soil properties changes and timely adjusting management patterns. Moreover, soil fertility evaluation within farmland is urgently needed for understanding soil fertility level and avoiding soil degradation. This study addressed the spatial and temporal changes of both soil properties and soil fertility, and their causes in a large grain-production area of China's subtropical plain, which has been used as the national commodity grain base for 35 years. In 2007 and 2017, 827 and 733 soil samples were collected across the area, respectively. Soil pH, organic matter (OM), available nitrogen (AN), available phosphorus (AP) and available potassium (AK) were measured. The changes of soil properties were analyzed with the descriptive statistics methods and geostatistical methods, while the soil integrated fertility index (IFI) was calculated by the integrated fertility index method combined with the minimum data set method. In general, 2017 had a lower variability than 2007 for all properties except AP, and pH had a lower variability than other indicators. Differences of all indexes among 6 cropping systems and 2 crop years were significant, and oilseed rape-soybean cropping system performed better than other cropping systems in most indexes. Moreover, in 2007 there was a stronger spatial dependence than in 2017, and AP and OM had a stronger spatial dependence than other indicators. Mapping indicated that a more patchy distribution of all indicators was found in 2007 than in 2017. In addition, OM and AK in soil decreased substantially, and AN decreased slightly, but AP and pH increased. By principal component analysis and correlation analysis selected OM, AP and AK as soil IFI evaluation indexes, and their weights were determined. The IFI results showed that soil fertility declined in all areas, while the double rotation systems had higher soil fertility levels than the continuous single cropping systems. These results confirm that long-term scale planting can reduce spatial variability and dependence of properties and homogenize soil properties. Our findings demonstrated that the changes of cropping systems, the differences of fertilization and the reduction of acid rain were the three important reasons for the changes of soil properties and soil fertility in Jianli county in recent years. In this study area, effective measures should be taken to increase OM, AP and AK contents in soil.