[Spatial and Temporal Variability of Soil C-to-N Ratio of Yugan County and Its Influencing Factors in the Past 30 Years].

The soil carbon-to-nitrogen (C/N) ratio of soils is a sensitive indicator of soil quality and an indicator for assessing the carbon and nitrogen nutrition balance of soils. Its variation is significant in reflecting the carbon and nitrogen cycling of soils. An accurate knowledge of how the C/N ratio varies spatially and temporally and the driving factors at county scale is of great significance to the extrapolation of balanced fertilization based on soil C/N ratio regulation as well as to the protection of the ecological environment. This study was based on 200 points of surface soil samples (0-20 cm) collected during the second National Soil Survey in 1982 and 423 points of surface soil samples (0-20 cm) collected during the soil test-based formulated fertilization project in 2012 in Yugan County. Combined with the soil parent material, soil type, farmland-use type, terrain factors, pH, straw incorporation pattern, and nitrogen fertilizer rate over the past 30 years, spatial and temporal variability characteristics of the soil C/N ratio were analyzed by using ordinary kriging methods, and the effects of the influencing factors were quantified by regression analysis. The results indicated that the mean value of the C/N ratio was 10.05 and 11.18 in 1982 and 2012, respectively. The coefficient of variation was 19.40% and 25.04%, respectively, which suggested the soil C/N ratio had moderate variability in the study area. The ratios of nugget to sill were 15.91% and 71.25% in 1982 and 2012, respectively. This means that the leading factor from the structural factors (parent material and soil type) into the stochastic factors (nitrogen fertilizer rate and straw incorporation pattern). In the past 30 years, most of the regional soil C/N ratio increased significantly especially the eastern region. The spatial variability of soil C/N in 1982 was mainly affected by soil parent material, soil type, terrain factor, and pH, with the degree of influence of each variable at 17.3%, 14.2%, 7.4%, and 2.3%. In 2012, the spatial variability was mainly affected by soil parent material, soil type, farmland-use type, terrain factor, straw incorporation pattern, and nitrogen fertilizer rate, with the degree of influence of each variable of 8.7%, 23.5% 28.2%, 12.2%, 12.6%, and 42.3%, respectively. To maintain the steady growth of the soil C/N ratio, it is suggested that the return of carbon be incorporated with the input of nitrogen, such as incorporating crop residues into the soil and inputting more organic fertilizers into the soil in future farming practices.