Spatio-temporal variability of farmland soil organic matter and total nitrogen in the southern Loess Plateau, China: a case study in Heyang County

Soil organic matter (SOM) and total nitrogen (STN) play an important role in terrestrial ecosystems. Knowledge of their regional distribution and dynamical change is an important basis for reasonable utilizing and protecting soil resources. However, very little attention has been paid to this in cultivated land of the southern Loess Plateau. In this study, Heyang County, an agricultural county located in the southern Loess Plateau, was chosen as study area. SOM and STN data were collected in 1983 and 2006. Spatial autocorrelation, geostatistics, and fractal methods were used to analyze their spatio-temporal variability. Results showed that mean contents of SOM and STN had significantly increased in the past 23 years. A relatively more increase was found in the southeast and relatively less increase in the northern half. Compared with those in 1983, SOM and STN contents in 2006 showed a higher nugget/sill ratio, shorter spatial autocorrelation distance, and larger fractal dimension, indicating that the spatial dependence of SOM and STN showed a weakening trend and a stronger spatial variability of the 2006 dataset. The main factors affecting SOM and STN changes were topography, soil type, and farm management practices. But, more and more impacts were derived from anthropogenic activities.