Spatial variability and distribution of soil organic matter and total nitrogen at different scales:a case study in Pinggu County,Beijing

Knowledge of spatial variability of soil organic matter (SOM) and total nitrogen (TN) at different scales is limited. The objective of this study was to assess the spatial variability of SOM and TN at two scales in the suburban of Beijing. Large-scale (county scale) topsoil samples (0-20 cm) were collected in a 400 m×400 m grid and a total of 1076 samples were taken within an area of 1075 km2 in Pinggu County. Small-scale (town scale) topsoil samples (0-20 cm) were collected in a 100 m×100 m grid and a total of 171 samples were taken within an area of 28.8 km2 in Machangying Town of the county. The contents of SOM and TN were determined for each soil sample. Descriptive statistics and geostatistics were used to analyze these two scales data. The results showed that the coefficients of variation (CV) for both SOM and TN variables at two scales were in the range of 0.31-0.40, which demonstrated a moderate variation among these. With the decreasing of scale, the CV for the TN content was reduced slightly. Semivariance analysis showed that both SOM and TN variables of the two different scales were spatially correlation at different lag distances. At the county scale, SOM and TN content had a large spatial correlation distance, which were 88.2 km and 4.9 km, respectively. However, the SOM and TN content at the town scale had the smaller spatial correlation distance, which were close to 0.7 km. The unobserved points were estimated and the contour map was obtained using the kriging method. The results indicated that the spatial distribution of SOM and TN content at two different scales was obviously different. The mainly affected factors were the topology, soil type, land use types and farming practices etc.