Spatial scaling effects on variability of soil organic matter and total nitrogen in suburban Beijing

- The spatial patterns of SOM and STN contents are different at three scales.
- The average values of SOM and STN decreased with the downscaling.
- The spatial correlation distances of SOM and STN decreased with the downscaling.
- Topography, soil types and land use were the main affecting factors at large scale.
- Farming practices were the main affecting factors at medium and small scales.

Understanding the spatial distribution of soil organic matter (SOM) and total nitrogen (STN) at different scales is helpful for elucidating relationships between soil properties, environmental factors and human activities. The objectives of this study were to compare the spatial patterns of SOM and STN and to explore the main factors affecting SOM and STN distribution in suburban Beijing at three spatial scales: large-scale (Pinggu County), medium scale (Plain area) and small-scale (Machangying town). For the county and plain scales, a total of 973 soil samples (0-20 cm) were collected on a 400 × 400 m grid across an area of 1075 km2. For the town scale, a total of 171 topsoil samples were collected on a 100 × 100 m grid within an area of 28.6 km2. The SOM and STN concentrations were determined for each soil sample. Descriptive statistics and geostatistical methods were used to analyze the data at the three spatial scales. The results showed that the mean values of SOM concentrations at large, medium and small scales were 14.88, 13.14 and 10.91 g kg− 1, respectively. The corresponding values for STN were 0.91, 0.79 and 0.66 g kg− 1, respectively, which also showed a decreasing trend with downscaling. The SOM and STN concentrations at the county scale had the largest spatial correlation distances, 88.2 km and 25.3 km respectively, while their spatial correlation distances at the town scale were the smallest, 2.5 km and 3.4 km, respectively. The spatial distribution patterns of SOM and STN were different. At county scale, the SOM and STN concentrations showed decreasing trends from the northeast to the southwest across the county, and topography, soil types, soil texture and land use types were the main influencing factors. At the plain scale, the SOM and STN exhibited a similar spatial distribution pattern as at the county scale, and soil types and farming practices were the main factors affecting the SOM and STN distribution patterns. At town scale, SOM and STN showed relatively uniform distributions, and soil texture and farming practices were the main affecting factors. It was concluded that manipulation of farming practices and land use types should be considered for improving SOM and STN levels in soils.