Multi-scale variability of soil carbon and nitrogen in the middle reaches of the Heihe River basin, northwestern China

Insight into the variability of soil carbon and nitrogen at multiple scales is essential for accurately recognizing their distribution and stocks in arid inland river basins where landscape patterns are complex. For this objective, soil sampling and vegetation survey were conducted in 2012 to estimate the regional distribution and analyze the differences of soil organic carbon (SOC), total carbon (TC, the summation of organic and inorganic carbon) and total nitrogen (TN) among landscapes (cropland, desert, woodland and grassland) and sub-regions in the middle reaches of the Heihe River basin, northwestern China. The effects of soil properties, vegetation conditions and management practices on soil C and TN were determined. The results showed that the average regional densities of SOC, TC and TN were 68.2, 216.9 and 6.90 Mg ha− 1 in the 0-80 cm soil profile, respectively, and approximately 16% and 31% were stored in the 0-10 and 0-20 cm layers, respectively. Cropland stored the highest SOC and TN, whereas grassland stored the highest TC and woodland had the lowest SOC and TC. Variability in soil texture, frequency and amount of irrigation, fertilizer type and fertilization rate contributed to the differences in SOC and TN densities among croplands in the three sub-regions. Cropland and woodland far from the river bank (approximately 16-18 km away) accumulated more SOC, TC and TN than those near the river bank (approximately 4 km away). Soil texture was the predominant factor influencing SOC and TN in the surface soil of woodland. Aboveground biomass of shrubs and herbs, especially fresh weight, was regarded as a dominant factor affecting TC in desert soil and SOC, TC and TN in grassland soil. The results of this study are essential for accurately recognizing the status and variability of soil C and TN in complex landscape patterns in arid regions.