Patterns of Soil 15N and Total N and Their Relationships with Environmental Factors on the Qinghai-Tibetan Plateau

The patterns of soil nitrogen (N) isotope composition at large spatial and temporal scales and their relationships to environmental factors illustrate N cycle and sources of N, and are integrative indicators of the terrestrial N cycle and its response to global change. The objectives of this study were: i) to investigate the patterns of soil N content and natural abundance of N-15 (delta N-15) values in different ecosystem types and soil profiles on the Qinghai-Tibetan Plateau; ii) to examine the effects of climatic factors and soil characteristics on the patterns of soil N content and soil delta N-15 values; and iii) to test the relationship between soil delta N-15 values and soil C/N ratios across ecosystems and soil profiles. Soil profiles were sampled at 51 sites along two transects 1 875 km in length and 200 km apart and distributed in forest, meadow and steppe on the Qinghai-Tibetan Plateau. Each site was sampled every 10 cm from a soil depth of 0 to 40 cm and each sample was analyzed for soil N content and delta N-15 values. Our results indicated that soil N and delta N-15 values (0-40 cm) in meadows were much higher than in desert steppe. Soil N decreased with soil depth for each ecosystem, while variations of soil delta N-15 values along soil profiles were not statistically significant among most ecosystems but for mountain meadow, lowland meadow, and temperate steppe where soil delta N-15 values tended to increase with soil depth. The parabolic relationship between soil delta N-15 values and mean annual precipitation indicated that soil delta N-15 values increased with increasing precipitation in desert steppe up to 500 mm, and then decreased with increasing precipitation across all other ecosystems. Moreover, the parabolic relationship between delta N-15 values and mean annual temperature existed in all individual ecosystem types. Soil N and delta N-15 values (0-40 cm) increased with an increase in soil silt and clay contents. Furthermore, a threshold of C/N ratio of about 11 divided the parabolic relationship between soil delta N-15 values and soil C/N ratios into positive (C/N 11) parts, which was valid across all ecosystems and soil profiles. The large explanatory power of soil C/N ratios for soil delta N-15 values suggested that C and N concentrations, being strongly controlled by precipitation and temperature, were the primary factors determining patterns of soil delta N-15 on the Qinghai-Tibetan Plateau.