Climatic, Edaphic and Biotic Controls over Soil δ13C and δ15N in Temperate Grasslands

Soils delta C-13 and delta N-15 are now regarded as useful indicators of nitrogen (N) status and dynamics of soil organic carbon (SOC). Numerous studies have explored the effects of various factors on soils delta C-13 and delta N-15 in terrestrial ecosystems on different scales, but it remains unclear how co-varying climatic, edaphic and biotic factors independently contribute to the variation in soil delta C-13 and delta N-15 in temperate grasslands on a large scale. To answer the above question, a large-scale soil collection was carried out along a vegetation transect across the temperate grasslands of Inner Mongolia. We found that mean annual precipitation (MAP) and mean annual temperature (MAT) do not correlate with soil delta N-15 along the transect, while soil delta C-13 linearly decreased with MAP and MAT. Soil delta N-15 logarithmically increased with concentrations of SOC, total N and total P. By comparison, soil delta C-13 linearly decreased with SOC, total N and total P. Soil delta N-15 logarithmically increased with microbial biomass C and microbial biomass N, while soil delta C-13 linearly decreased with microbial biomass C and microbial biomass N. Plant belowground biomass linearly increased with soil delta N-15 but decreased with soil delta C-13. Soil delta N-15 decreased with soil delta C-13 along the transect. Multiple linear regressions showed that biotic and edaphic factors such as microbial biomass C and total N exert more effect on soil delta N-15, whereas climatic and edaphic factors such as MAT and total P have more impact on soil delta C-13. These findings show that soil C and N cycles in temperate grasslands are, to some extent, decoupled and dominantly controlled by different factors. Further investigations should focus on those ecological processes leading to decoupling of C and N cycles in temperate grassland soils.