Correspondence between δ13C and δ15N in soils suggests coordinated fractionation processes for soil C and N

Abstract

Although a number of different factors influence C and N isotopic fractionation of organic matter, the δ13C and δ15N values of soil organic matter both tend to increase with soil depth, following similar trajectories. This similarity has not been investigated at the global scale. As microbial decomposition increases organic matter δ13C and δ15N values, soil isotopic values are hypothesized to generally increase with depth across local and global scales. Soil δ13C and δ15N values for 16 soil depth-profile sites were used for local-scale investigation, and 5447 global single-depth sites were used for global-scale investigation of the correspondence between δ13C and δ15N. Correlative and boosted regression tree analyses were used to determine the main drivers of the variance in soil δ15N globally and also the environmental association of variability in the correlation with depth between δ13C and δ15N at a number of sites. Strong positive correlations between δ13C and δ15N values through soil profiles were found at a number of sites and were found to be independent of vegetation type. Globally, soil δ13C and δ15N values were also found to be significantly positively correlated across a wide range of climates and biomes. The global correspondences between δ13C and δ15N values may suggest a mechanistic link between δ13C and δ15N through the process of SOM decomposition and microbial processing and highlight the importance of soil-related processes in determining isotopic signals in soils. The variability in these soil processes should be considered when interpreting soil isotopic values of δ13C and δ15N as indicators of ecosystem sources of soil C and N and inferring vegetation inputs.