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

Although a number of different factors influence C and N isotopic fractionation of organic matter, the delta C-13 and delta N-15 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 delta C-13 and delta N-15 values, soil isotopic values are hypothesized to generally increase with depth across local and global scales. Soil delta C-13 and delta N-15 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 delta C-13 and delta N-15. Correlative and boosted regression tree analyses were used to determine the main drivers of the variance in soil delta N-15 globally and also the environmental association of variability in the correlation with depth between delta C-13 and delta N-15 at a number of sites. Strong positive correlations between delta C-13 and delta N-15 values through soil profiles were found at a number of sites and were found to be independent of vegetation type. Globally, soil delta C-13 and delta N-15 values were also found to be significantly positively correlated across a wide range of climates and biomes. The global correspondences between delta C-13 and delta N-15 values may suggest a mechanistic link between delta C-13 and delta N-15 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 delta C-13 and delta N-15 as indicators of ecosystem sources of soil C and N and inferring vegetation inputs.