Depth trends of soil organic matter C:N and 15N natural abundance controlled by association with minerals

Plant residues show carbon:nitrogen (C:N) decreases, N-15 isotopic enrichment and preferential loss of labile substrates during microbial decay. In soil profiles, strikingly similar patterns of decreasing C:N and N-15 isotopic enrichment with increasing depth are well documented. The parallel trend in organic matter composition with soil depth and during plant residue decay has been used as evidence to suggest that organic products accumulate or develop in the subsoil due to increasing intensity of microbially-driven processing, although no studies to date have verified this. Here, by applying sequential density fractionation, specific surface area, oxalate extractable Fe and Al, C:N and delta N-15 measures with depth to soils with relatively uniform soil mineralogy (Oxisols), climates and vegetation we show that changes in organo-mineral associations drive subsoil C:N and delta N-15 and C:N depth patterns more than in situ organic matter decay. Our results provide the first direct evidence that soil depth trends could be driven by mineral association instead of in situ processing.