Mineral Organic Interaction Invited Speaker.

¹⁵N soil depth profiles have been widely observed to increase with depth, concomitantly with decreasing C/N ratios, and decreasing soil C content. However the mechanisms for these patterns remain unclear. One common interpretation is that increased microbial processing of the organic matter occurs with depth, resulting in higher ¹⁵N values and lower C/N ratios. We examined changes in the chemical composition of soil organic matter with depth for two Oxisol soil depth profiles. We examined changes in C and N content and ¹⁵N and ¹³C stable isotope composition of sequentially deeper and denser soil fractions. Four density fractions per soil depth for 7 depths (in 10 cm - 20 cm intervals) were examined up to 1.2 m within the soil profile. Density fractions were also examined for specific surface area, and oxalate extractable Fe and Al (an index of short range ordered mineral abundance). Bulk soil trends with depth showed a trend of strongly decreasing C/N values and C content with increasing ¹⁵N values in one soil and only slight in the other.  In both soils increasingly ¹⁵N enriched and low C/N ratio materials accumulated in the denser soil fractions throughout the entire soil profiles (all depths). However the ¹⁵N values changed comparatively little within any one particle density fraction through both soil profiles.  C/N ratio, C content and ¹³C enrichment patterns across each of the density fractions yielded similar trends. The mineralogy shifts with depth include denser mineral particles, lower specific surface area (SSA) and lower oxalate extractable Fe_o and Al_o. The results suggest shifts in soil particle density (and concomitant changes in soil mineralogy) explain the observed increase in ¹⁵N and decreasing C/N ratio depth trends with depth rather than through increasing transformations in soil organic matter that might occur due to increased decomposition or SOM alteration with depth. Overall, changes in the dynamics of soil organic matter chemistry with depth were found to be driven by more by preferential accumulation of specific biochemical compounds associated with specific surface areas, and soil mineralogy (particle density) rather than increasingly altered SOM compounds developing in the subsoil.