Nitrogen‐rich microbial products provide new organo‐mineral associations for the stabilization of soil organic matter

Understanding the cycling of C and N in soils is important for maintaining soil fertility while also decreasing greenhouse gas emissions, but much remains unknown about how organic matter (OM) is stabilized in soils. We used nano‐scale secondary ion mass spectrometry (NanoSIMS) to investigate the changes in C and N in a Vertisol and an Alfisol incubated for 365 days with 13C and 15N pulse labeled lucerne (Medicago sativa L.) to discriminate new inputs of OM from the existing soil OM. We found that almost all OM within the free stable microaggregates of the soil was associated with mineral particles, emphasizing the importance of organo‐mineral interactions for the stabilization of C. Of particular importance, it was also found that 15N‐rich microbial products originating from decomposition often sorbed directly to mineral surfaces not previously associated with OM. Thus, we have shown that N‐rich microbial products preferentially attach to distinct areas of mineral surfaces compared to C‐dominated moieties, demonstrating the ability of soils to store additional OM in newly formed organo‐mineral associations on previously OM‐free mineral surfaces. Furthermore, differences in 15N enrichment were observed between the Vertisol and Alfisol presumably due to differences in mineralogy (smectite‐dominated compared to kaolinite‐dominated), demonstrating the importance of mineralogy in regulating the sorption of microbial products. Overall, our findings have important implications for the fundamental understanding of OM cycling in soils, including the immobilization and storage of N‐rich compounds derived from microbial decomposition and subsequent N mineralization to sustain plant growth. Understanding how organic carbon (C) and nitrogen (N) are cycled in soils is important for decreasing emissions of greenhouse gases and for maintaining soil fertility. We used nano‐scale secondary ion mass spectrometry (NanoSIMS) to examine changes in C and N in two soils. We have found that N‐rich compounds derived from microbial degradation often sorb directly to mineral surfaces not previously associated with organic matter (OM). This is important as it shows the ability of soils to store additional OM in new associations, with this information also assisting in the development of models for OM and nutrient cycling.