Spatial pattern analysis of soil organic matter micropatches show distinct µm-sized C-rich and N-rich subunits

Mineral-associated organic matter (OM) exhibits a heterogeneous arrangement in soils at the microscale and nanoscale as revealed by high-resolution imaging techniques. The arrangement of OM at the microscale has broad implications for biogeochemical cycles of major elements such as C and N by compartmentalizing their dynamics into distinct micropatches and a few µm-sized hotspots. It is crucial to understand the organization of this heterogeneous microscale arrangement across diverse soil systems. Here, we present a meta-analysis of spatial patterns of OM patches based on unsupervised segmentation of nanoscale secondary ion mass spectrometry (NanoSIMS) measurements. Using a dataset of over 450 measurements of fine fractions from soils with different texture and C content, we evaluated the spatial coverage, clustering, and heterogeneity of OM micropatches across mineral surfaces. The OM coverage across mineral surfaces linearly correlated with the bulk soil C content, indicating a spatially expanding arrangement of OM whereas large parts of mineral-dominated surface remain. Higher OM coverage was related to more connected and more clustered OM patches. Within the OM patches, we found evidence of recurring µm-sized distinct C-rich and N-rich subunits based on a fractal geography approach. Within more homogeneous OM patches, subunits showed stronger differentiation in C and N composition, whereas subunits within more heterogeneous patches exhibited less differentiated C and N composition.  The distinct spatial organization of OM micropatches observed here suggests a compartmentalized framework of OM dynamics with implications for C and N cycling in soils.