Tracing litter-derived soil organic nitrogen across the intact soil structure at microscale

At the microscale, soil organic matter (OM) and the mineral matrix are highly heterogeneous, shaping microbial activity and nitrogen (N) transport within soils and thereby influencing detritusphere formation. Although native soil OM determines the stabilization of new organic inputs in bulk soil, it is unclear whether the native soil OM is also controlling processes at the microscale, within the detritusphere. Here, using Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS), we examine the microscale spatial expression of native soil OM effects on detritusphere formation through direct isotopic mapping. Intact soil macroaggregates (1–2 mm) from topsoil and subsoil differing only in native OM content and containing occluded ¹⁵N-labelled straw were analysed by NanoSIMS (30 µm fields of view; ~120 nm lateral resolution) after 51 days of incubation. Two-dimensional mosaic images show that litter-derived N is redistributed into the surrounding soil matrix as discrete, micrometre-scale hotspots extending up to 150 µm from particulate OM. In both topsoil and subsoil, the size, spatial separation, and persistence of these hotspots are consistent with biologically structured transfer pathways, potentially moving along the saprotrophic fungal hyphae and through micropores within macroaggregate. Hotspots were more abundant in subsoil than in topsoil, consistent with more mineral binding sites and greater microbial acquisition of scarce N resources in low-OM subsoils. The observed microscale heterogeneity in the redistribution of litter-derived N within the mineral matrix of the detritusphere illustrates the importance of spatially explicit biological processes and soil architecture in governing soil N dynamics within macroaggregates.