Mineral-associated organic matter (MAOM) stores the majority of soil carbon and nitrogen (N) and largely consists of N-rich microbial residues. However, the decomposition potential of MAOM and intrinsic properties regulating its degradation remain poorly understood. Here, we deliberately constructed ¹³C-labelled MAOM with varying molecular compositions and organic carbon (OC) loadings, and conducted three independent microcosm experiments to investigate how MAOM’s intrinsic properties influence its persistence in soils. Microbial decomposition of MAOM was monitored by measuring the rate and isotopic signature of respired CO₂ during 30-day incubations, while MAOM molecular composition was characterized using pyrolysis-gas chromatography/mass spectrometry. We found that MAOM decomposition was positively correlated with the abundance of N compounds, which emerged as the primary predictor of MAOM decomposition across all experiments. Notably, N-rich microbial residues preferentially stacked on minerals coated with pre-existing organics via organo-organic interactions, rather than on unoccupied mineral sites. This led to high OC loadings as well as high decomposition potentials for N-rich MAOM. These findings suggest that microbe-derived N-rich MAOM decomposes rapidly, challenging the prevailing view that microbe-derived compounds are strongly sorbed to mineral surfaces and resistant to degradation. Furthermore, the high decomposability of OC retained through organo-organic interactions potentially poses an upper limit of carbon retention on MAOM, offering an alternative explanation to carbon saturation other than mineral surface availability. Collectively, this study highlights the need to integrate MAOM properties (composition and OC loading) to better predict the carbon sequestration potentials and dynamics of MAOM in soils.
 

soil organic carbon, mineral-associated organic matter, microbial necromass, plant-derived organic matter, decomposition potential