How do minerals and organic substrates interact in controlling the efficiency of the formation of mineral-associated organic matter?

Soil organic matter (SOM) is the largest terrestrial carbon (C) pool and conceptually, it can be divided into mineral-associated organic matter (MAOM) and particulate organic matter (POM), with MAOM is regarded as more persistent than POM. MAOM is mainly formed by organic compounds attaching to the surfaces of reactive minerals, thus rendering the organic compounds less accessible to decomposers and their enzymes. Recent studies proposed that high-quality litter maximizes the synthesis of microbial products and residues, which then increasingly contribute to the formation of MAOM. However, the extent to which microbial or plant residues contribute more to MAOM formation is still under debate and may vary between mineral types. The accumulation of organic carbon and nutrients at mineral surfaces in the course of MAOM formation also changes the substrate availability to microbes, and thus, their community composition and traits, such as enzyme activities and carbon use efficiency (CUE). In order to better understand if MAOM formation is more efficient from microbial than plant-derived substrates, and how mineral types influence this process, we setup a one month incubation experiment with microbial and plant-derived substrates (bacterial residue (Bacillus subtilis), C:N = 3.7; fungal residue (Aspergillus niger), C:N = 12.5; maize litter (zuckermais yucon chief), C:N = 15.4) in presence of three minerals of different reactivity (pure quartz, SSA = 0.2 m² g^–1; quartz with 20 wt.% goethite, SSA = 15.4 m² g^–1; quartz with 20 wt.% illite , SSA = 34.6 m² g^–1) in a factorial design. A common inoculum from a German agricultural soil was added to each of these substrate–mineral mixtures. We found that both substrate quality and mineral type significantly influenced MAOM formation and microbial properties. Maize litter had the highest MAOM formation efficiency, followed by fungal residues and bacterial residues. We also found that the presence of minerals with higher reactivity reduced decomposition rates and increased MAOM formation efficiency (except for bacterial residue). Mineral type also affected the microbial community composition and its functioning, with higher enzyme activities in presence of goethite than illite and pure quartz. Accordingly, we found no evidence for preferential stabilization of microbial-derived over plant-derived residues on any of the tested minerals, particularly not for bacterial residues, which decomposed fastest and with the smallest CUE measured at the end of the experiment. Mineral type strongly influenced the microbial habitat with higher sorption leading to overall reduced decomposition in the presence of highly reactive (pristine) mineral surfaces.