Linking microbial community to soil water-stable aggregation during crop residue decomposition

The dynamics of soil water-stable aggregation (WSA) following organic matter (OM) addition are controlled by microbial activity, which in turn is influenced by carbon substrate quality and mineral N availability. However, the role of microbial communities in determining WSA at different stages of OM decomposition remains largely unknown. This study aimed at evaluating the role of microbial communities in WSA during OM decomposition as affected by mineral N. In a 35-day incubation experiment, we studied the decomposition of two high-C/N crop residues (miscanthus, C/N = 311.3; and wheat, C/N = 125.6) applied at 4 g C kg⁻¹ dry soil with or without mineral N addition (120 mg N kg⁻¹ dry soil). Microbial characteristics were measured at day 0, 7, and 35 of the experiment, and related to previous results of WSA. Early increase in WSA (at 7 days) was related to an overall increase of the microbial biomass (MBC) with wheat residues showing higher values in MBC and WSA than miscanthus. In the intermediate stage of decomposition (from day 7 to 35), the dynamics of WSA were more associated with the dynamics of microbial polysaccharides and greatly influenced by mineral N addition. Mineral N addition resulted in a decrease or leveling off of WSA whereas it increased in its absence. We suggest that opportunistic bacterial populations stimulated by N addition may have consumed binding agents which decreased WSA or prevented its increase. To the contrary, microbial polysaccharide production was high when no mineral N was added which led to the higher WSA in the late stage of decomposition in this treatment. The late stage of decomposition was associated with a particular fungal community also influenced by the mineral N treatment. We suggest that WSA dynamics in the late stage of decomposition can be considered as a « narrow process³ where the structure of the microbial community plays a greater role than during the initial stages.