The Structure and Functioning of the Soil Microbial Community as Indicators of Soil Organic Matter Stabilization Under Different Land Use Systems on Gray Forest Soils

Soil organic matter (SOM) stabilization is closely linked to microbial community structure and function, yet reliable biological indicators remain insufficiently defined. This study aimed to identify microbial and biochemical markers of SOM accumulation under different land use systems (cropland, mown with phytomass removal, mown without phytomass removal, and fallow) in gray forest soils. Soil profiles were investigated in four land use types (cropland, mown with phytomass removal, mown without phytomass removal, and fallow) in the Laishevsky District (Russia). Physicochemical properties, SOM fractions, basal respiration, substrate-induced respiration, Biolog EcoPlates, quantitative PCR, and metagenomic data were used to assess microbial diversity and activity. Microbial communities differed substantially among land use systems and soil horizons, with bacterial communities in fallow soils dominated by oligotrophic taxa, such as RB41, Candidatus Udaeobacter, and KD4-96, whereas arable and managed grassland soils showed increased relative abundance of copiotrophic genera, particularly Pseudomonas and Polaromonas. Fungal communities were primarily represented by Mortierella, Penicillium, Trechispora, and Metarhizium, while both bacterial and fungal diversity decreased with soil depth, and metabolic profiling indicated preferential utilization of carbohydrates and carboxylic acids across all land use types. The highest organic matter and total organic carbon (TOC) were in soils under mowing without phytomass removal and fallow land, while arable soils showed the lowest values. Microbial diversity decreased with soil depth across all variants. Hay meadow soils exhibited elevated metabolic activity and higher metabolic quotient (qCO₂), indicating intensified carbon turnover or microbial stress, whereas arable soils were characterized by reduced substrate utilization and simplified community structure. Oligotrophic bacterial taxa were associated with more stable SOM conditions, while copiotrophic dominance reflected rapid carbon turnover. The results demonstrate that microbial community composition, functional activity, and specific taxa (e.g., oligotrophic bacteria, saprotrophic fungi, arbuscular mycorrhizal fungi) can serve as sensitive indicators of SOM stabilization processes. These findings support the development of microbiome-based diagnostic tools for assessing soil carbon dynamics and guiding sustainable land management strategies.