How do arbuscular vs. ectomycorrhizal trees and site-specific variations affect soil organic matter pools?

The type of tree mycorrhizal association, together with the leaf type of the host, can influence carbon (C) pools and thus potentially C persistence in forest soils. In arbuscular mycorrhizal (AM) systems, litter tends to decompose rapidly with high C mineralization, thus favoring the formation of mineral-associated organic matter (MAOM). In ectomycorrhizal (EcM) systems, the litter decomposition is slower, which tends to result in the accumulation of particulate organic matter (POM). Yet, the effect of different mycorrhizal types associated with broadleaf trees on soil organic matter pools, and especially different fractions of POM (free: fPOM and occluded: oPOM), have rarely been explored. We quantified and characterized the soil organic matter (SOM) fractions within AM-associated and EcM-associated systems, on various sites. We collected soil samples (1-10 cm) on four sites in Sweden. Each site included broadleaf EcM-associated trees (Betula pendula), AM-associated trees (Fraxinus excelsior), and crop fields. We combined density and soil particle size fractionation to separate the soil into five organic matter (OM) fractions: fPOM, oPOM, oPOM_small (< 20 µm), MAOM (> 53µm), and MAOM_small (< 53 µm). We measured the C and N content, as well as δ¹³C values in all soil fractions and characterized the chemical composition of the POM fractions using ¹³C CP-MAS NMR spectroscopy. We also analyzed the fungal communities in the bulk soil using a sequencing approach.  As expected, forest soils contain higher amounts of POM, especially fPOM, than crop field soils. The fPOM in crop fields was less decomposed as in forest soils, as reflected by the lower alkyl C : O/N alkyl C ratio in the NMR spectra. Regardless of the vegetation and mycorrhizal types, the four sites presented oPOM and fPOM with similar chemical characteristics. Yet, the chemical composition of oPOM_small varied across sites, as reflected by contrasting alkyl C : O/N alkyl C ratio. While the vegetation type (forest versus crop field) tends to be an essential driver of SOM fraction mass distribution, site-specific variations, rather than vegetation and mycorrhizal types, seem to drive the chemical composition of oPOM_small fractions. As fungi are key decomposers of SOM we expect that differences in SOM fractions between vegetation types and sites will also be reflected in different fungal communities. However, we expect that differences in fungal communities between mycorrhizal types and vegetation types will be larger than between sites.