How tree species diversity and mycorrhizal association type influence contributions of plant and microbial residues to soil organic matter fractions in temperate forests

Forest soils play a key role in global carbon (C) storage. Important for long-term C storage in soil is the formation of mineral-associated organic matter (MAOM), which is protected through chemical bonds and occlusion from decomposition. Recent research demonstrated shifts in forest C cycles depending on tree diversity and mycorrhizal type, but we still lack mechanistic knowledge about the role of tree diversity and associated mycorrhizal symbiosis in soil C stabilization.

This study aims to quantify particulate organic matter (POM) and MAOM in relation to tree diversity and mycorrhizal type, i.e., ectomycorrhiza (ECM) and arbuscular mycorrhiza (AM) and to investigate the contributions of plant and microbial residues to POM and MAOM. It was conducted in the MyDiv tree experiment (Bad Lauchstädt, Germany), established in 2015 with 10 different deciduous tree species planted across a gradient of species richness. After density separation of POM and MAOM fractions, their mass, carbon and nitrogen (N) contents as well as ¹³C and ¹⁵N isotopic composition were determined. Soil samples were also taken from the grassland next to the site as background values. In addition, we measured ¹³C and ¹⁵N natural abundances of leaf litter, fine roots, saprotrophic and ectomycorrhizal sporocarps as well as ectomycorrhizal and soil hyphae.

There was no significant difference in the amount and C content of POM and MAOM between treatments. However, there was a tendency towards more POM (and POM-C) in diverse - especially ECM - systems. Compared to the grassland, C and N contents in POM were lower, which may result from reduced litter input after planting trees (soil was covered with a weed tarp until canopy closure). In line with this, POM of forest soil samples was enriched in ¹³C and ¹⁵N compared to grassland POM, suggesting a higher share of microbial residues in forest POM due to litter exclusion. Interestingly, also C content in MAOM declined due to litter exclusion, whereas ¹³C in MAOM seemed unaffected by the transition from grassland to forest.

The higher C/N ratio of POM compared to MAOM aligned with the expected greater contribution of plant residues to POM and microbial residues to MAOM. This is in accordance with a higher ¹³C-enrichment of MAOM compared to POM, especially in diverse systems and irrespective of the mycorrhizal type.

All in all, the analysis of natural stable isotope abundances is a powerful tool to elucidate the composition of POM and MAOM in temperate forests, which may change depending on forest age, mycorrhizal type and tree diversity.