Mycorrhizal Controls of Soil Aggregate Formation

Despite soil structure is important for land sustainability, resilience and fertility, the biological factors of aggregate formation and dynamics, especially the role of mycorrhiza fungi, are still unclear. Here, soils from secondary succession: grasslands → shrublands → secondary forests → primary forests in subalpine region were sampled to compare the contribution of mycorrhizal fungal communities to the aggregate structure. Four aggregate size classes (>2000 µm and 250-2000 µm, large and small macroaggregates, respectively; 53-250 µm microaggregates and <53 µm silt and clay fraction) were related to microbial community structure, root biomass and morphology, the mycorrhizal infection rate and the external mycorrhiza hyphae. The proportion of large macroaggregates increased by 10%, while of small ones decreased by 9% during vegetation succession, resulting in the raise of the mean weight diameter by 17%. The Shannon-Wiener index and Chaos of arbuscular mycorrhizal fungi in grassland were 3 and in shrubland 24 times higher than in the primary spruce forests. The Simpson index of arbuscular mycorrhizal fungi had positive linear relationship with the soil aggregate structure. The mycorrhizal hyphal density was maximal in primary forests, and determined aggregate size distribution before the arboreous layer was established. In contrast, dissolved organic carbon (DOC) and mycorrhizal hyphal diameter controlled the distribution of aggregates at the later succession stages. This indicates that the biological mechanisms of aggregation follow the ecosystem succession with dramatic changes from arbuscular mycorrhiza to ectomycorrhiza. Thus, mycorrhizal fungal communities are the main factor affecting the soil structure mainly during the grasslands and shrublands succession (AMF-stage), while at the ECM-stage the contribution of DOC to aggregate formation becomes more important.