Revealing the contributions of the belowground plant root associated microbiome to soil aggregation in agricultural soils

Soil aggregation is an important physical indicator of soil health. Aggregation result from biological and physicochemical processes in which primary soil particles and organic matter are bound. Biotic-mediated soil aggregation is mainly assisted by fungal hyphae facilitating macroaggregate formation through mechanical union and exudation of binding agents. In agricultural soils, many crops establish a symbiosis root- arbuscular mycorrhizal (AM) fungi where the extensive mycelium directly connects roots and soil aggregates. However, the relative importance and specific contributions of the belowground plant root associated microbiome to soil aggregation is still not known.

We set up a pot experiment under controlled conditions to investigate the aggregate stability of different crop species in an organic agricultural sandy soil. We manipulated the soil microbiome by planting 10 common crop species with different mycorrhizal status, 5 species with AM fungal symbiosis (barley, clover, maize, oat and wheat) and 5 non-mycorrhizal species (buckwheat, lupine, quinoa, rapeseed and spinach). We filled each pot with sandy soil sieved to 2 mm and added two sealed mesh bags, with mesh size of 40 µm to avoid roots but promoting hyphal entrance. Mesh bags were filled with the same sandy soil further sieved to 1 mm and 0.25 mm, respectively. All pots were harvested in the 10^th week and biomass, roots and soil samples were processed.

Soil aggregate stability, measured by wet sieving method, showed that soils with AM fungal symbiosis became more aggregated as these soils presented a higher % in soil fractions > 500 µm and > 250 µm. Moreover, non-mycorrhizal crops soils showed a significantly higher free mineral fraction (< 63 µm), i.e. a loss of soil aggregation. Still, our results showed that no crop species, independently their mycorrhizal status, were able to form aggregates above the sieving size and not all crop species conferred the same soil aggregation. Among mycorrhizal crop species, barley and wheat showed a higher soil aggregation compared to oat and maize. Curiously, barley and wheat were the crop species with the highest AM root colonization, 74 and 69 %, respectively, as well as the soils with the highest microbial biomass C, N and their ratio. Our results showed that exists a positive correlation between microbial biomass, root colonization and soil aggregation. Further analysis will provide data on soil mycelial length as well as fungal and bacterial community profiles.

In conclusion, the mycorrhizal status of different crop species revealed the key role of AM fungi in soil aggregation and its relationship with microbial biomass; however, not surprisingly, the effects are species dependent. Our forthcoming data of the soil microbial communities and their functionality will further reveal which groups have a direct effect on soil aggregation.