Contributions of biotic and abiotic factors to soil aggregation across a land use gradient

Soil aggregation is a major ecosystem process that can be impacted by intensified land use directly through soil disturbances, or indirectly through impacts on biotic and abiotic factors that affect soil aggregation. We collected soils from 27 grassland sites across a range of land use intensities including varying levels of mowing, grazing, and fertilization in order to test the importance of selected direct and indirect effects on soil aggregation. We measured root length and mass, root colonization by arbuscular mycorrhizal fungi (AMF), extraradical AMF hyphal length, soil aggregation, and soil hydrophobicity. We also quantified levels of phosphorus, nitrogen, organic carbon, carbonate carbon, and sand in the soil.As land use intensity (defined as a multivariate index combining mowing, grazing, and fertilization intensities) increased, root mass decreased and length of extraradical hyphae increased. Total colonization by AMF was unaffected by land use intensity, but vesicular colonization tended to increase while arbuscular colonization declined. Soil aggregation increased with increasing land use intensity. We used structural equation models to explore mechanisms of soil aggregation and found that extraradical AMF hyphal length contributed to soil aggregation in models containing only biotic explanatory factors. When we also included abiotic factors in the model, no biotic factor was significant, and soil aggregation decreased as levels of sand and carbonate increased, likely due to concurrent decreases in levels of clay in the soil.In summary, we have shown that agricultural measures such as mowing, grazing, and fertilization can increase soil aggregation in managed grasslands. Furthermore, abiotic factors can be more important for determining soil aggregation than biotic factors, especially in highly aggregated soils. Aggregate turnover may be reduced in such highly aggregated soils past the point required to ensure efficient integration of new labile C into stable aggregates.

Research highlights
► Root length decreased with increasing land use intensity (LUI).
► Extraradical hypha length of arbuscular mycorrhizal fungi went up as LUI increased.
► Soil aggregation increased with increasing LUI.
► Abiotic factors were more important that biotic factors for structuring soils.