The contribution from hyphae, roots and organic carbon constituents to the aggregation of a sandy loam under long‐term clover‐based and grass pastures

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Abstract
Summary: Water‐stable macro‐aggregate size fractions (>2.0 mm, 1.0–2.0 mm, 0.5–1.0 mm and 0.25–0.5 mm) and non‐aggregated soil from a sandy loam under long‐term clover‐based pasture and from grass pasture were analysed to determine the role of acid‐ and water‐extractable carbohydrate C, total hyphal length, microbial biomass, organic C and total and mycorrhizal root length in stabilization of the aggregates. Aggregates were examined by scanning electron microscopy (SEM) and the particle‐size distribution of the size fractions was also determined.Macro‐aggregation increased under grass, relative to clover‐based pasture; however, the properties of the aggregate fractions measured did not reflect this difference. Microbial‐biomass C, extractable‐carbohydrate C, hyphal length, total and mycorrhizal root length and organic C content of the soils were poorly correlated with macro‐aggregation. Within the aggregates, the proportion of 250–1000‐km sand was smaller and clay, silt and fine sand (20–250 μm) were greater relative to non‐aggregated soil, suggesting that the >250‐μm sand in the non‐aggregated soil limited the stabilization of macro‐aggregates. Under SEM, no enmeshment of aggregates by hyphae and roots was apparent. Although 50–160 m hyphae g−1 soil was found within the aggregates, calculations showed that on average only 5 to 13 lengths of hyphae were associated with each 250‐μm cube of soil within the aggregates, and suggested little potential to stabilize the aggregates by enmeshing. On average, all >2.0‐mm aggregates contained less than 3.6 mm of roots and less than 50% by weight of <2.0‐mm aggregates contained a single length of root. The findings cast doubt about the role of hyphae and fine roots in the stabilization of macro‐aggregates through an enmeshing mechanism in sandy soils.