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

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-mu m sand was smaller and clay, silt and fine sand (20-250 mu m) were greater relative to non-aggregated soil, suggesting that the >250-mu m 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 an average only 5 to 13 lengths of hyphae were associated with each 250-mu m 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.