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 unde r long-term clover-based pasture and from grass pasture were analysed to determine the role of acid- and water-extractable carbohydrate C, t otal hyphal length, microbial biomass, organic C and total and mycorrh izal root length in stabilization of the aggregates. Aggregates were e xamined by scanning electron microscopy (SEM) and the particle-size di stribution of the size fractions was also determined. Macro-aggregatio n increased under grass, relative to clover-based pasture; however, th e properties of the aggregate fractions measured did not reflect this difference. Microbial-biomass C, extractable-carbohydrate C, hyphal le ngth, total and mycorrhizal root length and organic C content of the s oils were poorly correlated with macro-aggregation. Within the aggrega tes, the proportion of 250-1000-mu m sand was smaller and clay, silt a nd fine sand (20-250 mu m) were greater relative to non-aggregated soi l, suggesting that the >250-mu m sand in the non-aggregated soil limit ed 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 a n average only 5 to 13 lengths of hyphae were associated with each 250 -mu m cube of soil within the aggregates, and suggested little potenti al 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 we ight of < 2.0-mm aggregates contained a single length of root. The fin dings cast doubt about the role of hyphae and fine roots in the stabil ization of macro-aggregates through an enmeshing mechanism in sandy so ils.