Interactions between earthworms and residues of differing quality affecting aggregate stability and microbial dynamics

Earthworm activity and the composition of organic inputs are known to affect soil aggregate dynamics. How earthworm interacts with different organic residues, and how these interactions vary with time is still unclear. Earthworm processing of residues may modify their effectiveness as stabilising agents through changes in residue composition and disposition with regard to mineral particles. A mesocosm (2 L) experiment was conducted to assess how Lumbricus terrestris L. and residues (grass-clover hay or wheat straw at 20 g kg−1) affected aggregate stability (measured by wet sieving) over 40, 80 and 100 days; several microbial and organic carbon indices likely to affect aggregation were also measured to indicate temporal changes in stabilisation mechanisms. FT-IR analyses investigated relationships between kaolinite and specific organic fractions to indicate the degree of residue-mineral mixing in aggregates. In the absence of earthworms, straw was more effective than hay in promoting stability (+21% vs +13%), especially later in the incubations. Hay increased microbial biomass-C (MBC +73 vs +58 mg kg−1) and K2SO4 extractable-C (KSE-C +44 vs −9 mg kg−1) more than straw, but the converse was true for hyphal length (+6 vs +7.9 m g−1). Earthworms reduced or eliminated these differences. Earthworms increased aggregation at 40 days (+17%), although aggregation at 100 days was not affected. Earthworms changed the organic composition of aggregates (e.g. a relative decrease in polysaccharides) and may have promoted adsorption of otherwise labile fractions onto clays. Soon after residue inputs, aggregation was more closely associated with MBC and KSE-C, whereas with time aggregation became more closely related to hyphal length. The extent and persistence of benefits associated with organic inputs of differing quality will be influenced by earthworms, with earthworm mediated incorporation of residues into aggregates particularly important for the effectiveness of readily decomposable residues.

► Less decomposable residues were more effective in promoting aggregation.
► Less decomposable residues favoured aggregate fungi and reduced soluble C.
► Earthworms improved aggregate stability with decomposable residues.
► Earthworms changed aggregate composition, especially for more labile residues.
► Earthworms enhanced clay–mineral associations in aggregates.