Response of C-cycling enzymes in soil aggregate-size fractions to slurry application over a one year period in two contrasting soil types

The distribution of enzymatic activity in soil aggregates is a key determinant in the turnover of organic matter. This study aimed to address how the application of a carbon-rich slurry influences extracellular enzymatic activity within aggregate-size fractions over a one-year period. Twenty-four pots of either a loam or sand soil, sown with Lolium perenne, were kept slurry-free (control), or were treated with a high dry matter slurry. Pots were sampled after 31, 137 and 361 days (n = 4). Soils were physically fractionated to obtain four aggregate-size fractions: ≥2 mm, ≥250 μm, ≤250 μm and ≤53 μm with enzyme assays of seven C-cycling enzymes conducted on each, and the potential extracellular enzymatic activity (pEEA) calculated. The strongest response in individual pEEA was seen after 137 days, where the pEEA of at least one C-cycling enzyme was significantly higher in the slurry treatment in each of the four aggregate-size fractions in both soil types (all at least P ≤ 0.05). Additionally, nearly all seven C-cycling enzymes were significantly higher in the slurry treatment compared to the control in the ≤53 μm fraction (all at least P ≤ 0.05) in both. No significant increase in pEEA was seen in the slurry treatment in any aggregate-size fraction after 361 days in the loam soil (all P > 0.05), and this was also largely the case for the sand soil. The results of this study clearly indicate that pEEA within aggregate-size fractions was strongly influenced by slurry application in the initial months, though this was not sustained.