Earthworm-induced distribution of organic matter in macro-aggregates from differently managed arable fields

To study the influence of soil structure on organic matter decomposition, and the possible role of earthworms therein, aggregates of the size of earthworm casts (3-4.8 mm) were sieved from air-dry soil of three arable fields. Due to different management histories (in terms of manuring and pesticide use), organic matter contents and earthworm population densities varied markedly between the fields. The fraction of aggregates that withstood wet sieving was determined and collected. Organic C content and the short-term C-mineralization rate of the organic matter was measured after grinding in the dry-sieved total 3-4.8 mm fraction and in the stable aggregates, and calculated for the unstable aggregates. C-mineralization of sonicated samples was used to evaluate effects of physical protection of organic matter. The percentage of water-stable aggregates increased with earthworm numbers, though stable aggregates were also found in the field without earthworms. In all cases, stable aggregates of all fields had significantly higher organic C-content than the total fraction. The C-content in stable aggregates from the field with high organic matter inputs was significantly higher than from the two fields with presently low organic inputs. These two fields did not differ in C-content of stable aggregates, even though they only shared the same treatment over the last 5 y, after 35 y of different organic management. On the other hand, after only 5 y of different management, fields that had shared 35 y of identical high organic matter inputs differed highly in earthworm numbers and C-content of stable aggregates. Organic matter appeared to be better protected in stable than in unstable aggregates. Yet, mineralization was higher in the ground material from stable than from unstable aggregates, at least in the presently low organic input fields with no or few earthworms. However, in the field with highest organic matter inputs and earthworm numbers, the decomposability of the organic matter in stable and unstable aggregates was comparable. The reasons for this remain unclear. Differences between the aggregate fractions of the three fields in decomposability might indicate that in the high organic input field, stable aggregates form more quickly or persist longer than in the presently low organic input fields. This study provides strong indications, but no proof, that earthworms stimulate these processes of C-enrichment and stabilization of macro-aggregates.