EARTHWORM-INDUCED DISTRIBUTION OF ORGANIC-MATTER IN MACRO-AGGREGATES FROM DIFFERENTLY MANAGED ARABLE FIELDS

To study the influence of soil structure on organic matter decompositi on, and the possible role of earthworms therein, aggregates of the siz e 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 man uring and pesticide use), organic matter contents and earthworm popula tion densities varied markedly between the fields. The fraction of agg regates that withstood wet sieving was determined and collected. Organ ic C content and the short-term C-mineralization rate of the organic m atter was measured after grinding in the dry-sieved total 3-4.8 mm fra ction and in the stable aggregates, and calculated for the unstable ag gregates. C-mineralization of sonicated samples was used to evaluate e ffects of physical protection of organic matter. The percentage of wat er-stable aggregates increased with earthworm numbers, though stable a ggregates were also found in the field without earthworms. In all case s, stable aggregates of all fields had significantly higher organic C- content than the total fraction. The C-content in stable aggregates fr om 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 t hey only shared the same treatment over the last 5 y, after 35 y of di fferent organic management. On the other hand, after only 5 y of diffe rent management, fields that had shared 35 y of identical high organic matter inputs differed highly in earthworm numbers and C-content of s table aggregates. Organic matter appeared to be better protected in st able than in unstable aggregates. Yet, mineralization was higher in th e ground material from stable than from unstable aggregates, at least in the presently low organic input fields with no or few earthworms. H owever, in the field with highest organic matter inputs and earthworm numbers, the decomposability of the organic matter in stable and unsta ble aggregates was comparable. The reasons for this remain unclear. Di fferences between the aggregate fractions of the three fields in decom posability 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 an d stabilization of macro-aggregates. (C) 1997 Elsevier Science Ltd.