EARTHWORMS, WATER INFILTRATION AND SOIL STABILITY - SOME NEW ASSESSMENTS

Water infiltration (by limiting surface water run-off) and stable crum b formation (by increasing top infiltration and decreasing slaking) ar e two key soil factors greatly affected by earthworms. Because of the great number of environmental variables controlling (1) earthworm popu lations; (2) their physical role behaviour; (3) their feeding behaviou r inducing faeces composition; and (4) the microbial activity stabiliz ing faeces to crumbs, we chose to make most measurements directly in s itu in various soils. The amount of infiltrated water was measured in various soils (n = 17) and gave a mean rate of 150 mm h(-1) per 100 g m(-2) of earthworms (P < 0.05) and more closely (n = 11) or 282 mm h(- 1) per 100 g m(-2) of anecic species (P < 0.01). A precise study was m ade in nine sites analyzing hydraulic (= active) burrows and their str uctural properties. Infiltration rate was correlated to earthworm biom ass (r = 0.975), burrow length, surface and volume (r = 0.99), but not with burrow diameter, tortuosity or with earthworm number and soil pr ofile depth. Faecal production was also estimated in two fractions: th e above-ground casts and the in soil cast fractions. The above-ground production is influenced by seasonal factors (moisture, temperature, p hotoperiod) and by controlled food types added on soil (natural herbs, wheat straw, lucerne and evergreen oak leaves). The total earthworm f aeces produced in the natural food treatment was 293.6 kg year(-1) +/- 10%, per 100 g m(-2) of earthworms. The amount varied with the food t ype added to soil. The above-ground fraction is rather small (8% of th e total). The stabilization of faeces in crumbs was studied during 536 days by incubation in situ (top soil to a 15 cm depth) and in laborat ory microcosms, versus the various food types. The crumb stability inc reased three-to four-fold during a year. The stability in the field de pended mainly on the organic food and less on the soil level. The mean turn over rate in the field of earthworm crumbs was 2.3 year. The sig nificance of microcosm studies is discussed and depends on the homeost asy and adaptive behaviour of earthworms (heterochresty). Microcosm re sults were validated for metabolism and unvalidated for soil bioturbat ion. (C) 1997 Elsevier Science Ltd.