Earthworms, water infiltration and soil stability: Some new assessments

Water infiltration (by limiting surface water run-off) and stable crumb formation (by increasing top infiltration and decreasing slaking) are two key soil factors greatly affected by earthworms. Because of the great number of environmental variables controlling (1) earthworm populations; (2) their physical role behaviour; (3) their feeding behaviour inducing faeces composition; and (4) the microbial activity stabilizing faeces to crumbs, we chose to make most measurements directly in situ 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 made in nine sites analyzing hydraulic (= active) burrows and their structural properties. Infiltration rate was correlated to earthworm biomass (r = 0.975), burrow length, surface and volume (r = 0.99), but not with burrow diameter, tortuosity or with earthworm number and soil profile depth. Faecal production was also estimated in two fractions: the above-ground casts and the in soil cast fractions. The above-ground production is influenced by seasonal factors (moisture, temperature, photoperiod) and by controlled food types added on soil (natural herbs, wheat straw, lucerne and evergreen oak leaves). The total earthworm faeces 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 type added to soil. The above-ground fraction is rather small (8% of the 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 laboratory microcosms, versus the various food types. The crumb stability increased three-to four-fold during a year. The stability in the field depended 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 significance of microcosm studies is discussed and depends on the homeostasy and adaptive behaviour of earthworms (heterochresty). Microcosm results were validated for metabolism and unvalidated for soil bioturbation. (C) 1997 Elsevier Science Ltd.