Production and nutrient content of earthworm casts in a tropical agrisilvicultural system

Earthworm surface cast production and nutrient turnover through casts were measured for 3 years in a 17-year-old timber plantation in southern Cameroo n after selective reduction to two timber stand densities (TSDs) and unders torey cropping with plantain and tannia. Neither understorey cropping nor t imber stand density treatments had significant effects upon cast production in any year. Mean cast production in cropped plots was 35.7 Mg ha(-1) in t he first year, 34.9 Mg ha(-1) in year 2 and 30.1 Mg ha(-1) in year 3. This was 63%, 84%, and 65%, respectively, of cast production in undisturbed, unt hinned timber plantation plots. In comparing cast nutrient concentrations b etween years 1, 2 and 3, there were highly significant correlations for nea rly all nutrients in control plots but fewer such correlations in the low T SD. In cropped plots, none of earthworm parameters was correlated with nutr ient concentrations of slash or amounts of slash applied to the soil surfac e at establishment. There were positive correlations between cast N and lit terfall N (kg ha(-1) y(-1)) in both year 2 (cast N = 0.49 x litterfall N 61.7, r(2) = 0.46) and year 3 (cast N = 0.38 x litterfall N + 55.6, r(2) = 0.42). The ratio of soil:cast nutrient concentrations were related by negat ive power functions to soil nutrient concentrations for all nutrients and o rganic carbon. This suggests that earthworm communities adjust to lower soi l nutrient concentrations by increasing their selectivity and thus produce relatively higher quality casts on poorer soil. (C) 2000 Elsevier Science L td. All rights reserved.