Quanti®cation of nitrogen assimilation ef®ciencies and their use to estimate organic matter consumption by the earthworms Aporrectodea tuberculata (Eisen) and Lumbricus terrestris L

Earthworms affect nitrogen cycling directly through the consumption and assimilation of organic N and the turnover of N through excretion and mortality. Although earthworms can process large quantities of organic matter, organic-matter consumption by earthworms based on their nitrogen requirements has not been quanti®ed. Organic-matter consumption and the ef®ciency of nitrogen assimilation by earthworms were determined using N-labelled litter-soil mixtures. Consumption rates were in ̄uenced by the type of mixture provided and ranged from 8.5 to 13.2 mg organic matter gÿ1 earthworm dayÿ1 for Aporrectodea tuberculata (Eisen) and from 1.4 to 2.7 mg gÿ1 dayÿ1 for Lumbricus terrestris L. Consumption rates of Nlabelled soyabean±soil mixtures (N-SOY) were higher than N-labelled ryegrass±soil mixtures (N-RYE). The addition of glucose to N-labelled soyabean±soil mixtures (N-SOY‡G) to stimulate microbial activity did not in ̄uence the consumption rate by A. tuberculata or L. terrestris compared to the N-SOY mixture. However, the addition of glucose to N-labelled ryegrass±soil mixtures (N-RYE‡G) signi®cantly increased the consumption rate of A. tuberculata by 36% compared to the N-RYE mixture. The ef®ciency of nitrogen assimilation from the N-labelled mixtures ranged from 10.0% to 25.8% for A. tuberculata and from 25.4% to 30.1% for L. terrestris. A. tuberculata had lower ef®ciency of nitrogen assimilation from the N-RYE mixture than from all other mixtures. However, there was no difference in the ef®ciency of nitrogen assimilation from N-labelled mixtures by L. terrestris. The ef®ciencies of nitrogen assimilation and estimates of the nitrogen ̄ux from A. tuberculata and L. terrestris populations in manure-amended and inorganically fertilized corn agroecosystems were used to estimate system-level consumption of organic matter by earthworms. Based on hypothetical diets, we estimated that earthworms consumed 11.8 to 17.1 Mg organic matter haÿ1 yearÿ1, which was equivalent to 19±24% of the total organic matter in crop residues and the top 15 cm of soil each year. # 1999 Elsevier Science B.V. All rights reserved.