Quantification of nitrogen assimilation efficiencies 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 ass imilation of organic N and the turnover of N through excretion and mortalit y. Although earthworms can process large quantities of organic matter, orga nic-matter consumption by earthworms based on their nitrogen requirements h as not been quantified. Organic-matter consumption and the efficiency of ni trogen assimilation by earthworms were determined using N-15-labelled litte r-soil mixtures. Consumption rates were influenced by the type of mixture p rovided and ranged from 8.5 to 13.2 mg organic matter g(-1) earthworm day(- 1) for Aporrectodea tuberculata (Eisen) and from 1.4 to 2.7 mg g(-1) day(-1 ) for Lumbricus terrestris L. Consumption rates of N-15-labelled soyabean-s oil mixtures (N-15-SOY) were higher than N-15-labelled ryegrass-soil mixtur es (N-15-RYE). The addition of glucose to N-15-labelled soyabean-soil mixtu res (N-15-SOY+G) to stimulate microbial activity did not influence the cons umption rate by A. tuberculata or L. terrestris compared to the N-15-SOY mi xture. However, the addition of glucose to N-15-labelled ryegrass-soil mixt ures (N-15-RYE+G) significantly increased the consumption rate of A. tuberc ulata by 36% compared to the N-15-RYE mixture. The efficiency of nitrogen a ssimilation from the N-15-labelled mixtures ranged from 10.0%, to 25.8% for A. tuberculata and from 25.4% to 30.1% for L. terrestris. A, tuberculata h ad lower efficiency of nitrogen assimilation from the N-15-RYE mixture than from all other mixtures. However, there was no difference in the efficienc y of nitrogen assimilation from N-15-labelled mixtures by L terrestris. The efficiencies of nitrogen assimilation and estimates of the nitrogen Bur fr om A. tuberculata acid L. terrestris populations in manure-amended and inor ganically fertilized corn agroecosystems were used to estimate: system-leve l consumption of organic matter by earthworms. Based on hypothetical diets, we estimated that earthworms consumed 11.8 to 17.1 Mg organic matter ha(-1 ) year(-1), which was equivalent to 19-24% of the total organic matter in c rop residues and the top 15 cm of soil each year. (C) 1999 Elsevier Science B.V. All rights reserved.