Quantification of nitrogen assimilation efficiencies and their use to estimate organic matter consumption by the earthworms Aporrectodea tuberculata (Eisen) and Lumbricus terrestris L.

Abstract

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 quantified. Organic-matter consumption and the efficiency of nitrogen assimilation by earthworms were determined using 15N-labelled litter-soil mixtures. Consumption rates were influenced by the type of mixture provided 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 15N-labelled soyabean–soil mixtures ( 15N-SOY) were higher than 15N-labelled ryegrass–soil mixtures ( 15N-RYE). The addition of glucose to 15N-labelled soyabean–soil mixtures ( 15N-SOY+G) to stimulate microbial activity did not influence the consumption rate by A. tuberculata or L. terrestris compared to the 15N-SOY mixture. However, the addition of glucose to 15N-labelled ryegrass–soil mixtures ( 15N-RYE+G) significantly increased the consumption rate of A. tuberculata by 36% compared to the 15N-RYE mixture. The efficiency of nitrogen assimilation from the 15N-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 efficiency of nitrogen assimilation from the 15N-RYE mixture than from all other mixtures. However, there was no difference in the efficiency of nitrogen assimilation from 15N-labelled mixtures by L. terrestris. The efficiencies of nitrogen assimilation and estimates of the nitrogen flux 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 ha −1 year −1, which was equivalent to 19–24% of the total organic matter in crop residues and the top 15 cm of soil each year.