Differential effects of earthworms on nitrogen cycling from various nitrogen-15-labeled substrates

Earthworms incorporate organic matter into soil but their influence on cycling of N from incorporated materials is not well understood. This study examined the role of earthworms in the turnover of N from different N-15-labeled inputs. We incubated intact soil cores with or without earthworms (Lumbricus terrestris) and with N-15-labeled KNO3, rye litter (Secale cereale L.), or cow manure, added to the soil surface at a rate of 150 kg N ha(-1). The cores were destructively sampled after 2, 6, and 10 wk and assayed for total soil N, KCl-extractable N, microbial-biomass N (MBN), two sire fractions (53-2000 mu m and >2000 mu m) of particulate organic matter (POM), and N-15 enrichment of all N poets. Earthworms increased the incorporation of N-15 into the soil in organically treated cores but bad little effect on the distribution of N-15 in the inorganically fertilized cores, The percentage of initial N-15 that was incorporated into the soil after 10 wk was 71 and 45% in cores with earthworms and 34 and 25% in cores without earthworms, in the manure and rye treatments, respectively. Earthworms incorporated the manure more rapidly than the rye, as shown bg temporal patterns of incorporation of total N and POM. Earthworms increased MBN in the manure treatment by about 20 to 30%, but slightly decreased MBN on one sample date (2 wk) in the rye treatment. Interactions between earthworms and organic materials of different quality influence both the rate at which the materials are incorporated into the soil and the subsequent mineralization of N from those materials.