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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Differential Effects of Earthworms on Nitrogen Cycling from Various Nitrogen-15-Labeled Substrates

^a MacArthur Agro-Ecology Research Center, 300 Buck Island Ranch Rd., Lake Placid, FL 33852 USA
^b Dep. of Entomology, Ohio State Univ., Columbus, OH 43210 USA
^c Environmental Sciences Graduate Program, Ohio State Univ., Columbus, OH 43210 USA

pbohlen{at}archbold-station.org

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 ¹⁵N-labeled inputs. We incubated intact soil cores with or without earthworms (Lumbricus terrestris) and with ¹⁵N-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 size fractions (53–2000 µm and >2000 µm) of particulate organic matter (POM), and ¹⁵N enrichment of all N pools. Earthworms increased the incorporation of ¹⁵N into the soil in organically treated cores but had little effect on the distribution of ¹⁵N in the inorganically fertilized cores. The percentage of initial ¹⁵N 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 by 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.

Abbreviations: MBN, microbial-biomass nitrogen • POM, particulate organic matter