Quantification of nitrogen excretion rates for three lumbricid earthworms using N-15

Nitrogen excretion rates of N-15-labeled earthworms and contributions of N- 15 excretion products to organic (dissolved organic N) and inorganic (NH4-N , NO3-N) soil N pools were determined at 10 degreesC and 18 degreesC under laboratory conditions. Juvenile and adult Lumbricus terrestris L., pre-clit ellate and adult Aporrectodea tuberculata (Eisen), and adult Lumbricus rube llus (Hoffmeister) were labeled with N-15 by providing earthworms with N-15 -labeled organic substrates for 5-6 weeks. The quantity of N-15 excreted in unlabeled soil was measured after 48 h, and daily N excretion rates were c alculated. N excretion rates ranged from 274.4 to 744 mug N g(-1) earthworm fresh weight day(-1), with a daily turnover of 0.3-0.9% of earthworm tissu e N. The N excretion rates of juvenile L. terrestris were significantly low er than adult L. terrestris, and there was no difference in the N excretion rates of pre-clitellate and adult A. tuberculata. Extractable N pools, par ticularly NH4-N, were greater in soils incubated with earthworms for 48 h t han soils incubated without earthworms. Between 13 and 40% of excreted N-15 was found in the N-15-mineral N (NH4-N+NO3-N) pool, and 13-23% was, in the N-15-DON pool, Other fates of excreted N-15 may have been incorporation in microbial biomass, chemical or physical protection in non-extractable N fo rms, or gaseous N losses. Earthworm excretion rates we:re combined with ear thworm biomass measurements to estimate N flux from earthworm populations t hrough excretion. Annual earthworm excretion was estimated at 41.5 kg N ha( -1) in an inorganically-fertilized corn agroecosystem, and was equivalent t o 22% of crop N uptake. Our results suggest that the earthworms could contr ibute significantly to N cycling in corn agroecosystems through excretion p rocesses.