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.