Plant N capture and microfaunal dynamics from decomposing grass and earthworm residues in soil

plant roots may be effective competitors with micro-organisms for the nutri ents released from decomposing organic patches buried in soil. We aimed to establish whether this was because they were more effective at acquiring nu trients or simply because they represent a slower turnover pool. Over 30 da ys we followed decomposition of, and plant N capture from, dual labelled (N -15/C-13) earthworms (Lumbricus terrestris L.) and grass (Lolium perenne L. shoots) added as discrete patches to soil microcosm units containing L. pe renne plants. Both patches decomposed rapidly as shown by the amounts of C- 13, as (CO2)-C-13, released into the soil atmosphere, which peaked after 8 h for the earthworm patches and 48 h for the grass patches. In the decompos ing grass patches the amounts of C-13 and N-15 remained co-varied and decli ned with time. No C-13 added in the earthworm patches was detected in the s oils, even after 3 days, confirming that decomposition of these patches was rapid. Grass patches supported greater microfaunal (nematode and protozoan ) biomass than the earthworm patches, and microfaunal biomass peaked at day 7 on both. Plant N capture from both patches increased with dry weight inc rement although N capture from the earthworm patch was greater than that fr om the grass patch. By day 30 plants had captured 29% (from earthworms) and 22% (from grass) of the N originally available in the patches. No C-13 enr ichments from the patches were detected in the plant tissues indicating tha t organic compounds were not being taken up by the plant roots. As plants o nly took up inorganic N from the patch, our results indicate that microbes initially out-compete plants for the added N, but with time, plants capture more of the N originally added as they represent a slower turnover pool. ( C) 2000 Elsevier Science Ltd. All rights reserved.