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 nutrients released from decomposing organic patches buried in soil. We aimed to establish whether this was because they were more effective at acquiring nutrients or simply because they represent a slower turnover pool. Over 30 days we followed decomposition of, and plant N capture from, dual labelled (15N/13C) earthworms (Lumbricus terrestris L.) and grass (Lolium perenne L. shoots) added as discrete patches to soil microcosm units containing L. perenne plants. Both patches decomposed rapidly as shown by the amounts of 13C, as 13CO2, released into the soil atmosphere, which peaked after 8 h for the earthworm patches and 48 h for the grass patches. In the decomposing grass patches the amounts of 13C and 15N remained co-varied and declined with time. No 13C added in the earthworm patches was detected in the soils, 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 increment although N capture from the earthworm patch was greater than that from 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 13C enrichments from the patches were detected in the plant tissues indicating that organic compounds were not being taken up by the plant roots. As plants only 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.