Earthworms increase nitrogen uptake by lettuce and change short-term soil nitrogen dynamics

Earthworms stimulate organic nitrogen (N) mineralization and this is linked to plant N uptake and N loss from soil. Earthworms may also simultaneously transfer N-rich residues into physically protected soil fractions and temporarily increase N stabilization. This study evaluated soil N transformations and lettuce N uptake in soil microcosms amended with N-15-labeled rice straw, with and without earthworms (Metaphire guillelmi), during a 45-day period. We measured the amount of straw N absorbed by lettuce, lost as N2O or in leachates, and remaining in bulk soil and aggregate fractions. Lettuce biomass increased significantly (P < 0.05) by 37% with earthworms, and a greater proportion of the N uptake by lettuce was from the soil native N pool rather than from straw N (soil-derived N in lettuce increased from 101 to 170 mg pot(-1) with earthworms). Earthworms did not change N losses by leaching. Total cumulative N2O emissions were 29% greater with earthworms than without earthworms (P 0.05), but the straw-derived cumulative N2O emission was three times lower with earthworms than without earthworms. Straw N was recovered in large macroaggregates (> 2 mm) and earthworm tissues in the microcosms with earthworms. There was 4.5% more straw N remaining in soil with earthworms, suggesting that earthworms temporarily increased the residence time of straw N. We propose that M. guillelmi stimulates short-term mineralization of soil native N, which is available for uptake by lettuce and increases the risk of N loss as N2O, while temporarily stabilizing straw N in the soil.