Nitrogen rhizodeposition assessed by a 15NH3 shoot pulse-labelling of Lolium perenne L. grown on soil exposed to 9 years of CO2 enrichment

The effects of elevated CO2 concentration upon rhizodeposition of nitrogen were investigated on field-grown Lolium perenne planted in soil cores set into the resident soil of a intensively managed ryegrass sward treated with elevated CO2 for nine consecutive years, under two contrasted N fertilisation regimes (Swiss FACE Experiment). The planted cores were excavated from the ambiant (35 Pa pCO2) and enriched (60 Pa pCO2) rings at two dates during the growing season (spring and early autumn). The cores were brought back to the laboratory for a pulse-labelling of ryegrass shoots with 15NH3, in order to quantify 15N-rhizodeposition.A recovery of 10–16% of the total 15N administred to the plant was recovered in the plant–soil system 48 h after the pulse-labelling; significant amounts of 15N were released into the soil adhering (adhering soil: AS) to the roots (0.44 μg 15N g AS−1 and 0.60 μg g AS−1 in the spring and the autumn samplings, respectively).In the spring sampling, there was no effect of atmospheric CO2 concentration on N rhizodeposition. In the autumn sampling, elevated CO2 stimulated N rhizodeposition that amounted to 7.2 and 5.2 mg 15N m−2, under elevated and ambient CO2, respectively. Nitrogen rhizodeposition was higher at high N (56 gN m−2) than at low N fertilisation (14 gN m−2), whatever the sampling date investigated.The mechanisms by which elevated atmospheric CO2 leads to a stimulation of the net root-released N flux remains to be investigated: was it caused by a higher nitrogen immobilisation by the microbial biomass and a reduced re-assimilation of mineralized N and/or by a stimulation of N efflux from roots? Concomitant to the observed reduction of C rhizodeposition, the stimulation of net N efflux suggests that the quality of root released compounds was modified under elevated CO2 concentration.