Rhizosphere processes in nitrate-rich barley soil tripled both N.sub.2O and N.sub.2 losses due to enhanced bacterial and fungal denitrification

Background and aims Plants can directly affect nitrogen (N) transformation processes at the micro-ecological scale when soil comes into contact with roots. Due to the methodological limitations in measuring direct N.sub.2 losses in plant-soil systems, however, the effect of rhizosphere processes on N.sub.2O production and reduction to N.sub.2 has rarely been quantified. Methods For the first time, we developed a robotic continuous flow plant-soil incubation system (using a He+O.sub.2 + CO.sub.2) combined with N.sub.2O .sup.15N site preference approach to examine the effect of plant root activity (barley - Hordeum vulgare L.) on: i) soil-borne N.sub.2O and N.sub.2 emissions, ii) the specific contribution of different pathways to N.sub.2O fluxes in moist soils (85% water holding capacity) receiving different inorganic N forms. Results Our results showed that when a nitrate-based N fertiliser was applied, the presence of plants tripled both N.sub.2O and N.sub.2 losses during the growth period but did not alter the N.sub.2O/(N.sub.2O + N.sub.2) product ratio. The .sup.15N site preference data indicated that bacterial denitrification was the dominant source contributing to the observed N.sub.2O fluxes in both nitrate and ammonium treated soils, whereas the presence of barley increased the contribution of fungal N.sub.2O in the nitrate treated soils. During the post-harvest period, N.sub.2O and N.sub.2 emissions significantly increased in the ammonium-fertilised treatment, being more pronounced in the soil with a senescing root system. Conclusion Overall, our study showed a significant interaction between rhizosphere processes and N forms on the magnitude, patterns, and sources of soil borne N.sub.2O and N.sub.2 emissions in moist agricultural soils.