Nitrate uptake and carbon exudation - do plant roots stimulate or inhibit denitrification?

Background and aims Plant growth affects soil moisture, mineral N and organic C availability in soil, all of which influence denitrification. With increasing plant growth, root exudation may stimulate denitrification, while N uptake restricts nitrate availability. Methods We conducted a double labeling pot experiment with either maize (Zea mays L.) or cup plant (Silphium perfoliatum L.) of the same age but differing in size of their shoot and root systems. The N-15 gas flux method was applied to directly quantify N2O and N-2 fluxes in situ. To link denitrification with available C in the rhizosphere, (CO2)-C-13 pulse labeling was used to trace C translocation from shoots to roots and its release by roots into the soil. Results Plant water and N uptake were the main factors controlling daily N2O + N-2 fluxes, cumulative N emissions, and N2O production pathways. Accordingly, pool-derived N2O + N-2 emissions were 30-40 times higher in the treatment with highest soil NO3- content and highest soil moisture. CO2 efflux from soil was positively correlated with root dry matter, but we could not detect any relationship between root-derived C and N2O + N-2 emissions. Conclusions Root-derived C may stimulate denitrification under small plants, while N and water uptake become the controlling factors with increasing plant and root growth.