Nitrate and water uptake, rather than rhizodeposition, control denitrification in the presence of growing plants

The main prerequisites for denitrification are availability of nitrate (NO₃^-) and easily decomposable organic substances, and oxygen deficiency. Growing plants modify all these parameters and may thus play an important role in regulating denitrification. Previous studies investigating plant root effects on denitrification have found contradictive results. Both increased and decreased denitrification in the presence of plants have been reported and were associated with higher C_org or lower NO₃^- availability, respectively. Accordingly, it is still unclear whether growing plants stimulate denitrification through root exudation or restrict it through NO₃^- uptake. Furthermore, reliable measurements of N₂ fluxes and N₂O/(N₂O+N₂) ratios in the presence of plants are scarce.

Therefore, 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 ¹⁵N gas flux method was applied to directly quantify N₂O and N₂ fluxes in situ. To link denitrification with available C in the rhizosphere, ¹³CO₂ pulse labeling was used to trace C translocation from shoots to roots and its release by roots into the soil.

Plant water uptake was a main factor controlling soil moisture and, thus, daily N₂O+N₂ fluxes, cumulative N emissions, and N₂O production pathways. However, N fluxes remained on a low level when NO₃^- availability was low due to rapid plant N uptake. Only when both N and water uptake were low, high NO₃^- availability and high soil moisture led to strongly increased denitrification-derived N losses.

Total CO₂ efflux was positively correlated with root dry matter, but there was no indication of any relationship between recovered ¹³C from root exudation and cumulative N emissions. We anticipate that higher C_org availability in pots with large root systems did not lead to higher denitrification rates, as NO₃^- was limiting denitrification due to plant N uptake. Overall, we conclude that root-derived C stimulates denitrification only when soil NO₃^- is not limited and low O₂ concentrations enable denitrification. Thus, root-derived C may stimulate denitrification under small plants, while N and water uptake become the controlling factors with increasing plant and root growth.