NO3- uptake and C exudation – do plant roots stimulate or inhibit denitrification?

Growing plants affect soil moisture, mineral N and organic C (C_org) availability in soil and may thus play an important role in regulating denitrification. The availability of the main substrates for denitrification (C_org and NO₃^-) is controlled by root activity and higher denitrification activity in rhizosphere soils has been reported. We hypothesized that (I) plant N uptake governs NO₃^- availability for denitrification leading to increased N₂O and N₂ emissions, when plant N uptake is low due to smaller root system or root senescence. (II) Denitrification is stimulated by higher C_org availability from root exudation or decaying roots increasing total gaseous N emissions while decreasing their N₂O/(N₂O+N₂) ratios.

We tested these assumptions in a double labeling pot experiment with maize (Zea mays L.) grown under three N fertilization levels S / M / L (no / moderate / high N fertilization) and with cup plant (Silphium perfoliatum L., moderate N fertilization). After 6 weeks, all plants were labeled with 0.1 g N kg^-1 (Ca(¹⁵NO₃)₂, 60 at%), and the ¹⁵N tracer method was applied to estimate plant N uptake, N₂O and N₂ emissions. To link denitrification with available C in the rhizosphere, ¹³CO₂ pulse labeling (5 g Na₂¹³CO₃, 99 at%) was used to trace C translocation from shoots to roots and its release by roots into the soil. CO₂ evolving from soil was trapped in NaOH for δ¹³C analyses, and gas samples were taken for analysis of N₂O and N₂ from the headspace above the soil surface every 12 h.

Although pots were irrigated, changing soil moisture through differences in plant water uptake was the main factor controlling daily N₂O+N₂ fluxes, cumulative N emissions, and N₂O production pathways. In addition, total N₂O+N₂ emissions were negatively correlated with plant N uptake and positively with soil N concentrations. Recently assimilated C released by roots (¹³C) was positively correlated with root dry matter, but we could not detect any relationship with 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 limited due to plant uptake. In conclusion, plant growth controlled water and NO₃^- uptake and, subsequently, formation of anaerobic hotspots for denitrification.