Carbon availability mediates differences in rhizosphere denitrification potential between intercropped plants

Denitrification is an important loss pathway for nitrogen (N) in agricultural systems; however, it is at times unpredictable. Some of the uncertainty surrounding denitrification rates is due to soil heterogeneity and spatial variability, especially with regard to carbon (C) availability, as it is the energy source fueling denitrification. Differences in C concentration and type are particularly large between the rhizosphere and bulk soil. The rhizosphere is a hotspot of microbial activity, where the chemical composition and quantity of C varies depending on both plant species and plant community diversity. However, we lack an understanding of how rhizosphere-driven differences in C impact denitrification in these different soil compartments. Here, we assessed denitrification and denitrifier gene abundance in bulk soils and rhizosphere soils of monoculture maize ( Zea mays L.) and maize interseeded with annual ryegrass ( Lolium multiflorum Lam.) and crimson clover ( Trifolium incarnatum L.). In addition, we measured potential denitrification and gene abundance in rhizosphere soils of ryegrass and clover. We found that denitrification potential was enhanced in all rhizosphere soils compared to bulk soil and this effect varied depending on the species of plant, with significantly increased rates of denitrification in maize rhizosphere compared to clover rhizosphere. The availability of dissolved organic C, but not microbial biomass C, accounted for differences in denitrification rates among rhizosphere soils from different plants. Although not consistently enhanced in the rhizosphere, the reduction of N 2 O in bulk soils was significantly lower when maize was intercropped with ryegrass compared to maize grown with clover or in monoculture, demonstrating the effects of increased plant diversity on denitrifiers. Of the denitrification genes assayed, the abundance of nirS was the most variable between soils, and gross N 2 O production was positively correlated with nirS but not nirK . This suggests that nirS denitrifiers may be more responsive to differences in available C between soils compared to nirK denitrifiers. This study demonstrates how denitrification rates can differ significantly within the rhizospheres of different plants and highlights the importance of C availability as a driver of differences in denitrification rates in the rhizospheres of maize and cover crops.