Lucerne (Medicago sativa) alters N2O-reducing communities associated with cocksfoot (Dactylis glomerata) roots and promotes N2O production in intercropping in a greenhouse experiment

Abstract

Lower emissions of the greenhouse gas nitrous oxide (N 2 O) are generally observed from intercropped compared to sole cropped systems. This could be due to better N-use efficiency, but differences in microbial communities establishing in the rhizosphere may also play a role as the only known biological sink for N 2 O is its reduction to nitrogen gas (N 2 ) by bacteria and archaea that possess the nosZ gene encoding the N 2 O reductase. Nitrous oxide reducing communities can be divided into two clades, I and II, and their relative abundance and diversity may have important consequences for N 2 O emissions. Here, we examine how intercropping with a legume ( Medicago sativa , “lucerne”) and a grass ( Dactylis glomerata , “cocksfoot”) species, compared to sole cropping of each species, affects the N 2 O emission potential, and the structure and abundance of root-associated N 2 O-reducing microbial communities. In a rhizobox experiment, we show that intercropping resulted in higher total shoot biomass compared to sole cropping. Further, N 2 O production rates were significantly higher in intercropped cocksfoot roots compared to sole cropping of either species. This coincided with lower abundances of nosZ clade II communities in intercropped compared to sole cropped cocksfoot roots, suggesting that these organisms likely act as a N 2 O sink. Phylogenetic placement of sequencing reads placed root-associated nosZ clade II reads close to Ignavibacteria and Opitutaceae, which harbour non-denitrifying N 2 O reducers with the genetic capacity to also perform dissimilatory nitrate reduction to ammonium (DNRA). We observed a shift in the composition of the cocksfoot root-associated nosZ I communities towards incomplete denitrifiers terminating with N 2 O in intercropped roots. Overall, we hypothesize that such alterations of plant-microbe and/or microbe-microbe interactions contributed to the higher potential N 2 O emission rate observed in intercropped cocksfoot roots. Understanding the nature of these interactions would represent an important step forward for the design of management practices that minimize N 2 O emissions. • Higher N 2 O production rates in intercropped cocksfoot roots than in sole cropping. • Intercropping favors the presence of cocksfoot root-associated N 2 O producers. • Lucerne exerts stronger effect on cocksfoot roots than vice versa in intercropping.