Disentangling direct and indirect effects of mycorrhiza on nitrous oxide activity and denitrification

Abstract

Abstract Nitrous oxide (N 2 O) is a potent greenhouse gas emitted at considerable rates from agricultural soils. A few recent studies have indicated the potential to reduce N 2 O emission rates in agricultural systems through management of soil microbiota, such as mycorrhizal fungi. Our limited understanding of how mycorrhiza influences N 2 O emission rates, however, forestalls such management practices. To address the knowledge gap regarding the mechanism of mycorrhizal modification of N 2 O formation and denitrification, we disentangle the direct effects of arbuscular mycorrhizal fungi (AMF) via hyphal growth on potential denitrification activity from those resulting from indirect AMF-induced changes in soil aggregation. We manipulated AMF status in a factorial experiment through adding or not AMF inoculum in the form of Rhizophagus irregularis and soil aggregation state through either crushing soil aggregates or using agricultural soil with intact soil aggregates. We then grew Zea mays (maize) in compartmentalized mesocosms for 4 months. At harvest, we observed approximately three-fold higher AMF root colonization and hyphal densities in the treatments where we had added Rhizophagus irregularis . Potential N 2 O activity rates and the denitrification ratio were higher in the undisturbed soil and in the mesocosms with a dense AMF hyphal network (direct effect of mycorrhiza). Further, AM-hyphae promoted soil aggregation while also altering potential denitrification activity (indirect effect of mycorrhiza). Overall, we present here a comprehensive case study on how mycorrhiza alters potential denitrification activity and related N 2 O activity from agricultural soils.