Soil properties impacting denitrifier community size, structure, and activity in New Zealand dairy-grazed pasture

Denitrification is an anaerobic respiration process that is the primary contributor of the nitrous oxide (N 2 O) production from grassland soils. Our objective was to gain insight to the relationships between denitrifier community size, structure, and activity for a range of pasture soils. We collected 10 dairy pasture soils with contrasting soil textures, drainage classes, management strategies (effluent irrigated or non-irrigated), and geographic locations in New Zealand, and measured their physicochemical characteristics. We measured denitrifier abundance by quantitative polymerase chain reaction (qPCR) and assessed community structure by terminal restriction fragment length polymorphism (T-RFLP) of the nitrite reductase ( nir S, nir K) and N2O reductase ( nos Z) genes. We quantified denitrifier enzyme activity (DEA) using acetylene inhibition technique. Differences in the physicochemical characteristics of the soils were driven mainly by soil mineralogy and the management practices of the farms. We found that nir S and nir K communities strongly structured along the gradients of soil water and phosphorus (P) contents. By contrast, the size and structure of the nos Z community was unrelated to any of the measured soil characteristics. In soils with high soil water content the richness and abundances of nir S, nir K and nos Z genes were significantly positively correlated with DEA. Our data suggest that soil moisture, microbial biomass, and fertility were the primary drivers of the structure and abundance of denitrifier communities across a wide range of geologic and geographical soil origins.