Changes in Bacterial Denitrifier Community Abundance over Time in an Agricultural Field and Their Relationship with Denitrification Activity

Abstract

This study measured total bacterial and denitrifier community abundances over time in an agricultural soil cropped to potatoes (Solanum tuberosum L.) by using quantitative PCR. Samples were collected on 10 dates from spring to autumn and from three spatial locations: in the potato “hill” between plants (H), close to the plant (H_p), and in the “furrow” (F). The denitrification rates, N₂O emissions, and environmental parameters were also measured. Changes in denitrifier abundance over time and spatial location were small (1.7- to 2.7-fold for the nirK, nosZ, and cnorB_B guilds), whereas the cnorB_P community (Pseudomonas mandelii and closely related spp.) showed an ∼4.6-fold change. The seasonal patterns of denitrifier gene numbers varied with the specific community: lower nosZ gene numbers in April and May than in June and July, higher cnorB_P gene numbers in May and June than in March and April and September and November, higher nirK gene numbers in early spring than in late autumn, and no change in cnorB_B gene numbers. Gene numbers were higher for the H_p than the H location for the nosZ and nirK communities and for the cnorB_P community on individual dates, presumably indicating an effect of the plant on denitrifier abundance. Higher cnorB_P gene numbers for the H location than the F location and for nosZ and cnorB_B on individual dates reflect the effect of spatial location on abundance. Denitrifier abundance changes were not related to any environmental parameter, although a weak relationship exists between cnorB_P gene numbers, extractable organic carbon values, and temperature. Denitrification and N₂O emissions were mostly regulated by inorganic nitrogen availability and water-filled pore space but were uncoupled from denitrifier community abundances measured in this system.