Seasonal changes in rates of potential denitrification in poorly-drained reseeded blanket peat

Abstract

Denitrification of added NO 3—N was estimated in peat cores from a poorly-drained reseeded area of blanket peat where grass growth did not respond well to early applications of NH 4NO 3. Potential denitrification rates (PDR) were estimated monthly by the acetylene block technique in cores from 0–5 cm depth incubated at 10°C for up to 10 days and receiving the equivalent of 112kg N ha −1 as NH 4NO 3. PDR showed a marked seasonal variation in cores incubated under either aerobic or anaerobic conditions. Maximum PDRs, equivalent to the loss of 6.7 kg N ha −1 day −1 under anaerobic and 4.2 kg N ha −1 day −1 under aerobic conditions, occurred in spring. During the summer months, N 2O production in aerobically incubated cores could not be detected, but rates increased again in the autumn. Although rates measured in summer under anaerobic conditions were greater than those estimated under aerobic conditions they did not attain the rates recorded in spring and autumn. Glucose additions suggested that microbial activity was carbon limited and increased PDR in summer, but values again did not attain those measured in spring and autumn. The reduction in PDR in summer did not correspond to changes in bacterial numbers, soil aeration or pH, but the rates correlated significantly and positively with CO 2 evolution. In spring and autumn, nitrate recovery after 10 days incubation was almost zero but, in periods of low PDR, 60–80% of added NO 3—N was recovered, indicating some assimilation of NO 3—N into the microbial biomass or removal along other pathways, but this was insufficient to explain the apparent suppression of PDR during the summer months.