Nitrous oxide (N2O) is an atmospheric constituent implicated in climate warming and stratospheric ozone depletion. Both bacteria and fungi participate in N2O production, but information is lacking with regard to the relative contribution of bacterial and fungal denitrifiers to the denitrification process in agricultural soils. The selective inhibition technique (SI) is widely used to assess the contribution of different groups of microbes to soil processes, but success of the technique depends on the effectiveness of the inhibitors.In this study, laboratory experiments were conducted to assess the contribution of bacteria and fungi to denitrification using soils from a woodlot, agricultural fields under conventional plowing (PT), and no-till for either 50 years (long-term) or 11 years (medium-term). A selective inhibition (SI) technique was developed using two bactericides (streptomycin, bronopol) and two fungicides (cycloheximide, captan) applied at different rates (0-32 mg g -1 soil). Regardless of application rate, streptomycin and cycloheximide were not effective inhibitors of denitrification, with degree of inhibition only between 2 and 20% relative to controls. These results are significant given the wide use of these products in SI studies. However, the bactericide bronopol and the fungicide captan effectively inhibited denitrification, with the strongest inhibition observed at an application rate of 16 mg g -1 soil. The ratio of fungal to bacterial denitrification activity (F:B) was generally less than 1, indicating a dominance of bacteria in denitrification activity in the soils investigated. However, an increase in F:B ratio from 0.24 in mediumterm NT to 0.87 in long-term NT soils was noted, suggesting perhaps a progressive increase in the role of fungal denitrifiers with longer duration of NT farming.3

Heavy application of nitrogen (N) fertilizers to agroecosystems contributes to water pollution and emission of nitrous oxide (N 2 O), an atmospheric constituent implicated in the greenhouse effect and stratospheric ozone depletion. The incorporation of cover crops biomass into agricultural soil is a practice that could provide a portion of the N needed by growing crops, and thereby could help reduce N fertilizer input and N loss from agroecosystems. Although the cover crop composition and the tillage management history of receiving soils could affect the pattern of mineral N release and its fates, these questions have not been addressed in previous research. A greenhouse experiment (85 d) was conducted to investigate the effects of two cover crops, hairy vetch (Vicia villosa, C/N: 11) and rye (Secale cereale, C/N: 82) on mineral N flux, N 2 O emission, and corn growth in soils under no-till (NT, 11 years) and plow-till (PT). Overall, tillage practice had only a marginal influence on N dynamics. Although NO 3 -flux was enhanced by both cover crops, the NO 3 -released was more efficiently utilized in the rye treatment. Corn growth and N uptake were significantly greater (P<0.02; 1.5 times)with rye compared to the vetch treatment. Conversely, N 2 O emission was significantly (P<0.03) higher with vetch than rye addition, with cumulative emission (across PT and NT) averaging 2.45 kg N ha -1 with vetch and 1.28 kg N ha -1 with rye addition. Positive relationships were found between NO 3 -flux and N 2 O emission, with the slopes of the regression lines suggesting a greater propensity of NO 3 -released from vetch to be converted into N 2 O. By leading to less N 2 O emission and better corn N uptake, the study results indicate that rye is a better cover crop option than vetch, regardless of tillage practice.

No abstract