Fungi are capable of nitrification and denitrification and often dominate the microbial biomass of temperate grassland soils. We determined the contributions of bacteria and fungi to N2O and N2 production in a grassland soil from Northern Ireland by combining the substrate‐induced respiration inhibition method and the 15N gas‐flux method. Streptomycin (C21H39N7O12) was used as the bacterial inhibitor and cycloheximide (C15H23NO4) as the fungal inhibitor. By labeling the NH4 and NO3 pools, we tested the hypothesis that fungi produce N2O and N2 solely by the reduction of NO3 Cycloheximide decreased the flux of N2O by 89% and streptomycin decreased the flux by 23%, indicating that fungi were responsible for most of the N2O production. All of the N2O was derived from NO3 reduction. Labeled N2 was only detected in control and streptomycin treatments. The distribution of the 15N atoms in the labeled N2 indicated that the source of the labeling was predominantly the NO3 pool, but that the process of formation was not dominated by denitrification. Codenitrification, where a 15N atom from labeled nitrogen dioxide (NO2) combines with a 14N atom from a natural abundance source, was proposed as the process forming labeled N2 About 92% of the labeled N2 was estimated to be due to codenitrification and 8% due to denitrification. The flux of N2O was always greater than the flux of N2, the mole fraction of N2O averaging 0.7. Fungal denitrification could be of ecological significance because N2O is the dominant gaseous end product.

No abstract

The objective of this study was to investigate how changes in soil pH affect the N 2 O and N 2 emissions, denitrification activity, and size of a denitrifier community. We established a field experiment, situated in a grassland area, which consisted of three treatments which were repeatedly amended with a KOH solution (alkaline soil), an H 2 SO 4 solution (acidic soil), or water (natural pH soil) over 10 months. At the site, we determined field N 2 O and N 2 emissions using the 15 N gas flux method and collected soil samples for the measurement of potential denitrification activity and quantification of the size of the denitrifying community by quantitative PCR of the narG, napA, nirS, nirK, and nosZ denitrification genes. Overall, our results indicate that soil pH is of importance in determining the nature of denitrification end products. Thus, we found that the N 2 O/(N 2 O ؉ N 2 ) ratio increased with decreasing pH due to changes in the total denitrification activity, while no changes in N 2 O production were observed. Denitrification activity and N 2 O emissions measured under laboratory conditions were correlated with N fluxes in situ and therefore reflected treatment differences in the field. The size of the denitrifying community was uncoupled from in situ N fluxes, but potential denitrification was correlated with the count of NirS denitrifiers. Significant relationships were observed between nirS, napA, and narG gene copy numbers and the N 2 O/(N 2 O ؉ N 2 ) ratio, which are difficult to explain. However, this highlights the need for further studies combining analysis of denitrifier ecology and quantification of denitrification end products for a comprehensive understanding of the regulation of N fluxes by denitrification.Denitrification is the microbial reduction of NO 3 Ϫ via NO 2 Ϫ to gaseous NO, N 2 O, and N 2 , which are then lost into the atmosphere (36). It therefore results in considerable loss of nitrogen, one of the most limiting nutrients for crop production in agriculture (20). Denitrification is also of environmental concern since, together with nitrification, it is the main biological process responsible for N 2 O emissions (7). N 2 O is a potent greenhouse gas which has a global warming potential about 320 times greater than that of CO 2 and has a lifetime of approximately 120 years (32). In the stratosphere, N 2 O can also react with O 2 to produce NO, which induces the destruction of stratospheric ozone (8). N 2 O can be released into the atmosphere by incomplete denitrification due to the effect of environmental conditions on the regulation of the different denitrification reductases (14, 41, 51), but it has recently been suggested that it could also be due to lack of nitrous oxide reductase in some denitrifiers (19,41). Since N 2 O is an intermediate in the denitrification pathway, both the amount of N 2 O produced and the N 2 O/(N 2 O ϩ N 2 ) ratio are important in understanding and predicting N 2 O fluxes from soils. The main environmental factors known to influence the N 2 O/(N 2 O...

No abstract

No abstract

No abstract

No abstract

The 15N content of N2 and N2O emitted from 15N‐enriched soils conveys information about their sources; such information can only be obtained by mass spectrometry. Conventional dual‐inlet isotoperatio mass spectrometry (IRMS) involve complex gas sampling and inlet systems, which limit the use of the technique. Continuous‐flow IRMS was evaluated for the automated analysis of 15N‐labeled N2 or N2O. A gas sample is flushed from a vial by a He carrier stream. Carbon dioxide, NO, and water are chemically removed prior to chromatographic separation of N2 from N2O. For N2 analysis, the carrier stream is passed through a Cu reduction column to remove O2. A triple‐collector isotope‐ratio mass spectrometer analyzes a portion of the carrier stream for N2 at m/z 28, 29, and 30. Accurate values for ratio (29/28 and 30/28) differences in normal and enriched atmospheres were obtained from enrichments up to 2 atom % 15N with coefficients of variation (CVs) ranging from 0.07 to 1.37%. The reduction column is bypassed for the analysis of N2O at m/z 44, 45, and 46. From the 45/44 ratios, the 15N content of the N2O could be determined accurately with enrichments up to 40 atom %. Precision depended on concentration and enrichment, with CVs ranging from 10% for 15 µL N2O L−1 at natural abundance to <1% for 100 µL N2O L−1 at 10 atom % 15N. The concentration of N2O could be quantified from the ion currents at m/z 44, 45, and 46 with a detection limit of 2.1 µL N2O L−1. A batch of 132 samples can be analyzed automatically for N2 or N2O at a rate of six determinations per hour. Automation of 15N determination in N2 and N2O should aid the study of the processes resulting in evolution of these gases from soil.