Bergsma, T.T., N.E. Ostrom and G.P. Robertson
Presented at the ASM at Snowbird (2000-08-02 to 2017-12-05 )
The nitrous oxide mole ratio (N2O / [N2O + N2]) represents the fraction of denitrification that results in N2O, a radiatively active trace gas that contributes to the destruction of stratospheric ozone. The ratio is difficult to investigate because of high spatial and temporal variability and because of the difficulty in making direct measurements of N2 flux from soil. We collected soil from three replicate 1-ha plots for each of two ecosystems — conventionally-tilled, high-input corn-wheat-soy rotation (‘cropped’) and 10-year-old native succession — at the W. K. Kellogg Biological Station LTER site. Soil was sieved, air-dried, and incubated in jars for 24 hours with nitrate (15N-labeled or unlabeled), glucose, and water (85% water-filled pore space) to stimulate denitrification. One set of jars received 80% of total water 48 hours before the start of the incubation. Gas chromatography (GC) with acetylene inhibition was used to assess N2O and N2 production. Isotope ratio mass spectrometry (IRMS) was used to assess N2 production and to estimate non-destructively the enrichment of the soil nitrate pool. For soil from cropped sites, mean N2O mole ratios measured by gas chromatography were 0.36 for the ‘pre-wet’ incubations and 0.90 for controls. For successional sites, ratios were 0.34 for pre-wet soils and 0.33 for controls. By ANOVA, the mean for cropped-site controls was significantly different from the others. There was a significant interaction of ecosystem and recent moisture history (P = 0.012). We attribute the interaction to differences in the abilities of the soils to maintain the status of the denitrifying enzyme nitrous oxide reductase (Nos). Apparently Nos status was maintained rather well in the successional soils, but declined with drying in the cropped soils and was re-induced quickly with the onset of anaerobic conditions. For N2 production, IRMS methods gave estimates that were about 0.34 of GC estimates. Yet estimates of enrichment of the soil mineral pool using N2 data agree well with those from N2O data and with mass-balance predictions. At present, we cannot explain unequivocally the discrepancy between the methods.
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