Controls on the Mole Ratio of Nitrous Oxide Flux from Agricultural Soils

Bergsma, T.T. and G.P. Robertson

Presented at the All Scientist Meeting (1996-07-16 to 1996-07-17 )

We are beginning some experiments to identify environmental and biological controls on nitrous oxide flux from soils. Nitrous oxide, an important radiatively-active trace gas in the upper atmosphere, is a potential contributor to global climate change (IPCC 1992). In soils, nitrous oxide is generated primarily by denitrification and nitrification. Denitrifiers reduce nitrate to nitrous oxide or (more completely) to dinitrogen. The mole ratio of nitrous oxide flux to dinitrogen flux is highly variable both temporally and spatially. N2O flux from soils depends (at least in part) on net denitrification rate and on the N2O mole ratio (cf. Firestone and Davidson 1989) . All though controls on denitrification rates are fairly well understood (e.g. Groffman and Tiedje 1989), controls on N2O flux are not. We hope to contribute to the success of N2O models by characterizing proximal controls on the N2O ratio.Our general approach is to use 15-N labeling and mass spectrometry to quantify N2 and N2O simultaneously. Lab incubations of soil cores will be used to explore the effects of WFPS, pO2, C:NO3-, etc., and to test the results in the field using modified chamber techniques. We expect to see a distinct trend in N2O ratio following real or simulated rainfall events. We will conduct other experiments to explore the effects of microbial community diversity and N2O production by nitrifiers.About half of the sources in the global N2O budget (needed to balance known sinks) are missing. Evidence suggests that land-use conversion (e.g., cultivation) may function as a long-term source of global N2O (Figure 1). Thus it is especially important to understand N2O flux in the agricultural context. The diversity of KBS treatments and controls will allow for rigorous tests of the hypotheses developed in this project.

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