Emissions of greenhouse gases CH4 and N2O from low-gradient streams in agriculturally developed catchments

Lentic wetlands are usually regarded as the most important natural freshwater sources of methane (CH4) and nitrous oxide (N2O) to the atmosphere, and very few studies have quantified the importance of lowland streams in trace gas emissions. In this study, we estimated fluxes of CH4 and N2O in three macrophyte-rich, lowland agricultural streams in New Zealand, to place their trace gas emissions in context with other sources and investigate the value of minimising their emissions from agricultural land. All three streams were net sources of both gases, with emission of CH4 ranging from <1 to 500 mu mol m(-2) h(-1) and of N2O ranging from <1 to 100 mu mol m(-2) h(-1) during mid-summer. For CH4, both turbulent diffusion across the surface and ebullition of sediment gas bubbles were important transport processes, with ebullition accounting for 20-60% of the emissions at different sites. The emissions were similar on a per area basis to other major global sources of CH4 and N2O. Although small on a catchment scale compared to emissions from intensively grazed pastures, they were significant relative to low-intensity pastures and other agricultural land uses. Because hydraulic variables (viz. depth, velocity and slope) strongly influence turbulent diffusion, complete denitrification can best proceed to N-2 as the dominant end-product (rather than N2O) in riparian wetlands, rather than in open stream channels where N2O fluxes are sometimes very large.