Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 18, 2006
Publication Date: January 31, 2007
Citation: Phillips, R.L. 2007. Organic agriculture and nitrous oxide emissions at sub-zero soil temperatures. J. Environ. Qual. 36:23-30. Interpretive Summary: Following harvest, autumn fertilizer application is a common practice among crop producers in the Red River Valley, USA. Organic producers, in particular, will apply manure prior to soil freezing with the expectation that nutrients applied will remain in the frozen soil until spring. Gaseous nitrogen (N) losses are perceived to be minimal for this climate region. The effect of autumn manure application on emissions of nitrous oxide was tested using soybean fields in production in the RRV. Approximately 1.5% of the N applied in dehydrated manure was emitted as nitrous oxide between November and early April, totaling 1 kg ha-1 more N as nitrous oxide than the adjacent, un-amended soybean field. Results suggest that soil microbes are active at sub-zero soil temperatures. The effect of spring manure application on gaseous N losses remains to be tested.
Technical Abstract: Intensive studies of nitrous oxide (N2O) fluxes from agricultural ecosystems indicate highly variable but significant responses to synthetic fertilizers, composts, and animal manures, with effects varying with form, timing, and rate of application. In the upper Midwestern U.S., where soils in winter remain below freezing for extended periods of time, manures are often applied in autumn under the assumption that over-winter denitrification and N2O fluxes are negligible. To test this hypothesis, N2O emissions for Red River Valley certified-organic agricultural soils were surveyed between November and April, following field application of dehydrated manure and compared with conventionally managed, un-amended soils. Fluxes measured at the soil-atmosphere interface when soil temperatures were below freezing exceeded those measured the previous August under soybeans. Laboratory tests for intact soils cores confirmed N2O emissions and denitrification at -2.5 C for both conventionally and organically managed fields. Cumulative N2O-N emissions for un-amended soils over-winter (between autumn and spring cultivation) averaged 0.64 kg ha-1, while cumulative emissions for manure-amended organic soils averaged 1.63 kg ha-1. Field and laboratory results suggest that autumn application of dehydrated manure enhanced N2O emissions from denitrification for organically managed soils, irrespective of sub-zero soil temperatures.