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TECHNICAL REPORTS

Organic Agriculture and Nitrous Oxide Emissions at Sub-Zero Soil Temperatures

USDA-ARS, Northern Great Plains Research Lab., Box 459, Mandan, ND 58554

^* Corresponding author (Phillips{at}mandan.ars.usda.gov)

Received for publication May 26, 2006. In the Red River Valley of the upper midwestern United States, soil temperatures often remain below freezing during winter and N₂O emissions from frozen cropland soils is assumed to be negligible. This study was conducted to determine the strength of N₂O emissions and denitrification when soil temperatures were below zero for a manure-amended, certified organic field (T2O) compared with an unamended, conventionally managed field (T2C). Before manure application, both fields were similar with respect to autotrophic and heterotrophic N₂O production and N₂O flux at the soil surface (0.15 ± 0.05 mg N₂O–N m^–2 d^–1 for T2O and 0.12 ± 0.06 mg N₂O–N m^–2 d^–1 for T2C). After application of pelletized, dehydrated manure, average daily flux (based on time-integrated fluxes from 20 November to 8 April), was 1.19 ± 0.34 mg N₂O–N m^–2 d^–1 for T2O and 0.47 ± 0.37 mg N₂O–N m^–2 d^–1 for T2C. Denitrification for intact cores measured in the laboratory at –2.5°C was greater for organically managed soils, although only marginally significant (p < 0.1). Cumulative emissions for all winter measurements (from 16 November to 8 April) averaged 1.63 kg N₂O–N ha^–1 for T2O and 0.64 kg N₂O–N ha^–1 for T2C. Biological N₂O production was evident at sub-zero soil temperatures, with winter emissions exceeding those measured in late summer. Late autumn manure application enhanced cumulative N₂O–N emissions by 0.9 kg ha^–1.

Abbreviations: POM, particulate organic matter • RRV, Red River Valley • WFPS, water-filled pore space