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

In the Red River Valley of the upper midwestern United States, soil temperatures often remain below freezing during winter and N 2 O emissions from frozen cropland soils is assumed to be negligible. This study was conducted to determine the strength of N 2 O 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 N 2 O production and N 2 O flux at the soil surface (0.15 ± 0.05 mg N 2 O–N m −2 d −1 for T2O and 0.12 ± 0.06 mg N 2 O–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 N 2 O–N m −2 d −1 for T2O and 0.47 ± 0.37 mg N 2 O–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 N 2 O–N ha −1 for T2O and 0.64 kg N 2 O–N ha −1 for T2C. Biological N 2 O production was evident at sub‐zero soil temperatures, with winter emissions exceeding those measured in late summer. Late autumn manure application enhanced cumulative N 2 O–N emissions by 0.9 kg ha −1