Use of laboratory incubation techniques to estimate greenhouse gas footprints from conventional and no-tillage organic agroecosystems

• Climate, C stocks and tillage duration determined GHG losses.
• AOB, amoA gene copies reflect shifts in temperature, WFPS, tillage and N source.
• Significant N2O can be emitted at a soil temperature of 10 °C if N is available.

Organic agroecological systems “produce goods using methods that preserve the environment” but can be a substantial source of greenhouse gases (GHG) if not managed properly. The objective of this experiment was to monitor nitrogen (N) and carbon (C) transformations in soils resulting from N additions, water filled pore space (WFPS) and prior tillage management during a simulated freeze-thaw. Incubated soils were taken from two USDA certified organic five-year small grain rotations with mixed legume cover crops that varied only in tillage, conventional (CT) vs. no-tillage (NT) for 3 yr prior to sampling. Soils incubated for 149 d were left unamended or amended with 15N labelled urea or sugar beet residue and maintained at 40, 60 and 80% WFPS. Non-metric multidimensional scaling (NMS) verified that soils amended with beet vs urea clustered separately in ordination space. A two way PerMANOVA analysis confirmed a significant interaction between WFPS and N amendment (p = 0.0002). Prior tillage management, N treatment and WFPS had a significant effect on GHG emission from soils. At 40% WFPS, soil previously in NT amended with beet residues emitted more nitrous oxide (N2O) and carbon dioxide (CO2) relative to soil previously in CT. At 60% WFPS, soil previously in CT amended with beet residues emitted greater N2O and less CO2 relative to soil previously in NT. Our research indicates that climate, carbon stocks and duration of prior tillage management determined the potential of no-till to reduce GHG emissions during a simulated freeze-thaw. Growers should note that as much as 4.51% of nitrates (NO3−) in residues can be lost as N2O at an average soil temperature of 10 °C.