Use of Laboratory Incubation Techniques to Estimate GHG Footprints from Clean and No-Tillage Organic Agroecosystems.

Organic agroecological systems “produce products using methods that preserve the environment” but can be a substantial source of greenhouse gases (GHG) (nitrous oxide (N₂O) and carbon dioxide (CO₂)) if not managed properly. The objective of this experiment is to monitor nitrogen (N) and carbon (C) transformations resulting from the effects of previous management (clean-till/no-till) and simulated freeze thaw events by measuring reactive N, CO₂ and gene copies of microorganisms involved in N cycling. Soils incubated for 149 d were amended with 5% ¹⁵N labelled urea, 1% ¹⁵N labelled sugar beet residue or left unamended and maintained at 40, 60 and 80% of water filled pore space (WFPS). Keeling plots containing atom % ¹⁵N-N₂O and 1/ N₂O were used to determine the isotopic composition of the source of N₂O. Completely randomized two-factor factorial ANOVA analyses confirmed soils taken from clean-till plots amended with beet tops emitted greater N₂O (P <0.001) and equivalent CO₂ relative to amended soil from no-till plots. Soils from clean-till plots with/without urea emitted less N₂O and more CO₂ relative to no-till soils with/without urea. Principal components analysis confirmed interactions among tillage, WFPS, N treatment and time of incubation. Measurements from urea amended soils clustered separately from those amended with beet tops. Preliminary data indicate that denitrifier nosZ gene copies increased in the 80% WFPS treatment amended with sugar beet. Soil previously under no-till management amended with beet residue produced less N₂O and CO₂ relative to the tilled system. Our research supports the potential of no-till organic systems to reduce GHG emissions when N is coupled with C and derived from organic materials. Growers should be aware that even at average air temperatures of 10°C, the presence/addition of organic residues or urea may lead to substantial N₂O emissions.