Gas flux from a fir bark substrate at an ornamental production nursery

Carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) flux from a fir bark substrate was estimated at an ornamental plant nursery in California during a typical 81-day production cycle. The soilless substrate consisted of 4:1 (v:v) fir bark:sand, incorporated with 3.47 kg m^‑3 Apex 9-2-0 sulfur-coated urea and 4.51 kg m^‑3 Osmocote Plus 15-9-12 controlled release fertilizer. On day zero, Lagerstroemia indica 'Whitt II' plants from number one containers were planted individually into number five containers filled with substrate. On day three, the substrate was topdressed with 34.7 g of 20-9-9 urea-formaldehyde fertilizer. Carbon dioxide and N₂O sampling occurred on four days during the first week and once each week thereafter for 14 total sampling days. Methane flux sampling occurred once each week starting on day 18, for nine total sampling days. Eight pots were randomly selected for sampling to measure CO₂ and N₂O flux and four of these were used to determine CH₄ flux as well. Each flux sample consisted of four gas samples taken at 10-min intervals from a static-chamber in each pot and held in Exetainer vials until analyzed by gas chromatography. Gas fluxes were calculated for each pot using the 'gasfluxes' package in R. Mean gas fluxes were -1.5 mg CH₄-C m^‑2 h^‑1, 467 mg CO₂-C m^‑2 h^‑1, and 3.6 mg N₂O-N m^‑2 h^‑1. Low methane concentration in the gas samples and high mean p-value (p=0.24) of methane flux indicated that the CH₄ fluxes were negligible. Carbon dioxide and N₂O fluxes were highly variable among samples and over the 81-day study. Future work to mitigate greenhouse gases from soilless substrate should focus on N₂O emissions since net CH₄ flux is negligible and CO₂ generated from the fir bark growing media is not considered a greenhouse gas.