Fertilizer Application Timing Influences Greenhouse Gas Fluxes Over a Growing Season.

Microbial production and consumption of greenhouse gases (GHG) is influenced by temperature and nutrients, especially during the first few weeks after agricultural fertilization.  The effect of fertilization on GHG fluxes should be sensitive to environmental conditions during and shortly after application, yet data indicating how application timing affects both GHG fluxes and crop yields during a growing season are lacking.  We designed a replicated (n=5) field experiment to test for the short-term effect of fertilizer application timing on fluxes of methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) over a growing season in the northern Great Plains.  Each 0.30-ha plot was planted to maize (Zea mays L.) and treated similarly with the exception of fertilizer timing: five plots were fertilized with urea in early-spring (1 April) and five plots were fertilized with urea in late-spring (13 May).  We hypothesized time-integrated fluxes over a growing season would be greater for the Late-Spring treatment, resulting in a greater net GHG flux, as compared to the Early-Spring treatment.  Data collected on 59 dates and integrated over a 5-month time course indicated CO₂ fluxes were greater (P<0.0001) and CH₄ fluxes were lower (P<0.05) for soils fertilized in Late-Spring.  Net GHG flux was also significantly affected by treatment, with 0.84 ±0.11 kg CO₂ equivalents m^-2 for Early-Spring and 1.04 ±0.13 kg CO₂ equivalents m^-2 for Late-Spring.  Nitrous oxide fluxes, however, were similar for both treatments.  Results indicate fertilizer application timing influences net GHG emissions in dryland cropping systems and additional research is needed.