Soil CO2, N2O and Nox Flux Responses to Biofuel Crop Plantation

Abstract:

Biofuel crops in high temperature environments, e.g, sorghum in southern California, USA, have a high capacity to assimilate atmospheric CO₂. Photosynthates from the canopy may provide extra labile carbon source to feed soil microorganisms and influence trace gas fluxes, including CO₂, N₂O and NOx. Understanding how soil microorganisms balance the carbon (energy) and nitrogen (nutrients) allocation between growing microbial biomass and respiration is critical for evaluating the GHG emissions and emissions of regional air quality pollutants. We conducted experiments in a high temperature agroecosystem both in fallow and sorghum production fields with an experimental nitrogen gradient (0,50 and 100 kg/ha, marked as control, low and high with triplicate repeat) to investigate the CO₂, N₂O and NOx flux responses. All gas fluxes were measured simultaneously from three replicate locations for each treatment in the field biweekly. Measurements were performed 2-5 days after irrigation. We found that planting sorghum has significant effects on soil CO₂ (p<0.0001), N₂O (p<0.0001) and NOx (p=0.04) fluxes, but nitrogen amendments only have marginally significant effects on CO₂ flux (p=0.07). Surprisingly, no significant response of N₂O (p=0.27) and NOx (p=0.61) were observed in responses to N amendments. Compared to the fallow field, the CO₂ flux in sorghum field increased 77%, 134% and 202% in control, low and high N level amendments, respectively. N₂O flux from the sorghum field are consistently higher than from fallow field, with 207%, 174% and 1064% increase in control, low and high N level amendments, respectively. For the NOx flux, no significant difference was found between fallow and sorghum field. Although nitrogen amendments did not show significant effects on CO₂, N₂O and NOx flux, the high N treatment in sorghum field continuously gains the highest flux rates. Our results suggested additional C inputs may be an important factor regulating CO₂, N₂O and NOx fluxes in high temperature agroecosystems.