Nitrous Oxide Emissions and Soil Respiration From Tomatoes Grown with Elevated Carbon Dioxide

Increase in atmospheric greenhouse gas (GHG) concentrations, including nitrous oxide (N₂O) and carbon dioxide (CO₂), has been partly attributed to agricultural activities.  N₂O is emitted from soil to the atmosphere as part of the nitrogen (N) cycle, and the addition of N through fertilizers increases N₂O emissions.  CO₂ enters the atmosphere as the result of fossil fuel, solid waste, wood burning and other industrial production processes.  However, CO₂ can also be sequestered by plants as part of the biological carbon cycle.  Sustainable agricultural practices are necessary to minimize GHG emissions while maintaining optimal crop production.  Particularly, it is important to quantify N₂O emissions and soil respiration in fields fertilized with N sources and subjected to elevated atmospheric CO₂ levels.  The objective of this study was to assess N₂O emissions and soil respiration in tomatoes grown under both ambient and elevated CO₂ levels and fertilized with Urea Ammonium Nitrate and Calcium Ammonium Nitrate.  During the summer of 2010, tomatoes were planted on a sandy loam soil and subjected to two CO₂ levels (ambient and 1.5 times ambient). The tomatoes were irrigated with a sub-surface drip irrigation system. Daily levels of atmospheric CO₂ within the plant canopy were monitored using CO₂ Analyzer. An EPA approved static flux chamber assembly and a Photoacoustic Field Gas-Monitor- INNOVA^® 1412 were used for N₂O emission measurements before and after fertilization events. A CIRAS-2 portable photosynthesis system with an attached CO₂ flux chamber was used to measure the soil respiration rates. Measurements were conducted at 84, 106 and 114 days after transplant (DAT). For the CO₂ enriched plots, mean daily CO₂ levels within the crop canopy ranged from 580ppm to 400 ppm during the 7 hours of application. Mean ambient CO₂ concentration was 358 ppm. For the CO₂ enriched plots, mean N₂O concentration decreased from 0.22 (±0.02) ppm on 84 DAT to 0.14 (±0.02) ppm on 114 DAT. In contrast, there was an increase in the N₂O levels from 0.04 (±0.01) to 0.17 (±0.03) ppm from the plots where tomatoes were exposed to ambient conditions. Generally, there was a moderate (r = 0.64) negative correlation between the N₂O levels measured in the CO₂ enriched plot versus those measured in the ambient plots. There was no significant difference (α ≥ 0.05) in both the soil respiration rates and tomato yields between CO₂ enriched and non-enriched plots. Generally, soil respiration rates decreased during the course of the study.