Understanding the impact of irrigation on CO₂ emissions from soils with different physical and chemical properties

Agricultural practices contribute to CO₂ emissions. Previous studies in our lab showed that application of irrigation water containing dissolved Ca²⁺, Mg²⁺, and HCO₃₋ contributed to CO₂ emissions and, at the same time, enriched soil inorganic carbon (SIC) content (i.e., soil carbonates). However, it is unclear how soil properties affect the quantity of CO₂ emitted when irrigation water containing dissolved inorganic carbon (e.g., bicarbonates) is applied to the soil surface. I conducted a series of laboratory experiments to examine how soil moisture, texture, organic matter (OM) content, and temperature affect the release of CO₂ during irrigation of soils with water drawn from Okanagan Lake, which contains dissolved inorganic carbon (DIC). A Picarro cavity ring-down spectrometer was used to monitor changes in CO₂ production and ¹³C of the emitted CO₂ following irrigation events. Organically derived CO₂ is depleted in ¹³C compared with DIC-derived CO₂, hence δ¹³CO₂ measurement was used to determine the source of CO₂ emitted from the surface of soil columns. As hypothesized, a greater proportion of DIC-derived CO₂ was released during irrigation of wetter soils than drier soils. Among the five gravimetric soil moisture levels tested (10%, 15%, 20%, 25%, 30%), CO₂ derived from DIC in irrigation water was detected only in soils maintained at 15% and 25% moisture. A greater proportion of DIC-derived CO₂ was released during irrigation of a silt loam (finer-textured) than a sandy loam (coarser-textured) soil; this effect was probably related to differences in the water holding capacity in the two soil types. Neither soil temperature (21 ℃, 30 ℃) nor soil OM content (7.7% and 3.3%) affected the proportion of DIC-derived CO₂ that was released during irrigation. Based on these findings, farmers who irrigate with DIC-rich water could use less frequent irrigation to reduce the production of DIC-derived CO₂. Nevertheless, the quantity of DIC-derived CO₂ released from these soils during irrigation was less than 10% of the total CO₂ emitted; this CO₂ production is far outweighed by the CO₂ removed from the atmosphere by irrigated crops via photosynthesis.Science, Irving K. Barber Faculty of (Okanagan)Biology, Department of (Okanagan)Graduat