Bicarbonates dissolved in irrigation water contribute to soil CO2 efflux

Understanding the processes that drive the release of carbon dioxide (CO2) from soil is essential to combat rising atmospheric greenhouse gases. Whilst significant research has focused on soil organic carbon (C) dynamics, soil inorganic C has received much less attention. In arid and semi-arid regions, crops are often irrigated with water containing inorganic (bicarbonate) C, which can be a source of CO2 emissions. CO2 released from dissolved bicarbonates is C-13 enriched compared to that from organic sources in the soil (e.g., organic matter and respiring roots). Measurement of delta(CO2)-C-13 at the soil surface can therefore be used to trace CO2 sources. Using on-line C-13 analysis, we monitored delta(CO2)-C-13 and rate of soil CO2 emissions in an apple orchard in British Columbia, Canada, irrigated with bicarbonate-containing irrigation water drawn from Okanagan Lake. In Experiment 1, we applied deionised or irrigation water to wet and dry soils; in Experiment 2, we applied irrigation water from Okanagan Lake to soils with and without a surface mulch. In both experiments, soil efflux responded within seconds to application of water. In Experiment 1, a 6 parts per thousand enrichment of (CO2)-C-13 followed the application of irrigation water, confirming the contribution of bicarbonate-C to soil surface efflux, whereas a 4 parts per thousand depletion of (CO2)-C-13 followed the application of deionised water, suggesting a stimulation of labile organic C mineralisation. Experiment 2 confirmed that bicarbonates in irrigation water contribute to soil CO2 efflux; surface mulch had no effect on the response of CO2 release following irrigation. Using an isotopic mass balance model, we calculated that bicarbonates dissolved in irrigation water accounted for between 9 and 15% of total soil surface efflux, and estimate that irrigation with Okanagan Lake water generates on average > 45,000 kg bicarbonate-derived CO2 each year.