Bicarbonates dissolved in irrigation water contribute to soil CO<inf>2</inf> efflux
Hannam, K.D., Midwood, A.J., Neilsen, D., Forge, T.A., Jones, M.D. (2019). Bicarbonates dissolved in irrigation water contribute to soil CO2 efflux. Geoderma, [online] 337 1097-1104. http://dx.doi.org/10.1016/j.geoderma.2018.10.040
Plain language summary
Carbon dioxide is an important greenhouse gas. Rising concentrations of carbon dioxide in the atmosphere are causing global climatic changes. Agricultural practices are an important source of carbon dioxide in the atmosphere. We need a better understanding of how and when carbon dioxide is released during agricultural practices so we can find ways to reduce carbon dioxide production. In hot dry regions, which rely on the use of irrigation water to maintain agricultural productivity, irrigation water often contains dissolved bicarbonates. Bicarbonates can be a source of carbon dioxide. The carbon in carbon dioxide exists as two main isotopes: C-12, which has 6 protons and 6 neutrons, and C-13, which is slightly heavier because it has 6 protons and 7 neutrons. Because C-13 is slightly heavier, it reacts more slowly during chemical processes and, as a result, it tends to accumulate in some carbon-containing substances. For example, C-13 is relatively more abundant in the atmosphere and less abundant in plant tissue. We measured the abundance of C-13 released from the soil surface in a drip-irrigated apple orchard to estimate the quantity of carbon dioxide released from decaying plant tissue (which produces carbon dioxide with relatively less C-13) and from bicarbonates dissolved in the irrigation water (which produce carbon dioxide with relatively more C-13). Using this method, we found that most of the carbon dioxide released from the soil surface is produced by living roots and decomposing plant tissue, but bicarbonates dissolved in the irrigation water accounted for between 9% and 15% of the carbon dioxide released during irrigation. We estimate that irrigation using water drawn from Okanagan Lake generates more than 45 000 kg bicarbonate-derived carbon dioxide each year, which is equivalent to more than 19 000 L of gasoline.
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 13C enriched compared to that from organic sources in the soil (e.g., organic matter and respiring roots). Measurement of δ13CO2 at the soil surface can therefore be used to trace CO2 sources. Using on-line 13C analysis, we monitored δ13CO2 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‰ enrichment of 13CO2 followed the application of irrigation water, confirming the contribution of bicarbonate-C to soil surface efflux, whereas a 4‰ depletion of 13CO2 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.