Measurement of soil CO2 efflux using soda lime absorption: both quantitative and reliable

Measurement of soil CO2 efflux using a non-flow-through steady-state (NFT-SS) chamber with alkali absorption of CO2 by soda lime was tested and compared with a flow-through non-steady-state (FT-NSS) IRGA method to assess suitability of using soda lime for field monitoring over large spatial scales and integrated over a day. Potential errors and artifacts associated with the soda lime chamber method were investigated and improvements made. The following issues relating to quantification and reliable measurement of soil CO2 efflux were evaluated: (i) absorption capacity of the soda lime, (ii) additional and thus artifactual absorption of CO2 by soda lime during the experimental procedure, (iii) variation in the CO2 concentration inside the chamber headspace, and (iv) effects of chamber closure on soil CO2 efflux. Soil CO2 efflux, as measured using soda lime (with a range of quantities: 50, 100, and 200 g per 0.082 m2 ground area enclosed in chamber), was compared with transient IRGA measurements as a reference method that is based on well-established physical principles, using several forms of spatial and temporal comparisons. Natural variation in efflux rates ranged from 2 to 5.5 g C m−2 day−1 between different chambers and over different days. A comparison of the IRGA-based assay with measurement based on soda lime yielded an overall correlation coefficient of 0.82. The slope of the regression line was not significantly different from the 1:1 line, and the intercept was not significantly different from the origin. This result indicated that measurement of CO2 efflux by soda lime absorption was quantitatively similar and unbiased in relation to the reference method. The soda lime method can be a highly practical method for field measurements if implemented with due care (in terms of drying and weighing soda lime, and in minimizing leakages), and validated for specific field conditions. A detailed protocol is presented for use of the soda lime method for measurement of CO2 efflux from field soils.