CO2 emission and source partitioning from carbonate and non-carbonate soils during incubation

The accurate quantification and source partitioning of CO2 emitted from carbonate (i.e., Haplustalf) and non-carbonate (i.e., Hapludult) soils are critically important for understanding terrestrial carbon (C) cycling. The two main methods to capture CO2 released from soils are the alkali trap method and the direct gas sampling method. A 25-d laboratory incubation experiment was conducted to compare the efficacies of these two methods to analyze CO2 emissions from the non-carbonate and carbonate-rich soils. An isotopic fraction was introduced into the calculations to determine the impacts on partitioning of the sources of CO2 into soil organic carbon (SOC) and soil inorganic carbon (SIC) and into C3 and/or C4 plant-derived SOC. The results indicated that CO2 emissions from the non-carbonate soil measured using the alkali trap and gas sampling methods were not significantly different. For the carbonate-rich soil, the CO2 emission measured using the alkali trap method was significantly higher than that measured using the gas sampling method from the 14th day of incubation onwards. Although SOC and SIC each accounted for about 50% of total soil C in the carbonate-rich soil, SOC decomposition contributed 57%–72% of the total CO2 emitted. For both non-carbonate and carbonate-rich soils, the SOC derived from C4 plants decomposed faster than that originated from C3 plants. We propose that for carbonate soil, CO2 emission may be overestimated using the alkali trap method because of decreasing CO2 pressure within the incubation jar, but underestimated using the direct gas sampling method. The gas sampling interval and ambient air may be important sources of error, and steps should be taken to mitigate errors related to these factors in soil incubation and CO2 quantification studies.