Soil surface CO2 flux as an index of soil respiration in situ: A comparison of two chamber methods

Predictions of global climate change have recently focused attention on soils as major sources and sinks for atmospheric CO2, and various methodologies exist for measuring soil surface CO2 flux. A static (passive CO2 absorption in an alkali trap over 24 h) and a dynamic (portable infra-red CO2 gas analyzer over 1–2 min) chamber method were compared. Both methods were used for 100 different site × treatment × time combinations in temperate arable, forest and pasture ecosystems. Soil surface CO2 flux estimates covered a wide range from 0 to ca. 300 mg CO2C m−2 h−1 by the static method and from 0 to ca. 2500 mg CO2C m−2 h−1 by the dynamic method. The relationship between results from the two methods was highly non-linear, and was best explained by an exponential equation. When compared to the dynamic method, the static method gave on average 12% higher flux rates below 100 mg CO2C m−2 h−1, but much lower flux rates above 100 mg CO2C m−2 h−1. Spatial variability was large for both methods, necessitating a large number of replicates for reliable field data, with typical coefficients of variation being in the range 10–60%, usually higher with the dynamic than the static method. Diurnal variability in soil surface CO2 flux was partly correlated with soil temperature, whereas day-to-day variability was more unpredictable. However, use of a mechanistic simulation model of CO2 transport in soil, SOILCO2, showed that very large day-to-day changes in soil surface CO2 flux can result from rainfall events causing relatively small changes in soil water content above field capacity (ca. −10 kPa), even if CO2 production rates remained relatively unaffected.