Relationships between co 2 evolution, moisture content and temperature for a range of soil types

Abstract

Soils were collected from two locations at six sites on each of eight soil types. After sieving, soil CO 2 evolution was measured at three moisture contents and at 5, 10, 15 and 20°C. Our objectives were to (i) establish a regression model for the relationships between CO 2 evolution, temperature and soil moisture content, and (ii) to investigate which aspects of this regression apply universally and which are specific to individual soil types. The same soil samples were used at the different temperatures, giving CO 2 data which were repeated or longitudinal measurements. This experimental structure has implications for the covariance structure of the data. A preliminary analysis of the data showed that a natural logarithmic transformation of CO 2 evolution rates was effective both in linearizing the mean response and in stabilizing its variance. When CO 2 evolution was expressed as ln(ml CO 2 kg −1 loss-on-ignition h −1) and the soil moisture content as a proportion of the water-holding capacity, the shapes of the curves of CO 2 evolution against moisture content and the positions of their maxima became more similar than when CO 2 evolution and soil moisture content were expressed on an oven-dry weight basis. The average responses of ln(ml CO 2 kg −1 LOI h −1) of the soil types to temperature over the range 5–20°C were remarkably close to a set of parallel straight lines. This resulted in the formulation of a model in which: (a) the mean value of ln(ml CO 2 kg −1 LOI h −1) at each site within a given soil type can be described by a regression which is linear on temperature and quadratic on soil moisture content; (b) the mean value of the linear coefficient of temperature is common to all the soil types studied; and (c) the mean values of the intercept, and of both the linear and quadratic coefficients of moisture, vary stochastically between soil types. A classification of soil types on the basis of their mean regression coefficients is: (i) brown calcareous earths, typical brown earths, (ii) mimic alluvial gley soils, (iii) typical brown podzolic soils, podzols, stagnohumic gley soils, stagnopodzols, (iv) raw peat soils.