In situ respiration determination as tool for classifying soils according to soil organic matter content

Soil respiration is indicative of biological status of the soil and high respiration is correlated to high contents of available carbon (C) in soil and/or organic matter content. Because of soil respiration's relationship to soil organic matter status and content, soil respiration is considered one measurement that could aid in determining the quality of soil. In the global scale, the cycling of C in soil is important because the rise in CO2 in the atmosphere is linked to global climate change. In situ measurement of CO2 using instruments that are portable and perform analyses quickly are important to obtain sufficient number of measurements in the field to overcome spatial variability. Soil respiration tests were conducted in plots amended with fertilizer or organic amendments of agricultural or municipal residues since 1994. Besides CO2 moisture and temperature were measured over a period where the moisture varied from near saturation to below wilting point. It was found that flux was curvilinearly related to moisture from 5 to 40% (v/v). Maximum flux occurred for all plots between 30 and 40% saturation. The ratio of flux normalized by temperature to the volumetric soil moisture divided soils into two categories, those with soil organic matter (SOM) content above or below 4.5%. The determinations of CO2 flux, moisture and temperature uses equipment that is portable so that several sites in a field can be analyzed to reduce spatial variation. The only limitation is that the determinations must be performed on soils with less than 40% saturation or 25% moisture (v/v) because the normalized function is no longer linear above this moisture content. More than two SOM categories might be found if studies are expanded to soils with a wider range of SOM content.