ASSESSING AVAILABLE CARBON - COMPARISON OF TECHNIQUES ACROSS SELECTED FOREST SOILS

Available carbon (C) is defined as soil organic C that heterotrophic microorganisms can readily utilize as an energy and C source. Several techniques for assessing available C have been described in the literature, but none has become standard. Four of these methods, (i) mineralizable C (Min-C), (ii) cold water soluble C (CWS-C), (iii) boiling water extractable C (BWE-C), (iv) and total C (Tot-C), were ocmpared to each other and to a denitrification potential (DP) bioassay of C availability. These comparisons were made across selected forest soils which exhibited wide ranges of textural composition and organic C content and which ranged in acidity from pH 3.1 to 5.4. Carbon mineralized to CO2 during a 7 day aerobic incubation of field-moist soil (Min-C) provided the best prediction of DP. Min-C also appeared to distinguish between available C and refractory C in a soil with high Tot-C. Air drying increased CWS-C levels 2- to 10-fold. The C made soluble by drying and rewetting was strongly correlated with Min-C in all but one soil sample. Although the quantity of CWS-C increased, drying and rewetting did not appear to affect the mineralizable quality of C extracted. None of the procedures studied provides an unequivocal measure of available C. Nevertheless, Min-C appears promising as a standard procedure because: (i) it mesured C presumably mineralized by microorganisms, (ii) it proved effective in predicting DP under incubation conditions that were mostly C limited, and (iii)it was the simplest of the procedures examined.