Assessing available carbon: Comparison of techniques across selected forest soils

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Abstract
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) mlneralizable C (Min‐C), (ii) cold water soluble C (CWS‐C), (iii) boiling water extractable C (BWE‐C), (iv) and total C (Tot‐C), were compared 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 measured 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.