ISOTOPE EVIDENCE FOR SOIL ORGANIC-CARBON POOLS WITH DISTINCT TURNOVERRATES .2. HUMIC SUBSTANCES

Two experiments using C-13-enriched substrates have been undertaken to evaluate the relative turnover rates of the main pools of soil organi c carbon namely bulk organic carbon, humin and humic acids. Firstly, s oil organic matter was labelled naturally during a 5 year field experi ment by cultivating Zea mays, a C-4 plant, on a soil that had previous ly grown isotopically distincts C-3 plants, e.g. wheat. Secondly, soil organic matter was labelled artificially with either D-glucose of C-1 3-enriched D-glucose during a 21 day laboratory experiment. Isotopic v ariations observed during both experiments demonstrated the existence of soil carbon pools of decreasing turnover rates: humic acids > bulk organic carbon > humin.