Isotope evidence for soil organic carbon pools with distinct turnover rates-II. Humic substances

Two experiments using ¹³C-enriched substrates have been undertaken to evaluate the relative turnover rates of the main pools of soil organic carbon namely bulk organic carbon, humin and humic acids. Firstly, soil organic matter was labelled naturally during a 5 year field experiment by cultivating Zea mays, a C₄ plant, on a soil that had previously grown isotopically distincts C₃ plants, e.g. wheat. Secondly, soil organic matter was labelled artificially with either d-glucose of ¹³C-enriched d-glucose during a 21 day laboratory experiment. Isotopic variations observed during both experiments demonstrated the existence of soil carbon pools of decreasing turnover rates: humic acids > bulk organic carbon > humin.