Characterization of biogenic organic matter by stepwise thermogravimetry (STG)

The STG method presented here is a simple approach facilitating the characterization of biogenic organic materials. Pre-dried (130°C and grounded (< 0.5 mm) samples were heated in a muffle furnace at 280 °C for 6 h and subsequently at 520 °C for 6 h. The weight loss in the temperature range, 130–280°C (PI) and 280–520 °C (PII), provided an index (Rp) defined as, Rp = PII/(PI + PII). Plant materials rich in structural carbohydrates generally showed a Rp index around 0.3, whereas Rp for animal tissues rich in proteins usually were around 0.6. A general relationship between Rp and C:N for living biogenic organic matter, ranging from leaves of terrestrial origin to marine invertebrate tissue, was described by the equation: Rp = 0.791 x (C:N)^-0.246 (n = 13, r² = 0.946). During biological decomposition of composting barley straw, Rp increased from 0.17 to 0.37 and C:N decreased from 87 to 16. A similar Rp-C:N pattern was observed with depth in the upper 2 cm of an organic poor marine lagoon sediment (Rp increased from 0.43 to 0.47; C:N decreased from 8.4 to 7.7); indicating that microbial protein synthesis may have occurred with depth in this layer. The observed increase in both Rp and C:N with depth from 2 to 8 cm (Rp increased from 0.47 to 0.52; C:N increased from 7.7 to 10.3) suggested that humification may predominate in this zone. Accordingly, humic acids are found to have a Rp as high as 0.64. The Rp-C:N relationship appears to be a powerful two-dimensional tool applicable to characterize the bulk composition of various biogenic organic materials at different stages of decomposition.