Soil organic matter quality interpreted thermodynamically

Soil organic matter quality in the sense of how easily carbon in the soil o rganic matter can be mineralised is a major determinant of soil carbon stor age and rate of mineralisation of nutrients. Its origin has so far remained elusive and a number of indices, such as C-to-N-ratio, lignin concentratio n and other combinations of chemical constituents have been used as substit utes for quality. We suggest here that quality is the number of enzymatic s teps required to release as carbon dioxide a carbon atom from an organic co mpound. The larger the number of steps the lower is the quality of the carb on atom. Such a measure connects quality to thermodynamics. It also explain s the rapid decrease in decomposition rate with decreasing quality suggeste d in the q-theory of organic matter dynamics and shows that the decompositi on rate of low quality substrates has a stronger temperature dependence tha n that of high quality substrates. (C) 1999 Elsevier Science Ltd. All right s reserved.