Modelling soil organic matter turnover

This thesis investigates various ways of determining and predicting soil carbon quality. The continuous-quality theory was used to study soil carbon quality changes under perturbations and to explain soil C:N ratios. Also, using the framework of the same theory a comparison of five commonly used soil organic carbon models were done. The results confirm that NIR spectra in combination with the theory can be used for defining soil organic carbon quality. Decreases in quality in treatments without organic amendments and increases in the treatments with organic amendments are both accurately predicted. The microbial biomass has been suggested as an indicator of change of soil properties. The continuous-quality theory tends to overestimate the microbial biomass and here the dependence of the mortality rate of microbes on the quality of substrate appears critical for correct predictions of microbial biomass. Nevertheless, the study showed that the amount of microbial biomass carbon tends to increase with increasing inputs of carbon in the soil and that the accumulation of microbial biomass also depends on the quality of these inputs. The C:N ratio in soils varies between ecosystems and there seems to be no accepted explanation for this variation. The study confirms that the combination of the continuous-quality theory and the growth rate hypothesis lead to predictions about soil C:N ratios that are compatible with both specific experimental evidence and general observations. The study of different SOC models showes that there is still a long way to go before we will find some generally accepted way of describing turnover of SOC. Not even such a fundamental process as decomposer efficiency can be assigned a value.