Export of dissolved organic carbon and nitrogen (DOC and DON) from terrestrial ecosystems to watercourses has increased in the boreal zone. However, the effects of decomposing material and soil food webs on the release rate and quality of DOC and DON are poorly known. The objective of this study was to quantify the release of carbon (C) and nitrogen (N) from mor and peat, which are the most common organic soil types in the boreal zone. The impact of soil fauna on the release processes was also estimated. Decomposition products were divided into intermediate products (DOC and DON) and mineralised end products, i.e. carbon dioxide (CO2-C) and ammonium (NH4+-N). The release of C and N into the soil solution, extractable pools, and CO2-C efflux were measured. DOC and DON were further divided into high molecular weight (HMW) and low molecular weight (LMW) fractions.
Carbon release was slightly different in each soil type. In absolute terms, the peat released less C and in relative terms more DOC than mor. Enchytraeids worms (Cognettia sphagnetorum) affected the quantity and composition of the decomposition products by enhancing mineralisation and the release of HMW-DOC.
In the slightly decomposed peat, the net release of N occurred in the form of DON, whereas the other soil types mainly released NH4+-N. The slightly decomposed peats are typically located close to water bodies, in the topmost soil layer, where the water discharge is the highest. Therefore, the slightly decomposed peats might have an important role in controlling the quality and quantity of N export.
The results were used to further develop the decomposition model ROMUL to account for DOC and DON processes and the division between intermediate and end products. The parameterisation and structure of the model was analysed by means of the GLUE (Generalised Likelihood Uncertainty Estimation) method, and several simultaneously measured variables, which is novel in this research field.