Epsilon Open Archive

Abstract

Decomposition is crucial in the circulation of global C as well as that of other elements like heavy metals. The aims of this work were to investigate and model how temperature (-4, 0.3, 5, 15, 25 and fluctuating -4/5°C) and moisture (four levels between wilting point and 100% WHC) influence the decomposition rate in a heavy clay soil incubated in the laboratory. Concentrations of Cd, Zn, Mn, Cu and Pb in spruce needles incubated in litter-bags for 6-8 years in five spruce stands were also measured and their effects on decomposition rates discussed. Decomposition rate increased significantly with increasing temperature and moisture. The decomposition rate was twice as high in the topsoil as in the subsoil samples. Temperature fluctuations did not increase decomposition rate compared with samples at constant +5°C. A two-component first-order model was superior to a single-component model to describe decomposition. Quadratic temperature and moisture response functions were used. The calculated minimum temperature (Tmin) for decomposition was -0.83°C and Q10 increased from 2.2 at 25°C to 12.7 at 0.3°C. When analysing decomposition data compiled from the literature, the two-component model was again more accurate than the single-component. The goodness of fit did not differ between the temperature response models tested, but was affected by the reference temperature. Between 5 and 35°C, a Q10-value of 2 was found to be adequate. Concentrations of Cd, Zn, Cu and Pb increased, while concentrations of Mn generally decreased during spruce needle decomposition. Concentrations considered toxic to micro-organisms were not observed. Total amounts of Cd, Zn, and Mn decreased by at least 15, 24 and 43% while total amounts of Cu and Pb increased by over 40 and 230% respectively, before they became more constant or started to decrease. A PCA indicated that dynamics of total amounts were more similar between adjacent sites than between more distant sites. The studies indicate that there is a need for more research, especially at low temperatures where decomposition rate is low. The paucity of high-precision measurements of decomposition below 5°C makes it difficult to draw conclusions about the temperature responses below this temperature. Concerning heavy metals, after about 8 years of decomposition in the field, between 20 and 40% of the litter still remained. We therefore lack a complete picture of heavy metal dynamics during the latter parts of the decomposition period.