Impacts of rotations and crop residue treatments on soil organic matter content in two Swedish long-term experiments

In two long-term field experiments started in 1951 in south Sweden, impacts of crop residue treatments in four course rotations were compared. The rotations included sugar beets, cereals, oil seeds and clover/grass. Crops were grown after each other. Effects on soil carbon (C) and yields from manuring, straw burning, removing or returning straw were studied. Yields of winter wheat were negatively affected in the rotation with manure, while sugar beet yields reacted positively. Soil C content averaged 0.1 – 0.2 percentage units higher in treatments with manure application than those without. Soil C differences between burning/removing or return of straw were not observed. There were considerable differences in soil C between the sites at the experimental start. A rapid decline in soil C took place where the C values were the highest. With low initial values only minor changes over time were observed. Assuming a steady-state situation, 15% of the applied organic C was accounted for as humified in the rotation with manure. Where crop residues were returned, only 2% of applied C was humified.