Late stage pine litter decomposition: Relationship to litter N, Mn, and acid unhydrolyzable residue (AUR) concentrations and climatic factors

• A method to study details in rate-regulating factors for litter decomposition.
• Evaluated influence of 3 substrate-quality factors (N, Mn, and AUR) and MAT.
• Nitrogen and AUR had a dominant influence on late-stage decomposition.
• Mn and MAT had weak influences on late-stage decomposition, Mn pos., MAT neg.

The aim of this paper is to evaluate relationships between decomposition rates of Scots pine (Pinus sylvestris) and lodgepole pine (Pinus contorta var. contorta) needle litter in the late stage of decomposition (>30% accumulated mass loss), and the progressively changing concentrations of manganese (Mn), nitrogen (N), and acid unhydrolyzable residue (AUR), as well as mean annual temperature (MAT) and mean annual precipitation (MAP). Using available long-term decomposition studies on pine needle litter in a climate gradient in Sweden, we calculated annual mass loss and related to concentrations of Mn, N, and AUR at the start of each one-year period as well as to MAT and MAP. We investigated these relationships for (i) all data on annual mass loss combined and (ii) annual mass loss for five different decomposition categories as defined by accumulated mass loss.We found highly significant, negative, and dominant relationships between annual mass loss and N (R2 = 0.39) and AUR (R2 = 0.39), a slight but significant positive relationship to Mn (R2 = 0.08) and a significant negative relationship to MAT (R2 = 0.06). The relationships were dynamic, and changed with accumulated mass loss. The rate-dampening effect of N decreased to be a rate-enhancing effect at c. 60–80% accumulated mass loss. A similar trend was found for AUR, becoming rate-enhancing at 70–80% accumulated mass loss. For Scots pine needle litter the effect of MAT on mass loss decreased with increasing accumulated mass loss and changed to a rate-dampening effect at c. 50–70% accumulated mass loss. Mn showed a stimulating effect on mass loss rate in all categories whereas MAP showed no effect in this mainly boreal climatic gradient.The current approach indicates a method for detailed studies of rate-regulating factors for litter decomposition.