Carbon (δ13C) and Nitrogen (δ15N) Isotope Dynamics during Decomposition of Norway Spruce and Scots Pine Litter

We studied the dynamics of stable carbon (delta C-13) and nitrogen (delta N-15) isotopes in litter from Norway spruce (NSL) (Picea abies) and Scots pine (SPL) (Pinus silvestris) during in situ decomposition over a period of more than 4 years. Relative to initial values, delta C-13(NSL) showed a weak enrichment (0.33 parts per thousand), whereas delta C-13(SPL) was depleted (-0.74 parts per thousand) at the end of decomposition. Both litter types experienced a depletion in delta N-15 during decomposition; delta N-15(NSL) decreased by -1.74 parts per thousand and delta N-15(SPL) decreased by -1.99 parts per thousand. The effect of the selective preservation of acid-unhydrolyzable residue (AUR) in lowering delta C-13 of the residual litter was evident only in SPL. In the NSL, only in the initial stage did C/N have a large effect on the delta C-13 values. In the later stages, there was a non-linear decrease in delta C-13(NSL) with a simultaneous increase in AUR concentrations, but the effect size was large, suggesting the role of lignin in driving delta C-13 of residues in later stages. Depletion in N-15 in the residual litters concomitant with the increase in N concentration suggests bacterial transformation of the litter over fungal components. A consistent decline in delta N-15 values further implies that bacterial dominance prompted this by immobilizing nitrate depleted in N-15 in the residual litter.