Carbon (δ 13 C) and Nitrogen (δ 15 N) Isotope Dynamics during Decomposition of Norway Spruce and Scots Pine Litter.

We studied the dynamics of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) 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, δ¹³C_NSL showed a weak enrichment (0.33‰), whereas δ¹³C_SPL was depleted (−0.74‰) at the end of decomposition. Both litter types experienced a depletion in δ¹⁵N during decomposition; δ¹⁵N_NSL decreased by −1.74‰ and δ¹⁵N_SPL decreased by −1.99‰. The effect of the selective preservation of acid-unhydrolyzable residue (AUR) in lowering δ¹³C 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 δ¹³C values. In the later stages, there was a non-linear decrease in δ¹³C_NSL with a simultaneous increase in AUR concentrations, but the effect size was large, suggesting the role of lignin in driving δ¹³C of residues in later stages. Depletion in ¹⁵N in the residual litters concomitant with the increase in N concentration suggests bacterial transformation of the litter over fungal components. A consistent decline in δ¹⁵N values further implies that bacterial dominance prompted this by immobilizing nitrate depleted in ¹⁵N in the residual litter. [ABSTRACT FROM AUTHOR]