Effects of reduced N deposition on litter decomposition and N cycling in two N saturated forests in The Netherlands

Chronic atmospheric N input may alter the rate of decomposition and N cycling during needle litter decomposition. Fresh needle litter from Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) and Scots pine (Pinus sylvestris L.) was exposed to high (>37 kg N ha−1 yr−1) and strongly attenuated rates of natural N deposition (4–6 kg N ha−1 yr−1), achieved by means of transparent roofs in the field. Needle litter was exposed in polyethylene litterbags for 2 yr in the field. The isotope 15N was added as a tracer with throughfall to study the fate of throughfall-N in the decomposing litter. The rate of decomposition was not affected by the amount of N deposition in the Douglas fir stand. In the Scots pine stand the rate of decomposition was temporarily reduced at the low-N input rates. Three years of reduced N input did not alter litter decomposition or N dynamics through a change in litter substrate quality (e.g. N concentration). However, effects of the amount of N deposition on litter substrate quality are likely to become evident after several more years of reduced N input. In both litter species total lignin increased initially (within 4 mo) to 50% of the total dry weight and remained at this level while the loss of mass continued. The amount of N deposition affected the N concentration during the first months of decomposition as characterized by a net N immobilization. Retention of the annual throughfall-NH4 input by the annual litterfall was 0.3 kg N ha−1 yr−1 at strongly attenuated N input rates (9% of throughfall-NH4 input) and 1.6–2.2 kg N ha−1 yr−1 at natural high rates (4–7% of throughfall-NH4 input). The 15N tracer results indicate that throughfall-N was retained mainly in easily-decomposable fractions that rapidly decomposed again after the 15N addition stopped. The results suggest a different critical C-to-N ratio in litter exposed to high and strongly reduced N input rates. This may result from a shift in decomposer communities or a change in the C and N metabolism of micro-organisms.