Degradation of hemicellulose, cellulose and lignin in decomposing spruce needle litter in relation to N

Abstract

Decomposing needles from a Norway spruce forest in southern Sweden were studied for 559 days under laboratory conditions. Falling needles were collected in control (Co) plots and plots that had received 100 kg Nha−1yr−1 as (NH4)2SO4 for 9 years under field conditions. One of the aims was to determine whether the previously documented low decomposition rate of the N fertilized (NS) needles could be explained by a lower degradation degree of lignin. The lignin content was studied using the alkaline CuO oxidation method, the Klason lignin method and CPMAS 13C NMR spectroscopy. The amounts of cellulose and hemicellulose were also determined.The fertilized needle litters initially decomposed faster than the unfertilized, but later this reaction reversed, so that at the end the mass loss was 45% initial C in the control and 35% initial C in NS. Klason lignin decreased with time in both treatments and overall, the change of Klason lignin mirrored the litter mass loss. No major difference as regards the decomposition of hemicellulose occurred between the treatments, whereas significantly lower concentrations of cellulose were found in NS needles throughout the incubation. The CuO derived compounds (VSC) were somewhat lower in NS needles throughout the decomposition time. Initially, VSC increased slightly in both treatments, which contradicts the Klason lignin data. There was a weak positive relationship (p>0.05) between VSC and Klason lignin. Both vanillyls compounds (V) and cinnamyl compounds (Ci) increased slightly during decomposition, whereas syringyl compounds (S) vanished entirely. The lignin degradation degree, i.e. the acid-to-aldehyde ratio of the vanillyl compounds expressed as (Ac/Al)v, showed no significant effect of treatment. The 13C NMR analyses of the combined samples showed increased content of aromatic C with increasing decomposition time. The carbohydrate content (O–alkyl C) was lower in the fertilized needle litter throughout the incubation time. The alkyl C content tended to increase with decomposition time and N fertilization. The alkyl C/O–alkyl C ratios increased in both treatments during the incubation. The NMR results were not tested statistically.In conclusion, no major difference concerning lignin degradation could be found between the unfertilized and N fertilized needle litter. Thus, the study contradicts the hypothesis that higher amounts of N reduce lignin degradation. The reduced biological activity is probably due to direct N effects on the microorganisms and their decomposing ability.