Assessing the quality of dissolved organic matter in forest soils using ultraviolet absorption spectrophotometry

Ultraviolet spectrophotometry was used to investigate the effects, 30 yr after planting, of tree species substitution on the aromatic C content and related properties of dissolved organic carbon (DOC). Precautions were taken to correct measurements for the absorbance of NO3 and dissolved Fe. In litter leachates, a significant reduction in the aromatic content of DOC was found in the Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantation but not in the beech (Fagus sylvatica L.) plantation. The disturbance of short-term C dynamics thus revealed agreed well with field observations. Significant differences in aromatic content were also found in capillary soil solutions from the two planted stands. Overall, these modifications, produced by the substitution of trees 30 yr previously, mostly concerned the beech plantation. Soil processes, and probably adsorption, played a central role in controlling the quality of DOC in this soil and appeared to be influenced by the species planted. In low-capillary solutions, located in larger pores, changes to the aromatic content were only detectable in the surface soil of the beech plantation. We found a more pronounced effect of tree substitution in high-capillary solutions filling soil micropores, where the aromatic content of DOC might be tightly controlled by soil organic matter. It is difficult to say, however, whether the planting of Douglas-fir had actually accelerated soil recovery, or whether there will be future changes to attain a completely new equilibrium.