Effects of Elevated CO2 and Temperature On Forest Floor Litter Decomposition and Chemistry.

Forest floor can be a major component of the carbon held in forested soils.  In mature forests it represents the balance between additions and decomposition under current climate conditions.    Because of its position at the soil surface, this reservoir of C is highly susceptible to disturbance and climate change, and may be a significant source of atmospheric C under conditions that favor decomposition over additions.  In this study, forest floor litter was collected in an old-growth Douglas-fir forest and added quantitatively as the O-horizon on top of reconstructed mineral soils in large, outdoor, sun-lit, mesocosms.  Douglas-fir seedlings were planted in the mesocosms and were grown under a two-by-two factorial experiment of climate conditions that included two levels of air temperature (ambient and ambient+4 °C) and two levels of CO₂ (ambient CO₂ and ambient CO₂+200 ppm CO₂).  These exposures ran continuously for 4 years.  Under ambient treatment 42% of the litter C was lost after 4 years.  Under elevated CO₂ and ambient temperature or ambient CO₂ and elevated temperature more than 50% was lost.  However, under the combined treatment of elevated CO₂ and elevated temperature only 40% of the C was lost.  The loss of C was accompanied by a decrease in the C/N ratio and an increase in the calculated oxidation state of the remaining litter.  NMR revealed that the relative abundance of O-alkyl C in the remaining litter decreased and alkyl C increased compared to the litter at the beginning of the study.  There was an effect of treatment on the ratio of alkyl C to O-alkyl C, indicating that climate change will likely affect the biochemistry and extent of litter decomposition.  The results of this research provide a basis for anticipating changes in forest floor litter and nutrient cycling under future climatic conditions.