INCREASED ATMOSPHERIC CO2 AND LITTER QUALITY - DECOMPOSITION OF SWEET CHESTNUT LEAF LITTER WITH ANIMAL FOOD WEBS OF DIFFERENT COMPLEXITIES

Two-year-old chestnut trees were gorwn for two yr under ambient (350 ppm) and enriched (700 ppm) CO2 concentrations, in two naturally lit growth chambers. The doubling of CO2 resulted in a dilution of the nitrogen concentration in the leaf litter, with C:N ratios of 40 and 75 for the ambient and enriched CO2 concentrations, respectively. The litter was sterilized and inoculated with microflora and animal groups of increasing complexity (microflora + Protozoa; + nematodes; + Collembola; + Isopoda) and incubated over 24 wk. Every two wk, the CO2 release was measured and the litter was leached with demineralized H2O. The following analyses were performed on the leachates: pH, total nitrogen, dissolved and particulate carbon, inorganic nitrogen (NH4+ and NO3-), phosphate, and biological counts (Protozoa, nematodes and Rotifera). The initial decomposition stages (the first 12 wk) were dominated by the litter quality factor: CO2 release and nitrogen losses in leachates were higher and carbon losses lower in water leaching from the litter with low C:N ratio. Towards the late stages, when carbon mineralization decreased in the control litter, the animal effect emerged in litter with a high C:N ratio. Two groups appeared: (1) In the microflora + Protozoa units, carbon mineralization was reduced by 60% compared with the control litter. (2) In the diversified food web combinations, it became progressively higher with increasing complexity of the animal community and was enhanced by 30% compared with the control litter. This unexpected fundamental difference was explained by a change in the composition and activity of the microflora. Litter bleaching, respiration, C and N leaching and acidification rose with increasing animal complexity of the systems. Biological and chemical reasons explaining the invasion by white-rot fungi and its activity only in the material with a high C:N ratio are discussed. During the 24 wk. nitrogen and phosphorus mineralization was very low, indicating a high incorporation of the nutrient in the soil biomass.