Effects of Elevated Carbon Dioxide and Nitrogen Fertilization on Mycorrhizal Fine Roots and the Soil Microbial Community in Beech-Spruce Ecosystems on Siliceous and Calcareous Soil.

Abstract

To study the responses of forests to global change, model ecosystems consisting of beech and spruce trees were maintained in open top chambers for 4 years under four conditions, namely with normal and elevated CO2 and with low and high nitrogen input, each replicated four times. Each open top chamber (height 3 m, diameter 3 m, soil depth 1.5 m) contained two separate soil compartments containing nutrient-poor siliceous and nutrient-rich calcareous soil. Here, we focus on the fine roots and the soil microbial community in these model ecosystems. At the time of planting, the fine roots were cut back according to forestry practice, and the newly formed roots were colonized by the indigenous soil microflora. After 4 years, the total biomass of fine roots, when averaged over all treatments, was almost the same in each of the two soil types; it was highest in the top 100 mm of soil (60%) and decreased sharply in deeper soil layers. Fungal biomass associated with the fine roots, consisting mainly of ectomycorrhizal fungi, was estimated using the ergosterol content as a marker. It was much higher in fine roots in the siliceous than in the calcareous soil, indicating considerably enhanced ectomycorrhiza formation in the nutrient-poor siliceous soil. Elevated atmospheric CO2 stimulated fine root production by ca. 85% and 43% in the top 100 mm of calcareous and siliceous soils respectively. Increased nitrogen input caused a slightly reduced production of fine root biomass in the calcareous soil but increased it by 33% in the siliceous soil. Marker substances for microorganisms were analyzed in the root-free soil. The amounts of carbon released by fumigation/extraction (a general marker for microbial biomass) and chitin (a marker for fungal biomass) were significantly higher in the top layer of the siliceous than of the calcareous soil, but they did not respond significantly to the treatments with elevated CO2 or the nitrogen fertilizer. The total concentration of the phospholipid fatty acids (PLFAs) and the number of the PLFAs did not differ between the two soil types. However, four of the eight most abundant PLFAs differed significantly between the two soil types. Principal component analysis revealed clearly separated clusters for the two soil types. Although analysis did not reveal any significant changes in response to the treatments, the concentration of the PLFA typical for ectomycorrhizal fungi was significantly higher under conditions of elevated CO2 in the nutrient-rich calcareous soil.