Characteristics and energetic strategies of the rhizosphere in ecosystems of the Bornhoved lake district

This paper reviews investigations on interactions between soil microbiota, and plants during the interdisciplinary program 'Ecosystem research in the Bornhoved Lake district'; it deals with processes associated with the rhizosphere at four agricultural, and forest ecosystems. Emphasis was placed on a black alder forest representing the interface between terrestrial and aquatic ecosystems. In the alder forest, more than 96% of the alder roots were colonised by ectomycorrhizal fungi. The diversity and vitality of these mycorrhizal rootlets exhibited local differences being generally lower at a dystric-dry site compared to an eutric-wet site close to the lake. Bacterial populations typical of well-aerated arable soils and rhizosphere soils were detected in samples taken from the soil at the eutric-wet site although unfavourable anoxic conditions were assumed to occur near to the lake. The composition of the bacterial populations seems to be profoundly influenced by the plant. Furthermore, the Alnus - Frankia-symbiosis showed a high spatial variability and was also more extensively established at the lake shore indicating both higher nitrogen requirement of the trees and higher energy supply for N₂-fixing actinomycetes by the tree. The high energy supply by the trees at the eutric-wet site could be confirmed by the soil carbon availability index. The role of plants on in situ soil respiration was estimated for agricultural and forest sites. The aggregated and modelled data for arable soil indicated that the respiration of roots and rhizosphere organisms contributed up to 40% of the total soil respiration during summer. An even higher contribution to soil respiration may have been derived from rhizomicrobial respiration in the alder forest. In conclusion, the amount of carbon delivered below ground seemed to be adjusted dependent on ecosystem type and environmental conditions. In particular, alder trees seemed to have established a beneficial environment for the microbiota in the rhizosphere. (C) 2000 Elsevier Science B.V.