Organic matter and soil microorganisms - Investigations from the micro- tothe macro-scale

The aim of the present study is to provide an overview of recent investigat ions on soil organic matter and soil microorganisms at different hierarchic al levels (pico-, nano-, micro-, macro- and regional-scale) and to elucidat e whether results at any one level can be up-scaled to higher hierarchical levels. Pico- and nano-scale investigations are used to reveal the structur e and chemical composition of organic substances and microorganisms as well as the interaction between biota and humic substances. Since the decomposi tion rate of residues in soils depends much on their location within the so il, studies on the micro-scale enable researchers to delineate the mechanis ms driving C and N turnover. During the last decade, micro-scale investigat ions concentrated either on aggregates yielded by different physical separa tion procedures or on different microhabitats characterized by high turnove r of organic material. Plot-scale investigations were mainly perfomed to un derstand the influence of soil management on soil organic matter turnover; the parameters considered were changes in the quantity and quality of plant residues entering the soil, their seasonal and spatial distribution, the r atio between above- and below-ground inputs, and changes in nutrient inputs . In addition, many plot-scale investigations of chemical and microbiologic al properties from the range of different soil ecosystems provide not only a useful database to explain potential changes within a single field or plo t, but also a database with which to model processes on the regional scale. Landscape-scale analyses by geostatistical methods are now recognized as a useful tool for identifying and explaining spatial relationships between s oil biochemical processes and site properties. In conclusion, investigation s on each level of resolution may answer specific questions, but a complete understanding of a soil ecosystem requires an integrative view of investig ations at all levels of resolution.