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

The aim of the present study is to provide an overview of recent investigations on soil organic matter and soil microorganisms at different hierarchical levels (pico-, nano-, micro-, macro- and regional-scale) and to elucidate whether results at any one level can be up-scaled to higher hierarchical levels. Pico- and nano-scale investigations are used to reveal the structure and chemical composition of organic substances and microorganisms as well as the interaction between biota and humic substances. Since the decomposition rate of residues in soils depends much on their location within the soil, studies on the micro-scale enable researchers to delineate the mechanisms driving C and N turnover. During the last decade, micro-scale investigations concentrated either on aggregates yielded by different physical separation procedures or on different microhabitats characterized by high turnover of organic material. Plot-scale investigations were mainly perfomed to understand 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 ratio between above- and below-ground inputs, and changes in nutrient inputs. In addition, many plot-scale investigations of chemical and microbiological properties from the range of different soil ecosystems provide not only a useful database to explain potential changes within a single field or plot, 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 soil biochemical processes and site properties. In conclusion, investigations on each level of resolution may answer specific questions, but a complete understanding of a soil ecosystem requires an integrative view of investigations at all levels of resolution.