Workshop

Soil supports plant growth, provides habitat for the largest diversity of soil (micro) organisms, and regulates water and gas fluxes at the interface between the atmosphere, hydrosphere and litho-sphere. Vital economic, environmental, and human health issues are dependent to the ecological and biophysical functionality of the soil.
Soil structure, i.e. the spatial arrangement of soil constituents and voids (soil pores) controls transport of fluids as water and gas, water storage, chemical reactions, and provides spaces for biological activity. These functions are related to a host of ecosystem services ranging from mainte-nance of biodiversity, to proper matrix for plant growth, controls over groundwater quality and recharge, and soil-atmosphere gas exchange. In contrast to soil texture (determined by primary particles), soil structure is a highly dynamic trait that is affected by climatic processes induced by wetting-drying and freezing-thawing phenomena, biotic processes (root growth and root water up-take, burrowing of earthworms and other fauna, microbiological activity), and soil management activities including soil tillage and soil compaction caused by agricultural vehicle traffic.
The interaction between soil biology and the physical environment constitutes a feedback cycle, as shown in Fig. 1. Soil structure defines the physical habitat of soil living organisms; the biological activity, in turn, modifies the soil physical environment, i.e. soil fauna, bacteria, fungi and plant roots change their physical habitat (Fig. 1), e.g. through creation of new pore spaces and modification of surface properties. However, knowledge of this biophysical feedback cycle is incom-plete (Bottinelli et al., 2014), especially, mechanistic understanding is limited. Most research fo-cusses on effects of changed soil physical conditions (e.g. due to soil compaction, as affected by tillage systems) on biological activity (e.g. plant growth, earthworm abundance, microbial biomass), and this research is predominantly empirical (Webb, 2002). Work on how biological activity impacts soil structure and soil physical functioning has only started to receive attention relatively recently, but new non-invasive imaging technologies and numerical models now offer possibilities for new insights (see Hallett et al., 2013). While the overwhelming majority of organisms inhabiting the soil operate at the microscale, there is a spectrum of activities by biological agents at scales ranging
Swiss National Science Foundation | 2
from millimetre (earthworms, ants, young roots) to many centimetres (rodents, ant colonies, ma-ture root zones), and in dense tropical forests, the scale of continuous subterranean biological activity may span extensive landscapes. According to Hallett et al. (2013) “a major challenge re-mains to develop a modelling framework that incorporates the key biophysical interactions ac-counting for soil structure dynamics”.