Soil porosity as a habitat for microorganisms

Summary

The soil pore system has to be characterized quantitatively in order to describe the soil as a habitat for microorganisms. Soil pore morphology as determined by soil structure may be as important as the size distribution for the transport of gases and nutrients. This study adresses the physical properties of differently textured soils in undisturbed and remoulded state and their effect on microbial activity.
Bulk soil was retrieved from 0-20 cm depth at six locations along a textural gradient in an arable field in Denmark. The samples ranged in clay content from 11 to 45%. The soils were crushed in air dry state, mixed, re-moistened, and exposed to a 9 month period of structure regeneration. Following application of 14C-labelled organic residues, the soils (labelled NA1 - NA6) were exposed to a further 8 month period of structure regeneration. Intact soil cores were then sampled and drained to four different matric potentials, analyzed for physical characteristics and subjected to a 15 weeks period of incubation with collection of evolved carbon dioxide. Cores of undisturbed but recently tilled topsoil from each sampling location in the field (labelled RE1 - RE6) were included as reference samples for the physical characteristics.
The previously disturbed (NA) soils regained some of their secondary structure during storage under field-like conditions, but they were still structurally different from undisturbed (RE) soils of similar texture. The habitable and the protective pore space defined as the volume of pores ranging in size from 0.8-30 µm and 0.8-3 µm, respectively, appeared to be nearly constant across soils with different clay content. NA soils had slightly smaller volume of protective pore space compared with RE soils. Although partly aggregated, disturbed clayey soils showed less continuous / more tortuous pore systems than the texturally corresponding and structurally intact soils. Generally, the disturbed NA soils had a pore system with relatively small pores 'enmeshed' in the soil matrix, whereas the undisturbed RE soils exhibited larger pores.
Measurements of oxygen diffusion confirmed that the relative water content - often quantified as the percentage of water filled pore space (WFPS) - may be a reasonable parameter in models simulating the activity of aerobic micro-organisms in remoulded soils. However, differences in the structure of undisturbed soils were shown to invalidate the use of WFPS as a universal parameter in simulation models applied to field conditions.
The results indicated that turnover of soil OM was mainly controlled by the soil volume occupied by water. Soil texture affected turnover indirectly through its effect on the soil water regime. The turnover of the native OM was regulated by the total volume of water, whereas water in micropores and cavities <0.2 µm in diameter was not involved in the decomposition process of recently added OM.
The studies encourage further investigations of the activity of soil microorganisms as related to the soil physical framework and the physical processes of importance for the function of the microbial community.