Alvin J. M. Smucker

Knowledge on the geometry of pore networks of intra-aggregate soil pore spaces are of great value for many soil environmental processes. Advances in non-invasive 3D imaging techniques such as synchrotron-radiation-based microtomography offer an excellent opportunity to study the interrelationship of the pore network geometry with physical processes at a spatial resolution of a few micrometers. This paper presents results of a quantitative 3D pore space geometry analysis of small scale (~5mm across) soil aggregates from contrasting soil management systems. Soil aggregates have been scanned at the SR-μCT facility operated by the GKSS Research Center at HASYLAB / DESY (Hamburger Synchrotron Strahlungslabor / Deutsches Elektronen Synchrotron) in Hamburg/Germany. The achieved isotropic voxel resolution of the scans ranged from 2.4 to 5.4 μm. Three-dimensional reconstructions of the soil aggregates were analysed for various pore space features using a suite of algorithms based on mathematical morphology. Results have shown expected differences in distributions of pore size, throat size, channel length and width as well as tortuosity and connectivity of the intra-aggregate pores with potential implications for soil functions. Underlying image transformations and methods of visualization and quantification of soil pore networks will be discussed in view of their robustness and possible application of such information in soil related research fields.

Soil biophysical transport mechanisms promoting biogeochemical sorption of soluble organic carbon (SOC) compounds within macroaggregates control the retention and release of most soil nutrients, C‐ and N‐based polysaccharides, and contaminants. Ecosystems containing continuous supplies of soluble root exudates and particulate organic matter (POM) provide a constant supply of mobile SOC compounds to surfaces and internal pore networks of soil aggregates. Intra‐aggregate pores, especially the ultrafine pores, appear to be developed, interconnected, and blocked or disconnected by repeated drying and wetting (DW) cycling with direct but unknown contributions to movement and retention of SOC compounds. There is evidence that the severity (e.g., range of soil water potential) and frequency of severe DW cycles control intra‐aggregate micro‐ and nanopore formation and function. Heterogeneously distributed microsites within aggregates contain microbial communities that readily mineralize available C and N compounds, producing mobile SOC that can be tightly sorbed to additional mineral surfaces made available within micro‐ and nanosized fissures during repeated DW cycling. Mechanical removal of concentric soil layers of aggregates, synchrotron imaging and computer microtomographic (CMT) image processing software of three‐dimensional pore networks and connectivities, coupled with synchrotron X‐ray small angle scattering to measure pore sizes. Natural isotopes of ¹³ C and ¹⁵ N to quantify C and N sorption and CO ₂ respiration provide new and integrated approaches for quantifying spatially heterogeneous changes of pore diameters, connectivities, and organo‐ion‐mineral sorption within intra‐aggregate pore networks. Net C and N alterations at surfaces and within aggregates appear to modify both the microbial activities and bacterial community structures, producing integrated feedback and feed‐forward processes between the soil biological and physical components of soil aggregates.

Although significant progress has been made in our ability to characterize and quantify the soil physical environment and processes, there remains a critical gap in our understanding of interactions between the soil physical and biological realms. This was the impetus for the special symposium entitled “Soil Biophysics: A Challenging Interface” we organized for the 2005 SSSA Annual Meeting to increase awareness for the need to improve understanding of interactions between the soil physical environment, soil microorganisms, and plant roots. The symposium included research on the fate and transport of microorganisms (microbes and viruses), control and optimization of bioremediation and phytoremediation, physical controls on microbial ecology, improved descriptions of water and nutrient uptake by roots, and rhizosphere processes. This special section includes five selected articles derived from presentations at the soil biophysics symposium.

Greater knowledge of intraaggregate porosity modifications by tillage conveys new information for identifying additional hydrologic, ion retention, and aggregate stability responses to specific management practices. Macroaggregates, 2 to 4, 4 to 6.3, and 6.3 to 9.5 mm across, were separated into multiple concentric layers and their porosities were determined. Saturated hydraulic conductivity ( K _s ) of multiple aggregate fractions from two soil types subjected to conventional tillage (CT), no tillage (NT), and native forest (NF) soils were measured individually to identify the effects of tillage on aggregate structure, porosity, and K _s. Intraaggregate porosities were the highest in NF aggregates. Greater porosities were identified in exterior layers of soil aggregates from all treatments. Lowest intraaggregate porosities were observed in the central regions of CT aggregates. Soil aggregates, 6.3 to 9.5 mm across, had the greatest total porosities, averaging 37.5% for both soil types. Long‐term CT reduced intraaggregate porosities and K _s within macroaggregates, of the same size fraction, from both the Hoytville silty clay loam and Wooster silt loam soil types. Values for K _s of NF aggregates, 5.0 × 10 ⁻⁵ cm s ⁻¹ , were reduced 50‐fold by long‐term CT treatments of the Hoytville series. The K _s values through Wooster aggregates from NF, 16.0 × 10 ⁻⁵ cm s ⁻¹ , were reduced 80‐fold by long‐term CT treatments. The K _s values through NF and NT aggregates were positively correlated with their intraaggregate porosities ( R ² = 0.84 for NF and R ² = 0.45 for NT at P < 0.005). Additional studies are needed to identify rates at which pore geometries within macroaggregates are degraded by CT or improved by NT.