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a Michigan State University, Crop and Soil Sciences, 530 Plant & Soil Sci. Bldg., East Lansing, MI 48824-1325
b University of California, 123 Veihmeyer Hall, Department LAWR, Davis, CA 95616
* Corresponding author (smucker{at}msu.edu).
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.
Received 27 March 2007.
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Inclusion of soil biophysical processes into current numerical vadose zone models could improve predictability of mass and energy fluxes and provide better links between water, solute, gaseous, and thermal fluxes within soil profiles. Although soil structure and pore geometry largely control transport processes in soils, experiments that study the effects of soil structure and pore space characteristics on gaseous and solute diffusion are lacking (Tuli et al., 2005). Soil aggregate complexes are among the most active biophysical and biogeochemical structures known (Young and Crawford, 2004). Their contributions to soil structure are a major controller of the formation and maintenance of soil biological activity by providing highly diversified habitats for soil organisms and determining the movement and transport of carbon and nitrogen substrates.
The primary controller of most biogeochemical processes within aggregates is the complex network of interconnected pores whose radii range from a few angstroms to several millimeters. The majority of pores are filled with interactive biological and mineral processes. These biophysically interactive processes form dynamic gradients of soil organic compounds, gases, bacteria, and mineral colloids among organo-mineral complexes that stabilize aggregate structures. Stable aggregates contain numerous site-specific niches that promote (hydrophilic) and exclude (hydrophobic) solution flow (Calamai et al., 2000; Kaiser and Guggenberger, 2003; Masaphy et al., 1996).
Temporal variations in soil hydraulic properties influenced by biological activity are not well understood (Butters et al., 2003). Plant root growth and subsequent decay create biopores and preferential flow pathways, promote soil aggregation processes, and change pore-size distribution. During their life, plant roots release large quantities of organic compounds into their surroundings (Smucker, 1984). These rhizodeposition processes are of ecological importance due to loss of reduced C from the plant and the resulting input flux to soil organic C pool that fuels soil microflora. These C substrates fuel biological activity, such as nutrient and pollutant cycling and the dynamics of soil-borne pathogens. Rhizo-deposits are released into the soil in the forms of root cap cells, the secretion of mucilage, and the passive and controlled diffusion of root exudates. From 17 to 60% of the plant C is lost to the soils by root invasion into soils (Pierret et al., 2007; Smucker, 1984). These large quantities of organic compounds develop gradients of soluble C and N from the root to soil aggregates and from surfaces to interiors of soil aggregates, producing exponentially higher microbial communities.
The functional complexity of soil ecosystems includes a plethora of interdependent biophysical and biochemical processes contributing to the formation and function of soil aggregates that are strongly linked to soil biological activities through feed-forward and feed-back linkages (Pierret et al., 2007; Smucker et al., 2007). Soil organo-ion-mineral complexes assemble into stable soil aggregates, which sorb additional C and plant nutrients, retain water, increase microbial community activity and plant biomass, and contribute to the stabilization of larger storage capacities within larger macroaggregates (Smucker et al., 2007). As soil aggregates become larger and more stable, residence times of C and N compounds, soil pollutants, and heavy metals increase with concomitant microbial degradation. Intra-aggregate fine pores contribute to advective–dispersive flow of solutes in micropore regions where soil water diffusivity is reduced. An understanding of root functional architecture can improve the integration of research advances from fields operating as independent disciplines and improve our understanding of soil ecosystems (Pierret et al., 2007). Finally, there is a general lack of understanding of the potential contributions by mycorrhizal fungi to plant water uptake under arid conditions, with their complex and extended networks (Allen, 2007), and that new technologies are badly needed to observe mycorrhizal functioning.
Soil bacterial cells are often found embedded in biosynthesized extra-cellular polymeric substances forming accumulations of stationary colonies attached to solid surfaces (Or et al., 2007). Soil bacterial aggregation and pooling of resources offer a successful adaptation to variations in hydration status and in nutrient availability, and enhance cooperative genetic and metabolic exchanges. Massoudieh et al. (2007) reviewed gene transfer processes with a primary focus on conjugation as performed by the F plasmid in E. coli, emphasizing the kinetics of these processes in multiphase systems. They also introduced a modeling framework for kinetics of horizontal gene transfer on surfaces and integrate these novel kinetics of gene transfer by conjugation with bacterial fate and transport processes, including advection, motility, and filtration.
This special section of Vadose Zone Journal on soil biophysics incorporates numerous references to quantities, gradients, and flux rates of root–bacteria–fungi–exudate interactions with soil physical parameters, and demonstrates the biophysical contributions by roots and soil organic matter to soil hydrology. It is our hope that the presented examples provide new incentives for some that seek research opportunities to vigorously study the interactions of soil physical and biological processes, thereby deepening the broad field of soil ecology.
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