The need to recycle and optimize water in agriculture generates a demand for tools to predict the long-term impact that those waters will have on the soil properties and soil productivity. Models exist to simulate transport of water and chemicals, but while the chemical reactions involved in soil are relatively well known, there is no direct information about the physical mechanisms taking place in the soil while these processes are occurring. Changes in soil structure are quantified with reduction functions or simple correlations, but they typically do not explicitly account for the effect of solution chemistry, organic matter content, or vegetation coverage on the arrangement of the soil particles and aggregates.

My research interest emphasizes the study of the soil aggregate size and geometry, the arrangement of the aggregates in the soil matrix and how the chemical and microbiological activity affect the hydraulic properties and soil carbon accumulation. Below is an outline of my research activities in the recent past as well as the direction proposed for the future.

Traditional methods to quantify aggregate size and aggregate stability in soils require the dislodging of the aggregates from the soil matrix. The tests, generally performed in dilute systems, have been questioned lately. Dilute systems may not properly represent the soil conditions in the field, as geometrical confinement has a dramatic effect in the pairwise double-layer interaction between two clay particles. As an alternative to the traditional methods to measure aggregate stability, I developed a new method based on the quantification of the aggregates using scanning electron microscopy (SEM), that together with image analysis provides the tools required to measure pore and aggregate size and shape.

Irreversible changes in soil structure may occur when clay particles become dislodged, if for example, the electrolyte level is decreased or the sodium fraction increases. Gypsum has been used extensively in reclamation of sodic soils with infiltration problems. It is well known that application of gypsum to sodic soils improves the soil physical conditions by promoting flocculation, enhancing aggregate stability and increasing the infiltration rate. These observations have no scientific documentation or quantification regarding the actual assembling of the soil particles at the aggregate level. In Lebron et al., 2002a, we quantify the changes that the pores and aggregates undergo when a sodic soil is reclaimed with application of gypsum. We also related the changes in size and shape of the aggregates with saturated hydraulic conductivity. This study is intended to establish the basis for a conceptual model to predict soil reclamation and salinization and sodification processes in soils.

Soil pore space and its intrinsic characteristics such as surface area, roughness, tortuosity, and connectivity are probably the most important factors controlling water movement and microbial activity in soils. Porosity is the result of the arrangement of the soil aggregates. We found that the pore size distribution in undisturbed soils is highly correlated with the aggregates size (Lebron et al., 2002b), this has implications for the modeling of water and solute transport, since many models use particle size distribution instead of aggregate size distribution in their inputs to make predictions.

Soil organic matter is the major sink/source pool for C in agroecosystems. Accumulation of carbon in soil is a microbiologically and physico-chemically driven process that contributes to the formation of soil structure, the accumulation of plant nutrients, and the generation of a complex physical habitat for plant, animal, and microbial communities. To better understand these processes, it is important to perform fundamental studies to examine the relationships among soil carbon, soil structure, management practices, and several microbial parameters including diversity, functional redundancy, and community composition. (This line is the subject of a cooperation of myself with microbiologists from UCR David Crowley and James Borneman).

According to our present knowledge, the first step in the retention of soil carbon occurs when plant-derived organic substances are degraded and transformed by bacteria and fungi into sugars, humic and fulvic acids, and a variety of other organic substances that accumulate on the surface of soil particles. These soil-organic matter complexes are then cemented together into microaggregates (<250:m) by biopolymers (more persistent organic matter), these are further knitted together into macroaggregates by root hairs and fungi (fresh, readily degradable organic matter).

In a feedback process, soil structure controls microbial activity, biodiversity, and the size of the soil biomass, thousands of bacterial and fungal species become spatially organized in relation to redox, oxygen, and chemical gradients. These chemical gradients are located in the pore space within and among aggregates. Pore space and pore shapes are critical in the water retention of a soil. Soils with higher clay or higher organic matter content typically have higher water retention capacity due to particle geometry and spacing.

Soil structure changes induced by organic substrate microbial transformations occur mainly at the microscopic level. Electron microscopy (EM) techniques have been proven to be adequate in describing the different types of clay-polysaccharide associations and the production of microstructures with specific physical properties.

The interactions between organic carbon, soil structure, availability of water, and soil production are very complex, and our understanding of their underlying mechanisms is incomplete. To improve and sustain crop productivity we need to understand how carbon is transformed and incorporated in the soil, the role that carbon has in soil structure, and the impact that changes in soil structure have in crop production in general. To approach these problems we need long-term field studies coupled with controlled and targeted laboratory experiments to collect, test, and validate long-term C accumulation, effects in soil structure and repercussions in crop productivity and microbial biodiversity.

An essential consideration when studying the pH in soils is the geochemistry of carbonates. When present, carbonates are key to determining the pH in soils and consequent phenomena like weathering and CO2 sequestration. Carbonate precipitation in natural environments is a kinetically controlled process. Predicting the rate of carbonate precipitation is essential in any attempt to model mineral-water interactions and to predict all the pH-dependent processes, such as transport, and bioavalability of micronutrients. (Lebron and Suarez, 1996, 1998a, and b, 1999a)

Because measurement of hydraulic properties is costly and time consuming, considerable efforts have been devoted to the indirect estimation of the hydraulic properties from data that can be measured more easily or are already available. Predictions are made with so-called pedotransfer functions (PTFs), these are based on regression equations or neural network models. There are few predictive models that include the effects of solution composition and external factors on the hydraulic properties, both having the common trait of affecting soil structure. Since salinization of land and water resources is a world-wide problem, we believe that predictive and descriptive models for the hydraulic properties of soils should include solution composition as well as external conditions. (This is a project in cooperation with co-worker Marcel Schaap). Neural network and bootstrap analysis have been used extensively and very successfully to predict multivariable non-linear relationships. Natural ecosystems with complex interaction among the different components of the microcosm seem like a good challenge to test these kind of predictive models.