Mineral and Organic-Matter Interactions in Soils

The purpose of this research is to develop techniques and protocols (primarily for X-ray diffraction (XRD) analysis) to accurately determine the mineralogy of soils and to investigate the specific relationships between the mineral and organic constituents in both natural and human-impacted soils. Soil is a diverse biogeochemical environment wherein plant, microbial and animal life interact with the organic and mineral components. It is a complex ecosystem of great importance to global processes (carbon and nitrogen cycling, biological productivity, erosion, etc.). The vast bulk of the organic carbon in soils is refractory soil organic matter (SOM). SOM is produced from generations of pre-existing life that has decomposed to form what is now often referred to as humic material. The balance of organic matter production and decay is tied to its interactions with minerals in the form of organic/mineral colloids. The mineral component of the colloids are clays, formed in situ or inherited from weathering reactions elsewhere, semi-amorphous phases such as allophane or imoglite formed in situ, and iron and manganese oxides, also formed in situ. Organo-mineral complexes are significant contributors to the organization and structure of the soil. SOM is involved in both mineral precipitation and degradation/transformation reactions. By the same token, laboratory experiments have shown that clay minerals are able to adsorb and catalyze humic compounds and that the clay mineralogy and chemistry significantly effects both the rate and the products of the polymerization reactions. Both the mineral and organic matter in soils is continuously impacted by anthropogenic activities as humans derive their sustenance from cultivation of crops and release of industrial/agricultural by-products into soil environments.

This is a collaborative long-term Mud Lab research project involving me and my students and Dr. Billy Kingery and his students in the Dept. of Plant and Soil Science at MSU. Over the next several years we plan to investigate mineral/organic relationships in different soils exposed to dissimilar anthropogenic influences. The first study site is a private poultry farm in Neshoba County in central Mississippi. The site is comprised of a littered pasture soil with more than 20 years of broiler litter (chicken poop) application and an adjacent forested soil with no previous history of broiler litter application. The pasture consists of a predominate bermudagrass stand with a mixture of other forage species in summer and spring that was grazed by cattle and harvested for hay once a year. The wooded soil, with 40-60 year-old loblolly pine, was chosen for comparison because of its similarity and proximity to the littered field. This area had remnants of crop rows that indicate cultivation within the past 100 years.

Preliminary analyses show that differences in the mineralogy do exist within and between the unimpacted forested soil and the human-impacted pasture soil. Figure 1 shows the relative abundances of the mineral components of unimpacted and impacted soils. While similar, there is considerably more scatter of the data from the unimpacted soils.

Figure 2 shows examples of XRD patterns of oriented, glycol-solvated <2 µm fraction samples. Both patterns show the presence of kaolinite (indicated by the diffraction peaks at 7.24 Å and 3.59 Å), illite (10.1 Å and 5.02 Å), and vermiculite (14.52 Å and 4.82 Å). The most obvious difference in the patterns is the presence of an expandable mineral (indicated by peaks at 16.23 Å and 5.36 Å) in pattern A. The preliminary analyses have not shown if this mineral is low-, intermediate-, or high- charge vermiculite, hydroxyinterlayered vermiculite, or if it is a mixed-layer illite/smectite. More than 70% of the surface soils from the unimpacted field contain a clay mineral suite similar to that shown in pattern A. Conversely, more than 70% of the surface samples from the impacted field are similar to that shown in pattern B, which contains little if any of the complex expandable clay.

Figure 3. shows that both unimpacted and impacted soils become more clay-rich with depth. The unusual expandable clay is present in the deepest samples of three of the four impacted soils, even though it is absent at the surface. The relative abundance of the expandable clay does not seem to be related to depth in the unimpacted field. The differences shown by the preliminary analyses may be due to a physical process such as clay infiltration, or may be due to dissimilar chemical reactions occurring in the different soils.

The plan of study for this project is five-fold beginning with (1) rapid analyses of the samples, which are already underway. (2) The main effort is a detailed mineralogical analysis of end-member samples. Fundamental to the success of the project is the ability to separate soils into fractions that can be analyzed more efficiently and accurately than can the whole soil. Careful separation of samples into nearly monomineralic fractions will make unnecessary the use of "operationally defined mineralogies", and will provide samples that can be accurately characterized by XRD and chemical methods. (3) Detailed organic chemical analyses of end-member samples including functional group characterization and quantification will be accomplished through nuclear magnetic resonance (NMR) spectroscopic experiments. (4) The spatial relationship between inorganic and organic phases will be investigated at the soil colloid and at the mineral/humic macro-molecule scales. (5) The final phase of this project will be to integrate our data with remote sensing analyses. In effect, our data will become the "ground truth" for the remote sensing analyses.

The study employs both master's degree students and undergraduate workers. The project will be incorporated into several Geoscience and Plant and Soil Science courses. The spatial distribution of the samples is such that differences in soil properties determined by this study can be applied to actual agricultural and other real land use problems. Dissemination of that data will be accomplished through the Mississippi State Extension Office.

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