Biogeochemical Processes That Produce Dissolved Organic Matter From Wheat Straw
By Robert L. Wershaw, David W. Rutherford, Jerry A. Leenheer, Kay R. Kennedy, Larry G. Cox, and Donald R. Koci
The chemical reactions that lead to the formation of dissolved organic matter (DOM) in natural waters are poorly understood. Studies on the formation of DOM generally are complicated because almost all DOM isolates have been derived from mixtures of plant species composed of a wide variety of different types of precursor compounds for DOM formation. This report describes a study of DOM derived mainly from bales of wheat straw that had been left in a field for several years. During this period of time, black water from the decomposing wheat straw accumulated in pools in the field. The nuclear magnetic resonance and infrared spectra of the black water DOM indicate that it is composed almost entirely of lignin and carbohydrate polymeric units. Analysis by high-performance size-exclusion chromatography with multi-angle laser-light scattering detection indicates that the number average molecular weight of the DOM is 124,000 daltons. The results presented in this report indicate that the black water DOM is composed of hemicellulose chains cross-linked to lignin oligomers. These types of structures have been shown to exist in the hemicellulose matrix of plant cell walls. The cross-linked lignin-hemicellulose complexes apparently were released from partially degraded wheat-straw cell walls with little alteration. In solution in the black water, these lignin-hemicellulose polymers fold into compact globular particles in which the nonpolar parts of the polymer form the interiors of the particles and the polar groups are on the exterior surfaces of the particles. The tightly folded, compact conformation of these particles probably renders them relatively resistant to microbial degradation. This should be especially the case for the aromatic lignin structures that will be buried in the interiors of the particles.
Natural organic matter (NOM) has been studied for over 200 years. Most workers have attempted to elucidate the structures of chemical components of NOM by using the same methods that have been used to elucidate the chemical structures of pure compounds. These methods, however, are unsuitable for this task because NOM isolates are complex mixtures of compounds that form molecular aggregates in solution. Wershaw (2000) has proposed that a more fruitful approach would be to study the biochemical reactions that the chemical components of plant tissue undergo as they degrade into NOM. As plant organs (e.g. leaves) degrade in natural systems, soluble material is leached from the tissue. The organic compounds in this leachate contribute to the water-soluble component of NOM found in streams and soil waters which is commonly designated as dissolved organic matter (DOM) or dissolved organic carbon (DOC). A comparison of the spectral features of the DOM with those of the components of the plant tissue from which it is derived should give some clues about the chemical reactions that produced the DOM.
Generally, such studies are complicated because almost all DOM has been derived from mixtures of plant species. Due to an unusual set of circumstances, DOM derived mainly from wheat straw was formed in a field in southeastern Kansas where bales of wheat straw had been stored for use in the manufacture of particle board. The particle board venture failed, and the bales of straw were left in the field for several years. During this period of time, black water from the decomposing wheat straw accumulated in pools in the field.
Wheat straw is composed of approximately 70 percent cellulose and hemicellulose, 20 percent lignin, 5 percent silicates, and 5 percent ethanol-benzene extractable material (Mckean and Jacobs, 1997). The aromatic structures in the DOM therefore will be derived mainly from lignin. The lignin polymers in vascular plants are composed of varying amounts of p-hydroxyphenylpropanoid, guaiacylpropanoid and syringylpropanoid monomeric units (Douglas, 1996; Saake and others, 1996). The most common linkages formed during lignin polymerization are β-O-4 ether linkages (fig. 1), however, Saake and others (1996) have shown that other linkages such as α-O-4, β-β, β-5, and 5-5 also are present in lignins. Camarero and others (1994) have pointed out that especially in grasses the C-4 also may be attached to a hydroxyl group (C-4 phenol). The enzymatic degradation of lignin by micro-organisms generally involves depolymerization and aromatic-ring cleavage (Crestini and others, 1998).
In a previous study Wershaw and others (1996) developed a procedure for the fractionation of leachates from senescent leaves. The nuclear magnetic resonance (NMR) spectra of the leachate fractions from different types of leaves were measured and compared with the spectra of the leaves from which they were derived. In general, the spectra of the leachate fractions can be correlated to different components in the leaves (Wershaw and other, 1996, 1998).
In this report the biogeochemical processes that produce DOM from wheat straw are investigated. The results of this investigation should provide a clearer picture of the incorporation of lignin-derived components into the DOM pool than has previously existed.
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