Water-Resources Investigations Report 03-4213

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

SUMMARY AND CONCLUSIONS

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 intrinsically unsuitable for this task because NOM isolates are complex mixtures of compounds that form molecular aggregates in solution. A more fruitful approach is to study the biochemical reactions that the chemical components of plant tissue undergo as they degrade into NOM. This type of study is generally unfeasible in natural systems because the NOM in these systems is normally derived from many different plant species. The results reported here are unique because the dissolved organic matter (DOM) component of NOM studied was derived mainly from a single species. Bales of wheat straw had been left in a field for several years; during this period of time, black water (BW) from the decomposing wheat straw accumulated in pools in the field.

The dissolved organic carbon content (DOC) and elemental composition of the BW were determined. Lowering the pH of the BW to pH 1 yielded a precipitate (BWP) that contained 84 percent of the total DOC in the BW. The ultraviolet (UV)-visible spectrum of the BW has a well-defined absorption band at 278 nanometers (nm) which has been shown to be characteristic of lignin.

The DOM from the BW was fractionated by polarity into four fractions on XAD-8 resin. The relative concentrations of the fractions were: fraction 1, 31 percent; fraction 2, 53 percent; fraction 3, 14 percent; and fraction 4, 2 percent where fraction 1 was the least polar and fraction 4 the most polar. The BW DOM was also fractionated by dialysis using 3,500 dalton (Da) and 12,000 Da membranes. The 3,500 Da membrane retained 90 percent of the total organic carbon (TOC) in the BW, and the 12,000 Da membrane retained 68 percent of the TOC.

The unfractionated DOM, the BWP, and the different fractions were characterized by infrared and nuclear magnetic resonance spectroscopy. The composition of the carbohydrates in the BWP was determined. The carbohydrate composition is typical of the composition of wheat straw hemicellulose.

The unfractionated BW and 12,000 Da retentate were analyzed by high-performance size-exclusion chromatography (HPSEC) with multi-angle laser-light scattering (MALLS) detection. The HPSEC/MALLS measurements indicate the majority of the DOM particles in the unfractionated BW have molecular weights on the order of 100,000 Da and root-mean-square (RMS) radii of about 22 nanometers (nm).

The results presented here indicate that the BW 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. These cross-linked complexes apparently were released from partially degraded wheat-straw cell walls with little alteration. 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.

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