Carbon Transformations in the Land Areas Receiving Organic Wastes in Relation to Nonpoint Source Pollution: A Conceptual Model I

A simple conceptual model based on current literature data was developed to describe organic carbon (C) loss from land areas receiving organic wastes. The model considers the decomposition of substrate C as represented by the evolution of COs. Decomposition of wastes was described in two or three phases, assuming first-order kinetics at each phase. The fraction decomposed at each phase was determined graphically. Rapid decomposition of an easily decomposable Cfraction in Phase I and II was followed by the decomposition of more resistant C fraction(s). For plant residues, the amount of C decomposed in Phase I was significantly related to the log C/N ratio of the residue, whereas, a similar relationship was not observed for animal wastes. Decomposition rates were about 6 to 10 times faster during Phase I and II decomposition, as compared to Phase 111 decomposition. Kinetic rate constants, calculated at each phase of decomposition were adjusted for the soil temperature, soil moisture, and method of application. For plant residues, simulated results were in close agreement with observed results. No field data are available to test the complete model for animal waste decomposition. Transport of soluble C (easily decomposable C fraction) in leaching and surface runoff was discussed in the paper. Equations were presented to calculate the transport of waste particles (resistant C fraction) and sediment-associated C in the runoff water. Future research needs identified include (i) a better understanding of C transformations; (ii) decomposition of individual C species, such as water-soluble C, cellulose, and lignin; (iii) a relationship between soluble and particulate C fractions in runoff water; (iv) mechanisms involved on the mode of C transport along with percolating water or in runoff water; and (v) extensive testing of existing models. Additional Index Words: decomposition, plant residues, animal wastes, water quality. Reddy, K. g., R. Khaleel, and M. g. Overcash. 1980. Carbon transformations in the land areas receiving organic wastes in relation to nonpoint source pollution: a conceptual model. J. Environ. Qual. 9: 434-442. Organic carbon (C) subjected to microbial decomposition in soil is derived from sources such as animal waste (land application sites and animal defecation); plant residues (after harvest of corn, wheat, soybeans, and others); and applied sewage effluents, sludges, and industrial wastes. Animal wastes have been traditionally applied with an objective to supply plant nutrients (loading rates from about 5 to 20 metric tons/ha). Land disposal of animal or other wastes at high loading rates (about 100 metric tons/ha or higher) has resulted in environmentally unacceptable conditions, and hence, is not recommended. Crop residues result in incorporation of approximately 8 to 10 metric tons dry matter/ha after ~ Paper no. 5856 of the Journal Series of the North Carolina Agric. Exp. Stn., Raleigh, N.C. The use of trade names in this publication does not imply endorsement by the North Carolina Agric. Exp. Stn. of the products named, nor criticism of similar ones not mentioned. This research was supported by the USEPA on Grant no. R-805011-01-0, and North Carolina Agric. Exp. Stn. Received 8 Sept. 1979. 2Assistant Research Scientist, Univ. of Florida, Agric. Res. and Educ. Center, Sanford, FL 32771; Research Associate and Associate Professor, Dep. of Biol. and Agric. Eng., North Carolina State Univ., Raleigh, NC 27650; respectively. CO 2 every growing season. The application of wastes for any purpose evidently increases soil C and improves the physical characteristics of the soil, thus, improving the infiltration capacity of the soil and reducing the amount of runoff water. The land application of organic waste does, however, increase the potential for runoff transport of C. During the period after waste application, but before rainfall-runoff, increased microbial activity reduces the potential pollution of water bodies. Thus, C-transformation kinetics are essential in assessing the nonpoint source pollutional impact of wastes applied to land (Fig. 1). Decomposition of wastes applied to the soil depends on the nature of the wastes and on several soil and environmental factors. The main components of the waste materials include: (i) carbohydrates (sugars, starches, hemicellulose, cellulose, pectins, gums, and mucilages); (ii) proteins, amino acids, and amines; (iii) fats, oils, waxes, and resins; (iv) alcohols, aldehydes, and ketones; (v) organic acids; (vi) lignin; (vii) cyclic or ring structure compounds (phenols, tannins, and cyclohydrocarbons); (viii) alkaloids and organic bases; and (ix) important miscellaneous substances present in very small amounts (antibiotics, auxins, vitamins, enzymes, and pigments) (Allison, 1973). Based on decreasing biodegradability, the waste C species can be grouped into (i) readily oxidizable soluble organic C, (ii) proteins, (iii) hemicellulose, (iv) cellulose, and (v) lignin. The composition organic wastes with respect to these C species is shown in Table 1 and the composition based on total C is shown in Table 2. Roughly 90% of the dry matter in animal wastes is organic material from animal digestion of feeds. Animal rations consist largely of carbohydrates (sugars, starches, celluloses, and hemicelluloses); some proteins; fats; small amounts of lignin; and numerous inorganic nutrients, such as nitrogen, phosphorus, potassium, and micronutrients, which finally reflect on the composition of animal waste. The decomposition of animal wastes or plant residues