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

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 CO₂. 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 C fraction 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 III 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.