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Decomposition of Fresh and Anaerobically Digested Plant Biomass in Soil¹

ABSTRACT

Using water hyacinth [Eichhornia crassipes (Mart.) Solms] for wastewater renovation produces biomass that must be disposed of. This biomass may be anaerobically digested to produce CH₄ or added to soil directly as an amendment. In this study, fresh and anaerobically digested water hyacinth biomass, with either low or high N tissue content, were added to soil to evaluate C and N mineralization characteristics. The plant biomass was labeled with ¹⁵N before digestion. The fresh plant biomass and digested biomass sludge were freeze-dried and ground to pass a 0.84-mm sieve. The materials were thoroughly mixed with a Kindrick fine sand (Arenic Paleudults) at a rate of 5 g kg^–1 soil and incubated for 90 d at 27 °C at a moisture content adjusted to 0.01 MPa. Decomposition was evaluated by CO₂ evolution and ¹⁵N mineralization. After 90 d, approximately 20% of the added C of the digested sludges had evolved as CO₂ compared to 39 and 50% of the added C of the fresh plant biomass with a low and high N content, respectively. First-order kinetics were used to describe decomposition stages. Mineralization of organic ¹⁵N to ¹⁵NO^TM₃-N accounted for 8% of applied N for both digested sludges at 90 d. Nitrogen mineralization accounted for 3 and 33% of the applied organic N for fresh plant biomass with a low and high N content, respectively.

Key Words: CO₂ evolution • N mineralization • digested sludge • first-order kinetics

¹ Florida Agric. Exp. Stn. Journal Series no. 7645.

² Graduate Research Assistant, Associate Proessor, and Professor of Soil Science, Inst. of Food and Agric. Sci., Univ. of Florida, Gainesville, FL 32611; and Central Florida Res. and Education Center, Sanford, FL 32771.

Received for publication August 19, 1986.