Decomposition of Fresh and Anaerobically Digested Plant Biomass in Soil

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 "N before digestion. The fresh plant biomass and digested biomass sludge were freezedried 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' 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 "N 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 "N to "NO;-N accounted for 8°7o 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. Additional Index Words: CO; evolution, N mineralization, digested sludge, first-order kinetics. Moorhead, K.K., D.A. Graetz, and K.R. Reddy. 1987. Decomposition of fresh and anaerobically digested plant biomass in soil. J. Environ. Qual. 16:25-28. Anaerobic digestion of organic materials such as plant biomass, sewage sludge, or animal wastes is used to generate CH4 and stabilized organic waste material. This process generates by-products (solid residue and effluent) that must be either disposed of, or preferably utilized, in an environmentally safe manner. Disposal of the anaerobically digested sludge by land application is an option often considered. Anaerobically digested sludge composition differs from fresh organic materials. A consequence of anaerobic digestion is a reduction of the readily decomposable C of the organic material during production of CH4 and CO2. Anaerobically digested sludge application rates for soil are often determined by nutrient-release potential during decomposition. The N-mineralization potential is commonly used to determine sludge application rates. Low rates of C and N mineralization have been reported ' Florida Agric. Exp. Stn. Journal Series no. 7645. Received 19 Aug. 1986. 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. during decomposition of anaerobically digested sewage sludge (Miller, 1974; Tester et al., 1977). Much of the available information deals with land application of anaerobically digested sewage sludge, and only limited data have been reported on the decomposition of sludge derived from the anaerobic digestion of plant biomass. Anaerobic digestion of plant biomass to produce CH4 was evaluated as a possible alternate energy source (Shiralipour and Smith, 1984). This form of energy appears to be particularly attractive in the southeastern USA where favorable growing conditions exist throughout most of the year. Among plant biomass that have received critical evaluation include aquatic plants such as water hyacinth (Eichhornia crassipes [Mart.] Solm), terrestrial plants such as napier grass (Pennisetum purpureum L.), and sorghum (Sorghum bicolor) (Shiralipour and Smith, 1984). The objective of this study was to evaluate the C and N mineralization of fresh and anaerobically digested water hyacinth added to soil. Four materials were evaluated, fresh plant biomass with low or high tissue N content and their respective anaerobically digested sludges. MATERIALS AND METHODS Water hyacinths were grown in tap water and sewage effluent to obtain low (10 g N kg" dry plant tissue) and high (34 g N kg"' dry plant tissue) tissue N content, respectively. After removal from their respective growth media, the hyacinths were grown in nutrient solution containing N-labeled (NH4)2SO4 for 2 weeks, frozen, and chopped to a 1.6-mm length using a Hobart T 215 food processor. A portion of the water hyacinths were anaerobically digested for 16 weeks in 55-L batch digesters. Each digester received 4.7 g (fresh wt) of the "N-labeled water hyacinth, 5 L of inoculum from anaerobic digesters receiving water hyacinth as feedstock, and were buffered with 210-g NaHCOj. After digestion, the biomass sludge was separated from the effluent using a 1.00-mm fiberglass screen. Samples of the fresh water hyacinth and anaerobically digested water hyacinth sludge were freeze-dried (Thermovac T) and ground through a 0.84-mm screen of a Wiley Mill. The freeze-dried materials were characterized for lignin, cellulose, and hemicellulose (Goering and Van Soest, 1970), ashed mineral constituents (Gaines and Mitchell, 1979), total solids (TS), volatile solids (VS), total C (TC) (LECO Induction Furnace 523-300, LECO Corp., St. Joseph, MI), and total Kjeldahl N (TKN) (Nelson and Sommers, 1973). Surface (0-15 cm depth) soil samples of a Kendrick fine sand (Arenic Paleudults) were collected at the Agronomy Farm, Univ. of Florida in Gainesville, FL. The soil was air-dried and passed through a 2-mm sieve. The soil had a particle size distribution of 92.9% sand, 4.6% silt, and 2.5% clay. The cation exchange capacity (CEC) was 3.44 cmolc kg"' soil with a base saturation of 47%. Fifty-gram soil samples were preincubated for 5 d at a water content adjusted to 0.01 MPa before addition of the residues. The freeze-dried materials were thoroughly mixed with the soil at a rate J. Environ. Qual., Vol. 16, no. 1, 1987 25 of 5 g (dry wt) -~ soil (e quivalent to10 Mg ha-’) andincubated for 90 d at 27 °C. Each treatment consisted of three replications. Water content was adjusted to 0.01 MPa every 15 d. Ambient laboratory air with CO2 and NH3 removed by 3 M NaOH and 4 M H2SO, traps, respectively, was passed through the incubation flasks with an aquarium pump at a rate of 50 mL min-k The CO2 evolved from the soil samples was collected in 0.1 M NaOH traps. The percentage C evolved with time was calculated by subtracting C evolved as COz of the control soil (no organic C amendment) from the various treatments and dividing by the amount of C added for each residue. Soil samples were analzyed at 0, 30, 60, and 90 d for 2 M KCl-extractable NH~ and NO; by steam distillation (Keeney and Nelson, 1982), and TKN by acid digestion (Nelson and Sommers, 1972). The ’ ~N analyses were conducted on a Micro Mass 602 spectrometer (VG Instruments, West Sussex, England). RESULTS AND DISCUSSION PLANT RESIDUE CHARACTERIZATION Characteristics of the fresh plant biomass and the anaerobically digested biomass sludge are presented in Table 1. The TC and TKN concentrations of the digested sludges were higher than their respective fresh plant materials. The C/N ratio of the fresh plant biomass with a low N content decreased from 35 to 16 after anaerobic digestion. The C/N ratio of the fresh plant biomass with a high N content did not change after digestion. The low N fresh plant biomass contained approximately twice as much lignin as the high N fresh plant biomass due to a larger rooting mass of plants grown in tap water. The shoot/root dry weight ratio was 1.6 for the low N fresh biomass compared to 4.2 for the high N fresh biomass. Moore and Bjorndal (1984, unpublished data) concluded that water hyacinth roots, in general, have higher lignin content than shoots. Lignin content was significantly higher in digested sludges due to loss of the more readily decomposable C during anaerobic digestion. The hemicellulose content was higher with the low N plant biomass than the high N biomass but did not change after digestion. The cellulose content decreased for low N fresh plant biomass after digestion but increased for high N fresh plant biomass. Anaerobic digestion of fresh plant biomass resulted in a large loss of plant K. Potassium is relatively soluble and is found in the effluent after anaerobic digestion (Atalay and Blanchar, 1984; Field et al., 1984). Additional K may be required to enhance the microbial degradation of Table 1. Characteristics of the fresh and digested plant biomass. Low N plant biomass High N plant biomass Chemical constituent Fresh Digested Fresh Digested