Mineralization of Carbon and Nitrogen from Freeze- and Oven-Dried Plant Material Added to Soil

Drying organic material before soil incorporation is a common procedure used in mineralization or decomposition studies. A laboratory study was conducted to determine the effect of drying methods on plant C and N and associated mineralization patterns in soil. Freezeand oven-dried water hyacinth (Eichhornia crassipes [Mart] Solms) was added to a Kendrick soil (loamy, siliceous, hyperthermic Arenic Paleudults) at a rate of 5 g kg~' and incubated in the dark at 27 °C for 90 d. Oven drying in paper bags significantly increased the lignin content and decreased the mineral content of the plant material compared to freeze drying. The total C and N was not significantly different for the two materials. The mineralization of C from freeze-dried plant material was more rapid during the initial stage of decomposition and remained significantly higher throughout the incubation. At 90 d, SO and 41% of the plant C had evolved as CO2 for the freezeand oven-dried plant material, respectively. Mineralization of "N from the plant material accounted for 33% of the applied N of the freeze-dried material and 23% of the applied N of the oven-dried material. Nitrogen mineralization and CO, evolution were linearly correlated (r = 0.998) for the oven-dried plant material, but less correlated (r = 0.770) for the freeze-dried material. D ORGANIC MATERIAL before soil incorporation is a standard procedure in mineralization or decomposition studies. Oven drying is still a common practice (Gilmour et al., 1985), but freeze drying (Jenkinson, 1965; Miller, 1974) and air drying (Herman et al., 1977) have been used. Dalai (1979) oven dried plant material at 60 °C in a forced draft oven to minimize changes in C and P compounds as compared to air drying. Miller (1974) assumed that freeze drying sludge would not chemically alter the organic compounds or influence the rate at which the micrqbial population would decompose the sludge after soil incorporation. A comparison of the effect of drying techniques on distribution of plant C and related mineralization patterns in soil has not been made. Van Soest (1965) reported erroneous measurements of lignin when forages were oven-dried. A procedure for measuring cell wall constitutents (Goering and Van Soest, 1970) stated that heat drying of forages at temperatures above 50 °C produced analytically significant increases in yield of lignin. Goering and Van Soest (1970) attributed the increase of lignin to the production of artifact lignin via a nonenzymatic browning reaction that involved plant N. This reaction required water and involved the condensation of carbohydrate degradation products with protein or amino acids to form an insoluble polymer (Donoso et al., 1962). If oven drying does convert a portion of the soluble C or N to a more decomposition-resistant form, then oven drying of organic material may lead to erroneous conclusions about subsequent mineralization in soil. Dep. of Soil Science, 106 Newell Hall, Univ. of Florida, Gainesville, FL 32611. Contribution from the Inst. of Food and Agricultural Sciences, Univ. of Florida. Florida Agric. Exp. Stn. Journal Series no. 9004. Received 7 Dec. 1987. *Corresponding author. Published in Soil Sci. Soc. Am. J. 52:1343-1346 (1988). The objective of this laboratory study was to compare mineralization of C and N of freezeand ovendried water hyacinth added to soil, as measured by CO2 evolution and NO3-N accumulation. Water hyacinths have been used extensively to treat wastewater due to their potential productivity in nutrient enriched waters (Wolverton and McDonald, 1979; Reddy et al., 1985). The plant material has been evaluated as an organic soil amendment (Parra and Hortenstine, 1976) and as feedstock for anaerobic digestion (Shiralipour and Smith, 1984). There is considerable interest in the decomposition characteristics of this plant. MATERIALS AND METHODS Water hyacinths were collected from a wastewater retention pond of the Reedy Creek Utility Company at Walt Disney World near Orlando, FL. The hyacinths were grown in a nutrient solution containing N labeled (NH4)SO4 for 2 wk to obtain N labeled plant material. The concentrations of added nutrients were: NH4-N = 20.0 mg L~'; K = 23.5 mg L-'; PO4-P = 3.1 mg L~'; Ca = 20.0 mg L~'; Mg = 4.8 mg L-'; SO4-S = 6.4 mg L~'; and Fe = 0.6 mg L~'. Micronutrients were applied through a commercially available liquid fertilizer (Nutrispray-Sunniland, Chase and Co., Sanford, FL) to obtain an initial concentration of 0.2 mg Cu L-', 1.5 mg Mn L-', 0.04 mg B L-', and 0.02 mg Mo L-'. The plants were frozen at —10 °C for 1 wk to facilitate chopping to a 1.6-mm length by passing the material through a Hobart T 215 food processor. This length was chosen for subsequent studies on anaerobic digestion of water hyacinth (K.K. Moprhead and R.A. Nordstedt, 1986, unpublished results, Univ. of Florida). The effects of freezing the plant material on plant C and N were not determined. A portion of the chopped water hyacinths was freeze-dried (Thermovac T) and another portion was oven-dried in a forced-air convection oven at 70 °C for 1 wk. Both sets of dried materials were ground to pass through the 0.84-mm screen of a Wiley mill. The freeze-dried and oven-dried plant materials were characterized for volatile solids and ash, total C (LECO Induction Furnace 523-300, LECO Corp., St. Joseph, MI), lignin, cellulose, and hemicellulose (Goering and Van Soest, 1970), total Kjeldahl nitrogen (TKN) (Nelson and Sommers, 1973), and ashed mineral constituents (Gaines and Mitchell, 1979). Volatile solids and ash were calculated from loss on ignition at 500 °C in a muffle furnace. Surface (0to 15-cm depth) soil samples of a Kendrick fine sand were collected from a fallow field at the Agronomy Farm, University of Florida, Gainesville. The soil was airdried and passed through a 2-mm sieve. The soil had a particle-size distribution of 929 g sand kg~' soil, 46 g silt kg~', and 25 g clay kg~'. The cation exchange capacity (CEC) was 3.44 cmolc kg"' soil, with a base saturation of 47%. The initial pH was 5.44 in water (1:2, w/v) and the organic C and TKN contents were 5.8 g and 0.38 g kg~' soil, respectively. Fifty-gram soil samples were preincubated for 5 d at a water content equivalent to 10 kPa suction before addition of the plant materials. The preincubation was to equilibrate the soil to a constant water content. The freezeand ovendried plant materials were thoroughly mixed with the soil at a rate of 5 g (dry wt.) kg~' soil (equivalent to 10 Mg ha~')They were incubated in the dark for 90 d at 27 °C. Each treatment consisted of three replications.