DIVISION S-3-SOIL MICROBIOLOGY & BIOCHEMISTRY Humic Substances Formed during the Composting of Organic Matter Y. Inbar, Y. Chen,* and Y. Hadar ABSTRACT Humic substances (HS) were extracted from separated cattle-man- ure compost (CSM) at various stages of decomposition. Carbon-13 nuclear magnetic resonance ( U C-NMR) and Fourier-transformin- frared (FTIR) spectroscopy measurements were performed on the purified humic acids (HA) as well as crude fiber and chemical anal- yses. Levels of HS extracted from the CSM samples doubled during the composting (from 377 to 710 g kg- 1 organic matter [OM|). The HS were divided into: (i) HA, which increased from 184 to 457 g kg- 1 OM; and (ii) a fulvic fraction (FF). The FF was separated on XAD-8 resin to: (i) fulvic acid (FA), which maintained an unchanged level during composting (80-100 g kg- 1 ); and (ii) a nonhumic fraction (NHF), which increased from 107 to 170 g kg- 1 OM during the process. The ratios between the concentrations of these fractions were tested as humification indexes. These indexes changed signif- icantly during the process and were found to correlate exponentially with the composting time, C/N ratio, or both. Quantities of low- molecular-weight components (FF and NHF) rapidly increased dur- ing the first 40 d of composting and were nearly constant thereafter. The HA and HS contents reached nearly constant values after ~90 d. Only slight compositional and structural changes in CSM HA were detected with cross-polarization magic-angle spinning (CPMAS) 13 C-NMR, FTIR spectra, and chemical analyses. The elemental composition and functional groups of CSM HA resembled those reported for HA of plant residues, peats, and incompletely humified materials. The CPMAS 13 C-NMR spectra of the CSM HA resembled that of lignin. The aromaticity of the HA was about 40% and the lignin content was approximately 60%. The NMR data cor- related well with results obtained by FTIR spectroscopy, both indicating that the CSM HA are composed of partially degraded constituents of plant tissue, which still retain to some extent their chemical structures. The major plant components identified in the HA were lignin, carbohydrates, and long-chain aliphatic structural groups. I N THE LAST DECADE, the demand for peat as a pot- ting medium in horticulture has increased, while its availability has decreased. A number of organic wastes, such as tree bark, leaf mold, city and urban refuse, sewage sludge, sawdust, and treated animal ex- creta, have been introduced as peat substitutes in sub- strates after proper composting (Verdonck, 1984; Ravivet al, 1986). In recent studies, compost pro- duced from the solid fraction of CSM was found to be a high-quality peat substitute for container-grown plants (Inbar et al, 1986; Chen et al., 1988). However, the chemical characteristics and the degree of decom- Y. Inbar and Y. Chen, Dep. of Soil and Water Sciences, and Y. Hadar, Dep. of Plant Pathology and Microbiology, Faculty of Ag- riculture, Hebrew Univ. of Jerusalem, Rehovot 76100, Israel. Con- tribution from the Seagram Center for Soil and Water Sciences. Received 11 Aug. 1989. "Corresponding author. Published in Soil Sci. Soc. Am. J. 54:1316-1323 (1990). position of the organic matter required further elab- oration. Recently, application of NMR spectroscopy in stud- ies on HA has gained considerable attention. Both 'H and 13 C-NMR, in the solid and liquid state, have been used. Studies have shown that the technique of solid- state CPMAS 13 C-NMR (Bartuska et al., 1977) can provide C "fingerprints" of such diverse solid samples as peats (Hatcher et al., 1986; Preston et al., 1987; Wilson et al., 1983), whole soils (Preston and Rip- meester, 1983; Wilson et al., 1983), composts (Pio- trowski et al., 1984; Gerasimowicz and Byler, 1985; Preston et al., 1986; Newman et al., 1987; Inbar et al., 1989), and HS isolated from a variety of sources (Hatcher et al., 1983; Malcolm, 1989). An in-depth review of the state-of-the-art of NMR spectroscopy theory, techniques, and applications to a wide range of organic and inorganic geochemistry and soil chem- istry has been published by Wilson (1987). Infrared (IR) analysis has contributed substantially to the knowledge of the chemistry of HA (Schnitzer and Khan, 1972; Stevenson, 1982; MacCarthy and Rice, 1985), and complements the information ob- tained from solid-state CPMAS 13 C-NMR spectros- copy (Lobartini and Tan, 1988; Gerasimowicz and Byler, 1985; Inbar et al., 1989). The objectives of this study were to measure and quantify the amounts of HS produced during the com- posting of CSM, to characterize the HA produced by chemical analysis and spectroscopical techniques, and to develop analytical methods for the determination of compost maturity. MATERIALS AND METHODS Materials A solid fibrous fraction was obtained from liquid cattle manure using a Lisep Alfa Laval separator (Tumba, Swe- den). The solid phase (cake) was composted in 2 by 1 m 3 perforated plastic boxes for 147 d. The material was turned, mixed thoroughly, and sampled after 0, 3, 10, 18, 26, 39, 60, 91, 116, and 147 d. Samples of 25 L were stored at -4 °C. Two-liter subsamples were taken, dried at 105 °C, ground to pass through 0.25 mm, and stored in a desiccator under vacuum. Quantitative Estimation of Extractable Humic Substances Two-gram samples of dried bulk compost at varying de- grees of decomposition were weighed into 50-mL centrifuge tubes, 20 to 25 mL of 0.1 M NaOH were added, and the mixture was shaken for 24 to 48 h under N 2 . The extract was separated from the solids by centrifugation at 30 OOOg for 30 min. The supernatant was transferred immediately into a plastic bottle, acidified to pH 2 with concentrated H 2 SO 4 , and stored at 4 °C. Twenty milliliters of 0.1 M NaOH 1316