Formation and Loss of Humic Substances During Decomposition in a Pine Forest Floor

subsequent accretion of layers of the complexes might represent a combination of chemical and physical proSince twice as much C is sequestered in soils as is contained in the tection from enzymes. atmosphere, the factors controlling the decomposition rate of soil C are important to the assessment of the effects of climatic change. Humic substances are operationally defined, but have The formation of chemically resistant humic substances might be an been demonstrated to have characteristics distinct from important process controlling recycling of CO2 to the atmosphere. compounds synthesized directly by plants (Stevenson, Our objectives were to measure the rate of formation and loss of 1994). There are several theories of how humic subhumic substances during 13 yr of litter decomposition. We placed nets stances are formed, but most studies agree that they are on the floor of a white pine (Pinus strobus) forest to separate each comprised of large ( 700 atomic mass units [AMU]) annual layer of litter for 13 yr and measured humic substance concenmolecules, with carboxylic acid, phenolic hydroxyl, and tration using NaOH extraction followed by chromatographic fractionaromatic functional groups (Stevenson, 1994). Operaation. The humic acid fraction increased from 2.1% of the C in litterfall tionally, humic substances have been defined as subto 15.7% after 1 yr. On a grams per square meter (g m 2) basis the stances extractable in NaOH. With the recognition that humic substance fraction increased during the first year and then declined, with a half decay time (t1/2) of 5.1 yr, which was significantly there might be NaOH soluble substances that are not slower than the bulk litter (t1/2 3.9 yr). The carboxylic C concentrahumic, subsequent steps involving precipitation of hution estimated from 13C nuclear magnetic resonance (NMR) increased mic acids at pH 1 and adsorption of fulvic acids to a in the litter over time, though total mass of carboxylic acid C in the hydrophobic resin have been added (Swift, 1996). Other forest floor also declined over the 13-yr period (t1/2 4.6 yr). While steps have been added by Leenheer (1981) to further humic substances in the forest floor decomposed at a somewhat slower separate aquatic dissolved organic matter into humic rate than bulk litter during Years 1 to 13, they decomposed much substances, hydrophylic acids, hydrophobic neutral subfaster than has been calculated from 14C dating of the refractory stances, hydrophilic bases, and hydrophilic neutral subfraction of organic matter in the mineral soil. stances. It is generally believed that during humification the carboxylic acid functional group concentration of orA twice as much C is stored in the soil ganic matter increases (Stevenson, 1994). The carboxylic as in the atmosphere (Lal et al., 1995). Conseacid functional groups in humic substances are particuquently, the factors that control the storage of C in the larly important in controlling acidity, cation exchange, soil are among those that play an important role in and translocation of metals in soils. Several studies have regulating the concentration of CO2 in the atmosphere. compared the humic substance concentration or funcThe first-order decomposition rate constant of lignin, tional group concentration (using cross-polarization, one of the slowest decomposing substances present in magic angle spinning [CP/MAS] 13C NMR) of various original plant material has been reported as 0.18 yr 1 a soil horizons that reflect increasing degrees of decompohalf decay time (t1/2) of 3.8 yr in a temperate soil (Lynch, sition (Kögel-Knabner et al., 1991; Zech et al., 1992; 1991). However, the observed residence time of C is on deMontigny et al., 1993). Other studies have reconthe order of hundreds or even thousands of years in structed long-term rates of decomposition from chronosome mineral soils (Campbell et al., 1967). The factors sequence studies (Harmon et al., 2000). The actual rate that retard the mineralization of C originating from of development and loss of humic substances or funcplant compounds are largely responsible for the storage tional group characteristics in forest soils have not, to of C in soil and the chemical transformation of C into our knowledge, been measured directly over the long humic substances may help explain this decrease in bioterm (e.g., 13 yr). degradability. Humic substances are regarded as more Our objectives were: (i) to measure the rate of formaresistant to decomposition than compounds directly syntion and loss of humic substances in fallen litter as a thesized by plants (Stevenson, 1994). Other hypotheses function of time, and (ii) to estimate the rate of formathat might explain the observed persistence of C in tion and loss of carboxylic functional groups in a forest soil include: physical protection from microbes within soil organic matter as a function of time. To accomplish interstices too small for access by microbial cells, and these objectives, we laid nets on the floor of a Pinus formation of organomineral complexes that tend to prostrobus forest for 13 yr to keep annual layers separate, tect against enzymatic attack. These hypotheses are not measured total C loss, analyzed the concentration of mutually exclusive. For example, adsorption of humic humic substances by NaOH extraction, and chromatosubstances to Fe oxyhydroxides (Stevenson, 1994) and graphic fractionation, and estimated the change in concentration of carboxyl C by CP/MAS 13C NMR. R.G. Qualls and A. Takiyama, Dep. of Environmental and Resource Sciences, Univ. of Nevada, Reno, NV 89557; R.L. Wershaw, U.S. Abbreviations: AMU, atomic mass unit; 13C NMR, carbon-13 nuclear Geological Survey, Box 25046, MS 408, Denver, CO 80225. Received magnetic resonance; CP/MAS, cross polarization, magic angle spin3 June 2002. *Corresponding author ([email protected]). ning; DOC, dissolved organic carbon; DP/MAS, direct polarization, magic angle spinning; t1/2, half decay time. Published in Soil Sci. Soc. Am. J. 67:899–909 (2003).