Biodegradability of Humic Substances and Other Fractions of Decomposing Leaf Litter

refers to molecular-level characteristics such as elemental composition, functional groups, and molecular conFormation of chemically resistant humic substances might be an formation that influence the inherent biodegradability important process controlling recycling of soil C to the atmosphere. of the molecule. Interactions refers to the intermolecular Humic substances are believed to be resistant to microbial decomposition because they accumulate in soil, but there is little direct experiinteractions with other inorganic or organic molecules mental evidence for their inherent recalcitrance. My objective was to that influence biodegradability—for example, complexcompare the microbial mineralization rates of the humic and fulvic ation with Al. Physical accessibility refers to the location acid fraction to other fractions of decayed plant matter in soil. Uniof the molecule with respect to enzymes or cells. For formly 14C labeled Populus fremontii leaf litter that had decomposed example, substrates may be located in submicrometer for 180 d was fractionated into the NaOH-insoluble residue, phenolic, pores too small for microbial cells to enter (Sollins et humic acid, fulvic acid, hydrophilic acid, and hydrophilic neutral fracal., 1996). These conditions do not necessarily occur extions. Humic acid comprised 22.7% of the C in the decomposed litter. clusively. These fractions were added to intact cores of soil or sand. Respired The inherent recalcitrance of humic substances might CO2 was collected in NaOH and the radioactivity counted. The be one reason that C tends to be stored for long periods substrate C mineralized in soil at the end of 1 yr was, in order from least to greatest percentage of added radioactivity mineralized: humic in the soil. Humic substances are complex macromoleacid (12.7%), fulvic acid (29.2%), phenolic (35.4%), ground litter cules modified from plant compounds or newly synthe(38.8%), hydrophilic acid (44.6%), hydrophilic neutral (51.3%), and sized during decomposition (Stevenson, 1994). Because the NaOH-insoluble residue (57.6%). In acid-washed, nutrientthey accumulate in soil (Stevenson, 1994), they are beamended sand, inoculated with soil microbes, the relative order of lieved to be resistant to microbial decomposition, but mineralization rates of the fractions was the same as in soil, but there is little direct experimental evidence for their inapproximately 10% less C was mineralized. Results supported the herent recalcitrance. One widely cited form of evidence hypothesis that humic substances are inherently difficult for microbes of the recalcitrance of humic substances comes from the to mineralize, and this property can contribute to the sequestration 14C dating of fractions of soil. In a Mollisol, the mean of C in soil. Results also supported the hypothesis that the fulvic acid residence time of the humic acid fraction was 1235 yr fraction is more rapidly mineralized than humic acid. (Campbell et al., 1967). However, the mean residence times of the unfractionated soil, fulvic acid fraction and humin fraction, were 870, 495, and 1140 yr, respectively. T dead plant materials that form soil organic Radio-labeled fungal melanins that resemble humic matter contain compounds that decompose with acids have been prepared from fungal cultures and were half-decay times (t1/2) that vary from days to a few years. found to be resistant to decomposition when incubated The decomposition rate of lignin, one of the slowestin soil (Linhares and Martin, 1978). Jandl and Sletten decomposing substances present in the original plant (1999) found that 5% of the C in the hydrophobic material, has been reported to be 0.18 yr 1 (correspondacid fraction of a water extract of litter was mineralized ing to a half time of decay, or t1/2, of 3.8 yr) in a temperate in a solution incubated 110 d. soil (Lynch, 1991). However, 14C dating shows the t1/2 Other studies suggest that humic substances are not of soil organic matter are on the order of hundreds or so resistant to decomposition. An alkaline extract of a thousands of years, indicating the accumulation of large forest soil was incubated in liquid culture containing amounts of soil C (Paul et al., 1997; Torn et al., 1997). growth media, and wood-degrading basidiomycetes This accumulation of C in soil is critical in regulating were able to reduce the color of the solution by 57% the concentration of CO2 in the atmosphere, since about in 21 d (Gramss et al., 1998). Likewise, a humic acid twice as much C is stored in soil as is in the atmosphere extracted from forest floor material was incubated in (Lal et al., 1995). liquid culture with glucose-containing growth media The conditions related to the tendency of soil C to with manganese, and a litter-degrading basidiomycete become stabilized or sequestered have been classified was able to reduce the color of the solution by 75% in as recalcitrance, interactions between molecules, and 42 d (Steffen et al., 2002). Up to 27% of aquatic humic physical accessibility (Sollins et al., 1996). Recalcitrance substances were removed from solution when incubated in a glucose-containing nutrient broth, but 7% were Dep. of Environmental and Resource Sciences, Univ. of Nevada, Reno, removed without the full glucose broth (Hertkorn et NV 89557. Received 11 July 2003. *Corresponding author (qualls@ al., 2002). A humic acid fraction from isolated lake sediunr.edu). ments incubated in lake water lost 60% of C from the Published in Soil Sci. Soc. Am. J. 68:1705–1712 (2004).  Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: t1/2, half-decay time; DOC, dissolved organic carbon.