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

ABSTRACT subsequent accretion of layers of the complexes might represent a combination of chemical and physical pro-Since 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 sub-humic 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 concen-molecules, with carboxylic acid, phenolic hydroxyl, andtration using NaOH extraction followed by chromatographic fraction-aromatic functional groups (Stevenson, 1994). Opera-ation. The humic acid fraction increased from 2.1 % of the C in litterfall tionally, humic substances have been defined as sub-to 15.7 % after 1 yr. On a grams per square meter (g m2) basis the stances extractable in NaOH. With the recognition thathumic 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 no