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

Since twice as much C is sequestered in soils as is contained in the atmosphere, the factors controlling the decomposition rate of soil C are important to the assessment of the effects of climatic change. The formation of chemically resistant humic substances might be an important process controlling recycling of CO 2 to the atmosphere. Our objectives were to measure the rate of formation and loss of humic substances during 13 yr of litter decomposition. We placed nets on the floor of a white pine ( Pinus strobus ) forest to separate each annual layer of litter for 13 yr and measured humic substance concentration using NaOH extraction followed by chromatographic fractionation. The humic acid fraction increased from 2.1% of the C in litterfall to 15.7% after 1 yr. On a grams per square meter (g m −2 ) basis the humic substance fraction increased during the first year and then declined, with a half decay time (t 1/2 ) of 5.1 yr, which was significantly slower than the bulk litter (t 1/2 = 3.9 yr). The carboxylic C concentration estimated from 13 C nuclear magnetic resonance (NMR) increased in the litter over time, though total mass of carboxylic acid C in the forest floor also declined over the 13‐yr period (t 1/2 = 4.6 yr). While humic substances in the forest floor decomposed at a somewhat slower rate than bulk litter during Years 1 to 13, they decomposed much faster than has been calculated from 14 C dating of the refractory fraction of organic matter in the mineral soil.