Soil organic matter formation and sequestration across a forested floodplain chronosequence

Successional changes in soil organic matter formation and carbon sequestrat ion across a forested floodplain chronosequence were studied at the Savanna h river site, National Environmental Research Park, SC, US. Four floodplain sites were selected for study, three of which are in various stages of rec overy from impact due to thermal effluent discharge. The fourth is a minima lly disturbed reference site. Forest Boor organic matter increases rapidly during early secondary succession, with a maximum of 657 g/m(2) and decreas ing to 338 g/m(2) during the later seral stages. Carbon content in the fore st floor also reflected this pattern, with Bevels greatest during early suc cession and declining thereafter. Changes in carbon pools of the forest flo or are primarily driven by changing levels of forest floor biomass in the v arious stages of succession, rather than element concentrations. The compos ition of the forest Boor from the various stages differed markedly. The per cent herbaceous material declined during succession from 74% in an early st age to <1% in the latest seral stage. Conversely, the amount of woody folia ge increased from 6.7 to more than 70% in late succession. Measures of the degree of transformation of forest floor litter to soil organic matter usin g the lignocellulose index (LCI) did not differ between stages of successio n. Percent lignin and percent cellulose of the forest floor were similar be tween stages and ranged from 13.8-16.3, and 30.4-32.5%, respectively. Carbo n content of the mineral soil increased with successional stage of the floo dplain chronosequence. Soil carbon content ranged from 15.6 kg/m(2) per 0.7 m in the earliest stage of succession to 55.9 kg/m(2) in late succession. Regression analyses indicated that it may take over 50 years for carbon lev els to reach 75% of that of the reference site. The evidence also suggests that soil structure was disrupted by the disturbance, producing a greater p roportion of microaggregates in early seral stages. The formation of soil m acroaggregate structure, which may facilitate the accrual of carbon, appear s to be occurring slowly. (C) 2000 Elsevier Science B.V. All rights reserve d.