Soil organic matter formation and sequestration across a forested floodplain chronosequence

Successional changes in soil organic matter formation and carbon sequestration across a forested floodplain chronosequence were studied at the Savannah river site, National Environmental Research Park, SC, US. Four floodplain sites were selected for study, three of which are in various stages of recovery from impact due to thermal effluent discharge. The fourth is a minimally disturbed reference site. Forest floor organic matter increases rapidly during early secondary succession, with a maximum of 657 g/m* and decreasing to 338 g/m2 during the later seral stages. Carbon content in the forest floor also reflected this pattern, with levels greatest during early succession and declining thereafter. Changes in carbon pools of the forest floor are primarily driven by changing levels of forest floor biomass in the various stages of succession, rather than element concentrations. The composition of the forest floor from the various stages differed markedly. The percent herbaceous material declined during succession from 74% in an early stage to < 1% in the latest seral stage. Conversely, the amount of woody foliage increased from 6.7 to more than 70% in late succession. Measures of the degree of transformation of forest f loor l i t ter to soil organic matter using the lignocellulose index (LCI) did not differ between stages of succession. Percent lignin and percent cellulose of the forest floor were similar between stages and ranged from 13.G 16.3, and 30.4-32.50/o, respectively. Carbon content of the mineral soil increased with successional stage of the floodplain chronosequence. Soil carbon content ranged from 15.6 kg/m* per 0.7 m in the earliest stage of succession to 55.9 kg/m2 in late succession. Regression analyses indicated that it may take over 50 years for carbon levels to reach 75% of that of the reference site. The evidence also suggests that soil structure was disrupted by the disturbance, producing a greater proportion of microaggregates in early seral stages. The formation of soil macroaggregate structure, which may facilitate the accrual of carbon, appears to be occurring slowly.