Soil organic matter formation and sequestration during floodplain forest succession.

摘要

Successional changes in soil organic matter (SOM) formation and sequestration were studied at the Savannah River Site, South Carolina. Four sites were studied, three of which were in various stages of recovery from 30--35 years of impact due to thermal effluent discharge during nuclear reactor operations. The fourth was a minimally disturbed reference site. Forest floor mass (as well as C, N, and P pools) increased rapidly during early secondary succession, with a maximum of 657 g/m2 and decreasing to 338 g/m 2 in late succession. Percent herbaceous material of the forest floor declined during succession from 74% in an early stage to less than 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 floor litter to SOM using the lignocellulose index did not differ between stages of succession, suggesting that any effect by the forest floor on the increase in soil C and N was a function of forest floor mass only. Percent lignin and cellulose of the forest floor were similar between stages, ranging from 13.8--16.3%, and 30.4--32.5%, respectively. C:N ratios of the forest floor were similar throughout succession, ranging from 41 to 48, as were lignin:N ratios, which ranged between 12 and 14.; Carbon and N content of the mineral soil increased with successional stage of the floodplain chronosequence, from 15.6 kg C/m2/0.7 m in the earliest stage to 56 kg C/m2 in the latest, and 0.6 to 3.6 kg/m2 for N. Regression analyses indicated that it may take over 50 years for C and N levels in the upper 0.7 m of mineral soil to reach 75% of that of the reference site. C:N ratios in mineral soil returned to pre-disturbance levels after about one decade. Analysis of the distribution of soil aggregates across four size classes suggested that soil structure was disrupted by the disturbance, producing a greater proportion of microaggregates (53--250 mum) in early stages. Also, small macroaggregates (250--2000 mum) were accruing C and N more slowly than other size fractions, suggesting that small macroaggregates harbored a fraction less resilient to disturbance than other fractions. The formation of soil aggregate structure, which may facilitate the accrual of C and N, was occurring slowly. SOM dynamics in small macroaggregates and microaggregates were examined further by using physical fractionation procedures to produce twenty and sixteen size-density fractions, respectively. For small macroaggregates and microaggregates, disturbed sites contained significantly smaller amounts of C and N in the 2.0--2.2 g/cm3 density fraction of the 25 mum size classes, also indicating the possibility of a fraction that is more sensitive to soil disturbance.