ORGANIC-MATTER ACCUMULATION FOLLOWING FIRES IN A MOORLAND SOIL CHRONOSEQUENCE

Severe fires in 1957 and 1976 removed the vegetation and soil organic matter from the litter layers and organic horizons of soils at two adj acent moorland sites leaving exposed the uppermost mineral horizon of the soil. In the period since, plant recolonization and soil organic m atter reaccumulation have occurred to give a chronosequence. Assuming no major changes in the carbon and nitrogen content of the unburned so il since 1957, the rates of accumulation of soil C and N were estimate d to be 0.035 kg C m(-2) y(-1) and 0.001 kg N m(-2) y(-1) over the fir st 19 years, and 0.50 kg C m(-2) y(-1) and 0.023 kg N m(-2) y(-1) over the period from 19 to 38 years after burning. Solid-state C-13 NMR (c ross-polarization, magic angle spinning C-13 nuclear magnetic resonanc e spectroscopy) showed that the ratio of alkyl-and methyl-C-to-O-alkyl -C increased with stage of decomposition and in the unburned soil with decreasing particle-size. For the organic matter that had reaccumulat ed in the 1957-burned soil, the alkyl-C-to-O-alkyl-C ratio of the > 20 00 mu m and 2000-250 mu m particle-size fractions were greater than th ose of the corresponding size fractions from the unburned soil, indica ting that the reaccumulated soil organic matter was subject to decompo sition but limited fragmentation or comminution.