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 adjacent moorland sites leaving exposed the uppermost mineral horizon of the soil. In the period since, plant recolonization and soil organic matter reaccumulation have occurred to give a chronosequence. Assuming no major changes in the carbon and nitrogen content of the unburned soil since 1957, the rates of accumulation of soil C and N were estimated to be 0.035 kg C m^-2 y^-1 and 0.001 kg N m^-2 y^-1 over the first 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 ¹³C NMR (cross-polarization, magic angle spinning ¹³C nuclear magnetic resonance 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 reaccumulated in the 1957-burned soil, the alkyl-C-to-O-alkyl-C ratio of the > 2000 μm and 2000-250 μm particle-size fractions were greater than those of the corresponding size fractions from the unburned soil, indicating that the reaccumulated soil organic matter was subject to decomposition but limited fragmentation or comminution.