Soil organic matter turnover in British deciduous woodlands, quantified with radiocarbon

Soil samples, collected to a depth of 15 cm from 24 plots at six UK deciduous woodland sites in 1971 and 2002, were analysed for 14C, and total soil carbon pools (gC m−2) were estimated. The results, together with data from a further woodland site, were interpreted using steady-state models, driven by the 14C content of atmospheric CO2. The average soil pool is 6700 (±1100)gC m−2, where the error term is the standard deviation. Application of a single soil pool model yielded a mean residence time (MRT) of 87 (±82)years, with an input of 113 (±52)gC m−2 a−1. This implies that the remainder of the total litter production, about 300 gC m−2 a−1, passes through a fast pool with a short MRT (i.e. rapid mineralisation occurs). Slightly better fits, with less bias, were obtained using a model that has two soil C pools with fixed MRTs of 15 (slow pool) and 350 (passive pool), based on the findings of DD Harkness, AF Harrison and PJ Bacon (Radiocarbon 28, 328–337, 1986). From the 21 soils that gave similar results with the two C pool model, it is calculated that, on average, the soil contains 3300 gC m−2 in the slow pool (input 215 gC m−2 a−1) and 3400 gC m−2 in the passive pool (10 gC m−2 a−1); about 200 gC m−2 a−1 of the litter input is rapidly mineralised. The slow pool is approximately constant for all the soils, variations in total soil C being due to the range of passive pools (1800 to 5900 gC m−2). Equally good fits of the bulk soil 14C data were obtained with different assumptions about the MRTs of the slow and passive pools; therefore additional information about soil fractions is needed for more exact characterisation of soil organic matter cycling on timescales of decades to millennia.