Three studies explored the potential of the conventional radiocarbon dating procedure combined with the analysis of the degree of incorporation of 14C derived from nuclear
weapons testing (14C-bomb) during the early 1960s, for determining carbon turnover in forest soils.
The first study examined the gross annual turnover rate of soil carbon within the tree rooting zone of a mixed deciduous oak/ash/birch woodland (Meathop Wood) situated on an acid brown earth soil overlying carboniferous limestone. The turnover was estimated by dividing the soil carbon content of several soil layers (O + Of 0-5,5-10,10-15,15-25,25-35, and 35-50 cm depths) by their respective mean carbon age (carbon mean residence time) derived from the 14C analyses. The total turnover of carbon, estimated as 3860 kg C/ha/yr, very closely agreed with the estimated total carbon inputs to soil as litter components (leaf litter, twigs, flowers and fruits, herb layer throughfall, and stem flow plus root decay) of 3895 kg C/ha/yr. The latter data were derived from intensive site studies carried out during the International Biological Programme. Close agreement between the estimate for carbon turnover derived from the 14C data and the estimate for the annual total litter input is considered to validate the use of the isotope approach. It is also clear that different components within the litter layer and roots within the mineral soil layers, have different 14C-bomb signatures, thus indicating the potential to determine mean residence times of different forest litter materials within the O-horizon or roots within the mineral soil.
The second case study investigated the effects of birch on carbon dynamics in acidic heather moorland soils. Birch has gained the reputation of being a soil improver and has been shown to increase earthworm numbers, pH and extractable calcium, and mineralisable nitrogen, and significantly decrease the C:N, C:P, and C:K ratios in surface soils. Though these changes may be induced by a number of differing and interacting processes, an increase in the rate of soil organic matter decomposition, through enhanced biological activity, was thought to be a dominant factor. Using a chronosequence of sites from heather moor to 90-year-old birch stands at a single site in Scotland showing characteristic changes in soil properties, 14C measurements were made in 1976 on different soil horizons down to 40 cm depth. These showed a clear pattern of increasing 14C enrichment and the isotope penetration deeper into the soil profile with increasing stand age. These results accord with the hypothesis that there is an increasingly rapid turnover of the moorland soil humus and its partial replacement with younger birch-derived organic matter, with increasing birch stand age up to approximately 40 years.
The potential use of these 14C techniques in research on the dynamics of carbon in upland and high latitude soils containing high amounts of organic matter, in respect to the possible effects of global warming, has been suggested by studies of brown earth soils at four sites in an altitudinal gradient on the Pennines in northern England. Results showed that the soil carbon at the coolest site at 747 m had lower 14C enrichment values than that at the warmest site at 425 m. The sites differed in mean annual temperature by approx. 2°C. Furthermore 14C-bomb, indicating younger contemporary carbon, had penetrated only to c.3 cm at the highest site but had been incorporated to >5 cm at the lowest site. The combined results indicated that soil carbon turnover is much slower at the highest than at the lowest altitude site.
These 14C techniques might be used to partially validate computer models of carbon dynamics in forest ecosystems and at different scales of resolution (process, ecosystem, and landscape) in environmental studies.