The turnover of organic carbon in subsoils. Part 1. Natural and bomb radiocarbon in soil profiles from the Rothamsted long-term field experiments

The Rothamsted long-term field experiments, started more than 150 years ago, provide unique material for the study of carbon turnover in subsoils. Total organic C, C-14 and C-13 were measured on soil profiles taken from these experiments, before and after the thermonuclear bomb tests of the mid-20th century. Four contrasting systems of land management were sampled: land cultivated every year for winter wheat; regenerating woodland on acid soil; regenerating woodland on calcareous soil; and old grassland. The mean radiocarbon ages of all the pre-bomb samples from cultivated land were 1210 years (0-23 cm), 2040 years (23-46 cm), 3610 years (46-69 cm) and 5520 years (69-92 cm). Bomb radiocarbon derived from thermonuclear tests was present throughout the profile in all the post-bomb samples, although below 23 cm the amounts were small and the pre- and post-bomb radiocarbon measurements were often not significantly different. Values of delta C-13 increased down the profile, from -26.3 parts per thousand (0-23 cm layer, mean of all measurements) to -25.2 parts per thousand for the 69-92 cm layer. The C/N ratios decreased with depth in virtually all of the profiles sampled. Excluding the surface (0-23 cm) soils from the old grassland, the hyperbola m = 152.1 - 2341/(1 + 0.264n) gave a close fit to the radiocarbon data from all depths, all sampling times and all sites, where n is the organic C content of the soil, in t ha(-1), and m is the radiocarbon content of the soil, in Delta C-14 units, corrected for expansion or contraction of soil layers with time. The aberrant grassland soils almost certainly contained coal: one of them was shown by C-13-NMR to contain 0.82% coal C. In Part 2 (this issue) of this pair of papers, these radiocarbon and total C measurements are used to develop and test a new model for the turnover of organic C in subsoils.