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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, 14C and 13C 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 δ13C increased down the profile, from −26.3‰ (0–23 cm layer, mean of all measurements) to −25.2‰ 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 Δ14C 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 13C‐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.