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

Abstract

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, 14 C and 13 C 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 d 13 C increased down the profile, from À26.3& (0-23 cm layer, mean of all measurements) to À25.2& 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 ¼ 152.1 À 2341/(1 þ 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 À1 , and m is the radiocarbon content of the soil, in D 14 C 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 13 C-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.

Biased 14C-derived organic carbon turnover estimates following black carbon input to soil: an exploration with RothC

“…Some long-term agricultural experiments in Europe indicated a comparatively low radiocarbon activity (Rethemeyer et al 2005;Helfrich et al 2007;Ludwig et al 2007;Jenkinson et al 2008), and it has been hypothesized that possible contamination by fossil BC or coal may have contributed to this (Schmidt et al 1996;Leifeld et al 2006;Jenkinson et al 2008). This study supports the view of a need for a more rigorous examination of the role of BC for reliable calculations of soil carbon dynamics.…”

Application of thermal analysis techniques in soil science

“…Thermal methods offer the unique opportunity to directly measure changes in thermal stability of SOM or litter as a function of fire and thus complement structural-chemical investigations. Furthermore, non-biogenic soil carbon with characteristically high thermal stability can originate from geogenic or atmospheric input of coal, lignite, or soot (Helfrich et al, 2007;Jenkinson et al, 2008). This material, abundant in many ecosystems, differs in structure and isotopic composition from SOM and thereby confounds the interpretation of data on SOM chemistry unless it is quantified or separated.…”

Quantifying and isolating stable soil organic carbon using long-term bare fallow experiments

“…Indeed, the presence of BC has been reported at Versailles (probably due to the initial clearing of the forest between the 17th and the 19th centuries) and might be also present at the other sites (BC has been reported in some soils of the Park Grass experiment at Rothamsted near the old Manor House (Jenkinson et al, 2008), but there has been no evidence for the presence of coal in the bare fallow experiment plots yet). The comparison of BC content in actual LTBF samples with stable C pool concentration estimates, will allow one to discuss the percentage of stable C that is BC which would be an important insight for improving the calibration of the stable C pool in soil C models.…”

“…Using 14 C measurements (Trumbore et al, 1997;Jenkinson et al, 2008) and long-term C 3 /C 4 vegetation changes (Balesdent et al, 1988), a fraction of the SOM has been shown to be hundreds to thousands of years old. In many respects, the stable SOM fraction remains terra incognita: mechanisms explaining its stability are still discussed (e.g.…”