Long-term bare fallows reveal centennially stable soil carbon pools under contrasting fertiliser and amendment regimes

Long-term bare fallow (LTBF) field experiments provide a unique framework to investigate soil organic carbon (SOC) persistence in the absence of fresh organic inputs. The Versailles 42-plots LTBF, established in 1928, is the oldest continuously managed bare fallow worldwide. Initially designed to assess the effects of fertilisers and amendments on loess-derived Luvisols, it offers a rare opportunity to quantify and isolate a centennially stable SOC pool. In 2008, total SOC in the reference plots was shown to equate estimated centennially stable SOC pool (Barré et al. 2010).

Here, we used repeated soil sampling to assess (i) whether the SOC content have stabilised over time and (ii) how long-term fertilisation and amendment practices affect the size of this pool. Treatments included mineral N fertilisers (ammonium, nitrate), basic amendments (lime, basic slag), mineral amendments containing P or K, an organic fertiliser (horse manure), and no-input reference plots. Topsoil (0–25 cm) was sampled in all 42 plots in 2008, 2014, 2017, 2021, and 2025 complemented by archived samples from 1929, 1949 and 1962.

In all plots except those receiving annual manure inputs, SOC contents have stabilised over recent decades, with no significant variation between 2008 and 2025. These steady values show that a centennially stable SOC pool has been reached but with different pool sizes across treatments. SOC contents were higher in plots receiving mineral N fertilisers or basic amendments than in no-input controls, whereas plots amended with monovalent cations (e.g. Na⁺, K⁺) or phosphates exhibited lower SOC levels.

These patterns suggest that long-term soil chemical and physical conditions, shaped by fertilisation and amendment regimes, influenced stabilisation processes and ultimately, SOC persistence. We suggest that low pHs (<5) resulting from with mineral N fertilisation may favour SOC stabilisation, while enhanced physical protection is promoted in lime- and carbonate-amended plots. Conversely, poor soil structure in monovalent cations amended plots may explain less SOC persistence.

These results underscore the high scientific value of long-term experiments, which need to be maintained and valorised, for understanding SOC dynamics and stabilisation.