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

Abstract

Abstract. The stability of soil organic matter (SOM) is a major source of uncertainty in predicting atmospheric CO 2 concentration during the 21st century. Isolating the stable soil carbon (C) from other, more labile, C fractions in soil is of prime importance for calibrating soil C simulation models, and gaining insights into the mechanisms that lead to soil C stability. Long-term experiments with continuous bare fallow (vegetation-free) treatments in which the decay of soil C is monitored for decades after all inputs of C have stopped, provide a unique opportunity to assess the quantity of stable soil C. We analyzed data from six bare fallow experiments of long-duration (>30 yrs), covering a range of soil types and climate conditions, and sited at Askov (Denmark), Grignon and Versailles (France), Kursk (Russia), Rothamsted (UK), and Ultuna (Sweden). A conceptual three pool model dividing soil C into a labile pool (turnover time of a several years), an intermediate pool (turnover time of a several decades) and a stable pool (turnover time of a several centuries or more) fits well with the long term C decline observed in the bare fallow soils. The estimate of stable C ranged from 2.7 g C kg −1 at Rothamsted to 6.8 g C kg −1 at Grignon. The uncertainty Correspondence to: P. Barré (barre@geologie.ens.fr) associated with estimates of the stable pool was large due to the short duration of the fallow treatments relative to the turnover time of stable soil C. At Versailles, where there is least uncertainty associated with the determination of a stable pool, the soil contains predominantly stable C after 80 years of continuous bare fallow. Such a site represents a unique research platform for characterization of the nature of stable SOM and its vulnerability to global change.

How to link soil C pools with CO<sub>2</sub> fluxes?

“…As there are only very few LTBF experiments (Askov, Bad Lauchstädt, Bushland, Drain Gauge, Grignon, Kursk, Moscow, Praha-Ruzine, Rothamsted, Puch, Stone Steppe/Voronezh Tamworth, Thyrow, Ultuna, Versailles) without any inputs over decades (see details by Ruhlmann, 1999;Barré et al, 2010), this approach can be applied only at these sites. To my knowledge this approach was used solely to calculate decomposition rates and MRT of SOM pools (Ruhlmann, 1999;Barré et al, 2010) and to verify SOM models (Ruhlmann, 1999;Foereid and Hogh-Jensen, 2004;Ludwig et al, 2007). These decomposition rates of pools, however, were calculated by fitting of one or two exponential equations based on the decrease of total C content only (not on separated pools) and the results were not linked with CO 2 fluxes.…”

“…Because of the slow decomposition rates, the significant decrease of the C pools can be measured only after many decades (Jenkinson and Coleman, 1994;Ruhlmann, 1999 (Ruhlmann, 1999;Barré et al, 2010) and to verify SOM models (Ruhlmann, 1999;Foereid and Hogh-Jensen, 2004;Ludwig et al, 2007). These decomposition rates of pools, however, were calculated by fitting of one or two exponential equations based on the decrease of total C content only (not on separated pools) and the results were not linked with CO 2 fluxes.…”

A model based on Rock-Eval thermal analysis to quantify the size of the centennially persistent organic carbon pool in temperate soils

“…The SOC concentration of LTBF treatments can be used to estimate the size of the persistent SOC pool of a particular site, as proposed by Rühlmann (1999) and Barré et al (2010). Here, we refined estimates of the persistent SOC concentration previously published by Barré et al (2010) for the four sites used in this study. We then used these values to estimate the proportion of centennially 15 persistent SOC in 118 archived soil samples from LTBF and non-LTBF treatments of these four sites.…”

Effect of soil saturation on denitrification in a grassland soil

“…An agricultural soil, under grassland management since at least 1838 (Barré et al, 2010), was collected from a location adjacent to a long-term ley-arable experiment at Rothamsted Research in Hertfordshire (Highfield; see soil properties in Table 1 and further details in Rothamsted Research, 2006;Gregory et al, 2010). The mixed sward is dominated by Lolium and Trifolium species and is cut two-three times a year.…”