Carbon sequestration and fertility after centennial time scale incorporation of charcoal into soil

Abstract

The addition of pyrogenic carbon (C) in the soil is considered a potential strategy to achieve direct C sequestration and potential reduction of non-CO2 greenhouse gas emissions. In this paper, we investigated the long term effects of charcoal addition on C sequestration and soil physico-chemical properties by studying a series of abandoned charcoal hearths in the Eastern Alps of Italy established in the XIX century. This natural setting can be seen as an analogue of a deliberate experiment with replications. Carbon sequestration was assessed indirectly by comparing the amount of pyrogenic C present in the hearths (23.3±4.7 kg C m(-2)) with the estimated amount of charcoal that was left on the soil after the carbonization (29.3±5.1 kg C m(-2)). After taking into account uncertainty associated with parameters' estimation, we were able to conclude that 80±21% of the C originally added to the soil via charcoal can still be found there and that charcoal has an overall Mean Residence Time of 650±139 years, thus supporting the view that charcoal incorporation is an effective way to sequester atmospheric CO2. We also observed an overall change in the physical properties (hydrophobicity and bulk density) of charcoal hearth soils and an accumulation of nutrients compared to the adjacent soil without charcoal. We caution, however, that our site-specific results should not be generalized without further study.

Figure 1. Soil profile at the control site (Panel a) and the charcoal hearth (Panel b).

The letters indicate different pedologic horizons. In the charcoal hearth the dark anthropogenic layer (Acoal; 0–10 cm) can be easily identified.

Figure 2. Correlation between average annual atmospheric deposition of P, K⁺, Ca²⁺, Mg²⁺, Na²⁺ (mg l⁻¹ y⁻¹) and the difference between the input of the same elements due to charcoal application in 1858 and the amount found today in hearth’s soils (Δelement, kg hearths⁻¹) (y = 2.50×–14.31, R² = 0.82, p = 0.035).

Dashed lines represent 95% confidence interval.

Figure 3. Keeling plots measured by CRDS showing the δ¹³C of respired CO₂ fluxes versus the reciprocal of CO₂ concentration for control and charcoal hearth incubated soils (δ¹³C_CONTROL = 7353*[CO₂]⁻¹−24.8, R² = 0.99; δ¹³C_{CHARCOAL HEARTH} = 7467*[CO₂]⁻¹−25.2, R² = 0.99).

Horizontal and vertical bars indicate standard deviations (n = 3).

Figure 4. SEM micrographs showing the inner morphology of charcoal fragments and the absence of any microbes or plant debris.

a) is a radial section b) a longitudinal section.

Figure 5. Carbon content of charcoal produced from larch wood at different temperatures.

Wood samples were collected in close proximity to the hearths. Charcoal was produced in a muffle furnace at 400°, 500° 600° and 860°C. Dashed lines represent 95% confidence interval. (Y = 26.9+0.15 X-9.2 10⁻⁵ X²; r² = 0.94; p<0.0001).