Abstract
Uncertainty in soil carbon (C) fluxes across different land‐use transitions is an issue that needs to be addressed for the further deployment of perennial bioenergy crops. A large‐scale short‐rotation coppice (SRC) site with poplar (Populus) and willow (Salix) was established to examine the land‐use transitions of arable and pasture to bioenergy. Soil C pools, output fluxes of soil CO 2, CH 4, dissolved organic carbon (DOC) and volatile organic compounds, as well as input fluxes from litter fall and from roots, were measured over a 4‐year period, along with environmental parameters. Three approaches were used to estimate changes in the soil C. The largest C pool in the soil was the soil organic carbon (SOC) pool and increased after four years of SRC from 10.9 to 13.9 kg C m−2. The belowground woody biomass (coarse roots) represented the second largest C pool, followed by the fine roots (Fr). The annual leaf fall represented the largest C input to the soil, followed by weeds and Fr. After the first harvest, we observed a very large C input into the soil from high Fr mortality. The weed inputs decreased as trees grew older and bigger. Soil respiration averaged 568.9 g C m−2 yr−1. Leaching of DOC increased over the three years from 7.9 to 14.5 g C m−2. The pool‐based approach indicated an increase of 3360 g C m−2 in the SOC pool over the 4‐year period, which was high when compared with the −27 g C m−2 estimated by the flux‐based approach and the −956 g C m−2 of the combined eddy‐covariance + biometric approach. High uncertainties were associated to the pool‐based approach. Our results suggest using the C flux approach for the assessment of the short‐/medium‐term SOC balance at our site, while SOC pool changes can only be used for long‐term C balance assessments.
Highlights
The cultivation of soils with arable crops produces a net carbon (C) flux from the soil to the atmosphere, contributing to the increased greenhouse effect (Le Quere, et al 2013) and reducing soil fertility and water quality (Lal, 2004)
In the first layer (0–15 cm), the C% was higher in previous pasture, while in the second layer (15–30 cm), the C% was higher in previous cropland (P < 0.05)
Our pool-based approach indicated an average increase of 3360 g C mÀ2 in the soil organic matter (SOM) pool, which is a large value when compared with the fluxbased and the combined eddy-covariance + biometric approaches (Fig. 2)
Summary
The cultivation of soils with arable crops produces a net carbon (C) flux from the soil to the atmosphere, contributing to the increased greenhouse effect (Le Quere, et al 2013) and reducing soil fertility and water quality (Lal, 2004). Short-rotation coppice (SRC) cultures are defined as high-density plantations of fast-growing trees for. There is still a lack of quantitative information on the changes in soil organic C (SOC) for a land-use change (LUC) to second-generation bioenergy crops with respect to historical land covers (i.e. arable, grass) and current land management practices (genotypes, planting density, harvest) (Harris et al, 2015). A large-scale operational SRC plantation in Belgium, that has been intensively studied within the POPFULL project, provided the opportunity to examine the soil C balance of SRC under the prevailing conditions. As the closure of the carbon balance is a complex and difficult task, we used three different methodologies to reach the primary objective
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