Carbon farming practices for European cropland: A review on the effect on soil organic carbon

Carbon farming has been recently proposed as an effective measure for climate change mitigation through carbon (C) sequestration or C emissions reduction. In order to identify and estimate the climate change mitigation potential of carbon farming practices on European croplands we conduct a systematic review on both relative and absolute annual soil organic carbon (SOC) stock change (ΔSOC_REL; ΔSOC_ABS) related to single and combined agroecological practices tested on mineral soils at a minimum of 0–30 cm and up to 150 cm soil depth whenever data were available. We used the term ΔSOC_REL for SOC stock changes determined by the paired comparison method and the term ΔSOC_ABS for those calculated using the SOC stock difference method. We compiled a dataset with more than 700 records on SOC change rates representing 12 carbon farming practices. Mean ΔSOC_REL in Mg C ha⁻¹ yr⁻¹ at 0–30 cm soil depth were collected for cover crops (0.40 ± 0.32), organic amendments (0.52 ± 0.47 and 0.38 ± 0.37 when the control is respectively unfertilized or liquid organic amendment), crop residue maintenance (0.14 ± 0.06), improved rotations (0.21 ± 0.16), reduced soil disturbance (0.24 ± 0.34), silvoarable systems (0.21 ± 0.08), organic (0.9 Mg ± 0.25) and conservation management (0.78 ± 0.62), set-aside (0.75 ± 0.68 and −0.39 ± 0.50 when the control is respectively cropland or pasture/grassland), cropland conversion into permanent grassland (0.79 ± 0.47), poplar plantations (0.25 ± 0.68 and −0.85 ± 0.53 when established on cropland or pasture/grassland). SOC sequestration was detected only for organic amendments, cover crops, poplar plantations, conservation management, organic management, and combined carbon farming practices for which we estimated a median ΔSOC_ABS ranging between 0.32 and 0.96 Mg C ha⁻¹ yr⁻¹ at 0–30 cm. The ΔSOC_ABS observed at 0–30 cm soil depth from cropland conversion into short rotation forestry resulted in an increase of C, while negative values were observed when the control was grassland. Cropland conversion into permanent grassland or pasture showed positive ΔSOC_REL at 0–30 and 0–90 and 0–100 cm soil depth. Reduced soil disturbance full soil profile assessment at 0–50 cm soil depth completely counterweighted any SOC stock increase found in topsoil at 0–30 and 0–40 cm soil depth, therefore resulting in no net climate benefit. Conservation management, organic management, and combining cover crops with organic amendments are the most effective strategies shifting arable land from C source to net sink, with median ΔSOC_ABS at 0–30 cm soil depth of 0.63, 0.91 and 0.96 Mg C ha⁻¹ yr⁻¹, respectively. Permanent grasslands and pastures were negatively affected by any type of land-use change, at least in topsoil. Natural ecological successions after cropland abandonment (20-year set-aside), or arable land conversion into poplar plantations and grassland promote relative SOC stock annual increase by 1.08, 0.77 and 0.33 at 0–30 cm respectively, while the net climate benefit remains unclear when subsoils are assessed.