Abstract. The management of agroecosystems plays a crucial role in the global carbon cycle with soil tillage leading to known organic carbon redistributions within soils and changes in soil CO 2 emissions. Yet, discrepancies exist on the impact of tillage on soil CO 2 emissions and on the main soil and environmental controls. A meta-analysis was conducted using 46 peer-reviewed publications totaling 174 paired observations comparing CO 2 emissions over entire seasons or years from tilled and untilled soils across different climates, crop types and soil conditions with the objective of quantifying tillage impact on CO 2 emissions and assessing the main controls. On average, tilled soils emitted 21 % more CO 2 than untilled soils, which corresponded to a significant difference at P <0.05. The difference increased to 29 % in sandy soils from arid climates with low soil organic carbon content (SOC C < 1 %) and low soil moisture, but tillage had no impact on CO 2 fluxes in clayey soils with high background SOC C (> 3 %). Finally, nitrogen fertilization and crop residue management had little effect on the CO 2 responses of soils to no-tillage. These results suggest no-tillage is an effective mitigation measure of carbon dioxide losses from dry land soils. They emphasize the importance of including information on soil factors such as texture, aggregate stability and organic carbon content in global models of the carbon cycle.

Grasslands have potential to mitigate against climate change because of their large capacity to store soil organic carbon (SOC). However, the long-term impact of grassland management such as burning, which is still common in many areas of the world, on SOC is still a matter of debate. The objective of this study was to quantify the longterm effects of annual burning on CO 2 output from soils and SOC stocks. The study was performed on a 62 years old field trial comparing annual burning (AB) to no burning associated with tree encroachment (NB), and to annual mowing (AM) with all treatments laid out in randomized block design with three replicates per treatment. CO 2 emissions from soil were continuously measured over two years and were correlated to soil chemical and physical properties. AB and AM produced 30 and 34% greater CO 2 emissions from soil than NB (1.80 ± 0.13 vs. 2.34 ± 0.18 and 2.41 ± 0.17 g C-CO 2 m −2 d −1 for NB, AB and AM respectively). AB and AM also produced greater CO 2 emissions from soil and per gram of soil carbon (1.32 ± 0.1 and 1.35 ± 0.1 mg C-CO 2 g C −1 d −1 , respectively) than NB (1.05 ± 0.07 mg C-CO 2 g C −1 d −1), which corresponded to significant differences of respectively 26% and 29%. Overall, CO 2 emissions from soil (per m 2) significantly increased with soil water content (r = 0.72) followed by SOC stocks (r = 0.59), SOC content (r = 0.50), soil bulk density (r = 0.49), soil temperature (r = 0.47), C:N ratio (r = 0.46) and mean weight diameter (r = 0.38). These findings suggest that long-term annual burning increases CO 2 output from soils. Additional greenhouse gases emissions from burning itself and alternative grassland management techniques were finally discussed.

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