Carbon Sequestration and Greenhouse Gas Emissions
This task aimed to evaluate the impacts of agricultural management practices on carbon sequestration and GHG emissions with respect to climate change (CC) mitigation. The evaluation was based on data, which mainly derived from European long-term field experiments (LTEs). Soil organic carbon (SOC) is the main component of soil organic matter (SOM), which influences chemical, physical and biological soil quality. Furthermore, an increase in SOC with agricultural management changes (e.g. reduced tillage measures, addition of organic material) may lead to carbon sequestration by enhancing SOC stocks, thus lowering the atmospheric CO2 concentration. On the other hand, those management changes may lead to shifts in other GHG fluxes. In general, the most important greenhouse gases, which are connected with agricultural management, are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O).
The application of external C-sources, such as compost, farm yard manure (FYM) and slurry resulted in the highest overall increases of SOC concentrations (between 37% and 21%) compared to similar mineral N fertilisation. The growing of catch crops and cover crops also led to increased SOC contents (RR 1.16). Reduced tillage (reduction of the number of tillage operations or of the depth of disturbance), the incorporation of crop residues and (high) mineral N fertilization led to a SOC increase of 7 to 8% compared to the respective baseline management practices. Similar increases could be detected for SOC stocks. However, CO2 emissions increased with the mentioned management practices as well, ranging between 4% and 64%. Crop residue incorporation even led to an average 5fold increase of CO2 emissions. Furthermore, the emissions of the potent GHG N2O have risen with most of the mentioned management practices that have been assumed to be “best management practices” (BMPs) so far. Based, however, on a very limited number of data, often derived from short-time experiments, the highest N2O increases occurred with the incorporation of crop residues > the application of compost > slurry > (high) mineral N fertilization > no-till (NT) > catch crops/ cover crops. Non-inversion tillage (NIT) did not change N2O emissions compared with conventional ploughing (CT), and with FYM the N2O emissions tended to decrease compared to similar N fertilisation. Again, based on very few experimental data, the CH4 emissions decreased with compost and FYM application and with NIT. CH4 emissions increased with slurry application, (high) mineral N fertilisation, catch/cover crop, NT and residue incorporation. The quantitative assessments were summarised and converted - according to expert knowledge - to a qualitative assessment. In summary, and if only the overall investigated CC indicators are considered, preferential management practices are:
• FYM application
• Crop rotation
• Non-inversion tillage
• Compost application
These evaluations could change depending on the respective influencing factors, such as environmental zone, clay content, depth of soil sampling and the duration of the applied practice.
Table 1: Summary of relative response rate (RR) and relative difference (rel DIFF) of SOC and GHG when applying potential BMPs. ++: very positive effect; +: positive effect; 0: neutral effect; -: negative effect; --: very negative effect. All compared to the baseline treatment indicated in grey.