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Agriculture is a global contributor to greenhouse gas emissions, causing climate change. Soil organic carbon (SOC) sequestration is seen as a pathway to climate change mitigation. But, long-term data on the actual contribution of tropical soils to SOC sequestration are largely absent. To contribute to filling this knowledge gap, we measured SOC in the top 15 cm over 12 years in two agronomic long-term trials in Western Kenya. These trials include various levels – from absence to full adoption – of two widely promoted sustainable agricultural management practices: Integrated Soil Fertility Management (ISFM; i.e. improved varieties, mineral fertilizer and organic matter/manure incorporation) and Conservation Agriculture (CA; improved varieties, mineral fertilizer, zero-tillage and crop residues retention). None of the tested ISFM and CA treatments turned out successful in sequestering SOC long-term. Instead, SOC decreased significantly over time in the vast majority of treatments. Expressed as annual averages, losses ranged between 0.11 and 0.37 t C ha −1 yr −1 in the CA long-term trial and 0.21 and 0.96 t C ha −1 yr −1 in the ISFM long-term trial. Long-term application of mineral N and P fertilizer did not mitigate SOC losses in both trials. Adopting zero-tillage and residue retention alone (as part of CA) could avoid SOC losses of on average 0.13 t C ha −1 yr −1 , while this was 0.26 t C ha −1 yr −1 in response to mere inclusion of manure as part of ISFM. However, cross-site comparison disclosed that initial SOC levels of the two trials were different, probably as a result of varying land use history. Such initial soil status was responsible for the bulk of the SOC losses and less so the various tested agronomic management practices. This means, while ISFM and CA in the humid tropical agro-ecosystem of Western Kenya contribute to climate change mitigation by reducing SOC losses, they do not help offsetting anthropogenic greenhouse gas emissions elsewhere. [ABSTRACT FROM AUTHOR]

Conservation agriculture (CA), comprising minimum soil disturbance, retention of crop residues and crop diversification, is widely promoted for reducing soil degradation and improving agricultural sustainability. It is also claimed to mitigate climate change through soil carbon sequestration: we conducted a meta-analysis of soil organic carbon (SOC) stock changes under CA practices in two tropical regions, the Indo-Gangetic Plains (IGP) and Sub-Saharan Africa (SSA), to quantify this. In IGP annual increases in SOC stock compared to conventional practice were between 0.16 and 0.49 Mg C ha −1 yr −1 . In SSA increases were between 0.28 and 0.96 Mg C ha −1 yr −1 , but with much greater variation and a significant number of cases with no measurable increase. Most reported SOC stock increases under CA are overestimates because of errors introduced by inappropriate soil sampling methodology. SOC increases require careful interpretation to assess whether or not they represent genuine climate change mitigation as opposed to redistribution of organic C within the landscape or soil profile. In smallholder farming in tropical regions social and economic barriers can greatly limit adoption of CA, further decreasing realistic mitigation potential. Comparison with the decreases in greenhouse gas emissions possible through improved management of nitrogen (N) fertilizer in regions such as IGP where N use is already high, suggests that this is a more effective and sustainable means of mitigating climate change. However the mitigation potential, and other benefits, from crop diversification are frequently overlooked when considering CA and warrant greater attention. Increases in SOC concentration (as opposed to stock) in near-surface soil from CA cause improvements in soil physical conditions; these are expected to contribute to increased sustainability and climate change adaptation, though not necessarily leading to consistently increased crop yields. CA should be promoted on the basis of these factors and any climate change mitigation regarded as an additional benefit, not a major policy driver for its adoption. [ABSTRACT FROM AUTHOR]

The development of methods for improving the assessment of resilience of socio-ecological systems has become increasingly important as the severity and variability of global climate patterns continue to compound and intensify. The present study was conducted with the aim of using a dynamically-coupled bio-physical and socioeconomic model to test the efficacy of stakeholder-defined policy measures in conferring resilience to two agroecological variables, farm income and water-table depth, experiencing both socio-ecological shocks and climatic stress in the Rechna Doab basin of northeastern Pakistan. The objective was accomplished by using a dynamically-coupled physical and group-built system dynamics modelling framework for scenario testing and output generation, including three NASA Earth Exchange Global Daily Downscaled Climate Projections (NEX-DGGP), two relevant socio-ecological shock scenarios (market inflation and canal water supply variability), and three stakeholder-defined policy measures. Resilience was assessed using the following system functionality metrics: 1) The degree of return for each variable after a perturbation, i.e., the extent to which the observed variable returns to baseline functioning; 2) The return time of the variable to baseline functioning; 3) The rate of variable return to baseline; 4) Overall perturbation of the system post-disturbance; and 5) The corrective impact of the shock on system functionality. Differences in the resilience metrics were subsequently compared based on the behavioral change(s) of the study variables in response to the application of the three selected policy scenarios. The results presented here indicate that rainwater harvesting is the most effective stakeholder-defined policy measure for improving or maintaining resilience of the tested study variables in the Rechna Doab agricultural basin; this holds true for every climate and shock scenario with the exception of water-table depth in the upper and mid-watershed regions under canal supply shock conditions, for which canal lining is the most effective policy measure. The irrigation improvement and canal lining policies were more effective in improving the resilience metrics of water-table depth than those of the farm income variable, as the water-table depth variable is not bound so tightly by economic constraints; although there were regional differences in policy efficacy, these trends hold true, on average, for the entire watershed. The results presented herein align well with previous studies conducted in the region which note similar patterns in spatial resilience of agroecosystem variables (i.e., upper to lower watershed) in the Recha Doab basin as well as regional differences in the efficacy of the stakeholder-defined policy measures. This unique approach for assessing socio-environmental policies can help improve decision-making processes with respect to agro-infrastructure and climate change mitigation strategies in vulnerable agroecosystems. [Display omitted] • Improving the resilience of socio-ecological systems has become crucial with the intensification of global climate patterns • Quantify resilience of climate-stressed socio-agroecological variables via application of stakeholder-defined policies • Rainwater harvesting is the most effective stakeholder-defined policy measure for achieving variable-level resilience • Methods can be utilized in single-system analyses as well as comparative studies across spatial and temporal boundaries • The methods used herein can help improve socio-ecological policy development and climate change mitigation strategy [ABSTRACT FROM AUTHOR]

Vineyards are usually cultivated in soils characterised by low soil organic carbon (SOC) content and have high risks of soil erosion and degradation. Increasing SOC stocks in these cropping systems has the potential to contribute to climate change mitigation through SOC sequestration and to enhance soil quality. We conducted a meta-analysis and compared the SOC stock response ratio, the SOC stock rate of change, and the SOC sequestration rate in vineyards under different SOC sequestration (SCS) practices relative to conventional management. SCS practices included organic amendments (OA), biochar amendments (BC), returning pruning residues to the soil (PR), no-tillage (NT), cover cropping (CC), and several combinations of these practices. The average SOC sequestration rate of SCS management was 7.53 Mg CO 2 -eq. ha−1 yr−1 to a 30-cm soil depth. The highest SOC sequestration rate (11.06 Mg CO 2 -eq. ha−1 yr−1) was achieved under a combination of OA+NT and the lowest (2.82 Mg CO 2 -eq. ha−1 yr−1) was observed under PR treatments. Field experiments performed in particularly hot and dry bioclimatic zones were associated with lower SOC sequestration rates relative to those performed in more temperate areas. The high SOC sequestration rates obtained for many SCS practices, and the large land area dedicated to viticulture worldwide (7.45 Mha), imply that the adoption of SCS practices in vineyards can contribute to the global efforts to offset atmospheric greenhouse gas concentrations via SOC sequestration to mitigate climate change. • Effects of soil carbon sequestration (SCS) practices were assessed in vineyards using a meta-analysis. • All SCS practices led to soil organic carbon (SOC) accumulation in viticultural soils. • The average SOC sequestration rate was 7.53 Mg CO 2 -eq. ha−1 yr−1. • The impact of SCS management on SOC stocks was climate-dependent. [ABSTRACT FROM AUTHOR]

• A methodology for estimating regional soil organic carbon content has been developed. • A national SOC estimate for forest and agro-ecosystems in Italy is provided. • Soil organic carbon content under climate change conditions has been calculated. • Climate change causes a moderate soil carbon loss for forested and tree crop soils. The soil organic carbon (SOC) is the largest carbon pool in the terrestrial biosphere, second only to oceans, containing twice as much carbon as the atmosphere and three times that stored in global vegetation. Climate change (CC) is expected to impact this carbon pool. To date, large uncertainties still persist on the effects of CC on SOC stocks. In addition, a shortage of data related to regional SOC quantities of tree-covered areas and future changes under CC conditions is recognized. In this work, we used a spatial-explicit modelling approach to estimate the current SOC stock (at 2005) and future changes (at 2095) under CC conditions of the whole forest, tree crop, grassland, and pasture covered areas of Italy. A methodology was preliminarily implemented to obtain spatialized SOC estimates at a regional scale by using the CENTURY 5 model coupled with spatialized vegetation, soil, and climate data. We ran both moderate (RCP4.5) and extreme (RCP8.5) climatic scenarios, and used three Global Circulation Models for each one of the four ecosystems described above. The current SOC stock estimates range from 51.3 (orchards) to 129.5 Mg carbon ha−1 (coniferous forests) and we found an overall SOC stock in Italy ranging from 1320.1 to 1425.1 Tg. SOC projections under CC showed a moderate carbon loss suggesting that forest, grassland, and permanent crop soils could provide an important contribution to climate change mitigation. [ABSTRACT FROM AUTHOR]