The rewetting of dry soils and the thawing of frozen soils are short-term, transitional phenomena in terms of hydrology and the thermodynamics of soil systems. The impact of these short-term phenomena on larger scale ecosystem fluxes is increasingly recognized, and a growing number of studies show that these events affect fluxes of soil gases such as carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), ammonia (NH₃) and nitric oxide (NO). Global climate models predict that future climatic change is likely to alter the frequency and intensity of drying-rewetting events and thawing of frozen soils. These future scenarios highlight the importance of understanding how rewetting and thawing will influence dynamics of these soil gases. This study summarizes findings using a new database containing 338 studies conducted from 1956 to 2011, and highlights open research questions. The database revealed conflicting results following rewetting and thawing in various terrestrial ecosystems and among soil gases, ranging from large increases in fluxes to non-significant changes. Studies reporting lower gas fluxes before rewetting tended to find higher post-rewetting fluxes for CO₂, N₂O and NO; in addition, increases in N₂O flux following thawing were greater in warmer climate regions. We discuss possible mechanisms and controls that regulate flux responses, and recommend that a high temporal resolution of flux measurements is critical to capture rapid changes in gas fluxes after these soil perturbations. Finally, we propose that future studies should investigate the interactions between biological (i.e., microbial community and gas production) and physical (i.e., porosity, diffusivity, dissolution) changes in soil gas fluxes, apply techniques to capture rapid changes (i.e., automated measurements), and explore synergistic experimental and modelling approaches.

Agriculture is the backbone of the Ethiopian economy, and the agricultural sector is dominated by smallholder farming systems. The farming systems are facing constraints such as small land size, lack of resources, and increasing degradation of soil quality that hamper sustainable crop production and food security. The effects of climate change (e.g., frequent occurrence of extreme weather events) exacerbate these problems. Applying appropriate technologies like climate-smart agriculture (CSA) can help to resolve the constraints of smallholder farming systems. This paper provides a comprehensive overview regarding opportunities and challenges of traditional and newly developed CSA practices in Ethiopia, such as integrated soil fertility management, water harvesting, and agroforestry. These practices are commonly related to drought resilience, stability of crop yields, carbon sequestration, greenhouse gas mitigation, and higher household income. However, the adoption of the practices by smallholder farmers is often limited, mainly due to shortage of cropland, land tenure issues, lack of adequate knowledge about CSA, slow return on investments, and insufficient policy and implementation schemes. It is suggested that additional measures be developed and made available to help CSA practices become more prevalent in smallholder farming systems. The measures should include the utilization of degraded and marginal lands, improvement of the soil organic matter management, provision of capacity-building opportunities and financial support, as well as the development of specific policies for smallholder farming.

This paper summarizes currently available data on greenhouse gas (GHG) emissions from African natural ecosystems and agricultural lands. The available data are used to synthesize current understanding of the drivers of change in GHG emissions, outline the knowledge gaps, and suggest future directions and strategies for GHG emission research. GHG emission data were collected from 75 studies conducted in 22 countries (n = 244) in sub-Saharan Africa (SSA). Carbon dioxide (CO₂) emissions were by far the largest contributor to GHG emissions and global warming potential (GWP) in SSA natural terrestrial systems. CO₂ emissions ranged from 3.3 to 57.0 Mg CO₂ ha⁻¹ yr⁻¹, methane (CH₄) emissions ranged from −4.8 to 3.5 kg ha⁻¹ yr⁻¹ (−0.16 to 0.12 Mg CO₂ equivalent (eq.) ha⁻¹ yr⁻¹), and nitrous oxide (N₂O) emissions ranged from −0.1 to 13.7 kg ha⁻¹ yr⁻¹ (−0.03 to 4.1 Mg CO₂ eq. ha⁻¹ yr⁻¹). Soil physical and chemical properties, rewetting, vegetation type, forest management, and land-use changes were all found to be important factors affecting soil GHG emissions from natural terrestrial systems. In aquatic systems, CO₂ was the largest contributor to total GHG emissions, ranging from 5.7 to 232.0 Mg CO₂ ha⁻¹ yr⁻¹, followed by −26.3 to 2741.9 kg CH₄ ha⁻¹ yr⁻¹ (−0.89 to 93.2 Mg CO₂ eq. ha⁻¹ yr⁻¹) and 0.2 to 3.5 kg N₂O ha⁻¹ yr⁻¹ (0.06 to 1.0 Mg CO₂ eq. ha⁻¹ yr⁻¹). Rates of all GHG emissions from aquatic systems were affected by type, location, hydrological characteristics, and water quality. In croplands, soil GHG emissions were also dominated by CO₂, ranging from 1.7 to 141.2 Mg CO₂ ha⁻¹ yr⁻¹, with −1.3 to 66.7 kg CH₄ ha⁻¹ yr⁻¹ (−0.04 to 2.3 Mg CO₂ eq. ha⁻¹ yr⁻¹) and 0.05 to 112.0 kg N₂O ha⁻¹ yr⁻¹ (0.015 to 33.4 Mg CO₂ eq. ha⁻¹ yr⁻¹). N₂O emission factors (EFs) ranged from 0.01 to 4.1 %. Incorporation of crop residues or manure with inorganic fertilizers invariably resulted in significant changes in GHG emissions, but results were inconsistent as the magnitude and direction of changes were differed by gas. Soil GHG emissions from vegetable gardens ranged from 73.3 to 132.0 Mg CO₂ ha⁻¹ yr⁻¹ and 53.4 to 177.6 kg N₂O ha⁻¹ yr⁻¹ (15.9 to 52.9 Mg CO₂ eq. ha⁻¹ yr⁻¹) and N₂O EFs ranged from 3 to 4 %. Soil CO₂ and N₂O emissions from agroforestry were 38.6 Mg CO₂ ha⁻¹ yr⁻¹ and 0.2 to 26.7 kg N₂O ha⁻¹ yr⁻¹ (0.06 to 8.0 Mg CO₂ eq. ha⁻¹ yr⁻¹), respectively. Improving fallow with nitrogen (N)-fixing trees led to increased CO₂ and N₂O emissions compared to conventional croplands. The type and quality of plant residue in the fallow is an important control on how CO₂ and N₂O emissions are affected. Throughout agricultural lands, N₂O emissions slowly increased with N inputs below 150 kg N ha⁻¹ yr⁻¹ and increased exponentially with N application rates up to 300 kg N ha⁻¹ yr⁻¹. The lowest yield-scaled N₂O emissions were reported with N application rates ranging between 100 and 150 kg N ha⁻¹. Overall, total CO₂ eq. emissions from SSA natural ecosystems and agricultural lands were 56.9 ± 12.7 × 10⁹ Mg CO₂ eq. yr⁻¹ with natural ecosystems and agricultural lands contributing 76.3 and 23.7 %, respectively. Additional GHG emission measurements are urgently required to reduce uncertainty on annual GHG emissions from the different land uses and identify major control factors and mitigation options for low-emission development. A common strategy for addressing this data gap may include identifying priorities for data acquisition, utilizing appropriate technologies, and involving international networks and collaboration.