Soils are the main sources of the greenhouse gas nitrous oxide (N 2 O). The N 2 O emission at the soil surface is the result of production and consumption processes. So far, research has concentrated on net N 2 O production. However, in the literature, there are numerous reports of net negative fluxes of N 2 O, (i.e. fluxes from the atmosphere to the soil). Such fluxes are frequent and substantial and cannot simply be dismissed as experimental noise.Net N 2 O consumption has been measured under various conditions from the tropics to temperate areas, in natural and agricultural systems. Low mineral N and large moisture contents have sometimes been found to favour N 2 O consumption. This fits in with denitrification as the responsible process, reducing N 2 O to N 2 . However, it has also been reported that nitrifiers consume N 2 O in nitrifier denitrification. A contribution of various processes could explain the wide range of conditions found to allow N 2 O consumption, ranging from low to high temperatures, wet to dry soils, and fertilized to unfertilized plots. Generally, conditions interfering with N 2 O diffusion in the soil seem to enhance N 2 O consumption. However, the factors regulating N 2 O consumption are not yet well understood and merit further study.Frequent literature reports of net N 2 O consumption suggest that a soil sink could help account for the current imbalance in estimated global budgets of N 2 O. Therefore, a systematic investigation into N 2 O consumption is necessary. This should concentrate on the organisms, reactions, and environmental factors involved.

At the 21st session of the United Nations Framework Convention on Climate Change (UNFCCC, COP21), a voluntary action plan, the '4 per 1000 Initiative: Soils for Food Security and Climate' was proposed under the Agenda for Action. The Initiative underlines the role of soil organic matter (SOM) in addressing the three-fold challenge of food and nutritional security, adaptation to climate change and mitigation of human-induced greenhouse gases (GHGs) emissions. It sets an ambitious aspirational target of a 4 per 1000 (i.e. 0.4%) rate of annual increase in global soil organic carbon (SOC) stocks, with a focus on agricultural lands where farmers would ensure the carbon stewardship of soils, like they manage day-to-day multipurpose production systems in a changing environment. In this paper, the opportunities and challenges for the 4 per 1000 initiative are discussed. We show that the 4 per 1000 target, calculated relative to global top soil SOC stocks, is consistent with literature estimates of the technical potential for SOC sequestration, though the achievable potential is likely to be substantially lower given socio-economic constraints. We calculate that land-based negative emissions from additional SOC sequestration could significantly contribute to reducing the anthropogenic CO 2 equivalent emission gap identified from Nationally Determined Contributions pledged by countries to stabilize global warming levels below 2°C or even 1.5°C under the Paris agreement on climate. The 4 per 1000 target could be implemented by taking into account differentiated SOC stock baselines, reversing the current trend of huge soil CO 2 losses, e.g. from agriculture encroaching peatland soils. We further discuss the potential benefits of SOC stewardship for both degraded and healthy soils along contrasting spatial scales (field, farm, landscape and country) and temporal (year to century) horizons. Last, we present some of the implications relative to non-CO 2 GHGs emissions, water and nutrients use as well as co-benefits for crop yields and climate change adaptation. We underline the considerable challenges associated with the non-permanence of SOC stocks and show how the rates of adoption and the duration of improved soil management practices could alter the global impacts of practices under the 4 per 1000 initiative. We conclude that the 4 per 1000 initiative has potential to support multiple sustainable development goals (SDGs) of the 2030 Agenda. It can be regarded as no-regret since increasing SOC in agricultural soils will contribute to food security benefits that will enhance resilience to climate change. However, social, economic and environmental safeguards will be needed to ensure an equitable and sustainable implementation of the 4 per 1000 target.

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Priming effect (PE) is defined as a stimulation of the mineralization of soil organic matter (SOM) following a supply of fresh organic matter. This process can have important consequences on the fate of SOM and on the management of residues in agricultural soils, especially in tropical regions where soil fertility is essentially based on the management of organic matter. Earthworms are ecosystem engineers known to affect the dynamics of SOM. Endogeic earthworms ingest large amounts of soil and assimilate a part of organic matter it contains. During gut transit, microorganisms are transported to new substrates and their activity is stimulated by (i) the production of readily assimilable organic matter (mucus) and (ii) the possible presence of fresh organic residues in the ingested soil. The objective of our study was to see (i) whether earthworms impact the PE intensity when a fresh residue is added to a tropical soil and (ii) whether this impact is linked to a stimulation/inhibition of bacterial taxa, and which taxa are affected. A tropical soil from Madagascar was incubated in the laboratory, with a 13 C wheat straw residue, in the presence or absence of a peregrine endogeic tropical earthworm, Pontoscolex corethrurus. Emissions of 12 CO 2 and 13 CO 2 were followed during 16 days. The coupling between DNA-SIP (stable isotope probing) and pyrosequencing showed that stimulation of both the mineralization of wheat residues and the PE can be linked to the stimulation of several groups especially belonging to the Bacteroidetes phylum. The ISME Journal (2012) 6, 213-222; doi:10.1038/ismej.2011; published online 14 July 2011 Subject Category: microbial ecology and functional diversity of natural habitats

The effect of agricultural management practices on geochemical cycles in moderate ecosystems is by far better understood than in semiarid regions, where fertilizer availability and climatic conditions are less favorable. We studied the impact of different fertilizer regimens in an agricultural long-term observatory in Burkina Faso at three different plant development stages (early leaf development, flowering, and senescence) of sorghum cultivars. Using real-time PCR, we investigated functional microbial communities involved in key processes of the nitrogen cycle (nitrogen fixation, ammonia oxidation, and denitrification) in the rhizosphere. The results indicate that fertilizer treatments and plant development stages combined with environmental factors affected the abundance of the targeted functional genes in the rhizosphere. While nitrogen-fixing populations dominated the investigated communities when organic fertilizers (manure and straw) were applied, their numbers were comparatively reduced in urea-treated plots. In contrast, ammonia-oxidizing bacteria (AOB) increased not only in absolute numbers but also in relation to the other bacterial groups investigated in the urea-amended plots. Ammonia-oxidizing archaea exhibited higher numbers compared to AOB independent of fertilizer application. Similarly, denitrifiers were also more abundant in the urea-treated plots. Our data imply as well that, more than in moderate regions, water availability might shape microbial communities in the rhizosphere, since low gene abundance data were obtained for all tested genes at the flowering stage, when water availability was very limited.Land degradation is one of the most serious threats to food production on the African continent. Soil erosion, nutrient depletion, low organic matter content, and unfavorable pH values are some of the reasons for a deficient soil fertility (30), mainly in Central African countries. Combined with high variability and irregular distribution of rainfall, these factors contribute to negative nutrient balances. For example, 4.4 million tons of nitrogen (N) are lost per year in African soils, but only 0.8 million tons are reapplied by fertilization (12, 34). Since nitrogen is a key nutrient determining the productivity of agroecosystems (7,11,43), it is of central importance to optimize the nitrogen balance in these countries, mainly by steering the genetic resources of soil microbes in a way that losses of applied nitrogen are minimized and biological nitrogen fixation is increased. The aim should be to obtain a highly efficient nitrogen turnover, with leaching of nitrate and losses of gaseous products such as nitrous oxide (N 2 O) or dinitrogen (N 2 ) as low as possible.Despite the importance of this issue, not much data are available on microbial community structure and function related to the nitrogen cycle in agroecosystems of Central Africa, and scenarios from moderate climatic regions cannot simply be transferred to tropical agroecosystems. Furthermore, the few studies published thus far on...

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There is increasing interest in agroecology as a way to move toward more sustainable agriculture and food systems. However, the evidence of agroecology's contribution to sustainability remains fragmented because of heterogeneous methods and data, differing scales and timeframes, and knowledge gaps. Facing these challenges, 70 representatives of agroecology-related organizations worldwide participated in the development of the Tool for Agroecology Performance Evaluation (TAPE), to produce and consolidate evidence on the multidimensional performances of agroecological systems. TAPE is composed of: Step 0, the preliminary step that includes a description of the main socio-economic and demographic characteristics of the agricultural and food systems and an analysis of the enabling environment in terms of relevant policy, market, technology, socio-cultural and/or historical drivers; Step 1, the Characterization of Agroecological Transitions (CAET), based on the 10 Elements of Agroecology adopted by FAO and its member countries, using descriptive scales to establish scores and assessing the degree of transition, with information from the farm/household and community/territory scale; Step 2, the Core Criteria of Performance listing the key dimensions considered relevant to address the Sustainable Development Goals (SDGs): Environment & climate change; Health & nutrition; Society & culture; Economy and Governance. Finally Step 3, a participatory validation of the results obtained from the previous steps with the producers and relevant stakeholders. TAPE can be used (i) to assess the extent of agroecological transition among agricultural producers in a community or a territory, (ii) to monitor and evaluate projects by characterizing the initial and subsequent steps in an agroecological transition, and/or (iii) to evaluate widely diverse agricultural systems against agroecological elements and how they contribute to the achievement of the SDGs. Its application can support the transition of all forms of agricultural systems toward more sustainable practices and the formulation of adequate policies to enable this transformation. Preliminary results from pilot applications show that TAPE can perform in a variety of geographic regions and agroecosystems and that it allows assessment of performances of various criteria that move beyond classic indicators to begin to build a global evidence base for agroecology and support transformation to sustainable agricultural production and food systems.

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