:The carbon sink capacity of the world’s agricultural and degraded soils is 50 to 66% of the historic carbon loss of 42 to 78 gigatons of carbon. The rate of soil organic carbon sequestration with adoption of recommended technologies depends on soil texture and structure, rainfall, temperature, farming system, and soil management. Strategies to increase the soil carbon pool include soil restoration and woodland regeneration, no-till farming, cover crops, nutrient management, manuring and sludge application, improved grazing, water conservation and harvesting, efficient irrigation, agroforestry practices, and growing energy crops on spare lands. An increase of 1 ton of soil carbon pool of degraded cropland soils may increase crop yield by 20 to 40 kilograms per hectare (kg/ha) for wheat, 10 to 20 kg/ha for maize, and 0.5 to 1 kg/ha for cowpeas. As well as enhancing food security, carbon sequestration has the potential to offset fossilfuel emissions by 0.4 to 1.2 gigatons of carbon per year, or 5 to 15% of the global fossil-fuel emissions.

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Soil carbon sequestration impacts on global climate change and food security.

Climate Change 2007 Synthesis report

Common understanding of the causes of land-use and land-cover change is dominated by simplifications which, in turn, underlie many environment-development policies. This article tracks some of the major myths on driving forces of land-cover change and proposes alternative pathways of change that are better supported by case study evidence. Cases reviewed support the conclusion that neither population nor poverty alone constitute the sole and major underlying causes of land-cover change worldwide. Rather, peoples’ responses to economic opportunities, as mediated by institutional factors, drive land-cover changes. Opportunities and

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The causes of land-use and land-cover change: moving beyond the myths

The relationship between soil structure and the ability of soil to stabilize soil organic matter (SOM) is a key element in soil C dynamics that has either been overlooked or treated in a cursory fashion when developing SOM models. The purpose of this paper is to review current knowledge of SOM dynamics within the framework of a newly proposed soil C saturation concept. Initially, we distinguish SOM that is protected against decomposition by various mechanisms from that which is not protected from decomposition. Methods of quantification and characteristics of three SOM pools defined as protected are discussed. Soil organic matter can be: (1) physically stabilized, or protected from decomposition, through microaggregation, or (2) intimate association with silt and clay particles, and (3) can be biochemically stabilized through the formation of recalcitrant SOM compounds. In addition to behavior of each SOM pool, we discuss implications of changes in land management on processes by which SOM compounds undergo protection and release. The characteristics and responses to changes in land use or land management are described for the light fraction (LF) and particulate organic matter (POM). We defined the LF and POM not occluded within microaggregates (53–250 μm sized aggregates as unprotected. Our conclusions are illustrated in a new conceptual SOM model that differs from most SOM models in that the model state variables are measurable SOM pools. We suggest that physicochemical characteristics inherent to soils define the maximum protective capacity of these pools, which limits increases in SOM (i.e. C sequestration) with increased organic residue inputs.

Stabilization mechanisms of soil organic matter: Implications for C-saturation of soils

The Intergovernmental Panel on Climate Change (IPCC) Technical Paper Climate Change and Water draws together and evaluates the information in IPCC Assessment and Special Reports concerning the impacts of climate change on hydrological processes and regimes, and on freshwater resources – their availability, quality, use and management. It takes into account current and projected regional key vulnerabilities, prospects for adaptation, and the relationships between climate change mitigation and water. Its objectives are:

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Climate change and water.

There is little quantitative information on the interrelatedness of soil erosion, soil quality and soil productivity. A simulated erosion approach was used to quantify erosion and amendment effects on soil quality and subsequent productivity at four southern Alberta sites. Zero, 5, 10, 15 and 20 cm of topsoil were removed (cuts) at each site and subplots were amended with N + P fertilizer, 5 cm topsoil, 70 Mg ha−1 cattle manure or left unamended. Wheat (Triticum aestivum L.) yields in the 2-yr study on the non-amended check plots showed significant correlations with organic C at three of the four sites, and extractable P and inorganic C at all four sites. While manure was the best amendment for enhancing soil productivity, the magnitude of its effect depended on the organic C content of the recipient soil. At an organic C content of 8 g kg−1 on the Lethbridge Dryland site, manure addition increased crop yield by 1.75 Mg ha−1, compared with only 0.27 Mg ha−1 at an organic C content of 15 g kg−1. Our result...

Soil quality and productivity responses to simulated erosion and restorative amendments

Eliminating summer fallow or establishing perennial grass elicited measurable gains in soil organic C (SOC) after just 6 yr in a crop rotation study on an Aridic Boroll in southern Alberta. This study was resampled after 12 yr to determine if SOC increases were continuing with time and to evaluate the impact of alternative crop rotation and fertilizer treatments on SOC. The crop rotation treatments included in this study were fallow-wheat (FW), fallow-wheat-wheat (FWW), fallow-flax-wheat (FXW), legume-wheat (LW), continuous wheat (W) and continuous grass (G). The gain in SOC due to the elimination of fallow was 1.5 Mg C ha -1 after 12 yr, no greater than that observed after 6 yr. Soil organic C was the same for all rotations that included fallow (FW, FWW, FXW). Fertilizer treatments that had the greatest benefit on grain yields of annual crops also tended to increase SOC, although differences were barely detectable. The gain in SOC of unfertilized grass compared to the FW rotation was no higher after 12 yr than after 6 yr (3 Mg C ha -1 ). Under fertilized grass, in contrast, SOC continued to increase at a rate of approximately 0.5 Mg C ha -1 yr -1 . Accumulation of light fraction C accounted for most of the gains in SOC that occurred with elimination of fallow or establishment of grass. These findings suggest that much of the SOC gain due to adoption of C-conserving practices in soils like those of this study may occur early, within the first decade, and consist primarily of decomposable soil fractions. If confirmed, this means that C sequestration in these soils may be comparatively short-lived and vulnerable to future loss.

Soil Organic Carbon after Twelve Years of Various Crop Rotations in an Aridic Boroll

Assessment of grazing management on farm greenhouse gas intensity of beef production systems in the Canadian Prairies using life cycle assessment

The Haber–Bosch process for synthesizing fertilizer nitrogen (N) is among the most important modern discoveries because it has enabled us to grow enough food for several billion more of us. At the same time, however, profusion of added “reactive” N has become a prominent ecological threat, globally, because a large fraction of applied N is lost from agricultural ecosystems. Nitrogen added to agricultural soils, in organic forms or as synthetic N fertilizers, has one of four fates; it can be assimilated by plants, lost to surface- or groundwater, retained in the soil, or lost to the atmosphere. The last of these is particularly worrisome because of links to climate change and other threats to the biosphere. Our aim is to summarize briefly the processes of atmospheric N emissions to the air from agricultural ecosystems, and to consider how management practices might reduce those emissions. Nitrogen gases emitted from soil emanate from naturally occurring biological processes regulated largely by three interactive factors: substrate availability, aeration, and temperature. Although these factors are partly dictated by weather and intrinsic soil properties, they are also influenced by management so that emissions can be heavily influenced by practices imposed on the land. Variables to consider in devising systems with reduced emissions include: forms, rate, and timing of fertilizer; tillage and residue management; crop rotation, including the use of legumes; and manuring practices. All of these need to be considered together to devise systems, tuned to local conditions, which not only reduce emissions but also meet growing demands for agricultural yields. Developing such systems, based on holistic understanding from many disciplines, is now critical to sustain the long-term productivity and vitality of our ecosystems.

Nitrogenous gas emissions from soils and greenhouse gas effects.

In monocropped cereal systems, annual N inputs from non-fertilizer sources may be more than 30 kg ha-1. We examined the possibility that these inputs are due to biological N2 fixation (BNF) associated with roots or decomposing residues. Wheat was grown under greenhouse conditions in pots (34 cm long by 10 cm diameter) containing soil from a plot cropped to spring wheat since 1911 without fertilization. The roots and soil were sealed from the atmosphere and exposed to a15N2-enriched atmosphere for three to four weeks during vegetative, reproductive or post-reproductive stages. This technique permitted detection of as little as 1 μg fixed N plant-1 in plant material and 40 μg fixed N plant-1 in soil. No fixation of15N2 occurred during either of the first two labelling periods. In the final labelling period, straw returned to the soil was significantly enriched in15N, especially in a pot with a higher soil moisture content. Total BNF in this pot was 13 μg N plant-1, or about 30 g N ha-1. In a separate experiment with soil from the same plot, we detected BNF only when soil was amended with glucose at a high soil moisture content. Measured associative BNF was insufficient to account for observed N gains under field conditions.

H. Henry Janzen

Papers

Soil quality and productivity responses to simulated erosion and restorative amendments

Soil Organic Carbon after Twelve Years of Various Crop Rotations in an Aridic Boroll

Assessment of grazing management on farm greenhouse gas intensity of beef production systems in the Canadian Prairies using life cycle assessment

Nitrogenous gas emissions from soils and greenhouse gas effects.

Evidence against associative N2 fixation as a significant N source in long-term wheat plots