Publications

Identifying land uses and management practices that maintain or enhance soil carbon storage are important for sequestering carbon from the atmosphere and improving soil ecosystem services (Herrero et al., 2016). There is debate about how much additional carbon can be stored annually in soil, and for how long, following change in land use or management (Smith, 2014) and resolving this question is important. This is particularly relevant now following the aspirational goal established in the 4 per mille initiative (4p1000, 2017). In a paper titled: “Continuous soil carbon storage of old permanent pastures in Amazonia”, Stahl et al. (2017) purported to show increases in soil carbon when tropical forests are converted to grazed pasture of about 5.3 t C ha−1 year−1 through stock measurements across a chronosequence of sites. They state in the abstract: “… our results show that if sustainable management is applied in tropical pastures coming from deforestation… they can ensure a continuous C storage”, in the discussion: “This result supports the idea that certain ecosystems such as permanent pasture continuously accumulate soil carbon in the long-term (ie several decades).” and later in the conclusions: “Moreover, the humified C stored in the deeper soil layers is likely to provide unlimited C accumulation and conservation of C in the long-term.” (Italics added). Continuous and unlimited accumulation of soil carbon would certainly be of great interest to researchers, policy makers and the broader public as a large contributor to mitigating greenhouse gas emissions, particularly at rates as high as 5 t C ha−1 year−1. However, continuous and unlimited carbon accumulation is not supported by the author's data or the general literature. Using measurements of soil carbon stocks in a chronosequence of sites converted from forest to pasture, they reported no change in soil carbon for the first 24 years followed by increases in soil carbon between 24 and 36 years at 5 t C ha−1 year−1. From these data, it is not reasonable to extrapolate these increases for 12 years as continuous or unlimited without further justification. In the broader literature, it has been widely observed that carbon reaches a plateau in roughly 30 to 50 years or perhaps 100 years after system perturbation as new equilibrium is reached, and that pastures are not perpetual sinks of carbon (Smith, 2014). Soil carbon accumulation rates of greater than 1 t C ha−1 year−1 are also rare (Conant, Cerri, Osborne, & Paustian, 2017). At face value, accumulation of 5 t ha−1 year−1 would also require a high proportion of the gross primary production (GPP) captured as soil carbon rather than lost as carbon dioxide. Assuming a GPP of about 22 t C ha−1 for a high productivity pasture (e.g., Rutledge et al., 2015) would mean that 20%–25% of GPP was stabilized in soil every year for ~12 years. Additionally, by the end of the chronosequence the soil had a C:N ratio of 20 and was gaining of 5 t C ha−1 year−1 which would require a net nitrogen storage in soil organic matter (not in plants) of 250 kg N ha−1 year−1. In the absence of nitrogen fertilizer inputs, gross biological nitrogen fixation rates would need to be in excess of 250 kg N ha−1 year−1 from a reported legume cover of <10%. So what could be an alternative explanation for the trends observed in the data of the authors? Parsimoniously, the chronosequence sites may not have been derived from the same starting point as they were situated some 200 km apart and matching sites across such a wide geographical space is difficult (Walker, Wardle, Bardgett, & Clarkson, 2010). The parent materials of soils were matched by examining geological outcrops on roads. This is a rather limited approach to ensuring the landscape started at the same carbon stocks, particularly as the chronosequence length is only 36 years. Indeed, inspection of the soil texture data in the author's supplemental material had % sand of between 11% and 73% (average 48%, SD 15%), which does not support the contention that these soils were initially the same. Alternatively, these pastures may not have reached a new steady state within the time frame of the chronosequence and rates of accumulation will decrease as this steady state is reached, consistent with all other long-term experiments in the literature (Smith, 2014). Certainly, 12 years is not long enough to determine whether a new equilibrium has been reached. The data presented provide evidence of increased soil carbon stocks when Amazonian forest is converted to pasture, confirming a trend shown by some (Neill et al., 1997) but not all (Don, Schumacher, & Freibauer, 2011). However, labelling any gains observed as a continuous sink of carbon could lead to false hope by land owners and incorrectly inform policy makers. It should further be noted that only about 10% of the total ecosystem carbon lost after deforestation (due to tree removal, burning etc.) can be recovered through soil carbon sequestration (Fearnside, 1997; Neill et al., 1997). It remains critical that we determine rates of long-term carbon accumulation and how long they are sustained if we are to make progress towards the 4 per mille aspirational goal but methodological approaches require careful consideration. Miko Kirschbaum is thanked for providing helpful comments on a draft. The input of PS contributes to RCUK funded projects N-Circle (BB/N013484/1), U-GRASS (NE/M016900/1), GREENHOUSE (NE/K002589/1) and Soils-R-GGREAT (NE/P019455/1).

Abstract. Carbon is a critical component of soil vitality and of our ability to produce food. Carbon sequestered in soils also provides a further regulating ecosystem service, valued as the avoided damage from global climate change. We consider the demand and supply attributes that underpin and constrain the emergence of a market value for this vital global ecosystem service: markets being what economists regard as the most efficient institutions for allocating scarce resources to the supply and consumption of valuable goods. This paper considers how a potentially large global supply of soil carbon sequestration is reduced by economic and behavioural constraints that impinge on the emergence of markets, and alternative public policies that can efficiently transact demand for the service from private and public sector agents. In essence this is a case of significant market failure. In the design of alternative policy options we consider whether soil carbon mitigation is actually cost-effective relative to other measures in agriculture and elsewhere in the economy, and the nature of behavioural incentives that hinder policy options. We suggest that reducing cost and uncertainties of mitigation through soil-based measures is crucial for improving uptake. Monitoring and auditing processes will also be required to eventually facilitate wide-scale adoption of these measures.

Il est bien connu que le Surmulot (Rattus norvegicus) est difficile à empoisonner. Ceci est dû à une combinaison de traits comportementaux et de caractéristiques physiologiques dont l’étude, en plus de son intérêt appliqué, présente un intérêt certain en écologie du comportement et en écologie génétique. L’aptitude des rats à détecter et éviter des substances empoisonnées a entraîné le développement de poisons tel que l’anticoagulant Warfarine dont le mode d’action déjoue les défenses comportementales du Surmulot. L’évolution ultérieure d’une résistance physiologique à cette Warfarine constitue un des meilleurs exemples de sélection naturelle en action. Malgré son avantage sélectif évident pour les individus qui en bénéficient, la résistance à la Warfarine peut cependant avoir des désavantages : les individus résistants peuvent souffrir d’une carence en vitamine K, d’un taux de croissance réduit, et d’une viabilité moindre. Dans un environnement où le raticide n’est pas utilisé, ces «coûts » entraînent une fitness réduite des génotypes résistants et peuvent ralentir la progression de la résistance. Il pourrait cependant exister un nouveau développement dans cette « course aux armements » entre l’Homme et le Surmulot. Des études récentes suggèrent l’existence d’une forme de résistance peu coûteuse et susceptible de conférer un avantage sélectif aux individus résistants, même en l’absence de poison.

Did the study involve field work?Yes NoReporting for specific materials, systems and methodsWe require information from authors about some types of materials, experimental systems and methods used in many studies.Here, indicate whether each material, system or method listed is relevant to your study.If you are not sure if a list item applies to your research, read the appropriate section before selecting a response.

Abstract. Carbon is a critical component of soil vitality and is crucial to our ability to produce food. Carbon sequestered in soils also provides a further regulating ecosystem service, valued as the avoided damage from global climate change. We consider the demand and supply attributes that underpin and constrain the emergence of a market value for this vital global ecosystem service: markets being what economists regard as the most efficient institutions for allocating scarce resources to the supply and consumption of valuable goods. This paper considers how a potentially large global supply of soil carbon sequestration is reduced by economic and behavioural constraints that impinge on the emergence of markets, and alternative public policies that can efficiently transact demand for the service from private and public sector agents. In essence, this is a case of significant market failure. In the design of alternative policy options, we consider whether soil carbon mitigation is actually cost-effective relative to other measures in agriculture and elsewhere in the economy, and the nature of behavioural incentives that hinder policy options. We suggest that reducing the cost and uncertainties of mitigation through soil-based measures is crucial for improving uptake. Monitoring and auditing processes will also be required to eventually facilitate wide-scale adoption of these measures.

SUMMARY There is a very significant, cost effective greenhouse gas (GHG) mitigation potential in agriculture. The annual mitigation potential in agriculture is estimated to be 4200, 2600 and 1600 Mt CO 2 equiv/yr at C prices of 100, 50 and 20 US$/t CO 2 equiv, respectively. The value of GHG mitigated each year is equivalent to 420 000, 130 000 and 32 000 million US$/yr for C prices of 100, 50 and 20 US$/t CO 2 equiv, respectively. From both the mitigation and economic perspectives, we cannot afford to miss out on this mitigation potential. The challenge of agriculture within the climate change context is two-fold, both to reduce emissions and to adapt to a changing and more variable climate. The primary aim of the mitigation options is to reduce emissions of methane or nitrous oxide or to increase soil carbon storage. All the mitigation options, therefore, affect the carbon and/or nitrogen cycle of the agroecosystem in some way. This often not only affects the GHG emissions but also the soil properties and nutrient cycling. Adaptation to increased variability of temperature and rainfall involves increasing the resilience of the production systems. This may be done by improving soil water holding capacities through adding crop residues and manure to arable soils or by adding diversity to the crop rotations. Though some mitigation measures may have negative impacts on the adaptive capacity of farming systems, most categories of adaptation options for climate change have positive impacts on mitigation. These include: (1) measures that reduce soil erosion, (2) measures that reduce leaching of nitrogen and phosphorus, (3) measures for conserving soil moisture, (4) increasing the diversity of crop rotations by choices of species or varieties, (5) modification of microclimate to reduce temperature extremes and provide shelter, (6) land use change involving abandonment or extensification of existing agricultural land, or avoidance of the cultivation of new land. These adaptation measures will in general, if properly applied, reduce GHG emissions, by improving nitrogen use efficiencies and improving soil carbon storage. There appears to be a large potential for synergies between mitigation and adaptation within agriculture. This needs to be incorporated into economic analyses of the mitigation costs. The inter-linkages between mitigation and adaptation are, however, not very well explored and further studies are warranted to better quantify short- and long-term effects on suitability for mitigation and adaptation to climate change. In order to realize the full potential for agriculture in a climate change context, new agricultural production systems need to be developed that integrate bioenergy and food and feed production systems. This may possibly be obtained with perennial crops having low-environmental impacts, and deliver feedstocks for biorefineries for the production of biofuels, biomaterials and feed for livestock.