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Biofuel systems may represent a promising strategy to combat climate change by replacing fossil fuels in electricity generation and transportation. First-generation biofuels from sugar and starch crops for ethanol (a gasoline substitute) and from oilseed crops for biodiesel (a petroleum diesel substitute) have come under increasing levels of scrutiny due to the uncertainty associated with the estimation of climate change impacts of biofuels, such as due to indirect effects on land use. This analysis estimates the magnitude of some uncertainty sources: i) crop/feedstock, ii) life cycle assessment (LCA) modelling approach, iii) land-use change (LUC), and iv) greenhouse gas (GHG) metrics. The metrics used for characterising the different GHGs (global warming potential-GWP and global temperature change potential-GTP at different time horizons) appeared not to play a significant role in explaining the variance in the carbon footprint of biofuels, as opposed to the crop/feedstock used, the inclusion/exclusion of LUC considerations, and the LCA modelling approach (p<0.001). The estimated climate footprint of biofuels is dependent on the latter three parameters and, thus, is context-specific. It is recommended that these parameters be dealt with in a manner consistent with the goal and scope of the study. In particular, it is essential to interpret the results of the carbon footprint of biofuel systems in light of the choices made in each of these sources of uncertainty, and sensitivity analysis is recommended to overcome their influence on the result. [ABSTRACT FROM AUTHOR]

Agriculture is the largest single source of global anthropogenic methane (CH4) emissions, with ruminants the dominant contributor. Livestock CH4 emissions are projected to grow another 30% by 2050 under current policies, yet few countries have set targets or are implementing policies to reduce emissions in absolute terms. The reason for this limited ambition may be linked not only to the underpinning role of livestock for nutrition and livelihoods in many countries but also diverging perspectives on the importance of mitigating these emissions, given the short atmospheric lifetime of CH4. Here, we show that in mitigation pathways that limit warming to 1.5°C, which include cost-effective reductions from all emission sources, the contribution of future livestock CH4 emissions to global warming in 2050 is about one-third of that from future net carbon dioxide emissions. Future livestock CH4 emissions, therefore, significantly constrain the remaining carbon budget and the ability to meet stringent temperature limits. We review options to address livestock CH4 emissions through more efficient production, technological advances and demand-side changes, and their interactions with land-based carbon sequestration. We conclude that bringing livestock into mainstream mitigation policies, while recognizing their unique social, cultural and economic roles, would make an important contribution towards reaching the temperature goal of the Paris Agreement and is vital for a limit of 1.5°C. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 1)'. [ABSTRACT FROM AUTHOR]

Many global climate change mitigation pathways presented in IPCC assessment reports rely heavily on the deployment of bioenergy, often used in conjunction with carbon capture and storage. We review the literature on bioenergy use for climate change mitigation, including studies that use top‐down integrated assessment models or bottom‐up modelling, and studies that do not rely on modelling. We summarize the state of knowledge concerning potential co‐benefits and adverse side effects of bioenergy systems and discuss limitations of modelling studies used to analyse consequences of bioenergy expansion. The implications of bioenergy supply on mitigation and other sustainability criteria are context dependent and influenced by feedstock, management regime, climatic region, scale of deployment and how bioenergy alters energy systems and land use. Depending on previous land use, widespread deployment of monoculture plantations may contribute to mitigation but can cause negative impacts across a range of other sustainability criteria. Strategic integration of new biomass supply systems into existing agriculture and forest landscapes may result in less mitigation but can contribute positively to other sustainability objectives. There is considerable variation in evaluations of how sustainability challenges evolve as the scale of bioenergy deployment increases, due to limitations of existing models, and uncertainty over the future context with respect to the many variables that influence alternative uses of biomass and land. Integrative policies, coordinated institutions and improved governance mechanisms to enhance co‐benefits and minimize adverse side effects can reduce the risks of large‐scale deployment of bioenergy. Further, conservation and efficiency measures for energy, land and biomass can support greater flexibility in achieving climate change mitigation and adaptation. [ABSTRACT FROM AUTHOR]

Soil is the most important terrestrial carbon (C) reservoir but is greatly impacted by land use change (LUC). Previous analyses of LUC impacts on soil C have focused on biophysical variables, leaving aside the influence of socioeconomics. The aim of our study was to determine global soil organic carbon (SOC) change patterns after LUC and to assess the impacts of both biophysical and socioeconomic factors that influence stocks of SOC after LUC simultaneously. This was done at a global scale using 817 sites from 99 peer-reviewed publications. We performed separate analyses for cases in which there were gains and losses of SOC. The best predictors of SOC stock changes were the type of LUC and predictors related to sampling depth, climate, biome, soil order, relief, geology, years since LUC, and primary productivity. However, also, socioeconomic variables such as indices of poverty, population growth, and levels of corruption were important. They explained 33% of the variability in SOC on their own and helped improve model accuracy from 42 to 53% when considered in combination with biophysical variables. SOC losses were highly correlated to the type of LUC and social variables, while SOC gains correlated most strongly with years since LUC and the biophysical variables. The analyses confirm that one of the biggest drivers of SOC loss is conversion to agroindustrial scale cropping, whereas with regard to the recuperation of SOC after LUC, the factor "time since conversion" emerged as the most important predictive variable, which must be better integrated in respective policy expectations. We conclude that policies should more than ever incentivize holistic approaches that prevent additional loss of native SOC, while at the same time promoting sustainable intensification of existing agricultural regions. Finally future investments on LUC to regain SOC should be aligned with efforts to alleviate poverty and corruption for their potential to achieve mutual gains in soil fertility and socio-economic parameters. [ABSTRACT FROM AUTHOR]

Intergovernmental Panel on Climate Change ( IPCC) Tier 1 methodologies commonly underpin project-scale carbon accounting for changes in land use and management and are used in frameworks for Life Cycle Assessment and carbon footprinting of food and energy crops. These methodologies were intended for use at large spatial scales. This can introduce error in predictions at finer spatial scales. There is an urgent need for development and implementation of higher tier methodologies that can be applied at fine spatial scales (e.g. farm/project/plantation) for food and bioenergy crop greenhouse gas (GHG) accounting to facilitate decision making in the land-based sectors. Higher tier methods have been defined by IPCC and must be well evaluated and operate across a range of domains (e.g. climate region, soil type, crop type, topography), and must account for land use transitions and management changes being implemented. Furthermore, the data required to calibrate and drive the models used at higher tiers need to be available and applicable at fine spatial resolution, covering the meteorological, soil, cropping system and management domains, with quantified uncertainties. Testing the reliability of the models will require data either from sites with repeated measurements or from chronosequences. We review current global capability for estimating changes in soil carbon at fine spatial scales and present a vision for a framework capable of quantifying land use change and management impacts on soil carbon, which could be used for addressing issues such as bioenergy and biofuel sustainability, food security, forest protection, and direct/indirect impacts of land use change. The aim of this framework is to provide a globally accepted standard of carbon measurement and modelling appropriate for GHG accounting that could be applied at project to national scales (allowing outputs to be scaled up to a country level), to address the impacts of land use and land management change on soil carbon. [ABSTRACT FROM AUTHOR]

The article presents the authors' reply to Matthew Brander's response to their paper on the necessity of a land-use baseline in attributional LCA where he raised concerns on the appropriateness of attributable LCA (ALCA) to support decision-making. They point out that postponement of natural regeneration can be attributed to any land use regardless of the length occupation and whether the temporal scope includes the initial land transformation.

Abstract: Conversion of pastures to plantation forests has been proposed as a means to increase rates of carbon (C) sequestration from the atmosphere thereby reducing net greenhouse gas emissions from human activities. However, several studies have indicated that soil C stocks decrease after planting conifer (mainly pine) trees into pasture. This loss of soil C detracts from the role that plantation forests can play in net C sequestration. Here, we used a paired site (a grazed native pasture with the C4 grass Themeda triandra dominant, and an adjacent 16-year-old Pinus radiata plantation) to compare all C and nitrogen (N) pools (including soil, litter on the floor, below-ground and above-ground biomass) in the two ecosystems and to estimate the rate of C sequestration after the land use change from the native pasture to the pine plantation. Soil C and N stocks from soil surface down to 1m under the pine plantation were significantly less than under the native pasture by 20% (57.3MgCha−1 vs. 71.6MgCha−1) and 15% (5.6MgNha−1 vs. 6.7MgNha−1), respectively. Much more C and N was stored in litter on the floor in the pine plantation than in the native pasture (8.0MgCha−1 vs. 0.03MgCha−1, and 119.0kgNha−1 vs. 0.9kgNha−1), and in biomass (95.0MgCha−1 vs. 2.5MgCha−1 and 411.5kgNha−1 vs. 62.8kgNha−1). Carbon stored in coarse tree roots was alone sufficient to compensate the C loss from soil after the land use change. Much more C and N was deposited annually to above-ground litter in the pine plantation than in the native pasture (2.18MgCha−1 year−1 vs. 0.22MgCha−1 year−1, and 32.8kgNha−1 year−1 vs. 5.9kgNha−1 year−1), but less to below-ground litter (through fine root death) (2.71MgCha−1 year−1 vs. 3.57MgCha−1 year−1 and 38.9kgNha−1 year−1 vs. 81.4kgNha−1 year−1). The shift in net primary production from below-ground dominance to above-ground dominance after planting trees onto the pasture, and the slower turnover of litter in the plantation, played a key role in the reduction in soil C in the plantation ecosystem. In conclusion, planting pine trees onto a native temperate Australian pasture sequestered a significant amount of C (net 86MgCha−1, averaging 5.4MgCha−1 year−1) from the atmosphere in 16 years despite the loss of 14MgCha−1 from the soil organic matter. [Copyright &y& Elsevier]

Interest in bioenergy is growing across the Western world in response to mounting concerns about climate change. There is a risk of depletion of soil carbon stocks in biomass production systems, because a higher proportion of the organic matter and nutrients are removed from the site, compared with conventional agricultural and forestry systems. This paper reviews the factors that influence soil carbon dynamics in bioenergy systems, and utilises the model Full CAM to investigate the likely magnitude of soil carbon change where bioenergy systems replace conventional land uses. Environmental and management factors govern the magnitude and direction of change. Soil C losses are most likely where soil C is initially high, such as where improved pasture is converted to biomass production. Bioenergy systems are likely to enhance soil C where these replace conventional cropping, as intensively cropped soils are generally depleted in soil C. Measures that enhance soil C include maintenance of productivity through application of fertilisers, inclusion of legumes, and retention of nutrient-rich foliage on-site. Modelling results demonstrate that loss of soil carbon in bioenergy systems is associated with declines in the resistant plant matter and humified soil C pools. However, published experimental data and modelling results indicate that total soil C loss in bioenergy systems is generally small. Thus, although there may be some decline in soil carbon associated with biomass production, this is negligible in comparison with the contribution of bioenergy systems towards greenhouse mitigation through avoided fossil fuel emissions. [ABSTRACT FROM AUTHOR]

This project aimed to identify land uses and soil management practices that have significant associations with soil organic carbon (SOC) stocks (0–0.3 m) in New South Wales (NSW), Australia. The work presented in this paper is based on a one-off survey targeting key land uses and management practices of eastern NSW. Because of the nature of the work, the land uses and management combinations surveyed in different soils and climatic conditions were significantly unbalanced, and separately analyzing associations after breaking the dataset into different land uses may lead to significant increases in Type errors. Therefore, redundancy analysis (RDA) was undertaken to explore the association between explanatory variables (i.e., land uses, soil management, soil properties and environmental variables) and the variation in stocks (mass per unit area) of particulate organic carbon (POC), humic organic carbon (HOC) and resistant organic carbon (ROC) across 780 sites in eastern NSW, south eastern Australia. Results indicated that soil properties, land uses, soil management and environmental variables together could explain 52% of total variation in stocks of the SOC fractions. Specifically soil properties and environmental variables explained 42.8%, whereas land uses and management practices together explained 9.2% of the total variation in SOC fractions. A forward selection RDA was also undertaken considering soil properties and environmental variables as covariates to assess the statistical significance of land uses and management practices on stocks of POC, HOC and ROC. We found that pasture had significant positive associations on stocks of carbon fractions. Among the soil properties and environmental variables rainfall, longitude and elevation had a significant positive influence while pH and bulk density had a significantly negative influence on the HOC, POC and ROC stocks. Using a novel multivariate technique, the current work identified the land uses and soil management that had significant impact on carbon stocks in soil after accounting for influences soil properties and environmental variables. [ABSTRACT FROM AUTHOR]

In order to understand the climate effects of a bioenergy system, a comparison between the bioenergy system and a reference system is required. The reference system describes the situation that occurs in the absence of the bioenergy system with respect to the use of land, energy, and materials. The importance of reference systems is discussed in the literature but guidance on choosing suitable reference systems for assessing climate effects of bioenergy is limited. The reference system should align with the purpose of the study. Transparency of reference system assumptions is essential for proper interpretation of bioenergy assessments. This paper presents guidance for selecting suitable reference systems. Particular attention is given to choosing the land reference. If the goal is to study the climate effects of bioenergy as a part of total anthropogenic activity the reference system should illustrate what is expected in the absence of human activities. In such a case the suitable land reference is natural regeneration, and energy or material reference systems are not relevant. If the goal is to assess the effect of a change in bioenergy use, the reference system should incorporate human activities. In this case suitable reference systems describe the most likely alternative uses of the land, energy and materials in the absence of the change in bioenergy use. The definition of the reference system is furthermore subject to the temporal scope of the study. In practice, selecting and characterizing reference systems will involve various choices and uncertainties which should be considered carefully. It can be instructive to consider how alternative reference systems influence the results and conclusions drawn from bioenergy assessments. [ABSTRACT FROM AUTHOR]

Understanding the interactions between the initial biochemical composition and subsequent decomposition of plant litter will improve our understanding of its influence on microbial substrate use to explain the flow of organic matter between soil carbon pools. We determined the effects of land use (cultivation/native woodland/native pasture), litter type (above and below ground) and their interaction on the initial biochemical composition (carbon, nitrogen, water soluble carbon, lignin, tannin and cellulose) and decomposition of litter. Litter decomposition was studied as the mineralization of C from litter by microbial respiration and was measured as CO2-C production during 105 d of laboratory incubation with soil. A two-pool model was used to quantify C mineralization kinetics. For all litter types, the active C pool decay rate constants ranged from 0.072 d−1 to 0.805 d−1 which represented relatively short half-lives of between 1 and 10 days, implying that this pool contained compounds that were rapidly mineralized by microbes during the initial stages of incubation. Conversely, the decay rate constants for the slow C pool varied widely between litter types within and among land uses ranging from 0.002 d−1 and 0.019 d−1 representing half-lives of between 37 and 446 days. In all litter types, the initial lignin:N ratio strongly and negatively influenced the decay rate of the slow C pool which implied that the interaction between these two litter quality variables had important controls over the decomposition of the litter slow C pool. We interpret our results to suggest that where the flow of C from the active pool to the slow pool is largely driven by microbial activity in soil, the rate of transfer of C will be largely controlled by the quality of litter under different land-use systems and particularly the initial lignin:N ratio of the litter. Compared with native pastures and cultivation, above and below ground litter from native woodland was characterized by higher lignin:N ratio and more slowly decomposing slow C pools which implies that litter is likely to persist in soils, however based on the sandy nature of the soils in this study, it is likely to lack protection from microbial degradation in the long term. [ABSTRACT FROM AUTHOR]

Bioenergy aims to reduce greenhouse gas (GHG) emissions and contribute to meeting global climate change mitigation targets. Nevertheless, several sustainability concerns are associated with bioenergy, especially related to the impacts of using land for dedicated energy crop production. Cultivating energy crops can result in synergies or trade-offs between GHG emission reductions and other sustainability effects depending on context-specific conditions. Using the United Nations Sustainable Development Goals (SDGs) framework, the main synergies and trade-offs associated with land use for dedicated energy crop production were identified. Furthermore, the context-specific conditions (i.e., biomass feedstock, previous land use, climate, soil type and agricultural management) which affect those synergies and trade-offs were also identified. The most recent literature was reviewed and a pairwise comparison between GHG emission reduction (SDG 13) and other SDGs was carried out. A total of 427 observations were classified as either synergy (170), trade-off (176), or no effect (81). Most synergies with environmentally-related SDGs, such as water quality and biodiversity conservation, were observed when perennial crops were produced on arable land, pasture or marginal land in the 'cool temperate moist' climate zone and 'high activity clay' soils. Most trade-offs were related to food security and water availability. Previous land use and feedstock type are more impactful in determining synergies and trade-offs than climatic zone and soil type. This study highlights the importance of considering context-specific conditions in evaluating synergies and trade-offs and their relevance for developing appropriate policies and practices to meet worldwide demand for bioenergy in a sustainable manner. • Synergies and trade-offs between SDGs from land use for bioenergy. • Synergies are related to water quality, soil quality and biodiversity conservation. • Trade-offs are related to water availability, food security and revenue. • Previous land use and feedstock are more relevant than other context conditions. [ABSTRACT FROM AUTHOR]