The AMG model coupled with Rock-Eval® analysis accurately predicts cropland soil organic carbon dynamics in the Tuojiang River Basin, Southwest China.

Accurate soil organic carbon models are key to understand the mechanisms governing carbon sequestration in soil and to help develop targeted management strategies to carbon budget. The accuracy and reliability of soil organic carbon (SOC) models remains strongly limited by incorrect initialization of the conceptual kinetic pools and lack of stringent model evaluation using time-series datasets. Notably, due to legacy effects of management and land use change, the traditional spin-up approach for initial allocation of SOC among kinetic pools can bring substantial uncertainties in predicting the evolution of SOC stocks. The AMG model can fulfill these conditions as it is a parsimonious yet accurate SOC model using widely-available input data. In this study, we first evaluated the performance of AMGv2 before and after optimizing the potential mineralization rate (k0) of SOC stock following a leave-one-site-out cross-validation based on 24 long-term field experiments (LTEs) in the Southwest of China. Then, we used Rock-Eval® thermal analysis results as input variables in the PARTYSOC machine learning model to estimate the initial stable SOC fraction (CS/C0) for the 14 LTEs where soil samples were available. The results showed that initializing the CS/C0 ratio using PARTYSOC combined with the optimized k0 further improved the accuracy of model simulations (R2 = 0.87, RMSE = 0.25, d = 0.90). Combining average measured CS/C0 and k0 optimization across all 24 LTEs also improved the model predictive capability by 25% compared to using default parameterization, thus suggesting promising avenue for upscaling model applications at the regional level where only a few measurement data on SOC stability can be available. In conclusion, the new version of the AMG model developed in the Tuojiang River Basin context exhibits excellent performance. This result paves the way for further calibration and validation of the AMG model in a wider set of contexts, with the potential to significantly improve confidence in SOC predictions in croplands over regional scales.

Incorporating agricultural practices in digital mapping improves prediction of cropland soil organic carbon content: The case of the Tuojiang River Basin.

Accurate mapping of soil organic carbon (SOC) in cropland is essential for improving soil management in agriculture and assessing the potential of different strategies aiming at climate change mitigation. Cropland management practices have large impacts on agricultural soils, but have rarely been considered in previous SOC mapping work. In this study, cropland management practices including carbon input (CI), length of cultivation (LC), and irrigation (Irri) were incorporated as agricultural management covariates and integrated with natural variables to predict the spatial distribution of SOC using the Extreme Gradient Boosting (XGBoost) model. Additionally, we evaluated the performance of incorporating agricultural management practice variables in the prediction of cropland topsoil SOC. A case study was carried out in a traditional agricultural area in the Tuojiang River Basin, China. We found that CI was the most important environmental covariate for predicting cropland SOC. Adding cropland management practices to natural variables improved prediction accuracy, with the coefficient of determination (R2), the root mean squared error (RMSE) and Lin's Concordance Correlation Coefficient (LCCC) improving by 16.67%, 17.75% and 5.62%, respectively. Our results highlight the effectiveness of incorporating agricultural management practice information into SOC prediction models. We conclude that the construction of spatio-temporal database of agricultural management practices derived from inventories is a research priority to improve the reliability of SOC model prediction.

Changes in perspective needed to forge 'no-regret' forest-based climate change mitigation strategies.

Forest-based mitigation strategies will play a pivotal role in achieving the rapid and deep net-emission reductions required to prevent catastrophic climate change. However, large disagreement prevails on how to forge forest-based mitigation strategies, in particular in regions where forests are currently growing in area and carbon density. Two opposing viewpoints prevail in the current discourse: (1) A widespread viewpoint, specifically in countries in the Global North, favours enhanced wood use, including bioenergy, for substitution of emissions-intensive products and processes. (2) Others instead focus on the biophysical, resource-efficiency and time-response advantages of forest conservation and restoration for carbon sequestration and biodiversity conservation, whilst often not explicitly specifying how much wood extraction can still safeguard these ecological benefits. We here argue for a new perspective in sustainable forest research that aims at forging "no-regret" forest-based climate change mitigation strategies. Based on the consideration of forest growth dynamics and the opportunity carbon cost associated with wood use, we suggest that, instead of taking (hypothetical) wood-for-fossil substitution as starting point in assessments of carbon implications of wood products and services, analyses should take the potential and desired carbon sequestration of forests as starting point and quantify sustainable yield potentials compatible with those carbon sequestration potentials. Such an approach explicitly addresses the possible benefits provided by forests as carbon sinks, brings research on the permanence and vulnerability of C-stocks in forests, of substitution effects, as well as explorations of demand-side strategies to the forefront of research and, in particular, aligns better with the urgency to find viable climate solutions.

Embodied HANPP of feed and animal products: Tracing pressure on ecosystems along trilateral livestock supply chains 1986-2013.

The global livestock system puts increasing pressures on ecosystems. Studies analyzing the ecological impacts of livestock supply chains often explain this pressure by the increasing demand for animal products. Food regime theory proposes a more nuanced perspective: it explains livestock-related pressures on ecosystems by systemic changes along the supply chains of feed and animal products, notably the liberalization of agricultural trade. This study proposes a framework supporting empirical analyses of such claims by differentiating several steps of livestock supply chains. We reconstructed "trilateral" livestock supply chains linking feed production, livestock farming, and final consumption, based on the global flows of 161 feed and 13 animal products between 222 countries from 1986 to 2013. We used the embodied Human Appropriation of Net Primary Production (eHANPP) indicator to quantify pressures on ecosystems linked to these trilateral livestock supply chains. We find that livestock induced 65 % of agriculture's pressure on ecosystems, mostly through cattle grazing. Between 1986 and 2013, the fraction of livestock-related eHANPP that was traded internationally doubled from 7.1 % to 15.6 %. eHANPP related to the trade of feed was mostly linked to soybean imported for pig meat production, whereas eHANPP associated to traded animal products was mostly linked to cattle meat. eHANPP of traded animal products was lower but increased faster than eHANPP of feed trade. eHANPP was highest at the feed production level in South and North America, and at the consumption level in Eastern Asia. In Northern Asia and Eastern Europe, eHANPP was lowest at the animal products production level. In Western Europe, the eHANPP was equal at the animal products production and consumption levels. Our findings suggest that options to reduce livestock's pressures on ecosystems exist at all levels of the supply chain, especially by reducing the production and consumption in high-consuming countries and regulating international supply chains.

Forest Transitions in the United States, France and Austria: dynamics of forest change and their socio- metabolic drivers.

Understanding the drivers of forest transitions is relevant to inform effective forest conservation. We investigate pathways of forest transitions in the United States (1920-2010), France (1850-2010), and Austria (1830-2010). By combining evidence from forest inventories with the forest model CRAFT, we first quantify how change in forest area (ΔA), maximum biomass density (ΔBdmax ), and actual biomass as fraction of maximum biomass (ΔFmax ) shaped forest dynamics. Second, to investigate the connections between forest change and societal resource use, or social metabolism, we quantify the importance of selected proximate and underlying socio-metabolic drivers. We find that agricultural intensification and reduced forest grazing correlated most with positive ΔA and ΔBdmax . By contrast, change in biomass imports or harvest did not explain forest change. Our findings highlight the importance of forest growth conditions in explaining long-term forest dynamics, and demonstrate the distinct ways in which resource use drove forest change.

Altered growth conditions more than reforestation counteracted forest biomass carbon emissions 1990-2020.

Understanding the carbon (C) balance in global forest is key for climate-change mitigation. However, land use and environmental drivers affecting global forest C fluxes remain poorly quantified. Here we show, following a counterfactual modelling approach based on global Forest Resource Assessments, that in 1990-2020 deforestation is the main driver of forest C emissions, partly counteracted by increased forest growth rates under altered conditions: In the hypothetical absence of changes in forest (i) area, (ii) harvest or (iii) burnt area, global forest biomass would reverse from an actual cumulative net C source of c. 0.74 GtC to a net C sink of 26.9, 4.9 and 0.63 GtC, respectively. In contrast, (iv) without growth rate changes, cumulative emissions would be 7.4 GtC, i.e., 10 times higher. Because this sink function may be discontinued in the future due to climate-change, ending deforestation and lowering wood harvest emerge here as key climate-change mitigation strategies.

Quantifying and attributing land use-induced carbon emissions to biomass consumption: A critical assessment of existing approaches.

Biomass production generates land use impacts in the form of emissions from Forestry and Other Land Use (FOLU), i.e. due to changes in ecosystem carbon stocks. Recently, consumption-based accounting (CBA) approaches have emerged as alternatives to conventional production-based accounts, quantifying FOLU emissions associated with biomass consumption, for example, of particular territories. However, the quantification and allocation of FOLU emissions to individual biomass products, a fundamental part of CBA approaches, is a complex endeavour. Existing studies make diverging methodological choices, which are rarely critically discussed. In this study, we provide a structured overview of existing CBA approaches to estimating FOLU emissions. We cluster the literature in a two-by-two grid, distinguishing the primary element under investigation (impacts of changing consumption patterns in a region vs. impacts of consumption on production landscapes) and the analytical lens (prospective vs retrospective). Further, we identify three distinct dimensions which characterise the way in which different studies allocate FOLU emissions to biomass products: the choice of reference system and the spatial and temporal scales. Finally, we identify three frontiers that require future attention: (1) overcoming structural biases which underestimate FOLU emissions from territories that experienced deforestation in the distant past, (2) explicitly tackling the interdependence of proximate causes and ultimate drivers of land use change, and (3) assessing uncertainties and understanding the effects of land management. In this way, we enable a critical assessment of appropriate methods, support a nuanced interpretation of results from particular approaches as well as enhance the informative value of CBA approaches related to FOLU emissions. Our analysis contributes to discussions on sustainable land use practices with respect to biomass consumption and has implications for informing international climate policy in scenarios where consumption-based approaches are adopted in practice.

Modeling and empirical validation of long-term carbon sequestration in forests (France, 1850-2015).

The development of appropriate tools to quantify long-term carbon (C) budgets following forest transitions, that is, shifts from deforestation to afforestation, and to identify their drivers are key issues for forging sustainable land-based climate-change mitigation strategies. Here, we develop a new modeling approach, CRAFT (CaRbon Accumulation in ForesTs) based on widely available input data to study the C dynamics in French forests at the regional scale from 1850 to 2015. The model is composed of two interconnected modules which integrate biomass stocks and flows (Module 1) with litter and soil organic C (Module 2) and build upon previously established coupled climate-vegetation models. Our model allows to develop a comprehensive understanding of forest C dynamics by systematically depicting the integrated impact of environmental changes and land use. Model outputs were compared to empirical data of C stocks in forest biomass and soils, available for recent decades from inventories, and to a long-term simulation using a bookkeeping model. The CRAFT model reliably simulates the C dynamics during France's forest transition and reproduces C-fluxes and stocks reported in the forest and soil inventories, in contrast to a widely used bookkeeping model which strictly only depicts C-fluxes due to wood extraction. Model results show that like in several other industrialized countries, a sharp increase in forest biomass and SOC stocks resulted from forest area expansion and, especially after 1960, from tree growth resulting in vegetation thickening (on average 7.8 Mt C/year over the whole period). The difference between the bookkeeping model, 0.3 Mt C/year in 1850 and 21 Mt C/year in 2015, can be attributed to environmental and land management changes. The CRAFT model opens new grounds for better quantifying long-term forest C dynamics and investigating the relative effects of land use, land management, and environmental change.

Long-term changes in greenhouse gas emissions from French agriculture and livestock (1852-2014): From traditional agriculture to conventional intensive systems.

France was a traditionally agricultural country until the first half of the 20th century. Today, it is the first European cereal producer, with cereal crops accounting for 40% of the agricultural surface area used, and is also a major country for livestock breeding with 25% of the European cattle livestock. This major socioecological transition, with rapid intensification and specialisation in an open global market, has been accompanied by deep environmental changes. To explore the changes in agricultural GHG emissions over the long term (1852-2014), we analysed the emission factors of N2O from field experiments covering major land uses, in a gradient of fertilisation and within a range of temperature and rainfall, and used CH4 emission coefficients for livestock categories, in terms of enteric and manure management, considering the historical changes in animal excretion rates. We also estimated indirect CO2 emissions, rarely accounted for in agricultural emissions, using coefficients found in the literature for the dominant energy consumption items (fertiliser production, field work and machinery, and feed import). From GHG emissions of ~30,000 ktons CO2 Eq yr-1 in 1852, reaching 54,000 ktons CO2 Eq yr-1 in 1955, emissions more than doubled during the 'Glorious thirties' (1950-1980), and peaked around 120,000 ktons CO2 Eq yr-1 in the early 2000s. For the 2010-2014 period, French agriculture GHG emissions stabilised at ~114,000 ktons CO2 Eq yr-1, distributed into 49% methane (CH4), 22% carbon dioxide (CO2) and 29% nitrous oxide (N2O). A regional approach through 33 regions in France shows a diversity of agriculture reflecting the hydro-ecoregion distribution and the agricultural specialisation of local areas. Exploring contrasting scenarios at the 2040 horizon suggests that only deep changes in the structure of the agro-food system would double the reduction of GHG emissions by the agricultural sector.

Two contrasted future scenarios for the French agro-food system.

Narratives of two prospective scenarios for the future of French agriculture were elaborated by pushing several trends already acting on the dynamics of the current system to their logical end. The first one pursues the opening and specialization characterizing the long-term evolution of the last 50 years of most French agricultural regions, while the second assumes a shift, already perceptible through weak signals, towards more autonomy at the farm and regional scales, a reconnection of crop and livestock farming and a more frugal human diet. A procedure is proposed to translate these qualitative narratives into a quantitative description of the corresponding nutrient fluxes using the GRAFS (Generalized Representation of Agro-Food Systems) methodology, thus allowing a comprehensive exploration of the agronomical and environmental performance of these two scenarios. The results show that the pursuit of the opening and specialization of French agriculture, even complying with regulations regarding reasoned fertilization, would result in considerable environmental burdens namely in terms of water pollution. The scenario generalizing organic farming practices, reconnection of crop and livestock farming systems and a demitarian human diet makes it possible to meet the future national food demand while still exporting significant amounts of cereals to the international market and ensuring better groundwater quality in most French regions.

The effect of nitrification inhibitors on NH3 and N2O emissions in highly N fertilized irrigated Mediterranean cropping systems.

There is an increasing concern about the negative impacts associated to the release of reactive nitrogen (N) from highly fertilized agro-ecosystems. Ammonia (NH3) and nitrous oxide (N2O) are harmful N pollutants that may contribute both directly and indirectly to global warming. Surface applied manure, urea and ammonium (NH4+) based fertilizers are important anthropogenic sources of these emissions. Nitrification inhibitors (NIs) have been proposed as a useful technological approach to reduce N2O emission although they can lead to large NH3 losses due to increasing NH4+ pool in soils. In this context, a field experiment was carried out in a maize field with aiming to simultaneously quantify NH3 volatilization and N2O emission, assessing the effect of two NIs 3,4­dimethilpyrazol phosphate (DMPP) and 3,4­dimethylpyrazole succinic acid (DMPSA). The first treatment was pig slurry (PS) before seeding (50 kg N ha-1) and calcium ammonium nitrate (CAN) at top-dressing (150 kg N ha-1), and the second was DMPP diluted in PS (PS + DMPP) (50 kg N ha-1) and CAN + DMPSA (150 kg N ha-1) also before seeding and at top-dressing, respectively. Ammonia emissions were quantified by a micrometeorological method during 20 days after fertilization and N2O emissions were assessed using manual static chambers during all crop period. The treatment with NIs was effective in reducing c. 30% cumulative N2O losses. However, considering only direct N2O emissions after second fertilization event, a significant reduction was not observed using CAN+DMPSA, probably because high WFPS of soil, driven by irrigation, favored denitrification. Cumulative NH3 losses were not significantly affected by NIs. Indeed, NH3 volatilization accounted 14% and 10% of N applied in PS + DMPP and PS plots, respectively and c. 2% of total N applied in CAN+DMPSA and CAN plots. Since important NH3 losses still exist even although abating strategies are implemented, structural and political initiatives are needed to face this issue.

Phosphorus management in cropping systems of the Paris Basin: From farm to regional scale.

The sustainability of phosphorus (P) fertilization in cropping systems is an important issue because P resources on earth are limited and excess P in soils can lead to ecological damage such as eutrophication. Worldwide, there is an increasing interest in organic farming (OF) due to its good environmental performance. However, organic cropping systems are suspected of generating negative P budgets, which questions their ability to provide sustainable P management. The design of agricultural systems at a broader scale also largely influences the shape of the P cycle and the possibility of its recycling to cropland. In this context, the aim of this study was to assess the relative influence of (i) OF versus conventional farming (CF) practices and (ii) the structure of agro-food systems at the regional scale, on P cycling and availability on cropland. For this purpose, we examined P budgets and soil P status of 14 organic and conventional cropping systems in commercial farms located in the Paris Basin. Available P was analyzed using two different methods: resin P and Olsen P. The results revealed no significant differences between CF and OF in available P stocks. Phosphorus budgets were always negative and significantly lower in CF systems, indicating that P was mined from soil reserves. In parallel, we estimated P budgets over cropland in all French regions for two distinct periods, 2004-2014 and 1970-1981, and showed that specialized intensive cropping systems in the Paris Basin led to a high, positive P budget in the latter period. However, this trend was reversed in the 2004-2014 period due to a sharp reduction of the mineral fertilizer application rate. The shift from very high P budgets to much lower and sometimes negative P budgets would not be a threat for agriculture due to the current high level of Olsen P in these regions, which was consistent with our measurements at the plot scale. Overall, these results suggest that OF would not lead to more P deficiency than CF. Instead, they emphasize that sustainable P management not only depends on farmers' choices but mainly on the structure and specialization of agro-food systems.

How the structure of agro-food systems shapes nitrogen, phosphorus, and carbon fluxes: The generalized representation of agro-food system applied at the regional scale in France.

The aim of the study was to develop a conceptual framework to analyze the agro-food system of French agricultural regions from the angle of N, P and C circulation through five major compartments (cropland, grassland, livestock biomass, local population and potential environmental losses). To reach that goal we extended the Generalized Representation of Agro-Food System approach to P and C and applied it to French regions. Using this methodology we analyzed the relation between production pattern and N surplus, P budget, and efficient organic carbon inputs (OCeff), assuming these three indicators to be good proxies for (i) N losses to waterbodies and the atmosphere, (ii) P accumulation or depletion in soils, and (iii) potential additional C sequestration in soils, respectively. A typology was then established, allowing for comparison between five types of agricultural systems. This made it possible to highlight that intensive specialized agricultural systems generate high environmental losses and resource consumption per unit of agricultural surface and present a very open nutrient cycle due to substantial trade flows. Conversely, mixed crop and livestock farming and extensive cropping systems had more limited N and P consumption and led to lower potential water and air contamination. However, this trend was reversed when expressing resource consumption and N and P budget on a pro rata basis of vegetal and animal product unit, reflecting the better nutrient use efficiency of specialized regions in their respective field of specialization. This study demonstrates the systemic impact of production patterns on environmental and agronomic performances at the regional scale.