Assessing the impacts of agricultural managements on soil carbon stocks, nitrogen loss, and crop production – a modelling study in eastern Africa

Improved agricultural management plays a vital role in protecting soils from degradation in eastern Africa. Changing practices such as reducing tillage, fertilizer use, or cover crops are expected to enhance soil organic carbon (SOC) storage, with climate change mitigation co-benefits, while increasing crop production. However, the quantification of cropland management effects on agricultural ecosystems remains inadequate in this region. Here, we explored seven management practices and their potential effects on soil carbon (C) pools, nitrogen (N) losses, and crop yields under different climate scenarios, using the dynamic vegetation model LPJ-GUESS. The model performance is evaluated against observations from two long-term maize field trials in western Kenya and reported estimates from published sources. LPJ-GUESS generally produces soil C stocks and maize productivity comparable with measurements and mostly captures the SOC decline under some management practices that is observed in the field experiments. We found that for large parts of Kenya and Ethiopia, an integrated conservation agriculture practice (no-tillage, residue and manure application, and cover crops) increases SOC levels in the long term (+11 % on average), accompanied by increased crop yields (+22 %) in comparison to the conventional management. Planting nitrogen-fixing cover crops in our simulations is also identified as a promising individual practice in eastern Africa to increase soil C storage (+4 %) and crop production (+18 %), with low environmental cost of N losses (+24 %). These management impacts are also sustained in simulations of three future climate pathways. This study highlights the possibilities of conservation agriculture when targeting long-term environmental sustainability and food security in crop ecosystems, particularly for those with poor soil conditions in tropical climates

Soil organic carbon accumulation under perennial forages in the Southern Highlands of Tanzania

Joint International Grassland & International Rangeland Kenya 2021 Virtual Congres

Soil organic carbon dynamics along chrono-sequence land-use systems in the highlands of Ethiopia

Soil organic carbon (SOC) dynamics along land-use changes influences the terrestrial and global carbon cycle, the climate, soil fertility, agricultural productivity, and food security. Taking soils under native forests as an appropriate ecological reference, we studied changes in soil organic carbon stock along eight land-use types in the highlands of Ethiopia. The general objective of the study was to investigate the dynamics of SOC stock following chrono-sequence land-use/cover systems in the highlands of Ethiopia. The specific objectives were to: (1) analyze loss due to land degradation; (2) analyze gain due to land restoration; and (3) estimate partial balance of SOC stock for the highlands of Ethiopia. The study followed the principle of the Forest Transition Theory (FTT). Eleven sub-areas were considered from the highlands of Ethiopia. A total of 241 auger composite samples from the topsoil (0−20 cm depth) were collected during December 2017 to June 2018, and analyzed at CropNut soil lab in Nairobi. The study results revealed that there were statistically significant variations (P < 0.05) across the land-use types with the mean stocks ranging from 31.4 Mg SOC ha−1 in soils of intensively grazed lands to 145.0 Mg SOC ha−1 in soils of guasa grasslands. Soils of natural/pristine vegetation and protected guasa grasslands contain the highest amount of SOC stock. Therefore, there should be more aggressive efforts towards an effective protection of these ecosystems. Soils under intensively used croplands and intensively grazed lands lost, respectively, 64.95% and 78.16%, SOC stocks originally accumulated in the top surface layers of the pristine forests. This points for the need to adopt locally feasible land management practices that lead to increased SOC stock and simultaneously reduced CO2 and greenhouse gas emissions from croplands and intensively grazed lands of the highlands of Ethiopia. Compared to stocks of SOC of intensively grazed lands (31.44 Mg SOC ha−1 ), the annual stock gains in soils of controlled grazing lands (4.60 Mg ha−1 ) were > gains in soils of enclosures (3.17 Mg ha−1 ) > gains in soils of afforestation (2.35 Mg SOC ha−1 ), which signifies that converting degraded lands to either controlled grazing lands, enclosures, or afforestation would be a promising practice for an enhanced carbon sequestration across the highlands of Ethiopia. This practice is in line with the United Nations’ Sustainable Development Goals. The estimated regional partial stock balance revealed that the loss and gain ratio was 35.1 in 1991, and it declined to 15.4 in 2001, 2.2 in 2011 and 1.8 in 2015. These decreasing ratios indicate the possibility of closing the gap between the losses and the gains in the near future, and eventually shifting to higher rates of gains than losses. It is also important to note that determined efforts towards the effective protection of natural forests and the creation of enclosures and reforestation areas by local communities for enhanced carbon sequestration will benefit them from payments of carbon emission reduction (CER) credits

Soil carbon response to land-use change: evaluation of a global vegetation model using observational meta-analyses

Global model estimates of soil carbon changes from past land-use changes remain uncertain. We develop an approach for evaluating dynamic global vegetation models (DGVMs) against existing observational meta-analyses of soil carbon changes following land-use change. Using the DGVM JSBACH, we perform idealized simulations where the entire globe is covered by one vegetation type, which then undergoes a land-use change to another vegetation type. We select the grid cells that represent the climatic conditions of the meta-analyses and compare the mean simulated soil carbon changes to the meta-analyses. Our simulated results show model agreement with the observational data on the direction of changes in soil carbon for some land-use changes, although the model simulated a generally smaller magnitude of changes. The conversion of crop to forest resulted in soil carbon gain of 10% compared to a gain of 42% in the data, whereas the forest-to-crop change resulted in a simulated loss of 15% compared to 40 %. The model and the observational data disagreed for the conversion of crop to grasslands. The model estimated a small soil carbon loss (4 %), while observational data indicate a 38% gain in soil carbon for the same land-use change. These model deviations from the observations are substantially reduced by explicitly accounting for crop harvesting and ignoring burning in grasslands in the model. We conclude that our idealized simulation approach provides an appropriate framework for evaluating DGVMs against meta-analyses and that this evaluation helps to identify the causes of deviation of simulated soil carbon changes from the meta-analyses

Simulating soil organic carbon in maize-based systems under improved agronomic management in Western Kenya

Improved management practices should be implemented in croplands in sub-Saharan Africa to enhance soil organic carbon (SOC) storage and/or reduce losses associated with land-use change, thereby addressing the challenge of ongoing soil degradation. DayCent, a process-based biogeochemical model, provides a useful tool for evaluating which management practices are most effective for SOC sequestration. Here, we used the DayCent model to simulate SOC using experimental data from two long-term field sites in western Kenya comprising of two widely promoted sustainable agricultural management practices: integrated nutrient management (i.e. mineral fertilizer and crop residues/farmyard manure incorporation) and conservation agriculture (i.e. minimum tillage and crop residue retention). At both sites, correlations between measured and simulated SOC were low to moderate (R2 of 0.25−0.55), and in most cases, the model produced fairly accurate prediction of the SOC trends with a low relative root mean squared error (RRMSE < 7%). Consistent with field measurements, simulated SOC declined under all improved management practices. The model projected annual SOC loss rates of between 0.32 to 0.35 Mg C ha-1 yr-1 in continuously tilled maize (Zea mays) systems without fertilizer or organic matter application over the period 2003–2050. The most effective practices in reducing the losses were the combined application of 4 Mg ha-1 of farmyard manure and 2 Mg ha-1 of maize residue retention (reducing losses up to 0.22 Mg C ha-1 yr-1), minimum tillage in combination with maize residue retention (0.21 Mg C ha-1 yr-1), and rotation of maize with soybean (Glycine max) under minimum tillage (0.17 Mg C ha-1 yr-1). Model results suggest that response of the passive SOC pool to the different management practices is a key driver of the long-term SOC trends at the two study sites. This study demonstrates the strength of the DayCent model in simulating SOC in maize systems under different agronomic management practices that are typical for western Kenya

Tapping into the environmental co-benefits of improved tropical forages for an agroecological transformation of livestock production systems

Livestock are critical for incomes, livelihoods, nutrition and ecosystems management throughout the global South. Livestock production and the consumption of livestock-based foods such as meat, cheese, and milk is, however, under global scrutiny for its contribution to global warming, deforestation, biodiversity loss, water use, pollution, and land/soil degradation. This paper argues that, although the environmental footprint of livestock production presents a real threat to planetary sustainability, also in the global south, this is highly contextual. Under certain context-specific management regimes livestock can deliver multiple benefits for people and planet. We provide evidence that a move toward sustainable intensification of livestock production is possible and could mitigate negative environmental impacts and even provide critical ecosystem services, such as improved soil health, carbon sequestration, and enhanced biodiversity on farms. The use of cultivated forages, many improved through selection or breeding and including grasses, legumes and trees, in integrated crop-tree-livestock systems is proposed as a stepping stone toward agroecological transformation. We introduce cultivated forages, explain their multi-functionality and provide an overview of where and to what extent the forages have been applied and how this has benefited people and the planet alike. We then examine their potential to contribute to the 13 principles of agroecology and find that integrating cultivated forages in mixed crop-tree-livestock systems follows a wide range of agroecological principles and increases the sustainability of livestock production across the globe. More research is, however, needed at the food system scale to fully understand the role of forages in the sociological and process aspects of agroecology. We make the case for further genetic improvement of cultivated forages and strong multi-disciplinary systems research to strengthen our understanding of the multidimensional impacts of forages and for managing agro-environmental trade-offs. We finish with a call for action, for the agroecological and livestock research and development communities to improve communication and join hands for a sustainable agri-food system transformation

Exploring new opportunities for improving AFOLU emissions reporting and the current status of AFOLU emissions for non-annex I countries

This policy brief has two main objectives. In a first instance, it aims to give an overview of how non-Annex I countries are currently communicating on the necessary elements (targets, monitoring plans, accounting mechanisms, implementation and achievements) for tracking their Nationally Determined Contribution (NDC) progress. Non-Annex I countries are to be understood as developing countries under the Kyoto Protocol. Contrary to Annex I countries, which are understood as developed countries and countries undergoing the process of transition to a market economy, non-Annex I countries do not have legally binding emissions reductions targets. With the existing transparency arrangements, Annex-I parties have demonstrated their ability in meeting the delivery of required information. On the other hand, non-Annex I parties have demonstrated limited experience in reporting mitigation action, which challenges not only tracking progress toward national targets but also towards global goals (Vaidyula & Rocha, 2018). In a second instance, this policy brief also aims to summarize the current status of CO2 emissions from the Agriculture, Forestry, and Other Land Uses (AFOLU) sector in regions covering non-Annex I countries. Given that the AFOLU sector is both a source and a sink of CO2 emissions, land-based mitigations are quickly being recognized as key measures to achieve the Paris Agreement’s (PA) goal of reducing global emissions. However, findings suggest that AFOLU activities have considerable variations among different data sources, with no clear patterns across considered regions, with some sources indicating an increase in some places and others indicating a decline over the period 2010-2015 (Nyawira et al 2022). In order to support realistic mitigation targets set under the PA, an in-depth analysis of existing AFOLU datasets across non-Annex I regions is necessary in order to understand these inconsistencies in sources and sinks of CO2

BNF in grain legumes in LPJ-GUESS

This file contains the input data for model runs on global scale and the measured results for BNF evaluation at site scales presented in Geoscientific Model Development (GMD) pape

Modeling symbiotic biological nitrogen fixation in grain legumes globally with LPJ-GUESS (v4.0, r10285)

Biological nitrogen fixation (BNF) from grain legumes is of significant importance in global agricultural ecosystems. Crops with BNF capability are expected to support the need to increase food production while reducing nitrogen (N) fertilizer input for agricultural sustainability, but quantification of N fixing rates and BNF crop yields remains inadequate on a global scale. Here we incorporate two legume crops (soybean and faba bean) with BNF into a dynamic vegetation model LPJ-GUESS (Lund-Potsdam-Jena General Ecosystem Simulator). The performance of this new implementation is evaluated against observations from a range of water and N management trials. LPJ-GUESS generally captures the observed response to these management practices for legume biomass production, soil N uptake, and N fixation, despite some deviations from observations in some cases. Globally, simulated BNF is dominated by soil moisture and temperature, as well as N fertilizer addition. Annual inputs through BNF are modeled to be 11.6±2.2ĝ€¯Tgĝ€¯N for soybean and 5.6±1.0ĝ€¯Tgĝ€¯N for all pulses, with a total fixation of 17.2±2.9ĝ€¯Tgĝ€¯Nĝ€¯yr-1 for all grain legumes during the period 1981-2016 on a global scale. Our estimates show good agreement with some previous statistical estimates but are relatively high compared to some estimates for pulses. This study highlights the importance of accounting for legume N fixation process when modeling C-N interactions in agricultural ecosystems, particularly when it comes to accounting for the combined effects of climate and land-use change on the global terrestrial N cycle

Assessing the impacts of agricultural managements on soil carbon stocks, nitrogen loss, and crop production - a modelling study in eastern Africa

Improved agricultural management plays a vital role in protecting soils from degradation in eastern Africa. Changing practices such as reducing tillage, fertilizer use, or cover crops are expected to enhance soil organic carbon (SOC) storage, with climate change mitigation co-benefits, while increasing crop production. However, the quantification of cropland management effects on agricultural ecosystems remains inadequate in this region. Here, we explored seven management practices and their potential effects on soil carbon (C) pools, nitrogen (N) losses, and crop yields under different climate scenarios, using the dynamic vegetation model LPJ-GUESS. The model performance is evaluated against observations from two long-term maize field trials in western Kenya and reported estimates from published sources. LPJ-GUESS generally produces soil C stocks and maize productivity comparable with measurements and mostly captures the SOC decline under some management practices that is observed in the field experiments. We found that for large parts of Kenya and Ethiopia, an integrated conservation agriculture practice (no-tillage, residue and manure application, and cover crops) increases SOC levels in the long term (+11g% on average), accompanied by increased crop yields (+22g%) in comparison to the conventional management. Planting nitrogen-fixing cover crops in our simulations is also identified as a promising individual practice in eastern Africa to increase soil C storage (+4g%) and crop production (+18g%), with low environmental cost of N losses (+24g%). These management impacts are also sustained in simulations of three future climate pathways. This study highlights the possibilities of conservation agriculture when targeting long-term environmental sustainability and food security in crop ecosystems, particularly for those with poor soil conditions in tropical climates