Abstract
This study provides estimates of the net GHG mitigation potential of a selected range of management practices in the world’s native and cultivated grazing lands. The Century and Daycent models are used to calculate the changes in soil carbon stocks, soil N2O emissions, and forage removals by ruminants associated with these practices. GLEAM is used in combination with these models to establish grazing area boundaries and to parameterize links between forage consumption, animal production and animal GHG emissions. This study provides an alternative to the usual approach of extrapolating from a small number of field studies and by modeling the linkage between soil, forage and animals it sheds new light on the net mitigation potential of C sequestration practices in the world’s grazing lands. Three different mitigation practices are assessed in this study, namely, improved grazing management, legume sowing and N fertilization. We estimate that optimization of grazing pressure could sequester 148 Tg CO2 yr-1. The soil C sequestration potential of 203 Tg CO2 yr¯¹ for legume sowing was higher than for improved grazing management, despite being applied over a much smaller total area. However, N2O emissions from legumes were estimated to offset 28% of its global C sequestration benefits, in CO2 equivalent terms. Conversely, N2O emissions from N fertilization exceeded soil C sequestration, in all regions. Our estimated potential for increasing C stocks though in grazing lands is lower than earlier worldwide estimates (Smith et al., 2007; Lal, 2004), mainly due to the much smaller grazing land area over which we estimate mitigation practices to be effective. A big concern is the high risk of the practices, particularly legumes, increasing soil-based GHGs if applied outside of this relatively small effective area. More work is needed to develop indicators, based on biophysical and management characteristics of grazing lands, to identify amenable areas before these practices can be considered ready for large scale implementation. The additional ruminant GHG emissions associated with higher forage output are likely to substantially reduce the mitigation potential of these practices, but could contribute to more GHG-efficient livestock production. ©2015 Elsevier B.V. All rights reserved.
Generated Summary
This study investigates the potential of greenhouse gas (GHG) mitigation practices in grazing lands globally, focusing on the modeling of soil carbon and nitrogen fluxes. Using the Century and Daycent models, the research assesses the impact of improved grazing management, legume sowing, and nitrogen fertilization on soil carbon stocks, N2O emissions, and forage production. The GLEAM model is integrated to determine grazing area boundaries and to parameterize the links between forage consumption, animal production, and GHG emissions. The study seeks to offer an alternative to traditional methods by modeling the interactions between soil, forage, and animals, providing new insights into the net mitigation potential of carbon sequestration practices. The research is based on a spatial resolution of 0.5 degrees and the findings offer insights into the effectiveness of different mitigation strategies across various regions and grazing land types.
Key Findings & Statistics
- The study estimates that optimization of grazing pressure could sequester 148 Tg CO2 yr-1.
- The soil C sequestration potential of 203 Tg CO2 yr¯¹ for legume sowing was higher than for improved grazing management.
- N2O emissions from legumes were estimated to offset 28% of its global C sequestration benefits, in CO2 equivalent terms.
- Conversely, N2O emissions from N fertilization exceeded soil C sequestration, in all regions.
- Our estimated potential for increasing C stocks though in grazing lands is lower than earlier worldwide estimates (Smith et al., 2007; Lal, 2004).
- The total grazing land area following this procedure was approximately 2.6 billion ha.
- Of the total 2.6 billion ha of grazing land over which the Century simulations were carried out, this practice was only found to be effective (i.e., changes in C stocks were estimated to be positive) in 28% of this area.
- Most of the C sequestration potential (~74%) was in rangelands.
- The regions with the largest sequestration potentials were Central/South America (26.7 Tg CO2 yr¯¹), Sub-Saharan Africa (24.3 Tg CO2 yr¯¹), Oceania (15.6 Tg CO2 yr¯¹), and East/Southeast Asia (13.7 Tg CO2 yr¯¹), collectively accounting for 73% of the total potential and 65% of the amenable rangeland area.
- We estimate that adjustments in grazing pressure, from current forage offtake rates to rates that maximize forage production, can sequester 148.4 Tg CO2 yr¯¹ in grazing lands worldwide.
- In the pastureland areas where net mitigation was positive, legume sowing increased N2O emissions by 122 Gg N₂O-Nyr-1 (56.9 Tg CO2-eqyr¯¹) offsetting 28% of the global GHG mitigation benefits of C sequestration, resulting in the global net mitigation of 146.5 Tg CO2 yr-1
- In the pastureland areas where net mitigation was positive, legume sowing increased N2O emissions by an average rate of 1.7 kg N2O—Nha¯¹yr¯¹, with substantial variation between regions ranging from 0.01 to 9.1 kg N2O-Nha¯¹ yr¯¹.
- As with improved grazing management, the average global rate of C sequestration (in the areas where net mitigation occurs) is highest in the humid pasturelands.
- In aggregate, our results show that sowing legumes would increase annual forage production on pasturelands by about 31.8 Tg DM from 71.8 million ha.
- The net increases in forage production from the practices assessed in this study involve tradeoffs with higher ruminant GHG emissions, because the consumption of this additional forage by grazing ruminants would require higher numbers of ruminants.
- For legumes, on the other hand, increases in forage consumption and associated ruminant emissions are estimated to only offset 26% of the net soil C sequestration potential of this practice at the global level.
- The study estimates the global potential for C sequestration in the soils of the world’s grazing lands is 352 Tg CO2 yr¯¹ through improved grazing management in rangelands and pasturelands, and the sowing of legumes in pasturelands.
- When subtracting the increase in soil N2O emissions associated with legume sowing, the global net sequestration potential of the assessed practices falls to 295 Tg CO2 yr¯¹.
Other Important Findings
- The study found that the amount of C that could be stored in grazing lands is considerable and presents a potentially large mitigation opportunity.
- Improved grazing management was applied to all grazing lands (i.e., native rangelands and pasturelands), but legume planting and fertilization were only considered to be feasible in pasturelands which are more amenable to agronomic inputs, because of their agroecological conditions (e.g., soil moisture availability).
- The study finds that there is a high risk of the practices, particularly legumes, increasing soil-based GHGs if applied outside of this relatively small effective area.
- The study notes that more work is needed to develop indicators, based on biophysical and management characteristics of grazing lands, to identify amenable areas before these practices can be considered ready for large scale implementation.
- The additional ruminant GHG emissions associated with higher forage output are likely to substantially reduce the mitigation potential of these practices, but could contribute to more GHG-efficient livestock production.
- The regions with the largest sequestration potentials were Central/South America, Sub-Saharan Africa, Oceania, and East/Southeast Asia, which collectively accounted for 73% of the total potential and 65% of the amenable rangeland area.
- For improved grazing management, the sequestration rate in the humid grazing areas was the highest, although for pasturelands temperate areas had an equally high global average rate.
- The results suggest the processes that led to soil C losses exceeded those that led to C gains in most cases.
- When normalized for N input rate, N2O emissions increases were greater at higher fertilization rates in all regions.
- Increases in forage consumption were correlated with increases in fertilization rates, and in the two regions where soil C stocks increased, fertilization increased forage production by an average of 0.2 Mg biomass ha¯¹ yr¯¹, summing to a total increase of 3.7 Tg biomass yr¯¹.
- The net increases in forage production from the practices assessed involve tradeoffs with higher ruminant GHG emissions.
- The implied growth in the animal numbers for each practice presents a useful development opportunity for extensive grazing systems, and has to be placed in the context of expected demand growth for animal products.
- The study showed that, improved grazing management resulted in a 10% reduction in the emission intensity of ruminant production in rangelands.
- For pasturelands, there is a 10% increase in emission intensity, despite reductions in each region, because production increases by much more in emission intensive regions (e.g., Sub-Saharan Africa and Central & South America) relative to other regions in this scenario.
- For legume sowing, a relatively smaller increase in forage production and animal numbers, combined much larger per hectare rates of C sequestration and slight improvements in animal productivity, resulted in a substantial 59% reduction in emission intensity.
Limitations Noted in the Document
- The study acknowledges the high risk of increasing soil-based GHGs when mitigation practices, especially legume sowing, are applied outside of a relatively small, effective area.
- The need for further research is emphasized to develop indicators based on biophysical and management characteristics to identify suitable grazing areas before large-scale implementation.
- The study’s reliance on simplified baseline assumptions about management practices and the lack of extensive global data on grazing land management practices are noted as limitations.
- The study’s findings are based on the Century and Daycent models, which, while representing multiple interactions between biophysical processes and management at a landscape scale, rely on emission factors generated from the synthesis or meta-analysis of published studies.
- The study’s assessment is confined to areas where livestock production is present and practice changes are likely to be effective, not assuming the blanket application of management practices across all or most of the world’s grazing lands, but the study notes that not all management changes are appropriate or possible for all grazing lands.
- The study’s estimates of mitigation potential are constructed using information about C storage and N2O emission rates given a change in land management and information about where land management changes are feasible, thus, not all management changes are appropriate or possible for all grazing lands, as their applicability and effectiveness depend on a range of factors such as accessibility, soil conditions, climate, and current and past management.
Conclusion
The study provides a comprehensive assessment of GHG mitigation potential in the world’s grazing lands. It underlines the significant opportunity for carbon sequestration through improved grazing management and legume sowing, while also highlighting the complexities and challenges associated with these practices. The findings reveal that the benefits of grazing management are substantial, with the potential to sequester large amounts of carbon. However, the study emphasizes the risks associated with the practices, particularly legume sowing, which can lead to increased soil-based GHG emissions if not implemented strategically. The analysis demonstrates the importance of targeted application of these practices, with the identification of amenable areas crucial for maximizing their effectiveness. The study underscores that the expansion of livestock production can generate substantial benefits for pastoral communities, while also creating a tradeoff with higher ruminant GHG emissions, thus requiring a balance. The study emphasizes the need for more detailed information to develop indicators for identifying grazing areas with high mitigation potential, and underlines the importance of further research, on-ground research, and piloting. The conclusions highlight the complex relationship between land management, carbon sequestration, and GHG emissions, calling for a strategic approach that considers both environmental and economic benefits. The authors suggest that the growth in less emission-intensive animal production associated with the assessed practices can provide important development opportunities for pastoralists, and has to be placed in the context of expected demand growth for animal products. The study calls for integrated approaches that prioritize sustainable management and emphasize the importance of ongoing research and policy interventions to enhance carbon sequestration in grazing lands.