# Ecology, Climate Change and Related News

1. ## Nature-based solutions can prevent $50 billion in Gulf Coast flood damages– Restoration of marshes and oyster reefs are among the most cost-effective solutions Leave a Comment • Wetland and reef restoration can yield benefit-to-cost ratios greater than seven to one, meaning more than$7 in direct flood-reduction benefits for every $1 spent on restoration. • Future flood risks from coastal hazards will grow, and that the major driver of risk in the Gulf is coastal development, particularly for the most extreme and costly events: the more people and property exposed to coastal hazards, the greater the flooding risk. Climate change, however, will result in more frequent losses. Events causing$100 billion in damages may become approximately three times more frequent in the future, the study found.

April 12, 2018 University of California – Santa Cruz Read full ScienceDaily article here

While coastal development and climate change are increasing the risk of flooding for communities along the US Gulf Coast, restoration of marshes and oyster reefs are among the most cost-effective solutions for reducing those risks, according to a new study.

…the study compares the cost effectiveness of nature-based and artificial solutions for flood reduction across the Gulf of Mexico. The results clearly demonstrate the value of nature-based solutions such as marsh and oyster-reef restoration. Overall, wetland and reef restoration can yield benefit-to-cost ratios greater than seven to one, meaning more than $7 in direct flood-reduction benefits for every$1 spent on restoration. Many artificial solutions (such as levees and home elevation) have benefit-to-cost ratios near or below one-to-one; their benefits can be high, but they are expensive to implement at scale.

The study was led by researchers at UC Santa Cruz, the Nature Conservancy, and the Swiss Federal Institute of Technology at ETH Zurich. It applied the Economics of Climate Adaptation (ECA) approach, which was developed by reinsurance company Swiss Re and partners to understand what drives coastal risk and to evaluate the cost-effectiveness of adaptation options…..

…The new study quantified the flood risks to people and property for the entire U.S. coast of the Gulf of Mexico under current and future climate scenarios and economic growth projections. It showed that future flood risks from coastal hazards will grow, and that the major driver of risk in the Gulf is coastal development, particularly for the most extreme and costly events: the more people and property exposed to coastal hazards, the greater the flooding risk. Climate change, however, will result in more frequent losses. Events causing \$100 billion in damages may become approximately three times more frequent in the future, the study found.

….”We show that nature-based measures for flood reduction can be considered right alongside artificial or gray measures such as seawalls in industry-based benefit-cost models. This removes a major impediment for engineers, insurers, and risk management agencies for building coastal resilience more naturally,” said project team leader Michael Beck, lead marine scientist at the Nature Conservancy and a research professor at UC Santa Cruz.

…The team developed open-source software based in part on Swiss Re’s natural catastrophe model to assess flood risks and adaptation solutions. All of the results and maps showing the cost effectiveness of adaptation solutions under future climate change and development scenarios are available in an interactive mapper available online at CoastalResilience.org.

Borja G. Reguero, Michael W. Beck, David N. Bresch, Juliano Calil, Imen Meliane. Comparing the cost effectiveness of nature-based and coastal adaptation: A case study from the Gulf Coast of the United States. PLOS ONE, 2018; 13 (4): e0192132 DOI: 10.1371/journal.pone.0192132

2. ## Wetlands provide landscape-scale reduction in nitrate pollution

• This research shows that wetland restoration could be one of the most effective methods for comprehensive improvement of water quality in the face of climate change and growing global demand for food.

January 29, 2018 University of Minnesota College of Science and Engineering read full ScienceDaily article here

A new study provides new insights to demonstrate that multiple wetlands or ‘wetland complexes’ within a watershed are extremely effective at reducing harmful nitrate in rivers and streams. These wetlands can be up to five times more efficient per unit area at reducing nitrate than the best land-based nitrogen mitigation strategies

…Significant research findings include:

• When stream flows are high, wetlands are five times more efficient per unit area at reducing nitrate than the best land-based conservation practices. Other common conservation practices are effective at lower flow conditions but overwhelmed with higher stream flows.
• The arrangement of wetlands within a watershed is a key predictor of the magnitude of nitrate reduction. If wetlands intercept 100 percent of the drainage area, they are three times more effective at nitrate removal compared to interception of 50 percent of the drainage area.
• Nitrate reduction due to ephemeral (temporary) wetlands, such as riparian floodplains and more geographically isolated wetlands (wetlands not connected to the river network by surface water), was measurable and was highest during high stream flows, when such features are hydrologically connected to surface water.

….Our work shows that wetland restoration could be one of the most effective methods for comprehensive improvement of water quality in the face of climate change and growing global demand for food,” said study co-author Jacques Finlay, a professor in the University of Minnesota’s Department of Ecology, Evolution and Behavior in the College of Biological Sciences….

Amy T. Hansen, Christine L. Dolph, Efi Foufoula-Georgiou, Jacques C. Finlay. Contribution of wetlands to nitrate removal at the watershed scale. Nature Geoscience, 2018; DOI: 10.1038/s41561-017-0056-6

3. ## New estimate of how much humans have transformed the planet; habitat restoration of degraded lands is key to sequestering carbon and reversing climate change

• Current human use of land is responsible for ~halving the potential storage of carbon by that land.
• Through large-scale grazing and other uses of grasslands, as well as forest “management,” humans have subtracted from Earth’s potential carbon sequestration in vegetation an amount equal to deforestation.
• Earth’s vegetation currently stores around 450 petagrams of carbon [450 billion tons (or gigatons Gt) of carbon or 1665 Gt of CO2e] and in a hypothetical without land use changes, potential vegetation would store around 916 petagrams of carbon, under current climate conditions.
• Avoiding deforestation is necessary but not enough to reverse climate change.
• Scenarios that limit global warming to 1.5 or 2 degrees [Celsius] require not only rapid cessation of greenhouse gas emissions but also removal of somewhere between about 100 and 300 billion tons of carbon [or 370 to 1110 billion tons (Gt) of CO2e] from the atmosphere; restoring vegetation is key contribution to controlling climate change
by Chris Moony Dec 20 2017  see full Washington Post article

In this age of climate change, we naturally train our attention on all the fossil fuels being combusted for human use — but scientists have long known that what’s happening is also all about the land.

Just as buried fossil fuels are filled with carbon from ancient plant and animal life, so too are living trees and vegetation on Earth’s surface today. Razing forests or plowing grasslands puts carbon in the atmosphere just like burning fossil fuels does.

Now, new research provides a surprisingly large estimate of just how consequential our treatment of land surfaces and vegetation has been for the planet and its atmosphere. If true, it’s a finding that could shape not only our response to climate change, but our understanding of ourselves as agents of planetary transformation….

….Using a series of detailed maps derived from satellite information and other types of ecological measurements, Erb and his colleagues estimated how much carbon is contained in Earth’s current vegetation. The number is massive: 450 billion tons of carbon, which, if it were to somehow arrive in the atmosphere as carbon dioxide, would amount to over a trillion tons of the gas. (The mass is greater due to the addition of oxygen molecules.)

But the study also presented an even larger and perhaps more consequential number: 916 billion tons. That’s the amount of carbon, the research calculated, that could reside in the world’s vegetation — so not in the atmosphere — if humans somehow entirely ceased all uses of land and allowed it to return to its natural state. The inference is that current human use of land is responsible for roughly halving the potential storage of carbon by that land….

…the impact calculation is so large because humans have done far more than just bring about deforestation, which Erb said accounts for about half of the loss of potential vegetation. … “But the other half, in most studies, is completely missing.”…

…The study found that there are two far-less-recognized components of how humans have subtracted from Earth’s potential vegetation — and that in combination they are just as substantial as deforestation. Those are large-scale grazing and other uses of grasslands, as well as forest “management.” With the latter, many trees and other types of vegetation are subtracted from forests — often the larger and older trees due to logging — but the forests as a whole don’t disappear. They’re just highly thinned out.

“This effect is quite massive, more massive than we expected actually,” Erb said….

….The research means that so-called degraded land — not fully deforested but not “natural” or whole, either — is a phenomenon to be reckoned with.

“It suggests that the amount of carbon released to the atmosphere from land use is approximately equal to the amount still retained,” said Tom Lovejoy, an ecologist at George Mason University who was not involved in the work. “That means the restoration agenda is even more important than previously thought and highlights the enormous amount of degraded land in the world.”…

….“Scenarios that limit global warming to 1.5 or 2 degrees [Celsius] require not only rapid cessation of greenhouse gas emissions but also removal of somewhere between about 100 and 300 billion tons of carbon [or 370 to 1110 billion tons (Gt) of CO2e] from the atmosphere,” Phil Duffy, president of the Woods Hole Research Center, said in an email.

This paper suggests that restoring vegetation around the world could in principle achieve that,” Duffy continued, noting that if all the potential vegetation were restored it would offset some 50 years of global carbon emissions. While “the full theoretical potential will never be realized in practice … this paper indicates that restoring vegetation could make an extremely important contribution to controlling global climate change.”

Karl-Heinz Erb et al. Unexpectedly large impact of forest management and grazing on global vegetation biomass. Nature  Dec 2017 doi:10.1038/nature25138

Abstract: Carbon stocks in vegetation have a key role in the climate system. However, the magnitude, patterns and uncertainties of carbon stocks and the effect of land use on the stocks remain poorly quantified. Here we show, using state-of-the-art datasets, that vegetation currently stores around 450 petagrams of carbon. In the hypothetical absence of land use, potential vegetation would store around 916 petagrams of
carbon, under current climate conditions. This difference highlights the massive effect of land use on biomass stocks. Deforestation and other land-cover changes are responsible for 53–58% of the difference between current and potential biomass stocks. Land management effects (the biomass stock changes induced by land use within the same land cover) contribute 42–47%, but have been underestimated in the literature. Therefore, avoiding deforestation is necessary but not sufficient for mitigation of climate change. Our results imply that trade-offs exist between conserving carbon stocks on managed land and raising the contribution of biomass to raw material and energy supply for the mitigation of climate change. Efforts to raise biomass stocks are currently verifiable only in temperate forests, where their potential is limited. By contrast, large uncertainties hinder verification in the tropical forest, where the largest potential is located, pointing to challenges for the upcoming stocktaking exercises under the Paris agreement.
4. ## Urban habitat restoration provides a human health benefit through microbiome rewilding: the Microbiome Rewilding Hypothesis

• We propose the Microbiome Rewilding Hypothesis, which specifically outlines that restoring biodiverse habitats in urban green spaces can rewild the environmental microbiome to a state that enhances primary prevention of human disease…

October 2017 Restoration Ecology

Abstract

Restoration aims to return ecosystem services, including the human health benefits of exposure to green space. The loss of such exposure with urbanization and industrialization has arguably contributed to an increase in human immune dysregulation. The Biodiversity and Old Friends hypotheses have described the possible mechanisms of this relationship, and suggest that reduced exposure to diverse, beneficial microorganisms can result in negative health consequences. However, it is unclear whether restoration of biodiverse habitat can reverse this effect, and what role the environmental microbiome might have in such recovery. Here, we propose the Microbiome Rewilding Hypothesis, which specifically outlines that restoring biodiverse habitats in urban green spaces can rewild the environmental microbiome to a state that enhances primary prevention of human disease. We support our hypothesis with examples from allied fields, including a case study of active restoration that reversed the degradation of the soil bacterial microbiome of a former pasture. This case study used high-throughput amplicon sequencing of environmental DNA to assess the quality of a restoration intervention in restoring the soil bacterial microbiome. The method is rapid, scalable, and standardizable, and has great potential as a monitoring tool to assess functional outcomes of green-space restoration. Evidence for the Microbiome Rewilding Hypothesis will help motivate health professionals, urban planners, and restoration practitioners to collaborate and achieve co-benefits. Co-benefits include improved human health outcomes and investment opportunities for biodiversity conservation and restoration.

Mills, J. G., Weinstein, P., Gellie, N. J. C., Weyrich, L. S., Lowe, A. J. and Breed, M. F. (2017), Urban habitat restoration provides a human health benefit through microbiome rewilding: the Microbiome Rewilding Hypothesis. Restor Ecol, 25: 866–872. doi:10.1111/rec.12610

5. ## How to Build a City That Doesn’t Flood? Turn it Into a Sponge

• There’s a global movement to build smarter and “spongier” cities that can absorb rainwater instead of letting it flow through miles of pavement and cause damaging floods.

From Iowa to Vermont and from San Francisco to Chicago, urban infrastructure is getting a reboot.

….Creating better stormwater management systems requires using green infrastructure elements in urban planning and restoring some of the rain-retention capacity that cities have lost to urbanization. These elements can be roughly broken into two categories: the man-made engineered replacements of the natural water pathways and the restorations of the original water routes that existed before a city was developed.

….Traditional road construction, made with asphalt, gravel and sand, is a very compacted structure that leaves little space between the particulates, and thus no room for the rainwater to seep through. In the construction industry that gap measure is described by the term “air void,” which is typically set at four percent for the traditional pavement mix, says Richard Willis, Director of Pavement Engineering and Innovation at National Asphalt Pavement Association.

One way to make cities spongier is to use permeable pavements, such as porous asphalt made with a lot of large stones rather than fine aggregates such as sand, and with added cellulose fibers to hold the porous asphalt together. This creates more pores, and increases the air void up to 15 or 20 percent, allowing more rainwater to seep through.

….Another way to make cities hold water is by building rain gardens and bioswales. A rain garden is a depression in the soil seeded with native plants that helps soak up rainwater. With that setup, house spouts can empty into a rain garden instead of a sewer, decreasing sewage overflows in heavy downpours. A bioswale is a rain garden on a larger, more engineered scale. It is constructed by creating deeper and larger depressions where water can temporarily accumulate and drain out slowly.

….Green infrastructure for sponge cities can also include non-engineered solutions—such as restoring urban forests and increasing their ability to absorb stormwater runoff. In Seattle, urban planners got rid of invasive species such as English Ivy and Himalayan blackberries and restored native evergreens that do a better job of stormwater retention.

….For countries in the developing world, which are on the frontlines of climate change, the problem is more urgent and monetary resources are a problem. In these countries, solutions that follow the Seattle model are increasingly being embraced, says Sarah Colenbrander, Senior Researcher at the London-based International Institute for Environment and Development. From Kampala, Uganda to Bangalore, India urban wetlands and woodlands are being restored in many cities. The biggest stumbling block, according to Postel, is scalability: can one-off examples work on a larger country-wide scale? That can only happen with a significant boost from policy implementation and top-down legislation, she says.
Studies found that local building codes often create needless impervious cover while giving developers little or no incentive to conserve the natural areas that are so important for the natural water flow. The world needs to rethink its cultural expectations of what a prosperous and successful city looks like, Colenbrander says: “Is it a city like Sydney or Los Angeles where everyone has a white picket fence and a nice garden? Or is it a city more like Hong Kong or even central London where people live much more densely and have a communal green space together so you have less of an ecological footprint?”….

A diagram of a water retention system. Credit: Pittsburgh Water and Sewer Authority

6. ## Riparian restoration’s leaf litter can reduce nitrate pollution from fertilizers

• Riparian restoration in agricultural landscapes can result in leaf litter that enhances microbial activity and reduction of polluting nitrates from fertilizers- and downstream impacts of the nitrates through eutrophication (major increases in algal growth that create dead zones without oxygen).
• Leaf inputs associated with increased riparian cover had the potential to double the catchment level rate of denitrification, offering a promising way to mitigate nitrate pollution in agricultural streams
• For the riparian plants to be effective in adding sufficient organic matter, the number of plants and species (e.g., leaf traits, quality) and ability of stream to retain organic matter would need to be addressed in riparian management plans.
• Riparian restoration has the greatest potential to remove nitrogen in comparison with hyporheic restoration or floodplain reconnection (Lammers and Bledsoe 2017).
• Riparian restoration is not a silver bullet and will only address some of the nitrogen problems, and a targeted approach to increasing denitrification needs to be combined with other land-based nutrient management practices, including reductions in fertilizer application (Newcomer Johnson et al. 2016).

O’Brien J et al. Leaf litter additions enhance stream metabolism, denitrification, and restoration prospects for agricultural catchments. Ecosphere Full publication history DOI: 10.1002/ecs2.201

Abstract: Globally intensive agriculture has both increased nitrogen pollution in adjacent waterways and decreased availability of terrestrially derived carbon frequently used by stream heterotrophs in nitrogen cycling. We tested the potential for carbon additions via leaf litter from riparian restoration plantings to act as a tool for enhancing denitrification in agricultural streams with relatively high concentrations of nitrate (1.3–8.1 mg/L) in Canterbury, New Zealand. Experimental additions of leaf packs (N = 200, mass = 350 g each) were carried out in 200-m reaches of three randomly selected treatment streams and compared to three control streams receiving no additional leaf carbon. Litter additions increased ecosystem respiration in treatment streams compared to control streams but did not affect gross primary production, indicating the carbon addition boosted heterotrophic activity, a useful gauge of the activities of microbes involved in denitrification. Bench-top assays with denitrifying enzymes using acetylene inhibition techniques also suggested that the coarse particulate organic matter added from leaf packs would have provided substrates suitable for high rates of denitrification. Quantifying denitrification directly in experimental reaches by open-channel methods based on membrane inlet mass spectrophotometry indicated that denitrification was around three times higher in treatment streams where litter was added compared to control streams. We further assessed the potential for riparian plantings to reduce large-scale downstream nitrogen losses through increasing in-stream denitrification by modeling the effects of increasing riparian vegetation cover on nitrogen fluxes. Here, we combined estimates of in-stream ecosystem processes derived from our experiment with a network model of catchment-scale nitrogen retention and removal based on empirical measurements of nitrogen flux in this typical agricultural catchment. Our model indicated leaf inputs associated with increased riparian cover had the potential to double the catchment level rate of denitrification, offering a promising way to mitigate nitrate pollution in agricultural streams. Altogether, our study indicates that overcoming carbon limitation and boosting heterotrophic processes will be important for reducing nitrogen pollution in agricultural streams and that combining empirical approaches for predictions suggests there are large potential benefits from riparian re-vegetation efforts at catchment scales.

7. ## Include Biodiversity in Habitat Restoration Policy to Facilitate Ecosystem Recovery

• Need to bridge the ‘practice – science gap’ between practitioners and biodiversity research to optimize restoration projects

November 27, 2017  Northeastern University College of Science Read full ScienceDaily article here

As restoration projects throughout the country focus on restoring natural ecosystems, researchers are looking for ways to better bridge the ‘practice science gap’ between practitioners and biodiversity research in an effort optimize these types of projects.

… there are more than two decades of research that show if you increase biodiversity — the living organisms that occupy an ecosystem — important ecosystem functions begin to see positive improvements….

Dr. Susan Williams, of the Bodega Marine Laboratory at University of California, Davis. “Even if we know the community is more diverse, we instinctively reach for an efficient restoration solution by focusing on a single species or the one that has been impacted most. Our instincts are often at odds with our growing understanding of the benefits of biodiversity.”…

….”There is reason to believe that biodiversity may be able to enhance the success of restoration, but we need more data, and the only way we’ll get that data is if more partnerships are formed between biodiversity scientists and restoration practitioners. It might be a relatively simple way to enhance the success of restoration projects,” she said.

A. Randall Hughes, Jonathan H. Grabowski, Heather M. Leslie, Steven Scyphers, Susan L. Williams. Inclusion of Biodiversity in Habitat Restoration Policy to Facilitate Ecosystem Recovery. Conservation Letters, 2017; DOI: 10.1111/conl.12419

8. ## Removing nitrate with buffer zones for healthier ecosystems

• For agricultural nitrogen, slow it down, buff it out
• Understanding where natural nitrate removal is highest can inform management of streams in agricultural settings
• Nitrate removal in buffer zones was significantly higher than in stream sediments.

September 27, 2017 American Society of Agronomy read full ScienceDaily article here

In a new study, researchers have identified nitrate removal hotspots in landscapes around agricultural streams.

Nitrogen can present a dilemma for farmers and land managers. On one hand, it is an essential nutrient for crops. However, excess nitrogen in fertilizers can enter groundwater and pollute aquatic systems. This nitrogen, usually in the form of nitrate, can cause algal blooms. Microbes that decompose these algae can ultimately remove oxygen from water bodies, causing dead zones and fish kills.

In a new study, researchers have identified nitrate removal hotspots in landscapes around agricultural streams. “Understanding where nitrate removal is highest can inform management of agricultural streams,” says Molly Welsh, lead author of the study. “This information can help us improve water quality more effectively.”…

….Nitrate removal in buffer zones was significantly higher than in stream sediments. “If nitrate removal is the goal of stream restoration, it is vital that we conserve existing buffer zones and reconnect streams to buffer zones,” says Welsh….

Molly K. Welsh, Sara K. McMillan, Philippe G. Vidon. Denitrification along the Stream-Riparian Continuum in Restored and Unrestored Agricultural Streams. Journal of Environment Quality, 2017; 46 (5): 1010 DOI: 10.2134/jeq2017.01.0006

9. ## Maximizing successful forest restoration in tropical dry forests

• tests maximizing success of tree replanting efforts in degraded soils in tropics
• tree species that were drought tolerant did better
• soil amendments only helped to get seedlings off to good start

September 21, 2017 University of Minnesota read full ScienceDaily article here

A new study has uncovered some valuable information on ways to maximize the success of replanting efforts [in tropical dry forests], bringing new hope for restoring these threatened ecosystems.

…Over the past century most of these forests, which help keep water clean and provide valuable habitat for wildlife, were replaced by farms and cattle pastures. Now, as conservationists work to replant deforested areas, they’re finding that the already challenging, high-clay soils underlying them have been degraded to an extent that makes it hard for tree seedlings to sink their roots.

…To find out what works best for reestablishing tropical dry forests, the researchers planted seedlings of 32 native tree species in degraded soil or degraded soil amended with sand, rice hulls, rice hull ash or hydrogel (an artificial water-holding material). After two years, they found that tree species known for traits that make them drought tolerant, such as enhanced ability to use water and capture sunlight, survived better than other species. Some of the soil amendments helped get seedlings off to a good start, but by the end of the experiment there was no difference in survival with respect to soil condition

Leland K. Werden, Pedro Alvarado J., Sebastian Zarges, Erick Calderón M., Erik M. Schilling, Milena Gutiérrez L., Jennifer S. Powers. Using soil amendments and plant functional traits to select native tropical dry forest species for the restoration of degraded Vertisols. Journal of Applied Ecology, 2017; DOI: 10.1111/1365-2664.12998

10. ## ﻿Soil carbon debt from 12,000 years of human land use

• global carbon debt due to agriculture of 133 Pg C [133 Gt (billion metric tons of C) or 488 Gt CO2e] for the top 2 m of soil, with the rate of loss increasing dramatically in the past 200 years
• assuming soil organic carbon (SOM) reaches a new steady state in 20 y (35, 44), this calculation suggests that 8 Pg C to 28 Pg C [up to 28 Gt (billion metric tons of C) or 103 Gt CO2e] or can be recaptured
• there are identifiable regions which can be targeted for SOC (soil organic carbon) restoration efforts
• [Note: Hansen et al 2017 calls for 150 Pg C or ~550 Gt CO2e extraction from atmosphere globally with massive greenhouse emissions reductions of 6%/year starting in 2021 to return to 350 PPM CO2 in atmosphere and to secure a safe climate (Holocene) for human civilization by 2100]
• [Sanderman high end scenario would be 19% of CO2e extraction needed to secure safe climate by 2100 per Hansen above]

Jonathan Sanderman, Tomislav Hengl and Gregory J. Fiske. Soil carbon debt of 12,000 years of human land use. PNAS September 5, 2017 vol. 114 no. 36 9575-9580

Abstract: Human appropriation of land for agriculture has greatly altered the terrestrial carbon balance, creating a large but uncertain carbon debt in soils. Estimating the size and spatial distribution of soil organic carbon (SOC) loss due to land use and land cover change has been difficult but is a critical step in understanding whether SOC sequestration can be an effective climate mitigation strategy. In this study, a machine learning-based model was fitted using a global compilation of SOC data and the History Database of the Global Environment (HYDE) land use data in combination with climatic, landform and lithology covariates. Model results compared favorably with a global compilation of paired plot studies. Projection of this model onto a world without agriculture indicated a global carbon debt due to agriculture of 133 Pg C for the top 2 m of soil, with the rate of loss increasing dramatically in the past 200 years. The HYDE classes “grazing” and “cropland” contributed nearly equally to the loss of SOC. There were higher percent SOC losses on cropland but since more than twice as much land is grazed, slightly higher total losses were found from grazing land. Important spatial patterns of SOC loss were found: Hotspots of SOC loss coincided
with some major cropping regions as well as semiarid grazing regions, while other major agricultural zones showed small losses and even net gains in SOC. This analysis has demonstrated that there are identifiable regions which can be targeted for SOC restoration efforts.

Implications: This analysis indicates that the majority of the used portions of planet Earth have lost SOC, resulting in a cumulative loss of ∼133 Pg C due to agricultural land use. These SOC losses are on par with estimates of carbon lost from living vegetation primarily due to deforestation (40) and are nearly 100 Pg C higher than earlier estimates of land use and land use change-driven losses of SOC (41). Importantly, as Fig. 1 demonstrates, there are hotspots of SOC loss, associated with extensive cropping regions but also with highly degraded grazing land (SI Appendix, Fig. S9), suggesting that there are identifiable regions which should be targets for SOC restoration efforts.

The potential to recover lost SOC may be more limited than is often assumed. The amount of SOC that has been lost historically can be thought of as the carbon sink potential of the soil (42). Our analysis has found that this sink potential is ∼133 Pg C (SI Appendix, Table S3). A widely repeated figure is that, with adoption of best management practices, two thirds of lost SOC can be recovered (42). If the two-thirds figure is accurate, then SOC sequestration has the potential to offset 88 Pg C (322 Pg CO2) of emissions. However, bottom-up estimates of the maximum biophysical potential for carbon sequestration on cropping and grazing land range from 0.4 Pg Cy−1 to 1.4 Pg Cy−1 (20, 43). Assuming SOC reaches a new steady state in 20 y (35, 44), this calculation suggests that 8 Pg C to 28 Pg C can be recaptured. Even the range of 8 Pg C to 28 Pg C is likely overly ambitious given the various social, economic, and technical constraints on universal adoption of best management practices (45), suggesting that the amount of the carbon sink that can be filled is on the order of, at best, 10 to 30% globally and may well be <10%.

Conclusions: Our data-driven statistical analysis confirms that agricultural land use is a significant driver of SOC levels. ….This analysis also demonstrated that not all land use is associated with large losses in SOC, particularly in regions with naturally infertile soils. These results provide a basis for national and international policies to target SOC restoration efforts but also suggest that more effort needs to be put into collecting, integrating, and using legacy soil profile data, especially historic data 50+ y old, so that even more reliable models of SOC dynamics can be produced.