Thank you to our panellists and to everyone who attended the event on 14 Sept and asked such good questions. Thanks to Rebecca Sanders, we’ve grouped some of these questions here, using the following themes:
- Stabilising Carbon in soil - the science
- NPK
- How much meat can we eat? How many livestock can we rear? What livestock should we rear and how?
- It’s complicated…
- Politics?
- How are GHG emissions from ruminants affected by their diet?
- Big ideas
- Changing Ecosystems/Rewilding/Ecosystem “baselines”/Land use change
You can find the event recording, along with the anonymized text from the Q&A: Event recording: Carbon sequesterers or climate trashers? What role for grazing ruminants in a 1.5°C world? | TABLE Debates
We welcome your thoughts.
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Stabilising Carbon in soil - the science
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Question for Francesca re: “carbon transformation”. Isn’t the transformation process inherently losing or releasing carbon? What is left might be in a more stable form but the more dynamic the soil system the more vulnerable or less stable carbon is released?
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Where is the evidence that introduction of short term grass/livestock into crop lands really increases carbon sequestration. A two year grass ley when its cultivated for 3 year cropping afterwards - why would that sequester and “save” carbon for a significant period?
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There’s a great deal of focus on C sequestration via grazing ruminants; could the panelists speak more to the potential of sequestration on croplands? What are the best practices that contribute to sequestration on croplands (composting? no-till?) and how does that potential compare to grazing?And does composting actually contribute to net sequestration or is it just cycling carbon?
- Depends; work I completed adding compost to grassland in Colorado (two, moderate additions over six years apart) did show we can sequester C in excess of C added from the compost
- Research from Whendee Silver’s lab suggests that the SOC sequestered in the Marin C Project was greater than just the C in the compost. That may not always be the case, but it certainly can sequester additional C
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What are the mechanisms by which the additional C (beyond what is in the compost) is incorporated into the soil?
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Change in plant production. in our case, we think it was a change in soil P rather than N because our grassland soil already had a low C:N ratio. it was also a hay system, so all the aboveground biomass removed and we still observed a ~ 0.3 Mg ha-1 yr-1 increase in soil C. So, we think it was driven by root inputs, as Francesca talked about earlier
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Here is our study: Infrequent compost applications increased plant productivity and soil organic carbon in irrigated pasture but not degraded rangeland - ScienceDirect
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Pete, you say that soils can reach saturation point. Does this account for the ability to convert existing subsoil into new topsoil?
- Francesca Cotrufo: In any soil mineral layer, the amount of soil carbon that is associated to mineral saturates (not the free particulate organic matter which however turns over more quickly). However deep soils are further from saturation
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Plants also release carbon compounds directly from their roots into the soil. What role does this play in the ability of the soil to sequester more carbon?.
- Pete Smith: they are part of the carbon input - so root exudates are important
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How long does it take for soils to reach saturation? I have read that deciduous woodland soils may keep accumulating soils for centuries. Is that true?
- Pete Smith: Depends on climate - faster in the tropics - slower in temperate regions. In temperate systems, it takes between 20-100 years. In the tropics can be much quicker
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(To Pro.Smith Pete) is it always a case that the soils reach equilibrium eventually? Any exceptions? And how many years [does] it generally take to reach equilibrium?
- Pete Smith: Peatlands continue to sequester carbon as long as they are waterlogged - the anaerobic conditions slows decomposition to the extend that peatlands continue to build (and don’t saturate) - so peats are the exception
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The theoretical potential of worldwide soil to sequester 30-40% of Co2-emissions may be misleading if we don’t also try to quantify what is technically possible. Were there attempts to quantify those more realistically achievable potentials, also considering the high variability in soil measurements, even in small spatial dimensions?
- Pete Smith: Quite agree - very variable - realistic potential is much lower that the technical potential
- Helen Breewood: Francesca’s recent paper looks at “theoretical, realistic and achievable” soil carbon sequestration potentials (https://www.science.org/doi/full/10.1126/science.abo2380 ).
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Should it be a surprise that the potential of soils in the transition to net zero (say about 30%) is about the same as the contribution of land use (change) to climate change, based on cumulative emissions (about 30%)? Fixing the broken C cycle is crucial to addressing the climate-nature crisis: which is about fossil fuels and land use…
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Is a bio-diverse ecosystem generally a better store of carbon than a less diverse ecosystem? Why?
- Pete Smith: Studies have shown that more biodiverse systems store more carbon - thought to be because they occupy more niches (e.g. roots at every depth)
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Again a practical question referring to Pete’s statement that croplands have the greater potential to sequestrate carbon. The potential lies in the change of land management by turning cropland into grassland, right? Or is there a promising crop land management?
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For example, continuous restorative cropping (understorey of white clover with grains) and/or no till/min till and/or organic?
- Pete Smith: There are lots of options in croplands without converting to grazing land, including less intensive tillage, inclusion of cover crops and agroforestry for example
- Okay, sure… I just heard that the humus content in the soil is subject to constant buildup and degradation, with climate being an important factor: the warmer, the higher the degradation, and that we need to make more efforts in the future to maintain humus content in agricultural soils. In addition, the humus content is said to be limited in time: as the content increases, the degradation increases. I.e. on arable soils there are limits of humus build-up that cannot be exceeded. so are these practices really enough?
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Pete used the term “degraded” grassland/grazing land. Could he expand on what the characteristics of “degraded” are from the carbon sequestration perspective? For example, wet fields with low productivity grass species might be regarded as degraded from a farming perspective but would it be good for carbon storage - also please more on “sequestration” and “storage” are they really the same thing?
- Pete Smith: Degraded usually means overgrazed - can be poached by overstocking or otherwise eroded by overgrazing. Leads to a loss of carbon inputs, and thereby to less soil organic carbon. So degraded soils have greater potential for carbon sequestration.
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When saying degraded land has more potential in C-seq, it kind of makes an impression that we are giving more credits to degraded land…whereas the farmers who manage their grasslands well do not get the credits, which might be tricky when it comes to policy incentives?
- Pete Smith: Agreed - would not want to create an incentive for land managers to deliberately degrade.
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Where does incorporating livestock in silvopasture systems fit in? How much potential is there for increasing C sequestration potential in grasslands by incorporating agroforestry (where ecologically appropriate)
- Pete Smith: My PhD student looked at this in Ecuador - the potential is quite significant
- Pete, can you expand on this further?
- Project Drawdown suggest 5-10x more C sequestration.
NPK (Nitrogen, Phosphorus, Potassium)
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How do NPK fertilisers and biocides affect soil’s ability to sequester carbon?
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@PeteSmith. Does the anaerobic activity of peatland mean it is emitting N2O?.
- Pete Smith: No - they emit methane, but N2O emissions are low, as peats are very nitrogen poor (exception is some fens near ag land)
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In the context of both grasslands/grazinglands with ruminants, as well as fertilizer inputs for croplands, can you speak to the role that Nitrogen might play in soil carbon sequestration?
- Francesca Cotrufo: soil organic matter has C:N ratios mostly ranging between 20 and 10, so for every 20 to 10 atoms of C we want to store , we’ll need two to one of N. There is no evidence of N shortage for C sequestration, which actually can improve internal N recycling and avoid deleterious N losses. Most of the N for C sequestration comes from the same plant inputs, but also integrating more legumes or free biological fixation can support the N inputs for C sequestration.
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The problem they have in the NL with livestock is that relevant for the rest of the EU?
- Walter Fraanje: Probably yes, but the current discussions around livestock in the Netherlands are very much about nitrogen (not so much about carbon sequestration). And the Netherlands comes with some context-specific things, not in the least it being a small country with huge pressures on land use from different sectors. There’s some info on the Dutch ‘nitrogen crisis’ here (WUR is one of the universities involved in TABLE): https://www.wur.nl/en/dossiers/file/nitrogen.htm
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C:N is a red herring as Pete describes it for C sequestration and storage but it is not in a mixed farming system where organic matter turnover is being used as a driver for cropping production
- Pete Smith: Agreed - the point I was making is that you don’t have to add synthetic nitrogen
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@Pete. Thanks for your answer on N&P. Is it fair to conclude that we do not need NPK fertilisers for the purposes of increasing SOC? THis seems to be conclusions from Mulvaney and Khan at Illinois Champaign.
- Pete Smith: You do not need to add additional NPK. However, if NPK leads to more vigorous plant growth, this will feed through in to higher soil carbon
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The argument for adding N, P fertiliser is that despite crop stubble (for example) already containing some N and P, the C:N:P ratio is so high in these materials, so by adding N/P can close this ratio leading to a greater digestibility/efficiency of conversion into SOM.
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Pete- SOC stocks are the balance of C input versus C mineralization. My perception is huge numbers of people believe that a positive relationship exists between C inputs and SOC. I wonder if the mineralization portion (is harder to measure) more important?
How much meat can we eat? How many livestock can we rear? What livestock should we rear and how?
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One important clarifying point here is that, though ruminants are a large global land user, the land use tradeoffs of ruminant grazing are not the same as other types of agricultural land uses. Though we do grow crops to support current livestock numbers, their diets can be sustained through grass (and even byproducts) only while maintaining other ecosystem features like biodiversity and food production, etc. That is not the case for any other category of livestock.
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Given that grazing lands already occupy such a large majority of lands globally, and animal agriculture is a leading contributor to agricultural emissions, could each of you share what you believe is the amount of red meat that CAN be produced globally in sustainable/regenerative ways? Is there a scientifically backed average number of acres of grassland required per cow, that allow the grazing to be regenerative, or at least neutral?
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That question—whether we have to live with zero livestock—seems awfully theoretical. What can sustainable livestock grazing support in dietary terms?
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How much ruminant meat could provide grasslands? [corrected: How much ruminant meat could grasslands provide?]
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Assuming we want to only graze native grassland (not converted forest), and are happy to feed ruminants on residues but not dedicated crops, how much beef can we eat as a % of today’s beef consumption?
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How to find solutions in both the ‘grazing’ element and the ‘farming’ element? So for typical US range cattle systems, most of the beef production (human edible food/protein) comes from the feedlot systems that take the weaned calves. feedlots import cereals and proteins from arable systems… Can we find/finetune the grazing system opportunities without finding a climate friendly solution for feedlots?
- thought that the cattle globally finished in feedlots was low, like 2-3%. maybe you are thinking a specific region?
- not sure about that statistic. the bulk of weaned calves from USA and canadian range systems go to finishing phases - feedlots. Much the same for lots of Brazil growing cattle…in UK the bulk of beef comes from small scale feedlots - the farmers here would call them that, but most come from housed phases…
- 10 - 4 my cattle finish in a lot [feedlot], but I think globally it is tiny percent. the big C02 emitters are hogs and poultry in concentrated housing taking concentrated feeds
- I echo this last question. In terms of translating this into advice for consumers, can we yet say anything about choosing 100% grazed ruminants vs. feedlot (or even smaller scale but grain-fed?).
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What about quality of meat vs the volume of meat?
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If there is more call for improved animal welfare, if ruminants help promote ecosystems, if diverse pasture produce more nutritious meat then surely eating less, but increased quality meat (omega in fat) is a better trade off than intensive meat ?
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What about total ruminant numbers? Is there data on that that include the former herds natural ruminants?
- Helen Breewood: FAOSTAT has data on livestock numbers by species - visualised by Our World in Data here: Livestock counts - Our World in Data
- Is that relative to the increase in the human population?
- Helen Breewood: No, I think it’s absolute livestock numbers.
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Pete - can you further expand on what meat produced in an environmentally friendly way would look like please?
- Pete Smith: “Less and better” meat. Reduce as much as possible, but the meat that remains in the system should be produced in a way that works with nature, rather than against it
- What does that look like Pete? You’ll know I’d say that would be 100% pasture fed and in a silvopasture system - does that reflect your understanding of less and better?
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Isn’t the dairy industry’s environmental impact just as bad as the beef industry? Shouldn’t we also be talking about limiting our dairy consumption? (And other animal products)
- Tamsin Blaxter: Partly because dairy animals produce milk and so calories over long periods, they generally produce more food (calories/protein) per environmental impacts than beef animals - milk protein or milk calories are still more carbon intensive than many plant sources, but generally much less so than beef. (Poore&Nemecek 2018 is an oft-cited review that covers this: https://www.science.org/doi/10.1126/science.aaq0216 )
- But dairy cows are then shipped off and killed for beef anyway. So it’s the same system.
- Helen Breewood: I believe the Poore and Nemecek paper distinguishes between beef from dairy herds, and beef from dedicated meat herds - the beef herds have notably higher carbon footprint and land use per gram of protein.
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Matthew said that ruminants were perhaps 40% of our meat consumption, so what is the other 60%? Presumably pigs and poultry, are you suggesting that these are pasture raised and if not then they will be eating grains which will be grown on land that could be used for grassland for ruminants or cereals for humans…
- Matthew Hayek: it’s only about 1/4 of our meat consumption! In the US and globally. Our greatest consumption is chicken, followed by pork. East and Southeast Asia eats more pork than chicken though, and Muslim countries eat little to no pork.
- These other livestock are also fed grains but have a much higher efficiency in converting a unit of grain into a unit of protein than large ruminants.
- Beef cattle can exist on marginal lands where crops can’t be grown and chickens/pigs might not flourish (e.g. semiarid rangeland). But these beef cattle are still often finished on grains (at least in US).
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If we’re talking about dramatically reducing meat consumption, when current consumption already makes disproportionately low dietary contributions, then why bother maintaining these industries at all given their climate and ecological impacts? And if production was made more extensive and less efficient, leading to higher prices, how would this affect the social distribution of meat consumption?
- Helen Breewood: One argument is that having multiple sources of food production adds resilience to the food supply - even for grain-fed livestock, the argument being that it’s better to keep production of grains higher than they would be in the absence of livestock, so that that grain can be redirected to human food supply in times of supply shocks. See section “Food waste as a buffer against food insecurity” of What is food loss and food waste? | TABLE Debates
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intensive beef production brings cattle to slaughter weight faster than grazing, with attendant reductions in CO2 emissions per animal. Is this important?
- Matthew Hayek: absolutely it is. It’s why a shift to exclusively grass fed beef production would result in less production and/or more GHG emissions
- Is that difference not partly diminished by other emissions from intensive beef productions, and assume that we must maintain current meat consumption?
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Could you speak to the Grazed and Confused report? Overall, it was not a positive assessment of grazing cattle’s contribution to carbon sequestration outside of highly specific degraded cropland restoration examples. Some regenerative grazing advocates say it’s been debunked. Is there an updated study or do the findings still stand?
- Pete Smith: I would say they still stands. The debunking has been debunked!! Oxford is revisiting this and will be producing a new “revisit” report - watch this space
Methane/ how are GHG emissions from ruminants affected by their diet?
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What is the net impact in terms of GHG if methane from the livestock is taken into account? - considering that the scale of wildfires appears to be reducing the way that methane is ‘neutralized’ within 10-12 years.
- Has this question been answered? GHG sequestration in soil is much less than is emitted as methane or NOx in my understanding
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@Francesca, are you considering the effects of methane emissions?
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Are all ruminants emitting the same amount of carbon roughly, or are some better than others?
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If all the domestic cattle in the world were replaced by “natural” herbivores, what effect would that have on methane output by said animals?
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If livestock numbers have been decreasing over the past 30-50 years, why are methane levels increasing? [as following comments make clear, this participants claim that livestock numbers have been decreasing is factually inaccurate]
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Also (layperson talking here though, so I don’t have a reference), permafrost in certain landscapes (tundra/ taiga) has warmed due to global warming and is emitting methane - or it could in the future. Sorry I don’t have more information on this, but worth taking into consideration as well.
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Methane can be rapidly absorbed by soils and broken down by atmospheric hydroxyl, processes that are most closely associated with the situations where many ruminants are grazed in more extensive systems. Does this mean that emissions from extensively grazing livestock are being significantly overestimated?
- Pete Smith: No - hydroxyl is atmospheric and not associated specifically with grazing land. Grazed livestock actually emit more methane (less digestible diets) than those fed on concentrates. An inconvenient truth!
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How are carbon emissions from ruminants affected by their diet? Specifically, how do ruminant carbon emissions differ between ruminants that are on a diet of biodiverse natural vegetation, those fed on a non-biodiverse pasture and those fed grains and other cropped outputs.
- Pete Smith: Lowest emissions from higher quality diets - so grain fed have lower emissions than grazed ruminants. But this uses crops that humans could eat…
- What about biodiverse diets e.g. on a natural landscape with a wide range of grass, herb, shrub and tree forage v. grass only e.g. a ryegrass pasture. Are carbon emissions from ruminants the same for both situations?
Changing Ecosystems/Rewilding/Ecosystem “baselines”/Land use change
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This is the most informative chart that shows how grazing land expansion is balanced against forests overtime. The world has lost one-third of its forest, but an end of deforestation is possible - Our World in Data
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There is increasing evidence now to suggest that in the UK at least the dominant vegetation would have been a mix of trees and grassland - wood pasture - rather than closed canopy woodland
- So; rewilding. In the context of increasing wild game and leaving grassland management to them. How will you keep the game away from the crops for human consumption? I may be narrow-minded but I do not see this as a realistic future.
- As climate changes, areas that would “naturally” be grasslands are changing to be forested. What, in this dynamic picture, would be the natural forest or grassland to be maintained in your views?
- This [???] is more or less what the independent English National Food Strategy proposed, but unfortunately the government then binned it
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How much of current grassland has been converted from forests or other land usage to provide area for grazing? Could it be converted back to natural state to benefit biodiversity and sequester more carbon?
- Helen Breewood: You can find a breakdown of global land use here (What is feed-food competition? | TABLE Debates ) although it doesn’t answer your question about how much of that area was converted from forests - one of the panellists might be able to answer that question.
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Globally, I’m pretty sure that most grazing lands are rangelands are low input (natural to semi-natural) arid and semi-arid grasslands (steppe), shrublands, and savannas.
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What share of the UK is native grassland suited to grazing vs native forest that ought not be grazed?
- From what I understand, a fairly low percentage… 10% ? Would be good to know this figure for sure.
- Pete Smith: Most of the UK would naturally revert to woodland (i.e. trees are the climax vegetation) - upland areas and peats are the exception
Yes, changing dietary patterns really needs a very holistic set of incentives/drivers.