Gaining an understanding of soil carbon and its management in a archaeological context

Speakers: Jim Williams, David Hopkins, Dan Miles

Speaker: So without further ado, I would like to introduce today's chair, Jim Williams. He is a science advisor here at Historic England.

Jim Williams: Hi, thanks Jess, and welcome! Thank you all for coming, it's wonderful to see so many of you here today!

I'm Jim Williams, I'm the Historic England Senior Science Advisor, and I'm going to be chairing this webinar which will be delivered by David, if you want to just introduce yourself.

David Hopkins: My name's David, yeah, good afternoon!

I'm David Hopkins. I'm not directly associated with Historic England, my background is as a researcher and a university academic, and I've worked in aspects of soil carbon, soil carbon management, soil science related to carbon for getting on for 40 years until I retired recently, so a little bit of background in the subject, and I'll be taking us through some slides and then dealing with some questions and discussion with Jim as we go along.

Jim Williams: Brilliant, and today we're also joined on this webinar by Dan Miles. Dan, if you want to turn your camera on and say hello as well.

Dan Miles: Hi, I'm Dan Miles, I'm the Senior Sector Development Advisor, and I'm responsible for the Sector Net Zero supporting heritage organisations on their journey to net zero.

Jim Williams: Brilliant, so this webinar is based on a report that Historic England commissioned from David last year, which Dan and I have had the pleasure of working with David on, and Jess is going to drop the chat in the room— in the link in the chat in a second. We commissioned this research as we felt that the archaeology sector was beginning to create multiple perspectives on soil carbon loss, with different organizations using different methods for calculating soil carbon loss and taking figures from previous studies sometimes with quite different soil types than are encountered on archaeological science.

As there was no general overview of soil carbon loss from soils due to archaeological activities, we felt it would be useful to create one, and we asked David to undertake this work because of his practical knowledge and experience in soil science, his previous work on aspects of preservation in situ, and most importantly, his long-standing research into carbon in soils. And as you'll learn from listening to him, his innate ability to make complex things sound simple.

So in a second, I'm going to hand over to David to run through his presentation, and then I'm going to ask David a few questions to expand on some of the key areas of work before we open up questions to the audience. But please do put your questions in the question box as the presentation goes on, and if there are any that David spots that he can work into the presentation as he goes, he's promised to do so.

So over to you, David! Thanks!

David Hopkins: Thank you! Thank you, Jim! Yes, and your kind words. I hope I don't disappoint.

We're actually ready to go to the slides now, so that's where we're going to start. You see the first one coming up any moment. Here we go. So this is just the introduction. It doesn't say anything that you haven't already heard or seen, so we'll move on to the next one.

I've broken this down into a handful of topics, and I'm going to spend a few minutes on each. I'm going to talk about carbon turnover, functionality, what happens when soils are disturbed. I'll make some comments about quantities and inherently why it is difficult to quantify soil carbon. I'll say something about what land managers and archaeologists and others can do to mitigate for carbon losses, and then I've got a very simple worked example which sets up a calculation of soil carbon loss in a particular archaeological context. It's only one example, and you can change the parameters and fiddle around with it, but it gives an illustration as to how you might go about thinking about quantifying soil carbon loss.

So this is my turnover slide. There's a lot of information on here, but I'll take you through it as easily as I can. First thing to bear in mind is the word up in the top left-hand corner. It's turnover. Carbon in soil is not a static pool that sits there and is always there. It is naturally turning over. It's naturally turning over because it enters soils as a result of photosynthesis, and leaves fall off trees, and branch— branches fall off trees, and animals eat bits of plants, and then they defecate and die. And all the residues and all of the organic detritus of life could end up in the soil.

It leaves— carbon leaves the soil because decomposer organisms in the soil break it down. They break it down because they use it as their own resources. They break it down because they use it as a source of energy. And all they're doing is, for the most part, respiring, just like you and I respire. Some of them slightly differently, but just like you and I respire, we release carbon dioxide back into the atmosphere.

And whether you've got a lot of carbon in the soil or not very much carbon in the soil, in essence, depends on historically what the balance has being between the incoming carbon coming out of the atmosphere, driven largely by photosynthesis, and the loss of carbon, which is the carbon which leaves the soil as a result of the decomposition activity, the respiration of the decomposer organisms.

The important thing to realize there is it's a complex set of processes. It's not a one single process that goes in a straight line. And that gives us a particular difficulty in understanding things like the turnover time. How much time does it take for all of the carbon that might be present at a particular time to be completely replaced by new atoms of carbon?

There's some fairly detailed work being done on this subject. On average, we reckon that it takes somewhere of the order of 1,000 to 1,500 years to replace all of the carbon. That's under normal circumstances. Now, that in itself presents an interesting challenge because if we do something which accelerates the turnover time, we're effectively undoing the result of the previous millennium or so.

So quick fixes often aren't all that quick. But within the total soil carbon, we've got some components which are very fast turning over. They're labile, they're easily degradable, they're accessible, and they may only last a few months before they're all replaced. and we've got other components that may last several thousand years to give us the average of between 1,000 to 1,500. So it's a complicated mixture of different components all turning over at their own characteristic rates.

If I move to the next slide, this is a set of— really enables me to make a set of comments about the function: What's the soil carbon doing for us? So, in modern parlance, I mean, this is something that's come along in the last 15 or 20 years or so, we talk about ecosystem goods and services. Ecosystem services are things that the ecosystem provides— nutrient cycling, water storage, and the like— because it's there. And soil carbon is critical for several of the key services which soils provide.

So nutrient supply is one of them. If soil organic matter did not turn over and break down and be replaced, we wouldn't get the release of all the other nutrients, particularly things like nitrogen and phosphorus.

Soil carbon also makes a major contribution to the physical stabilisation of soil, so holding it all together so that it doesn't slip down the hillside, holding it all together so it doesn't get washed away, holding it all together so it doesn't get blown away, is partly a function of carbon in soil because organic molecules, carbon-containing molecules, are naturally a bit sticky and they hold the mineral— they help hold the mineral components together.

And a consequence of the structural stability of the soil is that it also holds water. So organic molecules help stick the soil together. That creates a pore space, a void in the soil, which is where the water resides. And also organic molecules are themselves slightly absorbent of water, so it helps hold water in the soil, which is useful for plants that need the water. It's also helpful in things like flood management, and a whole range of reasons why we want to hold water in soil can be attributed in part to organic carbon in soil.

Now, each of those functions relies on turnover because it's the living organisms doing their stuff which are contributing to those ecosystem services. If the carbon wasn't turning over, the organisms wouldn't be alive. There is also a role for soil carbon in more modern times. We've come to recognise the importance of soils as a reservoir of carbon that is not in the atmosphere, and that's important because we've been putting carbon into the atmosphere very enthusiastically through fuel burning, and increasingly people have been looking at soils as a reservoir for holding back some of the carbon that might otherwise be in the atmosphere.

And if you think about it, there is a conflict here. Some of the key functions that we want soils to perform rely on the carbon turning over, in which case we're going to lose carbon to the atmosphere. This one, the sequestration of carbon, relies on it staying in the soil and not turning over. And somehow we've got to resolve that conflict. And it's in that context that people are increasingly working on the idea of stewardship of the soil carbon, accepting that we want to hold carbon in the soil, but also accepting that we've got to allow some of it to turn over in order to fulfill the other functions apart from sequestration that we expect of the soil. We move on.

Here's a picture, it's a diagram I drew a few years ago, which is an attempt to show at a fairly microscopic scale what soils look like. They're complex three-dimensional structures with mixtures of organisms and mineral particles and organic materials and water and air in a matrix which is not uniform. And whether organic matter decomposes in that matrix depends on a range of factors. The three ones that we commonly identify are the environmental conditions, so that's things like the temperature, the availability of oxygen, the availability of water. And those things are influenced by the three-dimensional structure. I'll come back to that in a moment.

There are then second set of properties are the properties of the organic material itself. You'll be entirely familiar, I suspect, with the idea that some compounds break down very easily, sometimes some compounds take a long time to break down, and that's partly to do with their chemical structure and how they're bonded together, and it's all part also partly to do with what value they have as a resource for the decomposer organisms. If the decomposer organisms have to invest a huge amount of physiological effort in breaking something down in order to gain not very much energy, or they have a choice, they can break something down which yields a lot of energy very quickly, they will do the latter, not the former.

And then the third factor is the presence and activity of the organisms. Now, we could usually assume that unless the soil has been really seriously abused, that most of the organisms to do most of the decomposition are going to be there, and if they're not there, they're probably likely to arrive. Whether they're active, however, is going to be determined by the environmental conditions.

So bringing these together, often it is the case that it's the environmental conditions that determine whether a particular organic residue decomposes or not— things like the oxygen availability, the water availability, the temperature, and the accessibility. So if there's plenty of oxygen, that implies the soil's porous, it's connected to the atmosphere, and oxygen can diffuse in and out. If there's plenty of water— well, let's start that again.

If it's waterlogged, then there may not be very much oxygen because the water and the oxygen are competing for the same space. And if there's a lot of water, there won't be very much oxygen. But water also has another effect because of the very high specific heat capacity of water. A soil which is waterlogged stays cold for longer, so water influences temperature.

Now, in the context of disturbance, what you're often doing is, you're disturbing the soil, allowing water to drain out, which means that actually the conditions will then favor gas influx, so oxygen will diffuse in, and also the disturbed soil will tend to warm up more rapidly and stay warmer for longer. And I think that's actually been captured in, in one of the questions that's just come up there. Essentially, yes, it does.

So the question that's come up is, does this mean trenching during the winter months when there's more moisture in the soil mean that water is— that the carbon is released? Well, it's the exact opposite of that. If you've got more water in the soil, there'll be less oxygen, and it'll usually be colder.

And then the final point here is the accessibility question. Disturbance will often break up some of the three-dimensional structure, and that will mean that organic components which are protected from decomposition because maybe they're locked away behind a shield of clay particles and sand and silt and the organisms can't get to them— if that breaks down, they become accessible.

So the physical accessibility is also affected by disturbance. If I come to another slide now, just changing the theme slightly now to look at quantity. So there's a lot of discussion, people say, well, we're going to lose X, Y, or Z percent of our soil carbon, and I often ask them, do they mean that percentage, or do they mean the mass of the soil, or are they talking about carbon, or are they talking about soil organic matter?

My point there is that there's a lot of talk about numbers from a perspective of loss without actually thinking through whether that's a lot or a small amount, or whether it's important or not. So here's a few things that I would just say about the soil carbon to start with.

First of all, the organic matter, the bit that contains the carbon in most cases, is often around about 1, 2, 3, 4, 5% in a mineral soil. It can go higher, And indeed, the example I'm going to use, I've used a higher value there, in part because it makes the arithmetic a bit easier. But, you know, 1 to 5% is not unusual in the topsoil of a mineral-dominated soil. Of course, peats are a different type of material altogether. They have very, very little, if any, mineral material in them, and maybe up to 100% of the dry weight is organic matter.

Now I've used organic matter in that little description there because the organic matter is not quite the same as the carbon. Organic matter contains carbon, but it does not only contain carbon. It contains oxygen and hydrogen and other elements as well. It's been long established, actually going back something of the order of 150 years now, that the carbon in soil organic matter is about 58%.

And people have asked me to justify that 58%. And I've often said, well, it just is, which isn't an entirely intellectually satisfactory answer. But actually, what I really mean is it's been measured so many times. And we've always come up with an answer that's remarkably close to 58%. It's just become accepted that it's 58%. There's a question here I'll just come back to. Is it worth mentioning that things work slightly different on peat soils due to waterlogged and anoxic conditions. Absolutely valid point.

If a peat— if a soil is waterlogged, whether it's a peat or not, but it's particularly the case for peats, when there is so much water that all of the oxygen is excluded, then respiration works in a very different way and different types of decomposition reactions occur. So other gases can be produced— methane, hydrogen sulfide, for example, can be produced under conditions with no oxygen.

Generally speaking, the loss of carbon as a result of anoxic processes is much slower, partly because it's less efficient, also because it's usually going on in places where the conditions are colder because the soils are wet. So yes, it's functionally different as well.

Anyway, coming back to the main slide here, one of the things that we need to be able to do, or at least conceptualize, is get an idea of actually how do we measure soil carbon. And there are 4 key pieces of data that are required. We need to know the concentration— that's the amount per unit mass of soil.

Now, soil scientists will always try and do something per unit dry soil because the water content can be so variable. So we need to have a constant value that we can go back to, so we tend to use per unit mass of dry soil. That's the first parameter. Actually, that one's not too difficult to measure.

The second one is we need to know the mass per unit area. Now, if you're scraping the soil off a 30-meter-long trench that's 1.8 meters wide, you will have a volume of soil that you've used. You will know the area and you will know the depth, so you've got a volume of soil. Those two aren't all that difficult to work out either. You can measure the length of trench, you can measure its depth, so you'll know the volume.

The more difficult one is the bulk density, because in order to get from a volume to a mass, or from a mass to a volume, we need to know the density. And the bulk density of soil is a very error-prone parameter. It's error-prone because it's difficult and messy to measure, it's time-consuming, it's laborious, it's also confounded by things like stones and roots and so on, which make it very difficult to get a very reliable, accurate measurement of bulk density. So there's an error in that term.

Now, that leads me neatly into the second, the right-hand column on this slide, which is asking the question about soil carbon change over time. Why is it difficult? Well, it's difficult partly because making the measurement of the amount of carbon you've got per unit area or unit volume is difficult because of this problem with the bulk density, but also for many investigations the need to do laboratory analysis could be prohibitive or expensive.

Second point is soil carbon is spatially variable and it varies on a very interesting set of scales. Clearly it varies on a scale which is regional or national and it will vary often from field to field. Fields are not accidents, Fields were often— the shape and size of original fields often tells you something about the soil in the field, because an area of uniform land would often be used for the same purpose, and therefore it would be regarded as a field.

And a field, you know, half a kilometre away may be different from it, so you get that scale of variation. But also you get spatial variation down at the metre and the sub-metre scale. And part of the problem with sampling soils is you're often making quite a small sample in a very variable landscape. So making sure that that sample is typical of the field is very challenging.

Third point here is, in order to know— in order to make an estimate of soil carbon in the depth in the soil, you need to know its depth. And actually, people are very poor at estimating the depth of soil, and most of the soil surveys make an arbitrary decision about depth. You could argue that you need to carry on digging until you reach the regolith, I would respectfully suggest that not very many people do that. So depth is a difficult one to estimate.

Bulk density, as I've already said, is error-prone. And the final result of all of this is that when we're looking to measure change in the amount of soil carbon we've got, we're trying to assess a small difference, relatively speaking, a small difference between two large numbers, and the errors on each of those large numbers is often going to be as big, if not bigger, than the change that we're looking for. So it's a statistical problem as well as a practical problem.

So let's change themes again. Let's think about what we can do to mitigate for carbon loss?

Well, on the right-hand side we've got a picture which shows a site that I dare say many of you will be— the sort of thing many of you will be familiar with. We've got an exposed soil surface, so there's soil that wasn't previously exposed to the atmosphere now warming up when it would otherwise not have done so. It's also subject to evaporation, so it's going to dry out, but noticeably it's got no plants on it, so the return mechanism, the mechanism that puts carbon into the soil, is not there either.

And then in the background, we've got a soil heap, and the soil heap is also an exposed surface, a big exposed surface that perhaps wasn't previously exposed. It's also been disturbed. The core of it right in the middle is probably going to be very different from the outer edges of it, and it may be anoxic at core, but we've got a very challenging environment here if we're thinking about soil carbon.

We've set up a set of circumstances here where some loss is going to be inevitable. So what can we do about it?

Well, minimizing the area that's excavated, notwithstanding whatever the archaeological considerations are— I mean, clearly that's the priority, so you can't just say, well, we'll just do a tiny area and then miss the archaeology. Similarly, minimizing the depth, notwithstanding what the archaeological priorities are. Minimizing the handling— every time soil is moved you get a bit more disturbance, a bit more breakdown of structure, a bit more ingress of oxygen, a bit more loss of water. Minimize the period between excavation and reinstatement— the longer it's left exposed, the more likely it is that carbon is going to be lost.

And then practical things that can be done are doing something to reinstate the carbon input mechanism, so growing plants on it, you know, if it's going to be exposed for a long time, cover it in plants. Plants will do a lot of the work, or will help undo a lot of the damage that losses— that are caused by losses. So those are a few suggestions. I think some of them are probably fairly obvious, to be honest, but there are a few suggestions.

So moving on to this slide, this slide concerns the calculation that I said I would do. So this is a very simplistic approach. There's a lot more detail in the report, the link to which has been posted in the chat, that goes through how we get to some of these ideas and some of these calculations. But I just want to take you through one simple example here. This is a 30-meter-long trench. It's 1.8 metres wide and it's 20 centimetres deep in this particular case, so each of those parameters that are showing in blue at the top can be measured relatively accurately, and that will tell us that we've got 10.8 cubic metres of soil involved.

If we assume the bulk density of 1,600 kilograms per metre cubed, which is fairly typical, ranges, you know, somewhere just below 2, down to just above 1, are fairly typical for soils. We can work out what the mass of that soil volume is, and it gives us 17,280 kilograms or 17 tonnes. I realise I can be accused of false accuracy here by including all the significant figures and decimal places, but I've kept them all in just so that people can follow the calculation.

If we assume that the concentration of carbon in that soil was 10%, so 0.1 kilograms, 100 grams per kilogram, quite a high value, but I use 10% to make the arithmetic a bit easier, then we can— then we know that we've got 1,728 or 1.7 kilograms or 1.7 tons of carbon.

The next thing we need to think about is how much of that carbon is labile, and this is a particularly tricky parameter to get a value for. Truth is, there are no straightforward measurements of— no straightforward ways of determining how much carbon in your soil organic matter is labile.

We'll come back to that a little bit later on. I've used an approach which involves simply measuring it. Putting some soil in a jar or putting a load of soils in some jars and measuring the carbon dioxide came out until it all but stopped. Now there's a bit of an arbitrary decision to be made there about all but stopped or stopped. Clearly with stuff that takes 1,000 or more years to turn over, it's going to take a lot more than my lifetime to wait for it all to stop.

What I'm saying is it got so slow that I could hardly detect it. That's not the same as stopped. And I've used that value in order to estimate the labile carbon, and I've assumed that the site was going to be open for 24 weeks, just under 6 months, and that was going to be the period that we're interested in. Of course, you can shorten that or lengthen it, and in the report there's a discussion of how you might go about that. What that does, you bring all of those pieces of information together, it shows under this particular rather extreme scenario— so this is a site with quite a lot of soil organic matter in the in the soil with quite a high proportion of labile carbon, which was kept open for a very long time, you get an estimate of 172 kilograms of carbon out of that, out of that trench.

Now you can apply that reasoning to all sorts of other scenarios, and you can change some of the parameters and make other estimates using that approach. And here we've got an image which is not quite the one I was expecting. Imagine this was a red bus, it doesn't matter what the colour is. The reason for showing you a picture of a bus is that people ask me sometimes, what does a tonne of carbon dioxide look like? Because it's a concept that is quite difficult to visualise, we're not used to looking, we're not used to weighing gases.

Well, the answer is it looks like about 5 double-decker buses. 1 tonne of carbon occupies about 500 cubic metres. 1 double-decker bus, if you're interested, is about 100 cubic metres. So that's just an idea to give you some idea of the sort of volume that we're looking at. And I'm going to move to my final slide, which says any questions on it? And I think at this point Jim, by the miracle of modern technology, is going to reappear.

Jim Williams: Yep, excellent. Thank you very much, David!

And we've got quite a few questions that have popped up in the chat and in the questions for presenters, which we'll come to in a second. I've just got a few just to try and tease out a little bit more from David on some specific topics.

So firstly, David, when calculating soil carbon and soil carbon loss, it's clearly even from that brief overview quite complicated. When we talked at the beginning of the project, you outlined 3 different options we could follow to come up with an understanding of soil carbon loss from archaeology.

Now the first of these was a desk-based exercise, which is what you've done that's in the report, and the other 2 being numerical modelling or monitoring and measuring of the soil carbon in the field. And I wonder if you could just kind of talk a little bit about what those options can tell us.

David Hopkins: Yeah, okay, thank you!

Well, let's start with the one that we did, because hopefully that's the one I have the best understanding of. What I did was I used a set of plausible assumptions, informed where possible by published research, to bring together a set of values that would enable us to put some value against each of the parameters that are needed, so depth, density, concentration, proportion, labile, and so on, to come up with some estimates, and then played those through different archaeological scenarios, you know, the size of trenches, depth of trenches, what would be the difference between the soil with high carbon, low carbon, what would happen if it was waterlogged, and things like that. So that was the approach of the desk-based study.

The modelling is an entirely alternative approach. Now modelling is used when there's complexity which is too great to be able perhaps to measure everything, and where the idea is to try and make some predictions Now there are several actually rather good models of soil carbon turnover. The restriction with using those in this context is that whilst they're rather good in the context in which they've been parameterised and for the purpose that they've been developed, you essentially have to re-parameterise them all, which means go and check all the coefficients in the model are correct for each new set of circumstances, and that's very rarely been done in soils which have been disturbed, whether for archaeological purposes or not.

So most of the models are parameterised according to climate, according to soil type, according to land use, vegetation, and things like that, but rarely where the soil goes through a deliberate management intervention or disturbance. So that restricts what we can get out of the modelling. It's not to say it's impossible, it's just that it's a different type of exercise.

And then the measurement one, well, I sort of touched on the measurement one previously. If you want to measure the soil carbon change, you have the difficulties that I've spoken about in terms of how do you quantify the carbon that you've got at the end of the process accurately and compare it with the carbon that you might have had at the start of the process.

Now, if you're in the fortunate position of being able to make pre-excavation measurements and then post-excavation measurements. In principle, you can do it, but actually that's often not the case. Now, you could alternatively go to soil survey data, and you could say, well, we could use soil survey data and try and get it, but then you've got the problem, and it's very unlikely that you're going to have accurate data for the precise location that you're working at.

So measurement is inherently difficult because you've got several parameters that you need to measure, and some of them you can't measure very accurately. But you've also got this difficulty of knowing what was there to start with if you hadn't already measured it. So the absence of historical data is often a problem with the direct measurement approach.

Jim Williams: Right, brilliant, thank you, David!

In the report, people will see that you've created a number of scenarios for soil carbon loss based around either high or low levels of carbon, and the example you gave was one with a high level of soil carbon, and also whether that carbon is more or less labile. Could you explain why it's difficult to be more precise than just kind of those created scenarios?

David Hopkins: Yeah, well, in part I've touched on it. It is because each site will have its own particular characteristics, and you can't always assume that just because the typical soil organic matter content for a permanent grassland is 5%, that a particular field is 5%, you know, it could still have quite a high variation within the field. So you've got that spatial variation problem.

And then the other issues, the difficulty in determining depth, in some cases, if you're really interested in the full depth of the soil, the difficulties in determining density in a bulk density in a representative way, and often the constraint of needing laboratory analysis in order to determine the carbon concentration.

Now, as I already indicated, all of this comes together to give you the challenge of trying to find the small difference between two relatively large numbers the before and the after value, when the error on the determination of those two large numbers is quite large and the difference may be similar in magnitude to the error.

So it's sort of analogous to the adage of trying to find out the weight of the ship's cat by measuring the displacement on the hull of the ship, the waterline if you like, with and without the cat on the ship. In theory, it's perfectly possible, but you will not get an accurate answer.

Jim Williams: Great, that's a good way of trying to understand the problem, I think. One of the aspects of the work that I think I probably have the most difficulty in understanding, and one that you promised to come back to, was when thinking about how labile the carbon is in the soil. I wonder if you can just expand a little bit more on that?

David Hopkins: Well, yes, it's a good question. It's a good question for two reasons. The first is that I know the answer to it. That makes it a good question. But the second one is actually a highly relevant question because it is actually the weakest point in the analysis here. So there is like it or not, no easy way of assessing the labile carbon without an empirical measurement just to see what happens when you let carbon decompose under ideal conditions. And that's a laborious process. It takes time because the process, you know, it can slow right down, but you've still got labile carbon which is being recycled. Which is, sorry, which is turning over.

So you've got a practical difficulty in determining. The second is that the research literature is badly cluttered up with alternative attempts to take a shortcut. So because it's slow and because it's laborious, people are often looking for shortcuts. So we've got thousands of research articles which claim to be able to show that if you use a particular solvent or if you use a particular extractant, you can extract the labile carbon from the soil. I would respectfully submit that you can't, because most of the solvents that are used are not ones that normally occur in the soil. I can understand the need to try and the legitimacy of trying to do it by some chemical extraction method, but the evidence that it works is not very convincing.

So labile is a difficult thing to measure. The concept is pretty simple. I think everybody will understand that there are some types of compounds which are easy to break down. That's because they're high resource value for decomposer organisms. It's because the decomposer organisms get easy access to them. You'll be familiar with things that decompose relatively rapidly, you know, from your own kitchen. I dare say you could even think of them.

On the other hand, you'll also be familiar with things that decompose very slowly because they are inherently complicated molecules and have great structural stability. But actually measuring that difference, that concept, is quite difficult. I used to direct a empirical approach. I think there's value in a direct empirical approach, but I absolutely accept it's laborious and time-consuming, and also that not everybody— you know, it's not going to be everybody's favorite technique.

Some people are still going to want to try and use an ex— you know, an extraction or some kind of shortcut, some kind of spectroscopic, fancy analytical approach. I can't say any of them are wrong, but they all have their limitations.

Jim Williams: Excellent! We've got a question in the chat which seems relevant to that. Hold on, that's just— all the questions just disappeared for a second, hold on. With issues around determining labile carbon, why not just use CO2 emissions directly? I think that says IR spectroscopy with mobile units would be able to measure this in real time.

David Hopkins: And that— so that's measuring the carbon dioxide flux out. Yep. And that tells you how much you're losing at a particular moment in time. Yep. And you can do that. Yeah. In order to do that across multiple sites, you're going to need to have multiple setups, multiple IR spectrometers set up. I mean, I've been involved in studies where we've done that with a handful of sites in the Arctic. With quite fancy devices that will— where the chambers will open and close and the carbon darts will flow through them, and you can collect the CO2 and measure it using an instrumental method.

The resource requirements in order to do that are significant because you need spectroscopy set up at hopefully several locations within an individual site to cater for the spatial variability but as a sophisticated instrumental method of doing it, well, absolutely, it'll work. And some very, very high-quality ecological studies use just that approach. But as a monitoring method and a method of capturing CO2 at small and medium-sized sites, it's quite challenging.

Jim Williams: Okay, thank you! I'm just trying to look at some of the other questions we've had come up here. There was one asking whether there's any programs or research running across the UK which are establishing regional soil carbon densities, i.e., creating a potential baseline that archaeology could tap into as a quick working guide for wherever excavation is happening, and I think that also links with a previous question that asked whether the UK Stationary Office map data for carbon is no use in allowing us to make estimates.

David Hopkins: Well, so I'll deal with the last of those first. It's a bit harsh to say no use because actually it is information and it was determined, you know, probably quite accurately with whatever methods were available at the time. It's a guide to historical information, historical data, and actually, if it so happens that some of the sites that were used in the original survey data coincide with sites that are now investigating it, that makes it a step better.

Now, the first part of the question was actually to do with, are there ongoing studies that could be helpful here? And yes, there are. Over the last few decades, the Soil Survey of England and Wales, Soil Survey of Scotland, and the Countryside Survey, which are run— which is run by CEH, has started to include more and more parameters which enable you to estimate soil carbon.

So some of the early versions, iterations of the surveys didn't include bulk density, for example. Some of them didn't include depth. Those are now being included. So as these sites are resurveyed, now, the Soil Survey of England and Wales is probably not going to do a resurvey according to my understanding. I'm happy to be corrected on that, but the Countryside Survey and the Soil Survey for Scotland are doing periodic resurveys, not always of every single site, but that does mean that they are starting to fill in important gaps in the data.

Now, it's important not to sound too critical here. When the soil surveys were set up, they were not set up with the intention of answering the sorts of questions that we're now asking about soil carbon. So although some of those data weren't collected, it wasn't an omission in the context of what the original objectives were. It seems as though it's an omission now, but we've changed the question.

And as I say, some of the studies are now starting to backfill some of that information, but you can't easily go back in time and work out something where the sample wasn't collected or wasn't stored in a way or the data weren't recorded. But yeah, there are new studies coming through.

Jim Williams: Excellent, thank you!

And apologies to those people who were trying to write something in the questions and they disappeared. If you do have a question, please put it in the webinar chat box. I've got one here which I think comes to kind of the issue of how much, how you count carbon, and I think maybe David, you could also talk a little bit about how we can't count it in the context of development.

So, where a site is trenched and then reinstated and then subsequently stripped for development, is the release of carbon from the archaeological work subtracted from the release by the development, or is it in addition to that? I.e., should we be measuring the development overall, should we not be measuring the development overall, rather than the archaeological work?

David Hopkins: So essentially you're dealing with two bits of disturbance, one on top of the other. The bit— so you'll lose some carbon through the archaeological stuff. If it's then reinstated, you'll get some recovery of that carbon. If it's then developed and the soil's disturbed again, you'll lose it all again.

So, the fact that you had some recovery in the middle is likely to be eliminated by the subsequent development. I think my answer to that question is, and this may not be entirely helpful from an archaeological context, but actually in terms of the overall carbon balance, what you really need to know is what was it before anybody started investigating compared with what is it after the development has been done. Because that's the difference that you're really interested in.

The fact that it might fall and rise a bit and then fall again as a result of the different steps that are taken during, you know, the planning and the development and the investigation and so on, it sort of gets lost in the detail. What was there before compared with what was there afterwards is what's really needed.

Jim Williams: Excellent! You've set the veritable cat amongst the pigeons with mention of cats at all within a webinar. Someone's asked here, if I'm understanding the analogy correctly, we know the cat is there but we can't measure it, i.e., we know the loss of carbon but we can't really say what it is.

David Hopkins: We can't say what it is, we can't say how much it is with a high degree of accuracy on every occasion. Now, I think the ship's cat analogy is a clearly extreme version, because the difference between the weight of a ship with and without the cat is absolutely minuscule by comparison with the weight of the ship. We're not looking at something where the difference is so small. It is measurable, but rather than the error being many, many, many times bigger than the change, which is what you've got with the ship's cat analogy, for soils the error is of a similar magnitude to the change.

So anything we can do to tighten up on the accuracy of the measurements before and after reduces the error and therefore increases the chances of us being able to measure the change accurately. You've still got the challenge of the spatial variability in the field, So the quality of the original samples that you take and how representative they are, and the error-prone nature of the bulk density. It used to be the case that people put a lot of effort into the accuracy of their instrumentation methods.

Well, actually, we now know that modern instruments are actually pretty valuable, pretty accurate. That's not where the error is anymore. The bigger errors are now in the quality of the sampling error. How representative is the sample in the first place, and the bulk density.

Jim Williams: Right, just a couple more questions. There's one here that Roger's asked in the chat. If we're estimating 178 kilograms of carbon for an evaluation trench 20 centimetres deep, which I think was what you had on your slide, this seems about the same figure as CO2 for diesel emissions for the excavation of that trench.

David Hopkins: Yes, so this is a topic we've, you and I have discussed at some length, and I think you, you need, you the archaeological community, I think need to put the carbon loss from soil figures into a context of all of the other carbon balances that are going on at the same time.

So, you know, that excavator is going to be burning fuel. In order to get that excavator on the site on the back of a loader, it probably had to be driven in a truck on the highway. In order for the archaeologists to get to the site, they'd probably have to travel from home by whatever, you know, there are going to be carbon losses at every stage or carbon fluxes at every stage in the process. The soil carbon one is only one of them, and it may not be the largest.

Jim Williams: And I think that's inherent in this question here about needing to understand the key driver for carbon loss is the development, not the archaeological work. We wouldn't be doing the archaeology without the application.

David Hopkins: Nothing I can add to that, yeah, absolutely. If you want to keep carbon in soil, leave it alone, let the plants do their stuff.

Jim Williams: And in terms of how do we defend against the topical soil carbon loss being weaponized against archaeology, especially in predetermination evaluation, I think it's just really coming back to that point that, you know, the evaluation is taking place because of the development, you know. Yes, yes, that's why it's happening.

I've got a question here which is, from my perspective, if we wanted to take some next steps on from this report, for example to provide some more geographical or geologically based recommendations, or wanted to test some of the assumptions in the field, what would you recommend we did after this point?

David Hopkins: So we touched on it in answer to an earlier question. You asked the question about were there other studies that were coming through, and actually there are. And I think one of the things that you could usefully do here is engage with the Countryside Survey that's being run by Centre for Ecology and Hydrology and the Soil Survey, because, you know, they are— those organisations are responsible for curation and collection of big datasets, and in particular the Countryside Survey has always been receptive to the changing research questions that are being asked of the data.

And if archaeology is one of the emerging sets of research, or if archaeology is supposedly one of the emerging sets of research questions, the people who are curating those data and doing the updates of those data may well want to engage with you, because actually you're doing each other a favor then. You're making their data collection more relevant, increasing their stakeholder base, and also hopefully getting answers to some of the questions which are relevant to the archaeologist. So there's one suggestion.

Second thing is, again, we just touched on it. I think a really pressing research requirement here is to set the soil carbon question in the context of all of the other carbon questions. So to look at the whole life cycle of the process and come up with a view as to whether the soil carbon is a big or a small part of the problem.

Now, I'm not saying, you know, if you come to the conclusion it's a small part of the problem, well, that's fine and good. You know that it's a small part of the problem. It doesn't absolve archaeologists or anybody else of responsible management of the land, because there are going to be some sites which get excavated and then aren't developed, or parts of sites that aren't developed, and you don't want to reinstate a site badly and then have its ecological functions impaired because it wasn't handled as intelligently as it might have done. But actually knowing the scale of the problem, I think, is part of the issue.

If you really then want to get right down into the detail of it, again, go back to the answer to an earlier question. Oh yeah, there are methods that will enable you to get a lot of very, very fine detail if you're prepared to put the investment in terms of people and instrumentation and analytical costs into collecting those data. It's all achievable. And I mentioned ecological investigations.

Well, you know, in ecological investigations, often you're looking at, you know, carbon flow through plants under particular contexts. You've got a very tightly defined experimental system where you've got lots of controls over what you're able to measure. Under those circumstances, it's worth using an expensive technique because you know that you've got so much control and so you've taken so much of the error out of the system that that sophisticated technique will give you an answer that's worth paying for.

Jim Williams: Yeah, brilliant! Thank you, David!

Thank you for answering those questions and for that really interesting and engaging presentation! And thank you everyone for attending!

Management of archaeological sites and processes

The Valletta Convention established the principle in favour of in situ preservation, and this is a principle built into the National Planning Framework 4 in Scotland.  Has there been any assessment of carbon value of preservation in situ, given that most archaeological sites have not been and may not ever be excavated, and may have higher carbon value than surrounding soils?

This is a good point – thank you for raising it.  As far as we are aware this hasn’t been given any significant consideration until now.

How do you envisage the sector using this information in respect of fieldwork methodologies? [another comment asked a poised a similar question - I'm finding this topic thought provoking, but ultimately see no reason to vary the way archaeological site investigations are undertaken, when it's acknowledged that it is the development itself that is responsible for the highest loss of carbon]

As archaeologists, it is important for us to understand and acknowledge our impact, even where we recognise that it is the development that is really contributing to the carbon release. This knowledge can also help us to improve how we do things to reduce our impact (where it doesn’t impact on the work we need to do). For example, if we know that proportionately more carbon is released if trial trenches are left for 4 weeks rather than 1 before being backfilled, then, where feasible (and you can take the term ‘feasible’ to include every single caveat as to why it needs to be 4 weeks that you can imagine!!), it would be beneficial to backfill at the earliest possible opportunity and to reseed the area to improve the rate of carbon sequestration. Equally, as another participant noted “I’d struggle to see why we should be held 'accountable' for carbon release for a residential development for example, but I could see an argument being made against us for solar farms, particularly when requiring trenching in the solar array”

Presumably there are variations on soil loss seasonably and as a result of (e.g.) agricultural activity. Would that affect how and when we may want to strip soil?

The first half of this question is a little unclear. We are going to assume that the question relates to whether undertaking archaeological work at different times of the year has an impact on the rate of soil carbon loss. In that case……

Other questions

You had any thoughts on the impact of nano or micro plastic on soil C? Apologies if this is out of context, it really bothers me in relation to artifact preservation

This isn’t something that was considered in the report. We would be interested in hearing more from you about what you feel are the principal threats to artefact preservation from nano and microplastics.

Are you linking archaeological soil carbon studies with those actively being undertaken in the geotechnical profession?

This isn’t something we’ve considered at this point. Would you be able to point us towards some of these studies being undertaken by geotechnical practitioners?

Presumably agricultural practices also lead to disturbance and loss of carbon (thinking about land uses prior to archaeological work let alone development)?

This question was answered in the chat by one of the other participants:  yes, minimum tillage is for example known to have lower soil carbon emissions than standard ploughing. Controlled traffic farming and other systems are being developed to try and manage soil carbon emissions. Natural England also has data on what habitat types sequester more or less carbon