Archives for posts with tag: soil science

After a long hiatus, I have returned to Ground to Sky! I have been very busy, dealing with finalising two research publications and spending every lunchtime in the university music practice rooms but now I am pleased to return to this blog. In this article I will provide a brief discussion of my latest research paper to be published. The (open access) paper is available online here.

The work that I will describe took place during my PhD and was located at West Sedgemoor in the (currently terribly flooded) Somerset Levels and Moors. This land is very low lying, and floods every winter as it is part of the floodplain of the River Parrett. This seasonal cycle creates a unique habitat for wetland birds, and the site is managed by the RSPB for their conservation. West Sedgemoor is a system of small fields that are separated by a series of interconnected drainage ditches. These are managed by the RSPB to ensure that the conditions are always good for wetland birds. Part of the management of West Sedgemoor involves short term grazing during the autumn months by young beef cattle. As part of my study into the greenhouse gas emissions from these seasonally waterlogged peatlands, I was interested to see how the cattle’s urine stimulated production of greenhouse gases inside the soil and their emission as the field went from dry to flooded.

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West Sedgemoor

To measure the greenhouse gas emissions (I was looking at carbon dioxide, CO2, methane, CH4 and nitrous oxide, N2O), I used ‘flux chambers’. These are boxes that are dug into the soil. A lid is put on the box and you wait a while for the gas to accumulate inside the box and take samples during this time. You can then calculate the emission of the gas from the rate of change of the gas inside the box. To measure greenhouse gases in the soil, I used ‘soil atmosphere collectors’. These are silicone tubes that are porous. Air from in the soil moves into these collectors as if they were a large soil pore and you can then take samples from the air inside through a cap accessible from the surface. Dipwells were used to measure the depth of the water-table from the surface.

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Equipment in the field.

Before we could start sampling, we needed some cattle urine. Although cattle were to be loose in the field at the time of sampling, we did not want them getting close to the equipment and actually we didn’t want them to pee near it either! For a controlled experiment, we needed to be sure that every plot (with box, soil atmosphere collector and dipwell) received the same amount of urine. This would be impossible letting the cows loose in the area, so we used urine from cows at the University of Reading farms and fenced the equipment away from the cows in the field. There were ten plots, five to be treated with the urine and five to act as controls and be treated only with water. This allowed us to be sure that it was the urine that caused any changes in the soil and not just the act of the soil getting wet. The experiment ran between September and November in 2010.

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This graph shows the effect of cattle urine and water application on CO2 emissions. There was a large emission of CO2 from the urine treated plots on the day that the cattle urine was applied. This is due to ‘hydrolysis’ of the urea in the urine when it impacts the soil. Bacteria make an enzyme called ‘urease’ which is found very commonly in soils and is catalyses the hydrolysis process. Despite this initial CO2 release, there was not much increase in CO2 due to the urine addition over the full period and there was no significant difference at all in CO2 in the soil atmosphere between urine treated and water treated plots.

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Here we look at the methane in the cattle urine plots and can see there was a substantial reduction in the methane a few days after applying urine. We are not sure what caused this; it is a very unusual finding. It happened in 3 of the 5 urine treated plots and none of the control plots. Overall, however, adding cattle urine increased the amount of methane that came out of the soil during the experiment. As you can see, the control plots remained sinks of methane – that is, soil bacteria were taking methane from the atmosphere and using it to metabolise. This is called methane oxidation. In the plots treated with urine, this activity was prevented as a result of the urine contents. This supports other studies for this is a known effect of adding urea to soils. Under the soil surface there was also evidence of increased methane in the urine treated soil relative to the controls.

n2oThe most profound effect of adding cattle urine to the peat soil was shown for nitrous oxide, N2O. Here we see that throughout the experiment, the urine caused large increase in N2O compared to the control. This peaked 12 days after application, following rainfall. This shows how important soil moisture and water-table can be in determining what happens to added nutrients in soil. Under the soil surface, the differences between control and urine treated plots were even more interesting.

n2oWhen you look at the above figure, notice the numbers on the y axis. On day 2 after the urine was applied, you could already see the difference in production of N2O in the urine treated soils. By the twelth day, the production in the control soils was dwarfed entirely, with production at 20cm depth dominating. By Day 56, the field was entirely flooded and N2O concentrations were very high. We believe that the reason why this happened so strongly at 20cm was due to the fact that the peat soil was covered by a layer of clay. Clay soils, when saturated, are not very good at letting air pass through them and therefore N2O that was produced at levels lower than 20cm, will also get trapped here.

For more information on this experiment, please see the full paper. This is only a short summary of all of the results that were presented there. But what does this mean for managing greenhouse gases in peat soils? Well, N2O and CH4 emissions will get worse after cattle have been on the field and they will get especially worse if the field then floods. This implies that if you are concerned about the greenhouse gas balance of the field, grazing cattle earlier rather than later is likely to reduce the emissions after the field floods. However, there are far more things to balance than just the greenhouse gas emissions; for example, managing the feed supply for the cattle, managing the field grass level and, in the case of RSPB West Sedgemoor, managing the land for wetland birds. Balancing all of these demands and best practice is never an easy task and will require a carefully considered compromise.

I will write again at Ground to Sky shortly, and will attempt to reduce the long time span between blog entries. Next time, I will write about using trace gases to help us to understand where greenhouse gas emissions come from, in particular the use of carbon monoxide as a tracer for fossil-fuel carbon dioxide in cities.

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It has been a while since my last blog article, apologies for that but I have been working hard preparing a couple of papers for publication and I’ve recommenced playing the piano after so many years so lunchtimes are often spent in the university practice rooms at the moment. Today though I’ll be taking a break from the literature and the keys to discuss the ways in which soil can be affected by the type of plant that grows within it. This is a bit of a topic of fascination for me, although admittedly no longer my chosen research area, during my undergraduate years I studied the ecological and agricultural effect of plant on soil and it is good to come back to this now.

As you might expect, plant and soil interaction is a two way process. Farmers and gardeners will tell you that you can’t just put any plant anywhere, even if the climate is correct, the soil must also be correct for the plant to thrive. You can adjust the soil sometimes a little more easily and less expensively than you can the plant’s surrounding climate (unless you have access to greenhouses or other such structures). For example, a wheat crop will need a range of nutrients if it is to grow to its full potential and therefore the farmer will add fertiliser to the field. Nowadays, in the advent of precision agriculture, it is no longer necessary to spread the same fertiliser over all your fields. This could be considered wasteful and it certainly can be expensive. Soils are known to be very different from each other even at small scales (heterogeneity) so why should all soil be given the same treatment? Precision agriculture aims to readdress the balance between soil and additive, to ensure better uniformity of nutrient availability across the field and save the farmer time and money on fertiliser applications.

Agriculture and forestry has done much to change the landscape and the soils that form it. By selecting what plants will grow on a patch of soil, people have made long-standing changes to the chemical, biological and physical nature of the soils below. For example, let’s take a fictional pine plantation. Forty years ago, a patch of deciduous woodland was chopped down for timber and because they are fast growing and easy to mill, tall pine trees were planted uniformly in its place. Quickly the acidic and slow to decompose pine needles get to work on changing the chemical balance of the soil. The efficient roots of the pine soon reduce the diversity of plants below the forest canopy and therefore addition of other types of decomposing plant matter to the topsoil diminishes. Eventually a new topsoil will develop that is shallow, acidic and slowly decomposing. The soil type will be very different from how it was when the naturally occuring trees occupied the soil forty years ago.

A reverse example is the reclaimation of heathland by deciduous woodland in the UK. Heathlands are a man-made landscape, created due to continuous grazing by livestock of previously wooded often sandy soils. Today, heathlands are less regularly grazed and slowly they revert to their natural state as trees seek to reinstate their claim on the soil. Managers of heathland have the perennial problem now of preventing this recession to woodland and they do so by various methods; burning, manual removal of trees and reintroduction of grazing to name the most common. Depending on the usage of the heathland site, each of these methodologies have their own pros and cons. It is important to remove the trees at an early stage partly because of the effect that the trees have on the underlying soil type. A good heathland needs nutrient poor, sandy and well drained soils to establish long-lasting heather and other heathland vegetation. An example of this type of soil is shown in the below picture.

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This was taken in 2008 on Chobham Common, an established heathland site in the South East of England. You can see there is a thin dark top layer and then a white-grey sandy layer. Below this is another shallow dark layer followed by gravelly sand.

The following graph shows a walk through a section of Chobham Common that is slowly reverting to birch woodland. The colour bars represent different ‘layers’ in the soil profile and the key thing to notice is that there are two different soil types here. The black-grey-black type (correctly called a ‘podzol’) as described above is represented by colour bars with light blue and yellow in them. A more uniform brown soil type with a thicker and lighter organic top layer is shown by bars that are missing the light blue and yellow.

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The research showed that the birch saplings at 0-2m and 6-10m had already had a noticeable impact on the soil type despite being young trees (probably less than 5 years old). Established birch trees were positioned at 18 to 20m into the ‘transect’ walk. A later study by a consultancy company looked into this data alongside their own investigations and provided advice to managers of Chobham Common to help them manage their heathland effectively. Chobham Common is now in the midst of grazing trials and hopefully, reintroduction of animals to the site will help to keep the trees back and protect the man-made environment that has become a home for so many specialised plants and animals.

In my next article on Ground to Sky, I will talk a little about one of my current research articles which focuses on the effect of cattle on the greenhouse gas emissions from a peatland. I will discuss how the experiment was designed and some of the conclusions, which give you a little more insight into how land management can affect the soil and atmosphere.

Soil is quite probably the resource that is most taken for granted by modern society. How many people who aren’t farmers or gardeners really think about soil and how important it is to the human race? Soil provides the nutrients for crops to grow, it’s the building block of forests, it provides a home for countless animals and micro organisms. It even has a part to play in regulating the climate. Soil is an invaluable resource that provides invaluable services and if we want to preserve it, we need to know where it comes from and how to make more of it.

When I was an undergraduate environmental scientist, I was taught the following equation as a way to visualise the many things that contribute to the formation of soil and determines what type of soil you get. I like the simplicity of this little function, so have a look at it and I’ll introduce all of its terms.

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Soil is a function of:

  • Climate
  • Parent Material
  • Topography
  • Biota
  • Time

Of these, I will begin with Parent Material. The parent material is the surface from which the soil will be formed. Soil doesn’t come out of nowhere and all soil has a ‘mineral component’ even if this is very small in the case of peat (will come back to these later). Parent materials are usually rocks, though in a few cases they could be animal or man made structures such as shells or concrete. The type of parent material is highly important in determining what the soil is going to be like. For example soil that forms from a red sandstone is likely to be red and gritty whereas soil that forms from a mudstone is likely to be dark and clammy. The sandstone soil probably drains better than the mudstone soil but bear in mind that the soil you get is a function of all of the contributing factors not just one or two.

So how much soil do you get for your parent material and how do you get it? Climate is an important control on the rate of the soil formation, Soil from rock is usually formed by weathering. Weathering can be physical (e.g. cracking from water seeping into cracks and freezing, or abrasion from water movement dragging pebbles over the parent material) or chemical (e.g. chemical reactions with air, water or some other chemical causing the parent material to break down into smaller components) or biological (e.g. ‘digestion’ of rock by lichens or pressure from plant roots). You can probably imagine how these weathering examples are variable according to the climate.

The topography of the land also controls the rate of soil formation and what type of soil you’re going to get. Imagine three different landscapes: a mountain range, a hilly heathland and a flat plain. The mountain range has steep slopes with very few places for soil to collect. The hilly heathland has a range of hills where the soil could be shallow and valleys where soil can accumulate. The flat plain is more uniform in soil depth.

The biota that are present in the soil forming environment is also key. Biota such as earthworms play an important part in moving established soil around and in aerating it (allowing oxygen to pass through it). Lichens have the remarkable ability to turn a rock into a soil and survive on bare rock alone. They are a symbiosis of two organisms; a plant that can photosynthesise and a fungus that clings to the rock and stores moisture. You can see soil making in action on old dry stone walls, where lichens first create a thin layer of soil and then mosses colonise this and as they eventually die away, through their decay become soil themselves. This, of course, is another major contribution of biota. Not only do they help to make soil from rock, they also become soil themselves when they die. Peats are soils that have a very high percentage of ‘organic matter’ as opposed to minerals. Peats usually form in waterlogged environments where the usual process of decay cannot occur as frequently due to a lack of oxygen. Therefore decay is very slow and soil accumulates over the years. The type of plant that grows on a soil also controls the soil type. For example, an oak woodland soil will have a rich ‘humus’ (organic) layer on the surface that is contributed to every autumn. A pine woodland will have a shallow organic layer that is highly acidic (because pine needles are acidic and because the trees don’t shed their leaves at a particular time of year).

Finally, time. The influence of time as a soil forming factor cannot be ignored. The amount of time that it takes to form a soil will depend on all the factors I have already discussed. What is the parent material? Does it break down quickly? Is the weather likely to help it break down? Are lichens likely to colonise it? What else grows nearby? Is it on a slope or in a hollow? Peat can accumulate fairly quickly by soil’s standards, it can take about 10 years to form a centimetre whereas from a bare rock, it could take between 200 and 500 years to accumulate the same amount.

When we think about the rate of soil erosion in the areas most prone, we should be concerned. Soil is lost much faster than it is replenished, so soil conservation is of major importance worldwide. In my next article, I will continue to discuss soil and will write some more about the influence of plants on soil type. I will use heathlands as an example to describe how drastically plants can change the soil and the effect that this can have on land use (this was the topic of my undergraduate dissertation).

Atop most of the world’s land surface, there is a layer of soil varying in depth from a mere few centimetres through to over ten metres in well established peat bogs. The majority of the world’s cities were founded in the most sensible places for human inhabitation – fertile lands close to a source of water. As time advanced, so did the extent and population density of many of these cities and towns and gradually bare and vegetated soils came to represent the minority land use within city borders.

Today, in many cities, parkland and other greenfield sites are considered premium resources. Townhouses bordering parks fetch the highest house prices and nearby green space is prerequisite for the majority of buildings of state and homes of the rich. Urban vegetation takes the form of trees planted by the roadsides, parks large and small, turf playing fields, urban farms and forestry and, importantly, residential gardens.

Across the world, city planners are coming to value the importance of greenery and soil within the city bounds and seek to maximise it. Why have they come to this conclusion? Not only to drive up the house prices and provide a pleasant environment to live in for city citizens but also because of the key influence that soils and vegetation have on natural cycles of heat, water and pollutants in the surface to atmosphere interface. These ‘services’ that the soils provide are termed “ecosystem services” and they are as important in the city as they are in natural environments. Ecosystem services might be cultural (house prices, space for recreational activity, aesthetic value) or provide habitats for animals and plants. They might be productive (city farms and allotments) or they might regulate the environment (take up carbon, capture pollutants such as heavy metals, cool the city down).

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In the bottom panel of the above graphic I have drawn three different land surfaces and how they impact three important things; heat, water and carbon dioxide. The size of the arrow is a rough indicator of the magnitude of the movement of that commodity. Outbound heat is much greater from the tarmac surface than from the vegetated or soil surfaces for several reasons. Firstly, tarmac is a dark surface and it absorbs a larger amount of incoming solar radiation. But soils can be dark too – so why no large heat loss from soil surfaces? Because soils are often wet as well as dark, in fact quite often a darker soil is a wetter one. And heat is required to evaporate this water, therefore less heat is absorbed and the warming effect is reduced. On vegetated soils the plants mediate this evaporation through their water-upake systems, a process known as transpiration. Back to the tarmac. The second reason it is a heating surface is that it does not absorb water, it pools on the surface. So instead of a consistent and steady evaporation flux you have either rapid evaporation from a pool on the surface, or if there is drainage, little cooling effect from water. In fact the tarmac is a barrier to downward water flow (infiltration) which is why urbanisation in areas that are naturally floodplains can cause serious flooding issues if nowhere is provided for displaced water to go. Finally, another important ecosystem service is the regulation of carbon dioxide by vegetation. This effect in some city neighbourhoods could be substantial enough in the summer to balance out the carbon dioxide from human sources (UC Santa Barbara, 2012) though it would take a lot of vegetation to do this at the city scale – to the extent that such an environment might no longer qualify as an urban space (University of Helsinki, 2012).

For further information on this fascinating and complex subject, I recommend this detailed review by Pataki et al (2011) which is available open-access here. This article provides analysis of key ecosystem services provided by urban ecosystems and their related costs (here termed ‘ecosystem disservices’).  It also provides a useful summary in Table 1. of the magnitude of impact of each of the discussed ecosystem services and the level of uncertainty associated with current knowledge.

In the next technical article on Ground to Sky, I will describe in more detail the influence of land surface type on heat, including the startling impact that hot cities have on the lower atmosphere. Before this, I will write the first of what I hope will be many insight articles into my life in science, focusing on taking field measurements of soil properties and greenhouse gases and why measurements are critically important now, as environmental science moves towards an entirely modelled world.

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There’s no hiding the impact of human life on the planet and for those in the know, the same thing goes for human impact on the soil. The way we use the land is a strong control on the ability of soils to sustain plant and animal life and the maintenance and development of those soil resources. Take the well-known issue of desertification for example: some soils are high in sand and low in organic matter and therefore don’t naturally bind together very well (organic matter acts a bit like glue for the mineral soil particles). Combine this with large trampling pressure from livestock and people and/or constant cropping whereby no residues are returned to the land and the outcome is loose soils, easily carried by air or water, leaving behind infertile bare rock.

A different example arises from my memories of my PhD studies on peatland environments. How we manage peatlands has a powerful influence on the capabilities of carbon storage, peat accumulation and fertility. Peatlands might be drained or excavated for a variety of reasons including agricultural, forestry, horticultural or fuel demands. When we drain a peatland, we allow respiring organisms to use the carbon locked away in that soil as ‘food’ and they release it as carbon dioxide. When we excavate a peatland we take away the ability of the landscape to create more peat (and therefore locking away more carbon). Striking a balance between the use of peatlands for exploitation and conservation is important for the future of these landscapes.

Soil management is particularly important for farmers, who are the only group of people (save perhaps very keen gardeners) that might be said to have an in-depth relationship with the soil, requiring substantial knowledge of the resource available on their land. Farmers on the whole are aware that it is a foolish move to disrespect the soil by overworking it. Sometimes the pressures of making a living from the land and the high reliance on factors outside of the control of the farmer (i.e. the weather, market prices, disease) can lead to a soil becoming more taxed than it can handle. This is particularly the case for intensified farms, where the land is in continual use and overworked soils are compensated with fertilisers. Fertilisers usually focus on the key nutrients (nitrogen, phosphorous, potassium) and exclude other important nutrients that are also required by plants (to a lesser degree than the aforementioned ‘macronutrients’). So follows a reduction in crop yield, for reasons that seem inexplicable without tests to confirm what is missing. In light of our understanding of the soil systems, few realise the careful balancing act that farmers must contend with when managing their soils. In the UK, there are a substantial range of consultancy business and governmental advice organisations that focus on providing a service to the farmer to help them to understand the impact they have on their soils and how to preserve them for future harvests.

Finally, what about the soils in the cities? We may pave over the land but below the tarmac, the soil is still there. In city parks, soils provide a valuable service; they feed plants that help to soak up some of the bustling city emissions (and give city residents a cool and pleasant space for recreation). These important functions are often termed ‘ecosystem services’ and even in our urban centres, they provide utility that has an influence on the surrounding environment and human quality of life. In the next article, I will focus on these ecosystem services in urban environments and talk a bit more about what makes the city different from the countryside in terms of understanding our ground to sky interface.

Soils are an important part of the complex and carefully balanced chemical cycles that keep the planet functioning in a manner that is comfortable to humans and human development. Soils are stores of key nutrients such as carbon, nitrogen and phosphorus which they pass on to plants and microorganisms to use. Most soils receive nutrients from elsewhere; from dead plant and animal matter, from the atmosphere, from rivers and groundwater, from the bedrock below and from animals moving about.

Some nutrients are used by microorganisms to produce or consume greenhouse gases. Organisms that live in the soil need a range of nutrients to survive and, like humans, they ‘breathe out’ waste products. Microorganisms that ‘breathe’ like us are called heterotrophic microorganisms. They survive in soils with plenty of oxygen which they use to decompose carbon forms in the soil and they release carbon dioxide. These organisms like drier soils, with a nice moisture film to give them the water they need (see my previous post) but enough oxygen to respire. When a soil is very rich in carbon, e.g. peat, allowing these organisms to function by draining the land means that more of that carbon is released into the atmosphere in the form of carbon dioxide and that an important carbon store is lost. Rather than taking in oxygen from the air, some other heterotrophic microorganisms use oxygen that is attached to nitrogen in a common nutrient form called nitrate. These are called denitrifying microorganisms and they can turn a nutrient that is useful for plants into a greenhouse gas which is far less useful in terms of land productivity.

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On the other hand, some microorganisms like to live without oxygen, and they ‘breathe’ in a different way, using chemical respiration (chemotrophs). Some examples of these are bacteria that produce methane (a greenhouse gas 25 times more powerful than carbon dioxide) and some that produce nitrous oxide (300 times more powerful a greenhouse gas than carbon dioxide). These bacteria are called methanogens (methane generators) and nitrifiers (bacteria that ‘breathe’ by turning ammonium ions into nitrous oxide) respectively.

There are far more bacteria types, that have all kinds of intesting metabolisms (iron oxidisers, sulphur oxidisers etc.) but I will continue to focus on the greenhouse gases for now. So, once a bacteria has made some gas – what happens to it? And how does this relate to soil porosity? Well, if a gas is produced in a wet pore – it may well stay there for some time, building up in a bubble until development of a pressure or concentration gradient releases it. Sometimes, if a gas stays in the soil long enough, other bacteria with different metabolisms from those that made the gas might want to use it. For example, some denitrifying bacteria can use nitrous oxide in their metabolism and they put out nitrogen gas instead. Nitrogen gas makes up nearly 80% of the atmosphere and is preferable in terms of global warming potential than nitrous oxide. Other bacteria consume methane, these are called methane oxidisers, so they need oxygen to function. So imagine you have a soil with a groundwater level of 10cm below the surface. In here, methane is being produced. On top, methane is being oxidised. So how much methane actually makes it out of the soil and into the atmosphere to contribute to greenhouse gas accumulation? Well, this depends on the extent of the oxidising contition and in turn on the soil porosity.

In my next article I will talk about how human land use change can affect the porosity of the soil and the nutrients that are available to bacteria. Soil’s contribution to greenhouse gas production is a natural process that can not (and should not) be artificially halted, however, when humans make substantial changes to the way the soil system naturally operates then the soil’s contribution is no longer in balance with other processes. This is where problems can arise.

When it comes to soil, space matters. The gaps between soil particles and aggregates provide channels for aeration, hydration and living space for organisms.

Soil porosity is a measure of this space between the particles. It is usually measured in cm^3 per cm^3. This is the volume of pores in cubic centimetres per cubic centimetre of total soil volume.

Porosity can vary massively between soil types. Take a clay for example, in this soil the pores are very small and not very well connected. When it rains the water takes some time to drain through the soil. This leads to features such as mottling – the green of reduced iron oxides speckled with patches of red oxidised iron – as water is not evenly distributed. Another issue with tight packed, non-porous clays is the difficulty roots have to penetrate through. It takes a water-loving plant with a shallow and efficient rooting system to thrive in clay. A sandy soil on the other hand has large and well connected pores for water, gas, plants and animals to travel through easily.

Soil pores can be full of either air or water or a mixture of the two. The water-filled pore space (WFPS) is a useful measure of how much of your soil’s pore volume is full of water. Even very dry soils retain some residual moisture, which clings to the particles in a film as a result of surface tension. This slick particle surface is the ideal living space for micro-organisms. As a soil drains, it becomes more and more difficult to pull more water out of it as dry particles hold on to the water very tightly. This is why plants start to struggle in a very dry soil, and why we water them.

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The gases that make up the atmosphere in the soil pores is quite different from the air that we breathe. There is far more carbon dioxide for example, thanks to the respiration efforts of millions of aerobic micro-organisms. There is also a little less oxygen, and sometimes there are higher amounts of nitrous oxide, methane and ammonia gas than in the Earth’s atmosphere.

These gases, after being produced by micro-organisms, have various ways of getting out of the soil. The key transport pathway is diffusion – simple passing of the gas through air or water according to a concentration gradient. The gas will always travel from a region of high concentration to a region of low concentration – hence the carbon dioxide making its way towards the surface where the concentration is much less.

Another way that gases travel is through convection. This is bulk flow that is driven by a difference in temperature or pressure from the pores to the surface. On a very windy day, pressure in the soil might end up higher than at the surface, as air is pushed away by the wind, and so, thanks to this gradient, the soil gas will travel out seeking equilibrium. In wet soils, gases that do not readily dissolve can accumulate in large bubbles, which are forced out of the soil by gradients in pressure or temperature. This process is known as ebullition.

Soil gases also travel through plants, particularly some reeds and sedges that have tissues specifically adapted to help them survive in wet soils where oxygen is limited.

So for transport of gases, soil porosity is very important, primarily in determining the rate of transport through diffusion. The amount of water in the soil pores is also a strong control on whether a gas produced at depth will make it to the surface and when that will happen.

In the next article, I will discuss some gas forming processes and talk a little bit more about what happens to a gas trapped in the soil by high amounts of water and/or low levels of pore space/connectivity.