#247      25 min 25 sec
Giving off gas: Agriculture's role in greenhouse emissions

Biogeochemist Prof William Horwath explains the impact that modern agriculture has on greenhouse gas emissions from the soil. Presented by Dr Shane Huntington.

"Agriculture is responsible for two-thirds of the nitrous oxide emissions across the planet, and nitrous oxide contributes close to 20 per cent or so of the total global warming potential of [greenhouse] gases combined." -- Prof. William Horwath




Prof William Horwath
Prof William Horwath

William Horwath, soil biogeochemist and the J. G. Boswell Endowed Chair in Soil Science at the University of California Davis is internationally known for his work in soil organic matter and sustainable agriculture. He is particularly focused on improving the understanding of nutrient release from crop and waste residues. His work is wide ranging, encompassing determining microbes as precursors to soil humic substances, effect of agricultural practices on water quality, role of agricultural practices on greenhouse gas emissions, to enzymic control of nitrogen uptake by microorganisms.
 
In 2009, he was elected 2009 fellow of the Soil Science Society of America Professor.  Horwath’s career can be summarized by his never-ending enthusiasm to conduct research in sustainable agriculture and forestry and to provide leadership on natural resource management and solve critical environmental issues related to food and fiber production.

Credits

Host: Dr Shane Huntington
Producers: Eric van Bemmel, Kelvin Param, Dyani Lewis
Audio Engineers: Gavin Nebauer (Melbourne) and Tim Kerbavas (Davis, CA)
Voiceover: Nerissa Hannink
Series Creators: Kelvin Param & Eric van Bemmel

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VOICEOVER 
Welcome to Up Close, the research talk show from the University of Melbourne, Australia. 

SHANE HUNTINGTON 
I’m Shane Huntington.  Thanks for joining us.  When we hear reports about climate change discussion is usually focused on the notion of carbon emissions and, in particular, the gas carbon dioxide or CO2. What's often overlooked in this simplified view is the potent impact that numerous other greenhouse gases have on global warming.Modern agriculture is one of the greatest sources of a range of greenhouse gases.  While belching cows might  be the first image that springs to mind, the problem is far more complex.  Indeed, plant crops are also culprits.  The way we use and care for the soil could be an important factor in controlling the impact of agriculture on our planet.To discuss the role that soil plays in agricultural emissions, we are joined from the studios of University of California Davis by William Horwath, Professor of Soils and Biogeochemistry.  Welcome to Up Close, Will.

WILLIAM HORWATH 
Thank you.  Good to be here.

SHANE HUNTINGTON 
When we think about greenhouse gases most of us think about carbon dioxide emissions from cars or emissions from coal burning or other industrial activities.  How does agriculture contribute to global greenhouse emissions?

WILLIAM HORWATH 
Well, agriculture contributes on average about seven per cent of the total greenhouse gas emissions across all economic sectors and it's pretty well standard for developed countries at least.  It may be more for developing countries for the lack of industry but agriculture produces methane, carbon dioxide and nitrous oxide, all these gases.  Methane comes from rice production and enteric fermentation.  Carbon dioxide comes from the loss of soil carbon and the use of fossil fuels to run equipment and nitrous oxide comes primarily from the application of nitrogen fertilisers and often nitrogen fertilisers are applied in the form of ammonium and in soil ammonium can convert to ammonia which is then a substrate that a very specialised group of organisms can utilise and convert to nitrate and upon its conversion it can lead to loss of nitrous oxide as an intermediate product.

SHANE HUNTINGTON 
When we talk about the importance of nitrous oxide and nitric oxide, I guess as well, as greenhouse gases, how do they rate compared to CO2 for example which everyone's well aware of?

WILLIAM HORWATH 
Well, agriculture produces all of the main greenhouse gases which are methane, carbon dioxide and nitrous oxide.  Methane comes from enteric fermentation which is sort of the belching of cows and carbon dioxide comes from a number of things, soil carbon depletion or use of diesel to run equipment but the use of fertilisers and nitrogen provides a substrate which is ammonia that can be oxidised into nitrous oxide which is the most potent greenhouse gas we know of which is 300 times more potent than carbon dioxide, for example.

SHANE HUNTINGTON 
When you speak about potency and that 300 times more powerful, what exactly are we referring to there?  Its ability to work as a greenhouse gas and lock in heat?

WILLIAM HORWATH 
Yes, its ability to re-radiate the heat radiation that bounces off the Earth's surface back into the atmosphere and if you have these gases in the atmosphere they bounce back to the Earth's surface keeping it warmer.

SHANE HUNTINGTON 
Why is then that we always hear so much about CO2 and not these other gases?

WILLIAM HORWATH 
Well, CO2 or carbon dioxide is often related to the combustion of fossil fuels which is sort of centre stage in the global warming story but agriculture is responsible for two-thirds of the nitrous oxide emissions across the planet and nitrous oxide contributes about - maybe less than 20 per cent or so of that total global warming potential of these gases combined.

SHANE HUNTINGTON 
And Will, if we consider - you know, we have very different agricultural set ups today than we did several thousand years ago, how do our modern sort of agricultural emissions compare to what you'd find just naturally where there are large amounts of trees, other life just existing in a natural state?  Why is it so much worse when we do it?

WILLIAM HORWATH 
That's a good question and the production of nitrous oxide from soils is a natural process.  All soils and ecosystems produce this gas.  It's a process done by a group of organisms called nitrifiers and they're a very undiverse group but yet very effective at what they do and so they take ammonia which could be from the mineralisation of soil organic matter or applied as a fertiliser and convert it to nitrate.  But that process is often not completely efficient and so you have these other by-products such as nitrous oxide that potentially are given off and that fertilisation process provides that substrate above and beyond what would be happening in a natural ecosystem.

SHANE HUNTINGTON 
So presumably we still have to worry about what's happening in the natural ecosystem though because that must be a contributor to these greenhouse gases, is that right?

WILLIAM HORWATH 
Well, it is a contributor.  We concentrate on agriculture as a source but we don't often look at what's happening in natural systems and so we don't really have that baseline and there's not much data on that.  Since it is a natural process, and it was probably an important process in earlier Earth's history when the sun was cooler, to keep these kind of greenhouse gases in the atmosphere but today as the sun warms and continues to warm as it gets old, we don't need all these greenhouse gases and nitrous oxide would be one of those that we could not have to deal with if we didn't have to.

SHANE HUNTINGTON 
If we were to compare the outputs that we get from your average crop field versus a livestock farm which one's sitting in the worst case for us?  Is there one that we should be particularly focusing on or are they both as bad as one another?

WILLIAM HORWATH 
Well, I don't world data but I'm working on reports for California which has actually an established climate change law and we have to develop inventories for all economic sectors and it turns out that enteric fermentation, which is cows and all livestock versus manure management versus fertiliser application, they all contribute about equally to the production of nitrous oxide and other greenhouse gases.

SHANE HUNTINGTON 
When we sort of dive down into the soil, is this primarily soil bacteria doing the job here or are there other processes occurring that are causing these outputs?

WILLIAM HORWATH 
It is primarily the result of microbial activity or bacteria and there are some non-biological pathways that have been demonstrated by most importantly are the biological processes.

SHANE HUNTINGTON 
Will, when farmer is looking for conditions that are good for growing a particular crop, what types of soil attributes are usually measured and determined as being optimum?

WILLIAM HORWATH 
Well, if you could only do one test I would say pH because pH so much controls the nutrient availability in soils.  It even controls the amount of nitrous oxide that can be produced but that's the simple test.  More often a farmer would be looking at soil texture.  A loamy soil would probably be better, for example, which is in between a clay and a sand.  They would be looking for carbon content or soil organic matter.  The more soil organic matter you have in soils, generally the more productive the soil can be or often is and things like that .

SHANE HUNTINGTON 
Then presumably this would be different for every different type of crop, is that correct?

WILLIAM HORWATH 
Yes, certain crops like certain soils.  Tree crops or perennial crops tend to like more looser soils like loams or sands and then row crops such as corns and grass crops or cereal crops can tolerate a wider range of soils.

SHANE HUNTINGTON 
I'm Shane Huntington and you're listening to Up Close.  In this episode we're talking about agriculture's contribution to climate change with soil scientist, Professor William Horwath.  So we have this incredible range of different parameters in the soil.  Now, in your lab in particular you've been looking at a number of factors that affect the emissions that we, as a result, get from these soils.  Can you speak a bit about what you're trying to look at there, in particular in terms of oxygen levels and other parameters, that might make these soils more susceptible to providing larger emissions?

WILLIAM HORWATH 
That's a good question because for the last 30 years it's been presumed that nitrous oxide production among other factors that affect it, that the oxygen content in soil is really the driver and that when you get to a water-filled pore space or how much water fills the pores in soil, you have to remember that soil is half air and half minerals, and if you get to the point where you fill up those pores with 70 per cent water it's always been assumed that that's when denitrification occurs and that's when the production of nitrous oxide can happen.  But there's been some interesting research in pure culture suggesting that the production of nitrous oxide can occur at very low oxygen levels which doesn't make sense for our understanding in soils and so that's what motivated this research that I'm talking about today.  Then when we looked at this we decided that would control for the oxygen content in soil by using three different soils, basically a sand, a loam and a clay and provided the same amount of water instead controlled the oxygen content.  By controlling the oxygen content we soon found that the pure culture studies were actually leading us in the right direction and the most nitrous oxide production from what we thought was an aerobic process was occurring at a point where oxygen was almost extinguished, almost disappeared from the soil system.
That's, in itself a major paradigm shift because most of our biogeochemical models assume that this is all one pathway which is denitrification but this research shows that maybe not.  It's not often that you can find a soil totally devoid of oxygen and so that's what motivated this research.

SHANE HUNTINGTON 
So, given that outcome, does that give us the opportunity to change some of the parameters of the soil to actually minimise the emissions that we would get or is this the worst case scenario we're expected it would be lower under those conditions but, in fact, it's still there?

WILLIAM HORWATH 
It's a process that, as I said, occurs in natural ecosystems or it's always occurring.  The reason it becomes a problem in agricultural soils is that we're adding the substrate.  Often, when we add fertilisers we add them in ammoniacal or ammonium form and the oxidation takes place when an enzyme called ammonia oxygenase grabs on to an ammonium and then starts the cycle of oxidising the ammonium into nitrate and the first product is hydroxylamine and then that is converted to nitric and nitrous oxide and then eventually could actually go all the way to dinitrogen or N2.  But the point being that if oxygen is controlling the production of nitrous oxide from the nitrification process then it seems like the soil should maintain a certain oxygen level above what we see here in this paper that we're discussing above three per cent and how do you do that?  You can do that by ensuring that there's sufficient soil carbon in the soil which gives soil structure.  It gives it larger pores.  Maybe even go to a no-till system where you would bring back earthworms so they can burrow and make larger pores so that the soil can breathe easier.  Those kinds of things that we talk about when we talk about sustainable agriculture. What can we do for the soil that can make it more productive?

SHANE HUNTINGTON 
Is there a possibility of a modification of the types of fertilisers that we would use that would also benefit this system?

WILLIAM HORWATH 
Yes, that was another interesting outcome of the study showing that different forms of fertiliser have a different propensity to produce this nitrous oxide and it, again, leads to the reaction that occurs when these fertilisers are added to soils and in this paper we've compared urea which is a standard fertiliser used throughout the world, with ammonium sulphate which is probably a little more expensive formulation that may not be used as much but the interesting thing is when you add something like urea or maybe  aqua ammonia or anhydrous ammonia, the initial reaction in the soil is to raise the pH slightly and as you raise the pH or the soil reaction just slightly, to a more alkaline reaction more of the ammonium is converted to ammonia and that ammonia, which is volatile is the substrate of these ammonia oxidisers or nitrifiers.  That doesn't happen with ammonium sulphate as much, for example. 

SHANE HUNTINGTON 
Now, I understand that you've also been looking at the iron content of soil.  Why would iron actually have an influence on the nitrogen cycle in these systems?

WILLIAM HORWATH 
Very good question and that also was very interesting to us because the literature really doesn't have very much information on that.  This project was motivated by the use of compost as a way to mitigate nitrous oxide emissions in soil but it turns out that there's not much effect of compost.  But we were looking at any parameter we could look at and one of them was iron and we looked at two different irons in the soil.  We looked a mineral iron which was extractible with a certain extractant and then another which was associated with the organic matter directly and it turns out the one that's associated with the organic matter is very highly correlated to nitrous oxide production and you might say, well, how does that relate?  It's not a substrate.  These ammonia oxidisers don't use carbon.  They're autotrophs.  They don't use carbon and so why would iron associated with organic matter have any impact on that?. Well, it turns out that maybe they can use this iron organic matter as sort of a battery, a sort of a way to shuffle electrons around and so that may be one way they can regenerate some energy and so it may not be a substrate but it may be a way for them to cycle electrons, for example.

SHANE HUNTINGTON 
Will, these sound like some amazing experiments that you're doing there.  You're going to have to explain for me the laboratory environment because I can't imagine you collecting these gas emissions out in the field so how do you set up a scenario where you can collect these various emissions in a very controlled way?

WILLIAM HORWATH 
In a laboratory it's much easier to do, of course, than in the field.  We do have a large field program as well because, as I said, the Global Climate Change Solution Act in California requires us to develop inventories for all economic sectors and so we have been spending a lot of time in the field developing emission factors for the top 10 crops in California and we have over 400 in California but that helps us to get inventories but it doesn't help us understand the actual processes leading to these emissions in the field and that's where a laboratory comes in where we can do highly controlled experiments.  Basically it's pretty simple, I mean, you just use hardware that you can control the oxygen content with.  You have certain chemical extraction procedures.  Things that you find in any standard lab basically and it's not really anything novel actually.

SHANE HUNTINGTON 
We're talking about fairly low concentrations of some of these gases, though, aren't we?

WILLIAM HORWATH 
Oh yes, we're talking about parts per billion in many cases but in laboratory incubations you can up that by adding that substrate and having smaller vessels and therefore you can get into the ppm range and therefore have much more accuracy in what you're looking at.

SHANE HUNTINGTON 
I'm Shane Huntington and my guest today is soil scientist, Professor William Horwath.  We're talking about greenhouse gas emissions and fertilisers here on Up Close.  Will, in these laboratory environments, have you been able to optimise the setup of the soil and the various conditions so that you minimise the amount of nitrous oxide that's actually coming out?

WILLIAM HORWATH 
Well, yes.  Often we look at these as ways to promote nitrous oxide.  We've often manipulate things to see what is actually causing its production but that indirectly then tells us what could be done to mitigate its production.   So yes, you can get that insight and I think as in the study we looked at, that when we looked at different levels of oxygen that gave us a clue that in the field if you can somehow make the soil breathe easier maybe it would produce less nitrous oxide.  So yes, there's direct connection between the laboratory environment and the field setting.

SHANE HUNTINGTON 
When you do that transition from the laboratory environment to the many of the actual farms there in California, what sort of management practices do you have to change in terms of the farms to get farmers to take this knowledge on board?  I can imagine the economic stresses might be a limiting factor there?

WILLIAM HORWATH 
That's a good question.  I can give you an anecdotal story here about a process that may help avoid some of the things that we find in the study.  For example, tomatoes is a very productive crop in California.  We produce over 90 per cent of tomatoes for the paste market in the United States and we export a lot too and the farmers 10 years ago were using a very old version of irrigation which was essentially furrow irrigation, just running water down furrows and fields.  Back then water was more available and it still is but it's becoming more limited.  So they moved towards new technology developed in Israel called sub-surface drip and essentially you put a drip line about a foot or 30 centimetres underneath the soil bed and plant the tomato crop on top of that and that has reduced nitrous oxide emissions dramatically in comparison to furrow irrigation.  One of the reasons for that is that we can now control the amount of fertiliser added to the crop when it needs it so a fertigation approach and also because the fertiliser is applied at a foot or 30 centimetres below the soil surface it now takes that potential nitrous oxide that's produced at that depth, now it has to move a very long way through the soil.  The longer the pathway is out of the soil the more potential that some other bacteria will grab it and convert it to dinitrogen.So just by changing irrigation approaches and not even worrying about soil type we can start to address nitrous oxide emissions.

SHANE HUNTINGTON 
As we know, around the world there's a large amount of interest in genetically modified crops.  Do these have the potential to substantially reduce the amount of emissions that we are getting on the farms?  Has that sort of parameter been investigated to this point?

WILLIAM HORWATH 
Yes.  It wouldn't be a direct effect of genetically modifying an organism but if we can crack trying to increase nitrogen use efficiency so any way to increase nitrogen use efficiency, if you can create a better crop to scavenge nitrogen out of the soil or if you can fertigate through sub-surface drip, any way that you can increase the uptake and the use of nitrogen and don't allow the microbial community to have that much access to the fertiliser then you can reduce nitrous oxide emissions.

SHANE HUNTINGTON 
Is another way to do that to avoid iron-rich soils as well?

WILLIAM HORWATH 
Yeah, and that's an issue.  I don't know how you can do that.  Some soils will be low in iron.  Some soils will be high in iron and I think you're just kind of stuck with that.  We don't really know where we go with that other than we know that iron is directly related to nitrous oxide production from this other study that we did, yes.

SHANE HUNTINGTON 
When we talk about all these changes that we can make and the various nitrous oxide emissions that we get as a result, what type of difference are we talking about for the global agricultural community in terms of those emissions? Are we just looking at a five to 10 per cent reduction or are we talking about something quite significant?

WILLIAM HORWATH 
Right now the estimate used in the Intergovernmental Panel on Climate Change is an estimate of about one to 1.2 or maybe a little larger per cent of the amount of nitrogen fertiliser after it's applied is converted to nitrous oxide.  If we could reduce that to less than one per cent and even lower that would be probably a milestone that would help a lot because agriculture produces two-thirds of the nitrous oxide on the planet.So, as I said, it's difficult to stop the process because the diversity of micro-organisms in the soil kind of prevents that.  We could add certain stabilisers, for example, like nitrification inhibitors.  They work to a certain extent.  I think the average functionality of them are a 30% reduction in nitrous oxide emissions but overall there are other approaches that can be used, for example, more complex crop rotations.  We've studied that here at Davis and putting in, for example, four year crop rotations or three year crop rotations that provide a diversity of crops that utilise nitrogen when other crops don't or when they do - or mop up after a very inefficient crop.  We have demonstrated very, very nice numbers of only two per cent loss of nitrogen over 10 years and that would dramatically reduce losses of nitrous oxides if we went that direction.But unfortunately, if you do the economics on that, it's not as profitable as a standard approach of a less complex rotation.  So we know how to do this, it's just that farmers are under certain pressures, economic pressures, that they have to deal with in the marketplace.  It doesn't allow them to use all these things that we do know about how to reduce nitrous oxide.

SHANE HUNTINGTON 
Will, farming seems to be one of the areas that many of the carbon taxing-type programs are not very effectively engaging with.  How do you see those programs as being an impetus to get these sorts of changes in place?  I can imagine especially in the third world where the drivers are a lot different, it would be very difficult to get these changes in place.

WILLIAM HORWATH 
It is and that's what we're battling with here in California since the passage of the Global Climate Change Solution Act and we've looked at carbon.  We looked at soil carbon sequestration and it's probably not going to pan out well for a grower to do that.  Yes, the grower does know that increasing soil organic matter is probably the best thing they can do for their soils.  It will increase cell productivity, it will increase aeration, it will probably reduce nitrous oxide emissions in the long run.  The issue is that you can only sequester so much carbon in the soils and, from our long-term studies, we can sequester about five tonnes of carbon.  In today's market that's not much money.  The average rate for a tonne of carbon I think has been somewhere around $20 or less per tonne and it would cost the grower a lot more to put that carbon in the soil than what they could get paid for on a cap and trade market, at least under these conditions today.If the value of the carbon increased dramatically in the future maybe that's more of an option.  The second problem with that is that you can only store so much carbon in your soils.  It's not like a saturation thing but it's kind of your environment, your soil type, what kind of crops you're growing.  You come up to a certain point and then that may be five tonnes, it may be 10 tonnes but it's always going to be finite and once you reach that point then you have to manage that soil the same way in perpetuity, forever and so that's not so appealing as a way to cap and trade a carbon type of product like soil carbon.

SHANE HUNTINGTON 
William Horwath, Professor of Soils and Biogeochemistry at the Department of Land Air and Water Resources at the University of California Davis, thank you for being our guest today on Up Close and talking with us about how agriculture contributes to global warning.

WILLIAM HORWATH 
I thank you for this invitation and I'd also like to thank my colleagues Xia Zhu, Martin Burger and Timothy Doane who helped me greatly in doing this research.

SHANE HUNTINGTON 
Relevant links, a full transcript and more info on this episode can be found on our website at upclose.unimelb.edu.au.  Up Close is a production of the University of Melbourne, Australia.  This episode was recorded on 2 May 2013.  Producers for this episode were Kelvin Param, Eric van Bemmel and Dyani Lewis.  Audio engineering by Gavin Nebauer here in Melbourne and Tim Kerbavas in Davis.  Up Close is created by Eric van Bemmel and Kelvin Param.  I'm Shane Huntington.  Until next time, good-bye.

VOICEOVER 
You've been listening to Up Close.  We're also on Twitter and Facebook.  For more info visit upclose.unimelb.edu.au.  Copyright 2013, the University of Melbourne.


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