Transcript of the reality of soil carbon: Insights from long-term trials webinar
Heather Field:
Okay. Hello everyone and welcome to today's webinar, which is the Reality of Soil Carbon: Insights from Long-term Trials. My name is Heather Field and I'm a Climate Change Service Development Officer with Agriculture Victoria and we'll be facilitating today's webinar.
Before our presenters begin, just a few housekeeping items. This webinar is being recorded and will be made available after today. You're currently all muted just to stop background noise. So if you do have a question, please use the chat function which is currently explained on your screen and we'll make some time at the end of the presentation for questions. There will be a quick survey following the webinar and that will just take a minute to complete and we greatly appreciate your assistance in completing that.
So before we commence, I'd like to acknowledge the traditional owners of the lands and water on which we are all meeting, and I pay my respects to elders past and present. And I'm tuning in from Ballarat, the lands of the Wathaurong People, and I'd like to acknowledge all the lands on which everyone is tuning in from today.
And as we work towards the challenge of climate change, including adaptation and emissions reduction, I recognise the leadership and wisdom of First People's and their knowledge and partnership as a vital way that we use to strive for more resilient and sustainable futures.
So today, we have the opportunity to hear more about the findings from Agriculture Victoria's long-term trials in the Wimmera and Mallee, which are the only long-term trial sites in Southern Australia. And if we move just to the next slide, I'll just also like to acknowledge that today's webinar is supported by the Federal Government's Carbon Farming Outreach Programme.
So I'm pleased to welcome our presenter today, Roger Armstrong. And Roger is a joint senior research scientist with Agriculture Victoria and a professor with Latrobe University based at the Grains Innovation Park in Horsham. And his research focuses on improving the sustained productivity of dry land farming systems in Southern Australia with a focus on managing soil constraints, improving fertiliser use, efficiency, and identifying strategies to both improve adaptation and mitigation of climate change. And here's the custodian of Agriculture Victoria's long-term experiments.
So today, Roger will present on a recent GRDC investment that is examining how long-term change in management can influence soil carbon and nitrogen stocks. So with that introduction, I will pass over to you, Roger, to get us started.
Roger Armstrong:
Thanks, Heather. Hopefully I'm coming through loud and clear.
Heather Field:
Yes, you are.
Roger Armstrong:
Okay. So I just would like to start by acknowledging numerous co-authors on this publication from AgVic who have contributed to this dataset now that's going back over 25 years. So several main messages from today's talk. Firstly, changes in soil organic carbon occur very slowly in modern cropping systems. The best options to increase or often just maintain salt organic carbon is to include a high proportion of legumes in the rotation, but just as importantly, do not fallow.
Growers have this sort of challenge of trying to balance short-term productivity gains from certain management systems versus long-term carbon sequestration. A really important message to take away from today is that environment and particularly rainfall and soil texture strongly influences soil organic carbon stocks.
And finally, because of the influence of soil bulk density changes associated with time of the year in climatic trends, these factors make it really difficult for growers to assess their soil organic carbon stocks on a commercial scale in a paddock.
Okay. So just a bit of background. Soil organic matter plays a really critical production services role in modern agriculture, particularly in the forms of water and nutrient cycling. There's been an increasing spotlight on agricultural environmental footprint and social licence. That's not just from the sustainability of hopping systems in Australia, but it's a really important factor from market access to a number of the Australian grain industries overseas markets.
That interest has also corresponded to, there's been increasing interest about carbon credits. And when I say increasing interest, that interest now has been going back probably over at least the last decade. But currently, a lot of the knowledge we have about agricultural management, particularly in cropping systems about soil carbon, it's often based on overseas data or modelling, and those models often aren't necessarily parameterized or validated against Australian conditions. And there's alternatively a range of research that's tried to look at soil carbon dynamics, but often that's what our short-term research. So the advantage, what we're going to do today is look at the evidence based on really a couple of long-term trials that Agriculture Victoria hosts.
So just a bit of a background about this really important consideration when we're thinking about soil organic carbon. One of the biggest challenges or message you want to get through today is about the challenge in actually assessing soil organic carbon stocks. So I pose the question, why is it difficult to assess stocks? So I'll give you this hypothetical scenario. Just say you have a four-tonne wheat crop in the Wimmera, so it's probably close to average. You're looking at a harvest index of about 0.4. You assume the amount of root biomass is 20% of the total biomass of that crop. So when you take into account the grain, the stove and the roots, only about 20% of the crop is below ground.
So if you look at the estimated residues of a four-tonne grain crop and a harvest index of 0.4, you're looking at about in the above ground, about six tonnes a hectare above ground and two tonnes below ground is roots. So that gives you a total of eight tonnes a hectare of residues. If we assume a carbon content of 40%, the potential carbon input from this system is 3.2 tonnes a hectare per year. But if we account for the fact that approximately 80% of the residue carbon is lost to respiration during the fallow, and that's based on a range of publications, both internationally and nationally.
Out of that 3.2 tonnes carbon per year, we are losing approximately 2.6 tonnes of carbon to respiration. So we're looking at a net carbon balance of 0.62 tonnes of carbon per hectare. If we look at the background soil organic carbon and some of the data we've got today, it's around about 25 to 30 tonnes a hectare. If you're looking at input of only 0.62 tonnes per hectare per year, the net change is approximately only 2% per year.
So what this sort of theoretical calculation highlights is we actually need direct assessments of how management alters carbon using long-term data where we've got the ability to detect these relatively small changes on a year-to-year basis.
All right. So from a national carbon accounting point of view, what are the potential land management methodologies that we could use? Well, the first one, I think one of the first ones used nationally was stubble retention. A lot of farmers these days are using zero-till systems, but there's still a number of farmers, particularly in the higher and medium rainfall zones that are forced to burn or cutting their residues for hay.
There's been increasing move towards zero tillage systems, but again, a number of farmers are probably in a hybrid system where they're using what we call reduced tillage or even some of them or actually use mechanical tillage. There's increasing interest in cover crops, particularly the use of green and brown manures, and probably the traditional one that's gained most focused nationally is inclusion of legume phases in the rotation, and they can either be pasture or pulses.
There's also been some recent interest in using subsoil amelioration, particularly the process called subsoil amelioration. A number of people have asked that does have an effect on productivity where there's subsoil constraints, but is there any carbon advantage in using this management strategy?
Okay. So the data I'm going to present today is derived from Agriculture Victoria's long-term experimental network. There's three trials in this network. I'm going to focus on two of those trials. The first one's called SCRIME, so that was established in 1998. It was based at Longerenong. It's in an average rainfall environment of 415 mil rainfall. It's based on a vertisol or cracking clay. So if you look in the top cell there, 65% clay content, pH 7.5, and it's got 10 treatments which are sort of phased and spatially replicated.
The second trial we're going to look at is what we call MC14. So it's based at Longerenong. It was established in the early 1980s. It's a 325 mil annual rainfall environment. It's based on the calcarosol. So basically, a calcarosol is a soil tend to be highly alkaline, particularly in the subsoil and a lower clay content than the vertisol. It has six treatments that correspond to different rotational systems and tillage practises. And similar to SCRIME, it's spatially phase and spatially replicated, so it allows us to have a good understanding of the natural variability in those trials.
What we've used to measure carbon in these presentations today are derived from the SCaRP protocol. So SCaRP was a nationally federally funded programme that started in the, I think it was about 2005, so it stands for Soil Carbon Research Programme. And what we've done is measured carbon in the zero to 10 to 20 and 20 to 30 centimetre increments. So internationally, it's the carbon in the top 30 centimetres is the recognised minimum for carbon storage.
Effectively, what we did to estimate soil carbon on a mass basis, so tonnes per hectare of carbon, we looked at the size of the depth increment by the concentration of carbon in that soil layer times bulk density on an oven dry basis. So that gives us stocks. We also accounted for the presence of carbonate and gravel if present. And for those who've seen previous talks, I would talk about stocks.
What I've done in this presentation is we've used a procedure called the equivalent soil mass procedure to account for changes in soil bulk density. And the reason that's important is when you go to sample soil, depending on the soil hardness and that, it's very hard to get consistent soil depths, so zero to 10, 10 to 20, 20 to 30.
The data I'm going to present today is I'll focus on the SCRIME trial at Longerenong and look at the changes in carbon from when we started the trial in 1998. We had a major sampling campaign in 2010. So 12 years after the trial commenced, we had data collected in ... Sorry. Its recent assessment was in 2024, so it's 26 years of data.
So this is a SCRIME trial here. It gives you an idea of setup 108 plots. Those plots, each 14 metres by 36 metres long. Typical vertisol profile. Compare that to MC14 on the calcarosol, slightly smaller plots, and again, a typical calcarosol profile.
Okay. So when we look at the soil carbon on a mass equivalent basis in SCRIME, and in this area here, I've just focused on the zero to 10 centimetre layer between 1998 and 2024. This is trend here. So what I'll do, I'll use these treatment acronyms. So in a case like this, WWW stands for wheat wheat wheat. PWB stands for pulse wheat barley rotation. Typically, the pulse we've used was field peas. GmWB stands for green manure. We've used vetch since the trial began wheat barley rotation. CWP stands for canola wheat pulse. LLLCWP is three years of lucerne followed by a canola wheat pulse rotation. And FWP is a fallow wheat pulse rotation.
This vertical bar here is just the LSD. It gives an idea of the differences between means, and these are the means here are actually real, statistically real or just random error. Main thing I wanted to show here is when we started the trial here, we measured carbon across all the plots, and that's an indication of the natural variability across this trial site. It's 108 plots, but eight hectare of trials. And what we can see here, for example, if we use the standard wheat wheat wheat treatment, you can see the line here, slow dip in 2001, levering off a bit in 2005.
It drops down in 2010. That rate of drop increases to 2016, and then it starts to slowly increase over the subsequent eight years. In contrast, if you look at the green manure, the rate of declines, much slower. And after about 10 to 15 years, you start seeing significant differences with that baseline continuous wheat treatment.
Okay. So that graph I just presented, that was the focus on zero to 10 centimetres. If you start looking at the soil organic carbon in the top 30 centimetres, and again, the reason that's significant is that, that's the standard international protocol looking at soil carbon, and I'll try and move this screen there.
What we can see here is most of the carbon stocks is in the zero to 10 centimetres, but when we start adding up the carbon from the 10 to 20 and 20 to 30 centimetres, we see similar trends, but slightly different. So the main ones to look here is particularly if you look at the green manure and the lucerne treatments, we see a significant increase in carbon stocks compared to the baseline in 1998 where what I call continuous cropping treatments such as the pulse wheat barley or the canola wheat pulse, or for that matter, continuous wheat, they tend to oscillate or slightly decrease over that period.
But the one real standout we've seen in SCRIME, and for that matter in all the Victorian long-term trials is the really negative effect that following has on soil carbon stocks. So that's a one-year and free fallow period. So a fallow wheat pulse, and that sort of trend became very obvious right from the first phase of the experiment.
So I'll summarise to a comparison here about when we talk about significant differences. So these are real effects as opposed to random chance. What I've done here, I've used the pulse wheat barley as a baseline because it's probably corresponds to the standard rotation that farmers would use compared to say something like continuous wheat. And what we're looking there for the total organic mass equivalent in 2024 in the top 30 centimetres, it was around about 24.7 tonnes per hectare.
When we look at what treatments resulted in any form of real increase, again, the increase there with agreement where we've got green and urine in the rotation, where we've had high incidence of legumes in the rotation. In this case here, three years of lucerne followed by a canola wheat rotation. This treatment here should explain is a green manure, canola, pulse, medic wheat, barley rotation. Again, it's small, but again, not statistically significant increases compared to the pulse wheat barley.
And this treatment here, about halfway through the trial, we inserted some pasture treatments. So the G+L is basically a grass plus legume pasture based around annual legumes. And again, we can see quite a statistically significant increase around about two tonnes a hectare compared to the baseline pulse wheat barley treatment. But most importantly, and again, the real standout is where we've got any form of following in the system, significant decreases compared to the continuous cropping treatments.
Okay. We took the opportunity when we measured carbon in these trials, also measured total or nitrogen, because nitrogen's a really important determinant of carbon stock. So this is the equivalent graph to what we presented for carbon this time, equivalent mass basis for zero to 10 soil carbon. Same treatments as before. What we can see here is initial starting base in 1998 and the gradual changes that have occurred over the subsequent 26 years.
Main standout here, not surprisingly, that the largest nitrogen stocks are where we've had the perennial pasture lucerne in the system and also the green manure. In terms of the continuous cropping ones, again, sort of fluctuations, small fluctuations up and down over the time period. But again, where we've got a fallow wheat pulse, so this treatment here in the solid blue line, even though there's a pulse there every three years, what we've seen, similar to carbon, are quite a significant decrease in total nitrogen stocks in the zero to 10 centimetres.
When we gain summary here, if we look at from a statistical point of view, again, using the pulse wheat barley as a benchmark. So when we started 2024, we're looking around about 2.34 tonnes a hectare of nitrogen in the top 30 centimetres. The only treatment there that's significantly different in terms of total nitrogen stock is a decrease in the fallow wheat plot. So we've lost around about 0.2 tonnes of hectare of nitrogen over that 26-year period.
Okay. I've had a real focus on changes in carbon and nitrogen stocks in SCRIME to date. How does that relate to some other indices such as the effect on mineral nitrogen and grain yield? This is just a snapshot of some data from 2023 and SCRIME. Again, you're looking at rotations here, continuous wheat, pulse wheat barley, et cetera. And what I'm showing here is the profile mineral N. We take that a month prior to sowing, so that's in the top 120 centimetres.
Looking at grain yield at harvest in treatments where we've applied additional N here and crop N uptake. So what we can see here when we focus on the profile N, so as you expect where there's continuous wheat, really low amounts of mineral N. Conversely, where we've got a green manure proceeding the wheat phase, really high levels, in this case, nearly 190 kilogrammes of nitrate N per hectare.
Similarly, surprisingly, where lucerne in the system, only about 80 kg of mineral N, which we thought tends to be higher. But interestingly, and you can see the attraction of following to, in some circumstances, prior to sowing wheat after a fallow, that system was still producing nearly 150 kg of mineral N across that site. So that stands in contrast, particularly this treatment here, that had the fallow wheat pulse ... Sorry. Lower soil organic carbon stocks system is a bit slow in the fallow wheat pulse, but one of the highest levels of mineral N. Okay.
All right. So I've tried to sort of, over the 26 years that SCRIME had been running for this dataset, I've graphed here what we've got growing season rainfall, annual rainfall, and the long-term average. So the long-term average here is 415 mils. We can see the very pronounced differences over that 26-year period in annual rainfall, growing season rainfall. And what are up here at the top are the four critical periods where we've measured carbon. So 1998, 2010, 2018 and 2024.
And for those were the older ones in the audience here, you can see that period here. So the first effectively 12 years of SCRIME corresponded to the millennium drought, so really critical deficiencies or rainfall throughout the growing season. But as the trial progressed, we turned to more normal season, and that was exemplified in 2016, which was quite a wet season. And finally, in '22 and '23 and '24, we had above average rainfall. But I suppose what this graph shows is if you look at the long-term average rainfall for the system over what, 130 years, you compare that period to the millennium drought where we average only 342 mil rainfall per annum, and then that period from 2017 to 2023 where there was nearly a considerable increase in total rainfall. So the system itself at this long and long site became much more productive.
So what I'm presenting here is looking at the net changes in soil organic carbon stocks, again, in the top 30 centimetres between 1998. So we're immediately prior to when the trial started in 2010 when we did the SCaRP measurement and then 2024. Again, if you look across all these treatments here, continuous wheat, pulse wheat barley, green manure, wheat barley, canola wheat pulse for zero till, reduced till, conventional till having lucerne in the system, fallow wheat pulse.
The main points that when we started that trial, so we had a mean of just under 25 tonnes a hectare, so basically no difference between treatments, which what we had hoped prior to starting the trial. And then if you look at changes in the latest stocks we measured in 2024, sort of have treatments ranging from a low of less than 22 tonnes per hectare, top 30 centimetres up to a high of 26.4 tonnes a hectare in the green manure treatment.
I suppose what a couple of things I want to emphasise here is firstly, I mentioned one of the land management practises about using different tillage systems. What we can see here is after 26 years of different tillage systems, there's actually no tillage made, no difference or very small, but statistically no difference in carbon stocks.
And the other thing that stands out here is that if you look at the percent change, even in the most of the continuous cropping ones compared to 1998, there's effective little or no change in your total carbon stocks over that 26-year period, even where the largest improvements in carbon stock were only about 5.5%. In this case, the green manure treatment. But where there was a fallow in the system, we lost approximately just under 14% of the initial carbon stock. In this case, around about 21.9 tonnes of hectare compared to the original 25.3 tonnes a hectare.
So the message this graph shows and what I'd like you to take away today, changes in soil organic carbon stocks occur very, very slowly, particularly in this medium rainfall zone cropping system.
Okay. I'll just briefly touch on some data we collected from the Walpeup trial. So this is a 415 mil cropping system. I'll go through the treatments here. FW stands for fallow wheat rotation. LFW stands for a legume fallow wheat rotation, and LW stands for legume wheat. The legume we used in the system up to 2019 was medic pasture. And since 2019, we've used vetch crop. And we've got two different tillage systems here, conventional, which it was conventional from when the trail started. So mechanical cultivation versus DD, which is direct drill.
The data I've got here is that the vertical bars are total organic carbon in the top 30 centimetres. So in this case here, if you look at the fallow wheat conventional, it's around about, I think from memory, about 12.5 tonnes a hectare increasing to about just over 16 tonnes per hectare where we've got a legume wheat rotation.
This red line here is that's the plot of soil total end stocks in the system. Again, notice the different axis. So they started at where we've got fallow wheat there around about 0.8 tonnes a hectare, increasing up to just over about one tonne a hectare of total nitrogen. So the key summary of what we've seen in this trial here is in terms of soil organic carbon is the lowest amount of carbon was where we had legume wheat. Sorry. Legume wheat had the highest stocks of soil organic carbon, which were higher than where we had a legion fallow wheat system, and they in turn were higher than we've got a fallow wheat system. And in contrast to the SCRIME system where tillage had no practise, what we're seeing here is where direct drill was used, significantly higher carbon stocks compared to a mechanical cultivation system.
In terms of total nitrogen, very similar pattern. No difference between the legume wheat and the legume fallow wheat system, but both of those rotations were significantly higher than the fallow wheat system. And similar to carbon direct drill, nitrogen stocks were higher than the conventional cultivation system.
A thought of focus here, similar to what I did to SCRIME, but this is a snapshot of the productivity indices in these different systems. The main things we want to show here is when we start looking at the profile nitrogen in the system is between these different rotations. So this profile nitrogen was again taken about a month prior to sowing the wheat phase in the rotation. And if we're looking at terms of nitrate nitrogen per hectare, we're a fallow wheat, we're looking at a range of about 50 to 93 kg and per hectare, depending on the tillage system.
Where we've got a legume prior to the wheat phase, seeing much, much higher amounts of profile nitrogen in the system, ranging in this case from 218 to approaching 370 kg mineral N per hectare and something intermediate where there's a legume wheat rotation. Grain yields are as expected. They're a function of both the mineral nitrogen in the system, but also much lower overall grain yields as you would expect in this 325 mil average rainfall zone system.
So what we're seeing here in contrast to SCRIME, there's a much better relationship between your carbon and nitrogen stocks and mineral N in the system. And those differences are corresponding to differences in improved not just grain yield, but protein and content and nitrogen uptake in the system.
Okay. One of the really interesting thing as a side, so this doesn't directly refer to carbon stocks. I mentioned those very high levels of mineral N, particularly where there's a legume in the system. So this is the distribution that mineral N down the profile, so down to 120 centimetres. So this treatment here is where there's a fallow wheat, so very low amounts of nitrogen. But conversely, where we've got legume in the system, in this case here, the legume fallow wheat, or in this case here, legume wheat, we're seeing very significant amounts of mineral N prior to the wheat phase in the subsoil.
And what we've seen repeatedly over multiple years where we've been monitoring this, those levels of mineral N in the profile actually have been even higher than I think the total here was around about 360. What's happening here is we strongly suspect that a lot of the mineral N produced by the legume phase is actually leaching out of the bottom of the profile. And the reason that's happening is there's a high level of subsoil constraints in this calcarosol soil. So quite simply, the wheat crop can't get its roots deep enough down in the subsoil to take full advantage of this mineral N. So the mineral N reflects high productivity potential, but it's at a significant environment cost in terms of mineral N leaching into the ground, potentially into the groundwater.
So just briefly, and I'm conscious of time. I mentioned right at the beginning about why is it so difficult to get accurate and reliable assessments of soil organic carbon, particularly if you're looking at a commercial scale? So just a bit of a refresher. So how most carbon credit schemes measure carbon stocks. So they need to account for the bulk density, which is the massive soil per unit area expresses kilogrammes per metre squared. The carbon stocks themselves are measured often by a LECO analysis. So the concentration of carbon times the bulk density will give you carbon stock in a given amount of soil.
And what we've got here is in 2010, over a 18-month period, there was measurements made of changes in soil organic carbon stocks in point out this was just in the top 20 centimetres, not top 30 centimetres. So what we see here is just in two treatments, the blue line here is the change in stocks in a just lucerne plot, sorry, and compared that to a fallow plot here. And these lines here are the bulk density.
So what we see over this period where we've got lucerne carbon stocks in the top 20 centimetres just after crop maturity, so beginning in January increased from around 10 tonnes a hectare and over the next 18 months increased by over two tonnes a hectare to 14 tonnes a hectare in the lucerne, similar increase in the fallow from just over eight tonnes of hectare in January to approaching 12 tonnes a hectare in the fallow plots.
So how can that happen? How can you get such a big increase in carbon stocks over a relatively short time period of 18 months, particularly in a fallow plot where there's no carbon going in, you could potentially see, understand lucerne, but again, there wasn't enough lucerne being produced over that period to explain that increase in soil carbon stocks. And the reason for that is when we look at the data here, changes in bulk density, so this is in January 2010, and we look over that period over the next 18 months, we see the bulk density increase from around about a bulk density of one gramme per cubic centimetre up to in the lucerne plots of nearly 1.45 grammes. So there's a 45% increase in bulk density.
And so the reason those carbon stocks increased were not because of any significant increase in soil carbon concentration, they reflect purely the increases in bulk density in that soil. So the point I make here is this was in a replicated experiment over relatively small plots by a specialist research team using the same equipment and just couldn't control for those changes in bulk density.
I suppose the question I'd pose for the audience here, if that's difficult to measure short-term changes in carbon stocks in really intense experimental conditions, how difficult would that be under commercial farming situation where you may have a 50 hectare paddock? Okay.
So in conclusion, so organic carbon stocks and total N stocks were lower in a Mallee environment than the Wimmera trials. This is a reflection of differences in productivity or primary productivity of the system, which is mainly due to rainfall, but also the lower course of soil texture in the Mallee trial. So the calcarosol has got a much sandier soil compared to the heavy clay vertisol. Really important message here is soil organic carbon stocks change very, very slowly in response to most management tech practises in modern cropping systems.
Quantifying soil organic carbon stocks depends on equal part of soil bulk density, the time of year and short-term climate trends, which has major implications for trying to measure it under commercial conditions, which is a prerequisite for soil carbon trading.
Next to organic carbon total nitrogen was concentrated in the topsoil. I didn't show this data, but that has really big implications for erosion events. And one of the cautions I would make about carbon trading is if most of your carbon's in the very topsoil and you lose that for some uncontrollable erosion event, be it flooding or wind erosion, you're going to lose a disproportionately high amount of your carbon out of the system.
Finally, growers have this sort of continued balance between short-term productivity gains and increases in profitability by using certain management practises, but that needs to be balanced against the long-term changes in your soil function, and particularly in this case, carbon nitrogen stocks.
And the question I pose here, what this presentation is, it shows what's happened in the carbon stocks over the last 25 years. Are those trends, particularly as climate slowly changes over the next 25 years, what can we expect in terms of carbon stocks under these different management systems?
And finally, I'll sort of make a plug for Agriculture Victoria. I need to point out that Agriculture Victoria has maintained all these trials over going back to MC14 for nearly 45 years, and hopefully I've showed you today the value of really what are term long-term trials in terms of assessing long-term sustainability and productivity of modern cropping systems. Thank you.
So just before I leave, just refer to this document here. It's online. It gives a really good snapshot of soil carbon. It was authored by Graeme Anderson, Melissa Cann and Heather. So that's online if you got ... It gives you a really good insight into carbon management.
And finally, I'd just like to acknowledge Ag Victoria, GRDC and the Wimmera CMA for funding this research. Really grateful from expert technical comments made recently by Ivanah Oliver from UNE and Sean Mason from Agronomy Solutions. And just acknowledge this webinar is funded as part of the Victorian Carbon Farming Outreach Programme, which is supported by DAF and the Landcare Victoria Group and Victoria's 10 Catchment Management Authorities. Thank you. All right. On that note, thanks, Heather.
Heather Field:
Great. Thanks, Roger. Really great presentation, lots of information and insightful look at some of the practical implications for growers working in challenging rainfall environments and impressive amount of data there for the long-term cropping trials.
So we do have time for questions, and we have had a few come in, and I've also popped in a few links into the chat with an article, a GRDC article that summarises some of the great information that you've heard today, and also that soil carbon snapshot. We have had a question and we have had a couple of questions around bulk density, but I'll get to those shortly. But first up, and my chat box keeps moving, we have had a question around earlier in your presentation, Roger, a question around long fallow as opposed to sowing a cover crop or similar between cash crops. Just a comment on that.
Roger Armstrong:
Okay. So I assume that the contrast there of having what a traditional whole seasoned fallow, whether it's done by a chemical fallow or mechanical fallow, what we saw was, yep, the really negative impact that had on carbon and nitrogen stocks, even though there was short-term, like what I call sugar hit in terms of mineral N.
You compare that to a green or brown manure such as the, in our case, we use vetch. The dramatic difference there in terms of the carbon nitrogen stocks is one of the really important principles about long-term productivity is to maintain your cover. And in this case, we had the green manure, it was fixing nitrogen. We retained that the vetch on plot. It wasn't exported as you would do in a hay crop and net result, we haven't directly measured it. We're in process now doing some calculations. You're potentially looking at up to about a couple of hundred kilogrammes of nitrogen fixed during that green manure phase, and conversely, a significant amount of extra carbon produced as a result of that extra nitrogen into the system. Does that answer the question?
Heather Field:
I think so. Yep. Thanks, Roger. We have had a few questions around long-term trials in pasture systems or grazing systems, particularly down at Hamilton and ways to increase soil organic carbon in pasture farming. So I'm just wondering, I know this presentation was on cropping. I just wonder if you've got any comments there, Roger.
Roger Armstrong:
I have to use my memory. So as part of the SCaRP programme, I think it was 2015, there was assessment made of changes in long-term carbon at the AgVic's long-term phosphate trial. I think what they found there from memory was where you increase productivity by overcoming that phosphorous deficiency that resulted in significant improvement in extra pasture, extra dry matter, and therefore extra carbon going into the system.
One thing I haven't addressed in this particular talk was what I've found in experiments, these were principally from Central Queensland, but I think they're equally applicable in Victoria. It was your best way of increasing your soil carbon is to have effectively a grassy pasture with some form of nitrogen input, preferably from a legume. So that maximises what the grass does. It sucks out any of the mineral N produced by the legume that keeps the legume producing more nitrogen by fixation, which in turns increases the productivity of the pasture.
So if from a purely productivity point of view, a nice, a good legume grass component in the pasture is probably definitely the best way of increasing soil organic carbon stocks in not just the long-term, but even the short medium-term phases, but that's not always applicable in modern cropping systems from an economic perspective.
Graeme Anderson:
Thanks, Roger. And Heather, I would just flag in the soil carbon snapshot, there is a section there that does look at all various research done on pasture-based systems and some of the key things that showed out, which really showed a strong, higher carbon in those perennial-based systems as opposed to the annual and things like that. But I guess Roger here, it's just so impressive to have such fine detailed analysis over such a long time in these plots for the cropping system. So, yeah, but the pasture stuff, have a look in the soil carbon snapshot. Thanks, Heather.
Heather Field:
Thanks, Graeme. Yeah, so we do have quite a few questions coming in and we have had about 200 people online, so we won't probably get to all questions, but we will definitely take a look at these after the webinar and summarise where we can. There was a few questions that have come in about bulk density, so I'll just try and summarise. We've got, how can bulk density changes be interpreted? Is this related to soil moisture changes? And there was another one which I've just lost. What increased the bulk density over time? And if SM was done, then it would adjust carbon stocks for those changes in bulk density.
Roger Armstrong:
Yep. So I'll start off that last comment. That's 100% correct that, that's the importance, it's internationally used now is using that equivalent mass equivalent for measuring carbon. And I think I've got this picture here on the slide. Why is it problematic measuring bulk density? And this is a crack in clay soil. So for the audience here, you can see that crack there.
Anyone who's tried to collect accurate bulk densities on a cracking clay, particularly after a harvest period when it's full of cracks, as opposed to when it's quite wet, it's very problematic to do it. It's very hard to get a reliable bulk density measurement. That data I presented from 2010, I wasn't directly involved in that as measurements, but it was all done by the same team, same equivalent. From memory, it corresponded to that period.
I think we went from an end of millennium drought and we had the flood. So there was something like 200 mils of rainfall recorded in the March period, and that had a dramatic effect on ability to measure bulk density in that system. But the person who made the point is equivalent mass, that is the only reliable way to estimate bulk density measurements or carbon stocks in the paddock. And that contrast, there's still a lot of carbon credit schemes around where people are just multiplying bulk density by your concentration, and it's ultimately going to give misleading measurements.
Heather Field:
Thanks, Roger. We had a question around, was green manure incorporated into the soil?
Roger Armstrong:
No, we just ... Oh, sorry. Actually, in the vetch green manure, we did, yes. So we'd typically take out of the green manure probably in early spring. And then in the first 20 years of the trial, we'll still do. We basically incorporate it within the next couple of months afterwards. So it wasn't cut. That vetch wasn't cut off as hay, but incorporated directly into the soil. So from that point of view, it's probably exaggerating the benefits in terms of nitrogen and carbon compared to it was used as a hay crop.
Heather Field:
Thanks, Roger. Just bouncing around here. There was a question around fallow, and I'm just trying to find it. Oh, here it is. At the very start, you referred to 80% loss of respiration during fallow. Is that for a year of fallow, not just summer?
Roger Armstrong:
It's over, from memory from the literature, it's just over that fallow period between harvest and crop, just residues sitting on the soil surface. I think the data, I averaged that 80%. I think the data shows direct measurements anywhere between about 72% and 90% of your cut residues sitting on the surface before it's incorporated doing anything. It's just simply lost through decomposition to the atmosphere as carbon dioxide. So that carbon is lost out of the system. Very little you can do about it. I think the only thing you'd probably potentially do it is if you incorporate it, you'd have to incorporate it depth and that's got implications too from a sustainability perspective.
Heather Field:
And there has been a few questions before the webinar about measurements down to depth and whether there's potential for these long-term trials to measure further down to depth and also the implications of bulk density with that as well.
Roger Armstrong:
That's a good point. So if you look at most, the more recent research ... Well, overseas and increasingly in Australia, there's a real tendency to measure carbon, not just down to the 30 centimetre international SCaRP standard, but down to a metre there. Funding opportunities came up. I'd love to do that.
What I would make a note of however, we've had other research from undertaken by AgVic where we've looked at the sort of where most of the roots are occurring in our medium and low rainfall zone cropping systems, particularly where you've got heavy clays. And the reason internationally people measure carbon down to, say, a metre or sometimes even metres down the soil in some environments, particularly where they're really high productivity such as North America, Central, North Europe, where there's a lot of rainfall, really, they're not just talking about aiming for four, five tonne wheat yield. They're looking at 15, 20 tonnes a hectare there from maize, whatever. The roots in those soils go down to a metre or deeper.
What we found in, and I didn't present the data here, is just the changes in our soil moisture on that long and on cracking clays, particularly not just over the millennium drought, but in subsequent years, what we're seeing in our modern continuous cropping systems is basically other than when there's flood events is often the soil water is very rarely getting past about 40 or 50 centimetres, because just the crops using it, a good winter rain for it. Longerenong could be 15 mils a week.
If the crop is using that 15 mils a week, it's falling on the surface, there's very little opportunity for recharge deeper down the profile. That's not necessarily the case in the fallow system, but in a continuous cropping. And it'd be safe to assume if there's no period of salt water below, say 40 or 50 centimetres, you're not going to have roots there, no roots, no increase in soil organic carbon stocks lower in the profile, but we won't know that until we do the actual measurements.
Heather Field:
Thanks, Roger. We've got time for another question or two. So we've got one on nitrogen. So Andrew says, "So I'm guessing the nitrogen levels in the soil in the rotation that had fallow were higher in these plots because there was mineralization occurring over the preceding year and no exporting by the growing of any crops." Comment on that?
Roger Armstrong:
Yep, correct. And so that's I suppose when I talk about short-term gains for long-term costs, it's like feeding. I use the analogy of feeding sugar to kids. So there's a short-term happy period. You could get all this free nitrogen, but conversely, that increase in mineral and nitrogen, two things to account for. In the short-term, you're potentially, in the case of Walpeup, you could lose it through leaching on some of the heavier soils, not necessarily well-structured crack and clays. You've got potential losing that significant amount of that mineral N build up the surface soil through denitrification.
And what the evidence from our day-to-day is in the long-term, you get a short-term increase in mineral N. In the long-term, your soil organic nitrogen and carbon stocks are decreasing. So it goes back to that question is, can farmers make tactical decisions to increase productivity, save on fertiliser costs by failing? Yes, but they need to balance that against the long-term costs in terms of sustainability and therefore profits.
Heather Field:
Great. Thanks, Roger. We are nearly at time, so I will pull up the questions there and we will have a look through if we've missed anything that we can follow up with later, but there has been some really great questions and lots of interest in this space. So thank you everyone for those questions. And we've had a lot of support in the chat there also around the continuation of the long-term trials as well, so absolutely. We're all very supportive of that as well.
So, unless Roger, you've got any final comments or Graeme, if not, I'll close this out. And I just want a big thank you to you, Roger, for sharing those valuable insights into soil carbon and nitrogen dynamics in a low and medium rainfall cropping system. Just want to remind those attendees that we do have a short survey when you close out of the webinar. If you're using the Zoom app, you'll see it pop up in the browser instead. So really appreciate you just giving us your feedback and any other questions you might have.
And if you'd like to revisit any part of today's webinar, the recording and the resources that I've popped into the link, into the chat will be circulated in the coming days, and feel free to reach out if you've got any follow-up questions. So with that, thanks again for your time and engagement, and we look forward to seeing you at the next AgVic climate webinar. So I'll leave it there and-