Increasing soil carbon by managing soil nutrition

So welcome everybody to the July Soils Knowledge,
Network of Knowledge webinar brought to you by the New South Wales DPI Soils Unit. This webinar series is part of a community
of interest around soil, so look out for other products you probably heard of like All The
Dirt, which you might like to subscribe to. If you’re listening but you’re not a subscriber,
the link to subscribe to the webinar invitation is on the front slide there at the bottom. The one with the eepurl letters in it. Well I’d really like to introduce Susan Orgill
who is presenting today’s webinar – Increasing soil carbon by managing soil nutrition. Susan has worked for NSW DPI since 2005 on
a range of soil health projects. Since 2012 she’s worked as a researcher with
the Soils Unit with the current focus on management strategies to increase soil carbon. Susan’s interested in carbon fractionation,
subsoil carbon sequestration, and the management of pastures to increase soil organic matter. And it’s my pleasure to give Susan the presenters’
rights. So over to you Susan. Good morning everyone thank you for joining
me for this presentation on soil carbon. Soil carbon is a topic that has received increasing
attention over the last couple of decades particularly with the role of soil for mitigating
climate change through increased carbon sequestration. However farmers have recognised the importance
of increasing carbon in soil for production for hundreds if not thousands of years so
it’s something that is important for policy and it’s certainly important for production
ability as well. Today I’d like to run through these six key
points. First of all what do we actually talked about
when we say soil organic carbon, so what is it? Secondly the importance of soil organic carbon
for production and farmer’s income in terms of production and farming resilience and sustainability. Thirdly the management practices to increase
carbon, so what can we actually do to get more carbon into the soil and in particular
soil nutrition for production and therefore organic matter supply. Then how do we keep carbon in the soil? So once we actually get carbon into the soil
how do we actually stabilise that carbon so it stays there for longer? And I’ll also run through some calculations
for soil organic carbon stocks as well, so how we actually measure them. So to begin with what do we actually mean
when we say soil organic matter and soil organic carbon? These two terms often get used interchangeably. So carbon is what we actually measure and
that’s the carbon component of organic matter in the soil and it’s approximately say fifty-eight
percent carbon by weight. So what’s the rest made up of? Well it’s pretty much what every plant is
made up of as well. So the rest is made up of Oxygen, so it ranges
from about ten to forty percent Hydrogen. So Oxygen and Hydrogen come from water and
also from the atmosphere as well. There’s Nitrogen, so about eight to ten percent
of soil organic matter is made up of Nitrogen, Phosphorus, Sulfur and then a range of other
nutrients and trace elements in minor amounts. So obviously while carbon is really important
we need to think about soil organic matter in terms of the other nutrients as well. When we actually look at what it is in the
soil, it’s been categorised into a range of different pools. So when we talk about organic matter in soil,
by definition in Australia it’s the less than 2 millimetre organic fraction of soil. Now it consists of partly decomposed organic
residues, microbes both alive and dead, humus which I’ll talk about later and also charcoal
as well. On the right-hand side of your screen you
can see what those fractions actually look like. So the particulate organic carbon is just
the small fractions of organic matter in the soil. Humus is the sticky glue that you can see
in the middle image. And on the right hand side you can see resistant
organic matter or charcoal, so very fine bits of charcoal. And in some Australian soils it’s been reported
to up to thirty-five percent of the soil organic matter actually consists of charcoal. Excuse me. So we need to think about soil organic matter
in terms of productivity as well, and productivity is actually linked to soil functions that
depend on the decomposition of organic matter. While we want to increase carbon in soil,
it’s also important to think about we want to be increasing the cycling of organic matter
through soil as well. So it’s a very important nutrient reservoir. Up to ninety five percent of soil Nitrogen
is kept in the organic matter pool. Up to forty percent of the available Phosphorus
is in that organic matter pool. We know that with farming a lot of our dollars
is actually spent on phosphorus and nitrogen. And up to ninety five percent of soil Sulfur
is also in organic matter. It’s important to think about the role of
organic matter for supplying nutrients. Now my example there where I say pasture phases
and cropping phases is about the role of pastures in increasing organic matter and also available
nutrients. Normally I would be presenting to people where
I could see them and do a demonstration on the board. But if we think about the pasture phase being
three years and a cropping phase being three years then going back into a pasture. During that three years of pasture phase I
want you to imagine that on the x-axis we’ve got time on the y-axis we’ve got carbon. As the pasture establishes we get an increase
in carbon over time, so we see that curve going up. Then we return to the cropping phase, if we
don’t apply mineral Nitrogen and Phosphorus and Sulfur, but the key is Nitrogen here,
when we return to that cropping phase that carbon level will actually decline over the
next three years. Now trials have supported this regardless
of pasture type. Once if we don’t apply a fertiliser, if we
go from a pasture phase to a cropping phase we get this kind of increase in carbon then
decrease. And then the role of the pasture phase again
is to increase carbon and then we get to the cropping phase and we decrease. That’s agronomically desirable because the
role of organic matter in that system, apart from other benefits such as enhanced soil
structure, is supplying Nitrogen. Therefore we’re not applying mineral nitrogen
to that cropping phase. But inherently were actually running carbon
up and down. If we wanted to maintain carbon in that system
when we return to the cropping phase, our trials have shown that you need to actually
apply mineral N. So the question then becomes what’s the purpose
of the pasture phase? If we ignore the benefits of carbon for climate
change mitigation, and if we’re looking at reducing mineral N and inputs in this system
and reducing costs than its agronomically desirable to actually use that organic matter
that’s been stored during the pasture phase in the cropping phase. So the point of this is that stores in a high
state of flux, so it’s turning over very rapidly. And mineral nutrients move between the organic
matter store and the soil solution through microbial mobilisation and immobilisation,
so mineralisation as well as production moves on. In addition to nutrients organic matter is
also very important for soil structure and increasing cation exchange capacity. Firstly increasing water holding capacity;
in a water limited environment such as Australia increasing water holding capacity is a really
important function of soil organic carbon. In a sand dominated soil we know that one
percent increase in soil organic carbon – so moving from one percent carbon to two percent
carbon represents about a third increase in water holding capacity. It’s less important for a clay loam but when
we’re looking at clay rich soils the increasing carbon is really important for the plant available
water. So making water that’s more available to plants. This is represented in a couple of examples
graphically on the right-hand side of your screen as well. We’re here on the x-axis we’ve got an increase
in soil organic carbon for a sand dominated soil and a silt loam soil. So two quite common New South Wales top soil
textures. And on the y-axis we’ve got soil-water content. Now this bottom line that we can see here
in both graphs is wilting point. The top line here is our field capacity so
whether soil can no longer hold any more water without deep drainage. And as we can see if we increase organic carbon
content in both these soils we get a significant increase in the amount of plant available
water, which is the difference between field capacity and wilting point. Also important is the role of soil organic
matter for increasing cation exchange capacity so the soils capacity to hold onto and exchange
nutrients. In sand dominated soils, organic matter accounts
for most of the cation exchange capacity, and that’s where that rule of thumb of two
percent organic carbon in your soils has actually come from. So if you’ve got a very sandy soil you need
at least two percent organic matter to actually support production through that role of cation
exchange. When we say it ranges from 25 to ninety percent
of the cation exchange capacity at the lower end of the scale, say twenty-five percent,
most of the cation exchange capacity is actually supplied by the clays in the soil. And at the higher end we’ve got it supplied
by organic matter in sand dominated soils. Now this is also dependent on pH because in
really acidic soils we have our humic and fulvic acids not dissociating, so they’re
not in solution and able to hold onto these cations that you can see hanging around here. As we increase the soil pH the dissociation
occurs and therefore we’ve got a higher capacity to store and exchange nutrients. So the point of this as well is that we’ve
got soils that vary in their capacity to accumulate and store carbon. So how do we actually
achieve charge as well. We’ve also got this inherent capacity that
we can see but ultimately what it’s about within a soil type is managing the supply
of organic matter and the loss of organic matter. If we think about this in terms of the soil
profile and the flux and flow of carbon around the landscape, now we have the carbon loss
via soil erosion, so we’ve got that red arrow there at the top right hand side. So if we just, that’s a given, we don’t know
necessarily that that returns to the atmosphere under all conditions, but it’s certainly a
loss from the particular site. We’ve got carbon being sequestered in the
soil through photosynthesis, so we’ve got this influx of carbon dioxide through the
process of photosynthesis into the soil. Once it’s within the plant it goes from the
gas to a liquid in a plant and then into a solid in terms of roots. So carbon gets into the soil through root
exudates, root sloughing and litter from above ground as well as dung as well. So we’ve got this above-ground contribution,
below ground contributions. Once it’s in that organic matter pool and
cycling around here we have organic carbon that can be protected within aggregates, so
it’s basically protected from soil organisms and it’s not readily accessible for decomposition. It can be chemically protected so that’s where
we get these organo-mineral associations and organic carbon being bonded with clays. And then we have this accumulation of microbes. So dead microbes is a way of sequestering
carbon but live microbes also respire carbon so this is where we get this soil respiration
and we also have plants respiring as well. So ultimately we just want the photosynthesis
carbon sequestration to be greater than the loss by soil and plant respiration. But as I said we know that there’s the influence
of soil type and of carbon. So here on the x-axis we have total carbon
that’s in grams per hundred grams so that’s equivalent to carbon percentage. On the y-axis we’ve got depth. So what I’m showing you now is to 70 centimetres
the carbon concentration on three different soil types in the Monaro region of southern
New South Wales. The first one is a basalt-derived soil the
second is a shallow granite derived soil, so where the C horizon or decomposing bedrock
is within 50 centimetres of the soil surface, and the third one is the Monaro deep granite
derived soil. All very productive as well. And as we can see and as you’d expect carbon
is highest in the surface soil layers and then decreases with an increase in depth. We can also see a very obvious difference
between a basalt derived soil with a high and reactive clay content compared with a
shallow soil and compared with the deep granite derived soil which is much sandier in those
surface soil layers. Interestingly for our native perennial pastures
and our introduced perennial pastures we can see little difference with perennial pasture
type. So these data comes from 32 survey sites in
the Monaro region. Now we’ve also got an influence of climate
as well. So what we can see now at the bottom of the
screen is the soil carbon concentration throughout the profile for a deep granite derived soil
in the Boorowa region. So we’ve got the Monaro region compared with
the Boorowa region and we can see that more than thirty percent more carbon was actually
stored in the profile from the deep granite derived store in the Monaro region and that’s
due to a difference in climate. So in the Boorowa region there’s an equi-seasonal
rainfall distribution and milder temperatures compared with the Monaro region where you’ve
got summer dominant rainfall and much cooler winters. So we’ve got a capacity to increase carbon
storage but also reduce the loss as well. If we look at what this means in terms of
carbon stocks, so the tons of carbon per hectare. These are the same sites – we’ve got our native
and our introduced perennial pastures and we can see there was no difference in carbon
stocks to 70 centimetres within a soil type but certainly differences within the region
between soil types, so twice as much in those basalt derived soils and far less in the shallow
granite derived soil where only native introduced pastures were considered. If we look at the Boorowa soils here we can
see the significant difference between the deep granite derived soils in the Monaro region
and the deep granite derived soils in the Boorowa region. But again no difference between introduced
and native perennial pastures. Now the CG and RG here refer to it continuously
grazed and rotationally grazed and there was actually significantly more carbon under continuously
grazed pastures in the Boorowa region compared with rotationally grazed when we look to 70
centimetres. This is contrary to some claims that have
been around for a long time about the role of rotational grazing on increasing soil carbon. But I’d like to point out here that this is
actually compounded by soil nutrition. So our rotationally grazed pastures in the
Boorowa region were low input pastures. So therefore they didn’t actually have fertiliser
as part of their management strategy. Whereas our continuously grazed pastures were
fertilised and it was the role of fertiliser in these systems that was actually increasing
carbon. So now we’ve got a table that looks at the
land management options to increase soil organic carbon. So we’ve got, on the first line we’ve got
within Australia looking at all the published data we’ve got sequestration rates reported
of up to 0.75 tons of carbon per hectare per year to 30 centimetres. So these are from replicated trials, not necessarily
field surveys, but replicated trials which have demonstrated increases in soil carbon. If we look at what the majority of those increases
in soil carbon associated with, they’re with the management of either soil nutrition or
with pastures. So we’ve got perennial annual pastures in
New South Wales sequestering up to half a tonne of carbon per hectare per year to 30
centimetres. Also the liming of pastures, pasture phases
in cropping rotations, nutrient management, grazing management and legumes in pastures. So supplying nitrogen in pastures as well. The highest rates reported are for legumes
in pastures and converting cultivated crops to pastures. So where you’re starting from a very low carbon
concentration and then establishing a fertilised perennial pasture to increase carbon. But if we have a look at some of these rates,
some of them are quite slow and unspectacular and that’s what we’ve seen some of our replicated
trials. So this is the MASTER trial which ran for
18 years in New South Wales at a place called Book Book. Now the soil type was a Sodasol, so it was
an acidic Sodasol, so it had acid soil problems and was also dispersive at depth. Now we’re looking at 18 years worth of data,
it was a continuously cropped paddock and there was treatments of lime and no lime and
also annual and perennial pastures. The annual pastures were sub-clover and annual
ryegrass. The perennial pasture was a mixture of phalaris,
cocksfoot with also an annual component of sub-clover. On the x-axis here we’ve got the organic carbon
concentration going up to three and a half percent. On the y-axis we’ve got depth descending to
1.2 metres. We’ve got our T0, so the start of the trial
marked and after 18 years worth of data marked here as well. So the circles are T0 the squares are after
18 years. Annual pastures are the solid symbols and
perennial pastures are the open symbols. And we can see after 18 years we were only
moving from a organic carbon concentration of 2.2 less than three so less than a one
percent increase. So it was quite slow and unspectacular in
that sense but there certainly was an increase. Interestingly though there was no difference
between annual and perennial pastures as both were fertilised and both were limed. So it was the role of the pasture in this
system, not necessarily the type of pasture in this study that was shown to be significantly
different, and the sequestration rates are up to a half a tonne of carbon per hectare
per year. So the question then becomes in some systems
why don’t we see an increase in soil carbon despite this management? And farmers often ask me the opposite. For example, the neighbour is doing ‘the
wrong thing’, cultivating aggressively or over grazing, why don’t they continually see
a decrease in organic carbon concentration? Well there’s a few reasons for this. First of all we could be looking at soil type
and climate. So these are our major drivers of net primary
productivity and decomposition. And if that’s overriding any influence of
management then we won’t see an increase or a decrease in carbon. Secondly it could be that it’s not necessarily
the land use, that is crop versus pasture, but it could be more the management of that
crop and pasture, and in particular ensuring that there’s adequate nutrition of the crop
or pasture to grow maximum amount of biomass. So the third point for the water limited environment
that that plant exists in. So where we’ve seen no difference between
annual and perennial pastures what that research actually showed was that both plants were
given ample opportunity to grow to their maximum amount for that given water limit within that
region. Secondly the other important thing was in
that particular trial they demonstrated that while seasonally the contributions of organic
matter were different between annual and perennial pastures, on an annual basis there was very
similar contributions of organic matter to soil. So annuals dumped it all at certain times
in the year, with perennials slowly distributed throughout the year. In pastures also we’ve got a very large background
level of carbon which means that detecting change might be challenging because you require
a bigger amount of change because of the amount of carbon that’s stored there and also similarly
with the spatial variability of carbon within a paddock. And in some cases perhaps soil organic carbon
is just the wrong metric for the benefits of the management that you want to look at. A classic example of this perhaps is when
we have started to look to 30 centimetres and we’re comparing conservation agriculture
with conventional cultivation, where you’ve got two different systems. So we’ve got minimum tillage or no-tillage
which has very important benefits for soil structure, reducing soil erosion, and increasing
soil water. But we’re not always seeing increases in soil
carbon when compared with continuously cropped paddocks and what we have is a redistribution
of carbon throughout the profile. And so it’s not saying that one is necessarily
better or worse than the other, but soil organic carbon is perhaps not the right metric in
that case. And in some cases maybe we just need to add
water. Left hand side of the screen now you can see
one of the granite derived sites from the Monaro region which was sampled in 2009. Now this particular site was an introduced
perennial pasture, this is the year before the drought broke, the Millennium drought,
and there was 40 tonnes of carbon per hectare in 30 centimetres. When we came back to this same site so this
arrow indicates the second point in the next photo in 2012 this is what the site looked
like. Now there’s been no additional inputs to this
site other than the drought breaking, it’s the same site, and we saw 47 tons of carbon
per hectare at this site here. So that was an increase of almost two and
a half tonnes per hectare per year to 30 centimetres which is considerable when we think about
the most we were achieving in replicated trials is about 0.7. So we’ve got this huge influence of climate. Another example for a basalt derived soil;
2009, they’ve had a hundred and four tonnes of carbon per hectare in 2009 to 30 cm and
again you can see that considerable influence of soil type for the same region. Coming back to the site in 2009, now in this
picture is is what I thought all sites looked like in the Monaro because it’s the first
time I’ve worked in this region – that is, during the drought. But, this is what I saw when I came back in
2012 and the site had a 131 tonnes of carbon per hectare. Now this increase is seemingly off the charts
because we’ve got almost nine tonnes of carbon per hectare per year being sequestered between
these two sampling points. And it was a huge amount of particulate organic
carbon that had been accumulated in that soil, so from fresh organic matter. If you can increase it that quickly you can
certainly decrease it that quickly as well, if not faster. So the way we measure carbon stocks is we
collect multiple samples to at least 30 centimetres, we calculate the soil bulk densities so that
we know the mass of soil per hectare, we measure the carbon concentration and then very simply
– this is more complicated when you’re comparing between sites and over time – we multiply
the carbon concentration by the bulk density by the depth of the sample and we want to
make sure that we just looking at soil, so we account for the gravel proportion. And that will give us the tons of carbon per
hectare. So if we just do a back of an envelope calculation
for how it would increase from two to three percent carbon in the top 10 centimetres. As I said it’s back-of-the-envelope, so I
literally wrote on the back of an envelope so it’s a bit rough. If we assume a bulk density of 1.2 – so remember
we’re looking at the previous equation and we’re looking for a 1% increase in carbon
over 10 centimetres. So it’s the one percent increase multiplied
by the 1.2 for bulk density multiplied 10 because that was the depth we were looking
at. Well that’s an extra 12 tonnes of carbon per
hectare added to the soil. Alright, so if we think that plant residues
contain about forty-five percent carbon, so that means for that top 10 centimetres we
need to add 25 tonnes of dry matter per hectare to achieve that increase. If we were going to achieve that over five
years it would be roughly about five tonnes of dry matter per hectare per year extra,
so above what’s being produced at that site now. But we know that about 50% of the added residues
decompose very rapidly and don’t move into that organic carbon, long term organic carbon
pool. So therefore we need to double it. So what we’re looking at is over 10 tonnes
of dry matter per hectare per year above what is currently added to get just a one-percent
increase in carbon. So it can be very slow and this is where soil
nutrition can certainly play a role in increasing biomass production, but this is also where
some claims of extraordinary increases in carbon just almost can’t be true. So if we think about grazing management to
increase soil carbon this was a replicated grazing trial in the Monaro region with three
grazing over five years. The site was fertilised with Phosphorus and
Sulfur so that the critical plant available nutrients were achieved. We’ve got ungrazed pastures, tactically grazed
pastures and cell grazed pasture so we’re looking at the influence of grazing management
over five years – this is at the end of the five-year trial, on total Carbon, Nitrogen
and labile Carbon – so fresh organic carbon – stocks. Now we’ve got the ungrazed pastures we can
see with significantly different to cell grazing, but there was no difference between the ungrazed
and tactical. So tactical was basically grazed to allow
seed set and the tactical and cell graze. So we can see that there is a very strong
influence of grazing management compared with if we just lock the pastures up on carbon
stocks, and this happened just over five years. And this was a replicated trial but there
was no difference between cell and tactical graze, so this difference was not significant. So ok well what does that tell us? It tells us that we can manipulate plants
through grazing management to achieve significant increases in soil carbon. There was no difference at the end of the
trial in total Nitrogen but we know that throughout the trial this would have varied. So we’ve got this strong influence of soil
nutrition on organic Carbon. This graph has been presented in many different
forms for a long time. So we’ve on the x-axis got Nitrogen on the
y-axis got Carbon concentration. We can see that there’s a correlation between
an increase in total nitrogen and an increase in carbon, so that’s well-known. We also know that there’s a link between phosphorus
Colwell phosphorus and total carbon and this is a positive and significant correlation
as well. Similarly we available sulfur. So therefore we could say well if we can increase
available phosphorus and sulfur we can a) grow more and maybe stabilised more which
will have a look at in a minute. So pasture management that’s increasing herbage
mass production is pretty much going to increase or give the soil the best chance of increasing
carbon accumulation as well. Our grass dominated pasture rely on nitrogen,
so we need to make sure that legumes have adequate phosphorus and sulfur for nodulation
so that we actually get this biologically fixed nitrogen as well. We know that both that legumes are sensitive
to pH but we also know that the Rhizobia species are also sensitive to pH as well. So we need healthy legumes for livestock production
but if we want to have healthy nitrogen fixation occurring so our legumes working harder we
need to make sure that pH actually is adequate for the Rhizobia species. So here’s our clovers here which is looking
at most of our sub-clovers. So they need a pH of above 5.5 to actually
be active and actively fixing nitrogen. So this is the key in terms of liming, not
necessarily for the pasture but more for the nodulation and the associated rhizobium species. I also said that nutrients are important for
carbon stabilisation and this is where I’m going to start ending up the presentation
on looking at the stabilisation of organic carbon in soil. Clive Kirkby, a scientist from CSIRO as well
as other international scientists have put a lot of research in to looking at the relationship
between nutrients and carbon. So if we look at the stable form of carbon
which in Australian soils is generally, in Australian agricultural soils, is generally
about 70-percent say humus, we know that for every tonne of carbon there’s 90 kilos of
nitrogen associated with it, about 20 kilos of phosphorus and about 14 kilos of sulfur. So we know that stable carbon which is actually
microbial detritus, so humus is basically an accumulation of dead soil organisms and
microbial products. It actually needs nitrogen, phosphorus and
sulfur as well and this is the most stable form of organic carbon in soil. If we take a grass for example such as wheat,
we know that a tonne of wheat has 17 kilos of phosphorus sorry of nitrogen, two of phosphorus,
three of sulfur. So there’s a significant gap here. So the nutrients are coming from somewhere. So we need to not only feed the crop or the
grass, the pasture, we also need to feed the soil organisms as well. And they’re very effective at feeding themselves
but we can manipulate this process through soil nutrition as well. If we look at the nutrient concentrations
between fungus and bacteria we can see that humus is roughly in-between the proportions
in the soil organisms and the concentration that’s actually in the grass. So this is where we know that these microbial
products are significantly contributing to the humic pool and this is where this theory
of microbial detritus representing humus comes from. So nutrients are important. So what can we do? We’ve got a trial starting in the field where
we’re looking at this recipe and we’ve certainly evidence from lab-based soil incubations as
well. Let’s look at this example now there’s two
different soil types, basalt and granite derived, we’ve got top soils 0 to 10 centimetres and
subsoils 40 to 50 centimetres. So looking at different soil depths, different
microbial communities as well as different clay contents saying if we increase carbon
which is the carbon added on the x-axis and we use these ratios here for carbon stabilisation. So basically a recipe saying we’ve got this
much carbon going in, this much nitrogen and this much sulfur, can we get a linear increase
in soil carbon, which is stable soil carbon so it’s going to be there for a long time,
providing those really important roles the cation exchange capacity, plant available
water soil structure and we can, it worked regardless of soil type and it worked regardless
of soil depth as well. This is in the lab and now we’re looking at
taking this to the field to say okay could we actually look at enhancing the stability
of carbon under pastures? But there’s obviously going to be a cost associated
with this and I wanted to think about the potential value of this as well. If we were going to purely take this recipe
approach in the field there’s going to be severe financial limitations, and I’m not
at all suggesting this is what we’d do. But if we’re going to put a value on those
nutrients, if we’ve got a system without legume which is the top part of this table. we’re actually applying bagged nitrogen these
are the cost per unit per kilo of this nitrogen added. It’s going to be about 216 dollars for one
tonne of carbon increase. With a legume in the system we can reduce
that down 81 dollars, but that’s still a considerable cost and so I’m not suggesting this what we
do. But taking the science I guess into how we
actually think of carbon in the soil and the associated benefits for production is certainly
a way forward for pasture science. So last slide. Can you trade carbon in soil in Australia? Well yes you can, there’s two methodologies
that are currently approved by the Australian government. So there’s a measurement based methodology
which is called ‘Sequestering carbon in soils in grazing systems’ and this requires baseline
measurements and then monitoring of these site. So basically collecting baseline data implementing
a grazing management change, it can be fertiliser and/or grazing management as well, and this
is just in grazing systems. And then you get paid based on the increase
in carbon. The second methodology is the model based
methodology and it’s called ‘Estimating sequestration of carbon in soil using default values.’ So basically modelled values. So based on your region, and a change in management
the model estimates your increase in soil carbon. It might be 0.2 tonnes per hectare. And that is how much you get credited for
if you can prove that that’s the management. So how do you prove that that’s the management
that you’ve adopted? Well you at the moment you get a stat dec
from an agronomist or local advisor or some other approved person that says that you’ve
adopted those management practices. No measurements required and you just collect
I guess some evidence that you’ve actually made that change. And those changes could be grazing management,
they can be fertiliser, they can be liming, improved pastures, renovating pastures. So there’s a lot of flexibility in that particular
methodology. But I hope I’ve shown you that our actual
sequestration rates can be quite limited by some factors that are outside the control
of management. So despite these policy tools I guess the
limited studies that we’ve got report variable soil organic carbon increases but there is
the opportunity to trade carbon in soil in Australia at the moment. So to finish off the key messages I’d like
to highlight is that both plants and soil microbes require nutrients and this I guess
is a key to increasing carbon in soils. And soil nutrition is important for plant
production and also nitrogen fixation. Ultimately my advice to farmers as always
no matter how you apply nutrients in terms of if its in organic form or mineral fertiliser
form, as long as you’re ensuring that plants have adequate nutrition to grow to their maximum
water limited potential for that particular region, then you’re pretty much ensuring that
you’ve got a maximum supply of organic matter for that site. Optimizing nutrient management increases production
and also organic matter stabilisation and some evidence suggests that strategic grazing
increases organic matter supply and reduces the loss of organic matter through maintaining
ground cover and reducing soil erosion. Thank you very much. Fantastic Susan, thanks very much for that
that’s certainly a an extremely comprehensive cover of a fairly difficult topic and you’ve
made it all you’ve made a lot of things that I know take a lot a lot of work to reach those
conclusions and make them sound quite quite simple, so thanks very much for that, you’ve
covered a lot in that time. So have we got any questions? I’ll throw it open to the audience now. I’ve actually got one for you Sue to start
off with. You mentioned, you touched on a little bit
on on tillage I just wondering whether you could expand on the role of strategic tillage
in the field of soil carbon, if there’s more you can say? Okay thanks Luke, so the question is about
strategic tillage. So in terms of carbon flux the role of tillage
can actually be an increase of carbon loss from the soil through enhanced decomposition. So basically when the soil is cultivated it
alters the air and moisture in the soil and mixes up organic matter so soil organisms
actually become more active and respire more. So I think ok well that’s where we get this
loss of carbon from the system and continued cultivation can also enhance soil loss through
soil erosion. The idea of strategic tillage is looking at
where tillage is going, the benefits of tillage are going to outweigh that short-term loss
of carbon. So if it’s going to reduce perhaps a pest
or a pathogen and increase ultimately increase biomass production and plant establishment
through – then we could actually get an increase in carbon. So Mark Conyer’s trial which on the GRDC strategic
tillage project has actually shown where you go in and actually strategically till a soil
within, and then establish a pasture within 2 to 3 years carbon levels actually go back
to their original level of the permanent pasture. So we’ve actually not necessarily detrimentally
decreased carbon in the soil, but we’ve perhaps in some cases even enhanced it if it means
that you can get a better pasture established a better crop established and therefore supply
more biomass to that site. So while continuous aggressive cultivations
is not necessarily recommended as a carbon management tool, obviously it wouldn’t be,
strategic tillage certainly offers opportunities to increase biomass and organic matter supply
to soil. Thanks Sue, we’ve got a question from Carol
Rose – just wondering what your take is on compost tea? I think that anything that you could do to
increase microbial activity in the soil could be beneficial for carbon sequestration. Compost teas I haven’t seen a considerable
amount of published data on the benefits in terms of nutrient cycling and increases in
soil carbon. The process I think theoretically on a smaller
scale could increase microbial activity but I haven’t seen any convincing evidence in
the published literature that it would change the microbial dynamics and I know you know
this Carol that we’ve got this huge abundance and diversity of organisms that are already
in the soil, introducing a small amount of specific organisms is unlikely to considerably
alter that balance. What they are going to be changed by I guess
is their environment, the habitat, air, moisture, shelter and food being carbon. I think certainly though, looking at microbes
in terms of nutrient supply and decomposition is an expanding field that needs more attention. Okay thanks Carol. We’ve got Lindsay Johnston with a question
I’ll just ask Lindsay’s question, it says is there a rule of thumb ratio of nutrient
levels required per unit of rainfall in different pasture systems to maximise growth? Do you want me to repeat the question? I’d only say I want you to repeat it so I
can think about it! It’s a very good question about a rule of
thumb for increases in nutrients per millimetre of rainfall. It certainly, we’ve got a project that starting
and that’s looking at carbon sequestration or across a climate gradient, or a proposal
in, across the climatic gradient looking at nutrient availability. So at the moment we, I don’t have that information
at hand, but it’s certainly an output that we hope to get from a current project. In terms of nutrients required for the humus,
that’s kind of universal, doesn’t matter on climate, doesn’t matter with soil type, it’s
just looking at those ratios that’s actually in that humic pool. But your question is really important because
what it does is I guess it overlays that matrix of nutrients required for pasture growth stability
and that role of nutrient cycling. And that role of nutrient cycling is going
to be different under different pasture types under different climate scenarios, so it’s
really an insightful question, so thank you I’m sorry I don’t have a better answer. Okay we got Jeremy Gittings. On one of the earlier charts you had a graph,
on one of the earlier slides you had a graph of water holding capacity improvements. Were the units metres cubed of water per metre
cubed of soil? Is that right? Yeah that’s correct and I think these slides
will be made available as well. So I mean this is an older paper and it’s
actually very challenging to find increases. Lots of people talk about increases in water
holding capacity and plant available water with increases in carbon and it’s very hard
to find published data actually on what those rules of thumbs are. But you’re correct. Thanks Sue, Thanks Jeremy. Now I’ve also got from Carol which is; do
you see a risk in carbon trading for those years when cropping or drought decreases carbon
in soil? Very good question, yes I do. They put a buffer into the contract for climate
and drought so there’s something like a 20-percent buffer that they take from your carbon credits
and park it somewhere due to these natural risks. Importantly, with carbon tradingit’s either
a 25 or a hundred year contract that you enter into which is tied to the title of the property. I think the benefits of increasing carbon
outside of carbon trading far outweigh the risk that you take on with carbon trading
in most of the agricultural systems. But the real opportunities are where carbon
trading can incentivise land-use change or better land use management. So where there is perhaps a degraded paddock
it can offer some incentive, financial incentive, to perhaps plant a permanent perennial pasture
and do it well in terms of establishment. But what you need to think about, and thinking
about that table that I put up before with the sequestration values, is you need to account
for additional emissions. So if you grow more pasture you are likely
to graze more livestock and then therefore that additional amount of livestock that you
graze, you need to account for those emissions in terms of how much carbon is sequestered. So if you sequester one for example but you
admit to two therefore you don’t get any money because you’ve actually emitted more than
you sequestered. If you’re not part of a trading agreement
though those emissions at the moment as it stands aren’t taken into account. The same goes for if you apply fertiliser. So if you can increase your soil organic carbon,
and even the model-based one will actually generate this for you, you can have a play
with it on the Internet, the tool that they’ve got there, if you increase soil carbon but
you increase your emissions because you’re using a nitrogenous base fertiliser, limes,
because when you apply lime the way that reacts in most soils produces carbon dioxide, so
they account for all that. So it makes the amount that you can actually
increase even less. And remember the last option I think carbon
per tonne was about twelve dollars a tonne, and it’s a bid, yep cheapest price carbon. Great thanks Sue, thanks Carol. I’ve got another question for you and it’s
relating to the variation in carbon within the year and how big this could be and what
the limitations are for the time of sampling, is there a best time of you to sample? Ok so first of all the variation within the
year is very significant obviously it depends on what system you’re in if you’re in a cropping
based system or if you’re under a perennial pasture or an annual pasture and if you’re
cultivating or not. But there is more variation within the year
generally then there is sequestered over the year. So it’s very important to get the same time
of year sampling or under the same conditions. And the classic example of this I think is
where we’ve had a scientific discussion about this is within the Technical Working Group
for soil carbon at the Australian government level because they had set there that you
had to sample within the same month when you go back and resample to show that you’ve either
increased or decreased carbon. Now we challenged this quite strongly because
it’s not necessarily about a month, or necessarily a season, but it’s about soil conditions. So roughly you could say alright if we’re
always going to sample before you maximum biomass production in perhaps April or spring
or whatever system that you’re actually working in, as long as the conditions are always the
same when you go back to have you continual monitoring, then that’s the most important
thing. If you pick a time of the year where there’s
more fluctuations, so kind of mid/peak growth then you might actually see more fluctuations
within your results depending on when you sew or pasture biomass or crop curves. Thank you. We’ve got, it’s more of a comment I think
from Douglas Fox. So Douglas has typed in the largest failure
in applying living entities to the soil is the failure to evaluate if there is a suitable
tucker and accommodation to sustain them as a viable living entity. I couldn’t agree more. I think by the time you start tinkering with
the actual organisms that are there, you’ve spent a long time making sure that everything
else is right and I don’t think very many people are at that point. Terrific. A wrap up comment from Rebecca Lines-Kelly,
so Rebecca has said Congratulations on an excellent presentation your grasp of the topic
is formidable and your summary is succinct and informative, easy to follow and understand,
and your future research looks fascinating. Thank you so much Rebecca Lines-Kelly. Thanks for that Rebecca, it mightn’t be a
bad note on which to end. So thank you so much Susan for a really informative
webinar. So thanks everybody, thanks Sue. increases
in soil carbon? Well it’s about the balance. So we need to increase the supply of organic
matter and the best thing about this is what we actually ultimately aim to achieve in production
so to grow more biomass. An alternative to this is to apply biomass
that’s grown elsewhere to the site, so in terms of compost or by biochar. So we need to supply more organic matter but
it’s the balance because we’ve also got a loss of the carbon that’s occurring as well. So we’ve got a loss of carbon through the
decomposition of organic matter. So as soil organisms breakdown organic matter
they also respire just like we do, and so you’ve got a loss of carbon from the system. Carbon also moves around the landscape with
soil erosion and different soils have different capacities to store carbon. So if we look at the screen now on the left-hand
side we’ve got a dark dermosol soil on the right – sorry a soil with enhanced soil structure
throughout the soil profile. We can see that this has ultimately got a
higher clay content and higher soil organic content compared with the kandosol on the
right-hand side of the screen. Now the soils capacity to store carbon is
firstly determined I guess by clay content because clay has a higher surface area then
the coarser fractions of soil. So it can actually hold onto and carbon through
organo- mineral association so basically organic carbon sticking to clay surfaces and the ability
of organic carbon to stick to clay surfaces depends on the type of clay as well because
they have different reactivity and differ.

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