By Patrick Francis
The value of optimising soil organic carbon (SOC) to farming ecosystems and plant and livestock productivity is unquestionable. In paddocks where soil organic carbon has been depleted which is the common scenario across most of Australia’s cropping zones and in grazing land which suffered soil erosion, returning carbon onto the surface as herbage and into the soil as organic matter produces significant beneficial responses.
But in 21st century farming in Australia optimising SOC for a particular soil type, rainfall, agricultural enterprise and farming methodology has become a far more contentious issue as the more science delves into organic carbon composition and dynamics the more scientists and farmers realise how little is known about it.
Three streams of understanding SOC have been emerging in Australia over the last two decades:
- Peer reviewed science which is monitoring SOC in agricultural soils and is characterized by recent papers such as “Soil organic carbon in cropping and pasture systems of Victoria, Australia” by Dr Fiona Robertson et al 2012, Victorian DPI; “Soil carbon sequestration in cool-temperate dryland – pastures: mechanisms and management options” by Dr Alieta Eyles et al 2015, University of Tasmania; “Effect of pasture systems on carbon sequestration”, by Dr John Graham et al, 2005, Victorian DPI; and “Soils and climate change: potential impacts on carbon stocks and greenhouse gas emissions, and future research for Australian agriculture, by Dr Jeff Baldock et al, CSIRO, 2012.
- Farmer directed reviews such as: “Soil Carbon Sequestration Potential: A review for Australian agriculture”, By Dr Jonathan Sanderman et al, 2010, CSIRO; “A farmer’s guide to increasing soil organic carbon under pastures” By Chan et al 2010, NSW Department of Agriculture; “Managing soil organic matter – a practical guide”, by Dr Frances Hoyle, 2013, for GRDC; “Soil carbon sequestration – myths and mysteries” by M. Bell and D. Lawrence, 2009, Queensland DPI; “Carbon Grazing – The Missing Link” by Alan Lauder 2001.
- “Grey literature” (a term coined by Alieta Eyles) referring to farmers and consultants personal opinions, and on-farm trials and observations published on line and in the farming press such as: “The Quest for a Practical, Predictable and Reliable Method to Build Soil Carbon in Agricultural Soils: Mr. Guy Webb” by Christopher Johns, Future Directions International, May 2015; “Race to measure carbon’s new job: by Matthew Cawood, Fairfax Agricultural Media May 2015; “Phillip Island cattle stud declared carbon neutral” By Kellie Nichols, February 2015, Victorian Landcare & Catchment Management.
The interest in soil carbon levels has risen considerably since July 2014, not so much for its inherent benefits to soil health, soil water holding capacity and pasture/crop[ productivity but for its income generating potential through the Emission Reduction Fund’s (ERF) decision to approve a methodology for “Sequestering Carbon in Soils in Grazing Systems”. In April this year this methodology hit pay-dirt with a number of grazing system projects winning Federal government’s Clean Energy Regulator carbon abatement contracts.
One soil carbon abatement via grazing project has committed under contract to deliver 3.4 million Australian Carbon Credit Units (one unit is one tonne of CO2e) over 10 years at an average price of around $13.95 per unit (this is average for the 144 contracts awarded in April 2015). For the farmers involved in this and other soil carbon abatement via grazing projects, this represents a potential income boost providing they can deliver the soil carbon volume under contract to the government. As yet, a methodology for measuring the TOC level increases across the grazing areas involved has not been developed by the project’s contractor. It is interesting that the Regulator’s existing approved methodology for sequestering CO2 in soil using grazing has not been adopted by this particular project’s contractor.
For most professional farmers giving priority to management that maintains or increases soil organic carbon if it has been depleted by previous practices, has become standard practice. Numerous scientific papers confirm higher levels of soil carbon improves paddock soil health, pasture and crop productivity, animal health, greenhouse gas abatement and ecosystem services. But when it comes to making decisions about “Sequestering Carbon in Soils in Grazing Systems” for an Emissions Reduction Fund project then the science and extension data is unconvincing for permanent native and introduced pastures and positive for land previously cropped for decades which is returned to pastures.. Figure 1 and 2 are typical of the hypothetical and actual data available for cropped paddocks returned to pasture.
Figure 1: Hypothetical or modelled example of how soil organic carbon (SOC) changes with time during cropping then return to pasture.
Figure 2: Typical data to show how returning a paddock to pasture can lift soil organic carbon level to native pasture state.
Such a clear and documented SOC response is not apparent in the scientific literature for different pasture types and grazing methods on farms where soil erosion is not part of the annual cycle of natural events.
This is an important point to make because soil erosion in pasture paddocks is far less common than it used to be prior to landcare starting in 1990. While it does happen particularly in autumn on pastures in southern Australia managed for high sub clover content, approximately 75% of the 85,000 pasture farmers in Australia’s 88 million hectare agricultural zone have taken steps to ensure sufficient ground cover is present to prevent it happening (source “Land management practice trends in Australia’s grazing (beef cattle/sheep) industries”, DAFF 2011).
Sanderman et al highlight the lack of data available for “improved” grazing practices in Australia’s permanent perennial pasture areas.
“While increases in dry matter production, especially when herbage is maintained through dry summer months, and turnover of plant material due to increased grazing should translate into SOC stock gains, this remains to be conclusively demonstrated.”
And in the rangeland the situation is not very different with most of existing scientific data about impacts of changing grazing practices impact on SOC based on North American data where soils generally contain significantly more organic matter than typically found in Australian rangelands.
“The applicability of these results to Australia is, therefore, questionable,” Sanderman says.
But the “grey” literature is positive about using “improved” grazing practices for increasing soil organic carbon in permanent native and introduced pastures across Australia’s agricultural zones and rangelands. Here in lies the dilemma for farmers with scientist, extension and grey literature all having different opinions about the magnitude of change in soil organic carbon levels derived by implementing different farming practices.
Science is confusing
The most confusing aspect for farmers surrounding organic carbon build-up or depletion in soil is interpreting how organic matter transitions into various components (also called pools), each with a dramatically different life span or turnover rate. Virtually every publication about SOC provides a table which describes its pools and how long each survives before decomposing in the soil. Table 1 and Figure 3 and 4 show typical examples.
Comparison of the data in table 1, figure 3 and figure 4 shows some significant organic carbon pools variations:
- Fresh residues <1% versus <10% versus no mention
- Particulate organic carbon: 2- 25% versus 10 – 50% versus <10%
- Humus: >50% versus 33 -50% versus 40 – 80%
- Resistant organic carbon: up to 30% versus 1-30% versus 10 – 50%
Table 1: Size, composition, turnover rate and decomposition stage of the four soil organic carbon fractions (in this case referred to as soil organic matter).
Figure 3: The five major soil organic carbon pools.
Figure 4: The three major soil organic carbon pools
To further confuse the picture, each SOC pool is given a turnover time range which lasts from months, to decades, to centuries. The questions arising for the farmer who is managing pastures to increase TOC are: which perennial herbages and grazing management method provide the particulate organic carbon which lasts for months versus decades and which produce humus which lasts for decades versus centuries. Being able to answer both questions is important for both ecosystem function and grassland productivity and critical for an Emissions Reduction Fund soil carbon project. In the latter case if the herbage and grazing management produces a bulk of humus which has a 10 to 15 year life span, then the probability of achieving a statistically significant permanent increase in soil organic carbon is low.
Sanderman et al highlights the importance of recognising the different soil organic carbon pools with varying turnover times. In a hypothetical example they show that depending on the fate (which pool it becomes) of new carbon sequestered in soil, there is a three fold difference in the amount of new carbon ultimately sequestered.
“An important collorary to this concept is that if the new inputs cease, the C stocks will return to their previous level rapidly if this new C has not become physically protected,” Sanderman says.
The Baldock et al 2012 paper makes some important points about gaining a greater knowledge about the SOC pools and their life span. He says “…an understanding of SOC composition will provide an assessment of the vulnerability of SOC stocks to subsequent changes in management practice. Factors defining the biological stability of SOC can be divided into two types: those responsible for defining whether or not a particular form of SOC can be decomposed (biological capability), and those defining the rate of decomposition (biological capacity).”
Baldock recognises “grey” literature claims about innovative farming practices such as introducing summer active perennial grasses into annual pastures, rotational grazing and pasture cropping impacting SOC but suggests some caution.
“Further definition of the mechanisms and quantification of rates of SOC change under these innovative systems are required. Additionally, the extent of soil carbon decline induced by agricultural production prior to the introduction of alternative ‘SOC friendly’ management practices requires consideration.”
He makes the point that sampling methods that are statistically robust to measure changes in SOC are still in their infancy in Australia. Measurements focussed on quantification of the total amount of organic carbon present in soil are too costly and not comprehensive enough when it comes to assessing impacts of new farming methods. However, he says the use of mid-infrared spectroscopy is showing promise.
“It has been shown to allow rapid and cost-effective predictions of not only the total organic carbon content of soils, but also the allocation of this carbon to component fractions with a defined confidence.”
The Graham et al paper while not isolating SOC pools does highlight the issue of making a statistically significant increase in paddock total organic carbon level. The paper is an analysis of SOC in the long-term phosphate trial paddocks which were managed at the DPI’s research farm near Hamilton Victoria for 25 years. While the research found TOC was not significantly affected by phosphorus fertiliser rate or stocking rate, over the year TOC in the 0-30cm depth increased by 216kg C per kg P applied annually. But:
“Field variability in soil organic C (TOC measured using the Walkley Black method) was such that an increase of around 9 t C/ha would be required to be detectable at the commonly accepted 5% level of significance (P<0.05). At a P application rate of 13 kg P/ha/year (average for western Victoria); this would take about 80 years, and with a P application rate of 33 kg P/ha/year, it would take about 30 years (assuming the response remained linear).”
These time estimates are questionable because the authors have not isolated the pools of the 216 kg C annual increase, either particulate or humus, or their life spans.
Even more unsettling is the fact that the Emissions Reduction Fund’s methodology for “Sequestering Carbon in Soils in Grazing Systems” calculates Australian Carbon Credit Units using TOC without reference to its pools and their turnover time. This could mean that measured increases in soil TOC in a project are principally particulate organic carbon with a short life span which is primarily influenced by annual rainfall.
A warning about the reliability of using TOC without reference to its pools was contained in James Walcott et al’s 2009 Bureau of Rural Sciences paper “Soil carbon for carbon sequestration and trading: a review of issues for agriculture and forestry”. He said:
“Depending on the lifespan of the carbon trade, only a proportion of soil carbon is sufficiently stable (humus) to be considered in carbon trading, and increasing the amount of carbon in this stable fraction is generally a long-term process.”
Dr Fiona Robertson et al identified soil organic carbon pools for different regions across Victoria. There was no data provided about the these pools turnover times. Robertson’s paper confirmed the link between TOC with annual rainfall. She concluded:
“Most of this (SOC) variation was attributable to climate; almost 80% of the variation in SOC stock was related to annual rainfall or vapour pressure deficit (i.e. humidity). After accounting for climate, differences in SOC between management classes (beef/sheep grazing versus dairy versus mixed farming) were small and often not significant. Management practices such as …perennial pasture species, rotational grazing and fertiliser inputs were not significantly related to SOC stock.”
Soil carbon to depth
Another important consideration associated with TOC components and their turnover times is their position in the soil profile. The scientific and extension literature usually only refers to TOC down to 30cm depth, the “grey” literature especially that related to an ERF soil carbon project is interested in TOC down 90 to 100cm.
The theory behind measuring to this depth is that while TOC below 30cm depth is lower percentage wise, if it can be increased by a small amount, the actual increase in carbon in the larger volume of soil can be significantly increased. As well, organic carbon below 30 cm is less subject to erosion and to mineralisation by soil organism so is a much more “protected” component. But the scientific data demonstrating significantly increasing TOC below 30cm in Australian pasture soils is virtually non existent. Dr John Graham provided some depth data down to 80 cm, but there was no significant change over the 25 year period of the trial which involved set stocking grazing management and keeping pasture availability in all treatment at between 1000 and 2000 kg dry matter per hectare for most of the year, figure 5.
Figure 5: Soil Total Organic Carbon (measured by Walkely Black (CWB method) level to 80cm did not change over 25 years under different levels of pasture productivity generated by increasing phosphorus fertiliser applications.
Dr Alieta Eyles et al hypothesise that increasing the humic pool in pasture soil represents the most likely method of increasing TOC. But she says how to do this is complex:
“The stability or persistence of SOM depends on complex interactions between the biological stability of SOM, which is influenced by the physical and chemical soil environment (e.g. moisture, temperature, pH, aeration and various organo-mineral interactions), and the physical and chemical accessibility of the SOM to microbes and enzymes. Recent research has shown that SOM stability is not due to the intrinsic chemical recalcitrance of the SOM itself, but rather it is the physiochemical and biological properties of the surrounding soil environment that influence the rate of decomposition.”
Eyles went on to conclude that:
“In terms of promoting SOC accrual, given that organic material bound in aggregates decomposes more slowly, increased aggregation should increase the retention time of SOC and therefore could increase SOC.”
Just how this would be best achieved in south eastern Australia perennial pasture soils was not demonstrated but she hypothesised that using deeper rooting plants could be one answer. And deeper roots in perennial grass species are considered to be related to the frequency of defoliation by livestock.
Root depth considered important
The extension and “grey” literature often uses an image of three plants (species usually unspecified) grazed (or cut) at different intervals to demonstrate this effect, figure 6. But Eyles’ review of scientific research found that frequency of defoliation was not consistently correlated with root depth:
“These variable findings suggest that the effect of grazing on root growth is complex and complicated by a range of factors including soil type, climate, fertilisation and species composition”.
Figure 6: Typical illustrations of the impact of different intervals for grazing or cutting leaves on root growth
An omission in the discussion of grazing method on TOC in the scientific literature is lack of reference to paddock herbage matter per hectare at livestock entry and exit in rotational grazing methods. Farmers were not asked for this data at the 615 sites measured in the Robertson et al research. This can lead to a conclusion as in Eyles paper that can be misleading for southern Australia perennial pastures:
“Regardless of the grazing system practised, grazing at a high stocking rate is expected to lead to greater SOC loss than at a lower stocking rate because of (i) the removal of a larger amount of aboveground C inputs, (ii) a reduction in below ground C inputs through lower root production and higher root litter turnover, and (iii) erosion.”
Understand stocking rate impacts
Firstly, high stocking rate is not a problem in itself, in some circumstances, especially in non-fragile perennial pastures it can be used to stimulate herbage growth, utilisation and increase organic matter return to the soil. This was demonstrated in the Long term phosphate trial at Hamilton, figure 4.
The problem with stocking rate happens when, at any point over a growing season, it exceeds paddock carrying capacity which leads to loss of a protective herbage and plant crown cover over the soil and in turn a loss of ecosystem functions. That’s why herbage present at livestock entry and minimum level at exit (say above 1200kg herbage dry matter per hectare) is so important to know and manage to maintain and possibly increase TOC.
Figure 7: Soil total organic carbon levels in perennial pastures are a function of previous management history in particular cultivation and subsequent farmers’ management and how it takes account of stocking rate, carrying capacity and ecosystem function. Photos: Patrick Francis
Eyles refers to the importance of the plant cover and persistence but in a minimalist sense, that is overstocking causes loss of TOC through soil erosion. Until scientists can document perennial pasture (native or introduced) herbage growth and utilisation dynamics with TOC and in particular its humus proportion and turnover rate, then grazing strategies are unlikely to be ‘bankable’ in terms of soil Australia Carbon Credit Units
Secondly, the evidence from Robertson’s analysis of TOC across Victoria, is that higher stocking rate livestock enterprises in the same soil type and environment are usually associated with similar or higher TOC. This was best illustrated by dairy enterprises on the Victoria volcanic plain which had higher TOC than beef or sheep enterprises in the same districts. Dairy enterprises usually run livestock at approximately twice the carrying capacity of beef/sheep enterprise.
Eyles didn’t join the dots between stocking rate, carrying capacity and pasture fertility as demonstrated in dairy farm TOC levels. Yet she notes:
“Grazing animals … play an important role in nutrient cycling and pasture fertility. In a 12-year trial, pastures grazed by cattle had significantly higher levels of SOC than either ungrazed or hayed pastures, highlighting the importance of animal waste products in SOC accrual.”
SOC components turnover time is given considerable exposure in : “Managing soil organic matter – a practical guide”, by Dr Frances Hoyle. The obvious influences are mentioned such as organic matter’s carbon to nitrogen ratio; its location – on the surface or incorporated; and degree of soil aggregation. However the information is so general in nature, it provides the farmer with little practical assistance for achieving a measureable increase in the SOC humic pool which has a multi-decade turnover time. Apart from that most of the factors involved such as temperature, rainfall and soil type are outside farmers ability to control.
Take home message
The reality of influencing SOC levels in native or introduced perennial pasture soils under grazing is that the relevant strategies for its improvement are unsubstantiated outside of repairing and avoiding degrading factors or when converting cropping land back to pasture.
The suggestion that a particular farm’s pasture SOC percentage can be significantly and permanently increased via grazing management above it’s annual temperature and rainfall influenced variation is currently not supported by scientific data.
In fact the scientific literature reviewing SOC is highly imprecise about proportions of different organic carbon pools found in agricultural soils and has even more vague estimates about their lifespans.
Robinson et al points to the fact that, “…in some regions much of the variation in SOC stock remained unexplained,”
Eyles et al conclude that, “The complex and largely unknown nature of C sequestration in pasture soils, combined with a lack of long-term studies under Australian conditions, limits our ability to predict the impact of various pasture management options on SOC accrual on a farm-by-farm or region-by-region basis.”
Bell and Lawrence in their extension article seem to have the most appropriate perspective for farmers about managing SOC while its science is being investigated. They say “it is very dangerous to consider soil and soil organic matter simply as a potential sink for impounding excess CO2. ….It is the flow of C through soils, rather than it’s sequestration in soils, that are the keys to healthy soils and sustainable land use systems. We therefore will gain greatest benefits out of increasing the inputs of carbon and organic matter to soils by growing better crops and pastures more often.”
Baldock et al give similar advice: “The guiding principal to enhance carbon capture in soils under any climate-change scenario will be to maximise carbon inputs. Where the ability of a soil to protect organic carbon against decomposition is not saturated and/or where inefficiencies in resource use (water and nutrients) can be improved by altered management to allow plants to capture additional atmospheric CO2, the potential exists to increase SOC and enhance soil resilience and productivity. This potential will vary from location to location as a function of soil type, environmental conditions, and past management regimes (how much carbon has been lost due to past management). “
There is an additional factor that needs to be embraced by farmers when considering SOC in agricultural soils and that is ensuring the paddocks ecosystem functions are maintained. Without managing pastures for year round persistence and groundcover irrespective of rainfall, soil organic carbon maintenance or increase will always be out of reach.
Alan Lauder’s makes an important conclusion in this regard in his book “Carbon Grazing”. He says: “New highly productive topsoil can be made if enough carbon is introduced via green plants. … There are many things which are interacting with each other (to achieve this) that are beyond the control of rural producers. What they have to manage is what they can control. Producers do have long-term control over carbon flows. …Ongoing carbon introduction during good and average seasons ensure less carbon is lost during droughts, because the landscape is more resilient.”