Building soil carbon for your business

Take home messages

• Growers should build soil organic matter for the right reasons.
• Growers should bank the inherent productivity benefit of improved soil health and not sell their soil carbon, as they will need this asset for the day when they might need to table it against the balance of their greenhouse gas emissions to meet supply chain demands.

Introduction

Since the Paris climate agreement (COP21), and more recently the Glasgow COP26 meeting, there is rising interest in the role that soils can play in helping Australia meet its greenhouse gas reduction targets. Under the Australian Emission Reduction Fund, there are two soil carbon offset methods available, although there are also a number of international voluntary soil carbon methods. To engage in these soil carbon offset markets, growers must be able to demonstrate they are undertaking activities which are in addition to their normal practice. For example, a grower who changes to zero till practices will be rewarded if they have registered the paddock (that is, defined a Carbon Estimation Area) and can show a measurable change in soil organic carbon in the top 30cm or deeper. A grower who has employed zero till for many years is unlikely to be rewarded as this is business as usual.

Unfortunately, placing a price on soil carbon has skewed the discussion away from what really matters to growers, which is soil health and productivity. Soil organic matter, of which only half (~58%) is soil organic carbon, is the engine room of soils, maintaining nutrient supply and soil structure. Soil organic carbon is usually only about 1 to 5% of the total soil mass, with the higher concentrations normally under long-term grasslands or crop rotations with substantial pasture phases.

What is soil organic carbon

There is some confusion about what constitutes soil organic carbon. Plant residues on the soil surface, roots and buried plant residues (>2mm) are not considered soil organic carbon. These first need to be broken down into smaller fractions to be considered soil organic carbon, which is why the soils are first sieved to two millimetres before an analysis, to remove all these larger fractions. Fractions considered to be part of the soil organic carbon (as per a soil analysis) would be particulate organic carbon (POC; 2.0 – 0.05mm) and humus (<0.05mm), with resistant organic carbon (ROC) being historic charcoal from fires or burning of stubbles. In other words, we must not confuse roots with soil organic carbon.

For sustained productivity, increasing the relative amount of POC is beneficial as this is readily decomposable and supplying nutrients. To have confidence to sell soil carbon, farmers want a substantial amount of carbon in a more recalcitrant (decomposing over decades) form, namely humus, so that you have confidence that the carbon sold will still be there in 25 to 100 years. These permanence time frames are required to engage in carbon markets.

The inherent benefits soil organic matter

The inherent benefits of building soil organic matter are outlined in Table 1. In a modelling experiment in western Victoria, we quantified the inherent productivity benefits of just two of these attributes, being nitrogen mineralisation potential and water holding capacity (Meyer et al.2015). In a permanent grassland situation, high soil organic matter (long term grassland) conferred between $100 - $150 of additional productivity value per hectare per year, when compared with a soil with low organic matter (long term cultivation then converted to grassland). Also noted in this research, the potential to increase soil organic matter was higher (0.3 - 0.5t soil C/ha/year sequestered) when moving out of long-term cultivation into a permanent pasture phase under high rainfall, that is, high potential to increase soil organic matter. Conversely, under the same conditions but with a high soil organic matter status due to long-term pasture, the potential to increase soil organic matter was largely determined by rainfall.

Table 1: The inherent biological, physical and chemical co-benefits that high soil organic matter may confer to an agricultural production system.

(Source: Jeff Baldock)

Building soil organic carbon

Building soil organic carbon is basically an input-output equation. The inputs are from decaying crop and pasture residues and roots. The outputs are CO₂ from microbes which are actively decomposing and transforming the carbon, using it as energy, but in the process releasing nutrients back to the soil to support plant growth. In a good rainfall year, the inputs increase in response to plant growth with a subsequent increase in outputs and thus a more rapid accumulation of soil carbon i.e. carbon inputs exceed outputs. In a drought, carbon inputs drop dramatically in response to reduced plant growth, but the outputs remain constant because the microbes respond to episodic wetting events and soil carbon decreases i.e. carbon outputs exceed inputs. Fallow years are good example of major losses in soil carbon as there is no addition.

In Australia, rainfall determines the majority of soil carbon change in a stable management system (see Meyer et al. 2015). Unless there is a dramatic change in management or land use, such as moving out of conventional cultivation into permanent pasture in a high rainfall zone, the majority of the annual change in soil carbon is a function of rainfall. Change in soil carbon in mixed cropping systems can often be large and unpredictable, particularly from labile pools (Badgery et al. 2020). In a country like Australia that has 23% more rainfall variability than most countries in the world (Love 2005), banking on selling soil carbon is therefore high risk given the frequency of drought. For example, Badgery et al. (2020) reported what to 12 years of increase in soil carbon was reversed in the following 3 years. Concluding that 12 years was not enough time to be confident that soil carbon sequestration was permanent. In contrast, our recent research showed that just two of the co-benefits of high soil organic matter, nitrogen mineralisation and water retention, could confer as much as $150/ha/year productivity value in a pasture system in western Victoria over the long term, when the carbon trading value under the same scenario is less than $20/t/ha/year. This raises the question, should growers focus on trading soil carbon, or just bank the inherent productivity benefit of having higher soil organic matter, as there is no paperwork, no contracts, no liabilities, but all the productivity benefits can be banked? In addition, when the farm needs to demonstrate carbon neutral production which is highly likely standard by 2030, this soil carbon will be essential to offset the balance of the grower greenhouse gas emissions – double-dipping is allowed.

How much soil carbon can be accumulated

Over the past few years there has been an increase in the number of growers and carbon aggregators making claims of increases in soil carbon that do not align with the published peer-reviewed science. Although conservative, the values presented in Table 2 are those estimated by the Australian Government official carbon model (FullCAM), showing likely increase in soil carbon in response to management. What is also seemingly ignored in some less scientific claims of soil carbon increase, is the assumption this can continue in perpetuity, which defies the law of diminishing returns. The more carbon you sequester, the more carbon inputs you then are then required to maintain. Where soil has a low organic matter content, but high clay content and good rainfall (namely, a high potential to increase soil organic matter), it is possible to achieve rates of soil carbon sequestration that exceed those presented in Table 2. The initial high carbon sequestration rates (that is, the first 5 to 10 years with rates from 0.7 to 1t C/ha/year in the top 30cm when converting cropland to pasture (Meyer et al. 2015; Robertson and Nash 2013)) will result in a new steady state after 10 years that matches the rainfall and management imposed. In contrast, the same conditions but with a high soil organic matter starting point, would only vary in direct relation to annual rainfall and distribution.

Table 2: Modelled soil carbon sequestration potential as stipulated by the Australian government ERF Offset method: Estimating Sequestration of Carbon in Soil Using Default Values, Methodology Determination 2015.

Robertson and Nash (2013) report similar soil carbon sequestration values, with the average change in soil organic carbon in the top 30cm after 50 years across Victoria ranging from 21t/ha under perennial pasture to 6.5t/ha under a zero till grain rotation without fallow. Any reversion to conventional practices, even for a short period of time has a major impact on the magnitude of soil carbon sequestration (Figure 1).

Figure 1. The impact of occasional fallow and stubble burning on soil organic carbon accumulation in a cereal rotation over 50 years at Bendigo (Vic) (Robertson and Nash 2013).

The SATWAGL long-term trial at Wagga (Chan et al. 2011) also demonstrated the clear benefits of stubble retention, zero tillage and pasture phases for increasing soil carbon (Table 3). Over a 25-year period, stubble retention compared to burning was 2.2t C/ha higher, zero tillage compared to conventional cultivation was 3.6t C/ha higher, and a pasture rotation every second year was between 4.2 and 11.5t C/ha higher than continuous cropping.

Table 3: Change in soil organic carbon (SOC, kg C/ha over 0–0.30m soil depth) and final stock (t C/ha) under different rotation, tillage, and stubble and pasture management in the SATWAGL long-term field experiment (1979–2004) (adapted from Chan et al. 2011).

NT, no tillage; CC, 3-pass tillage; SR, stubble retained; SB, stubble burnt; W/L, wheat/lupin rotation; W/C, wheat/clover rotation; W/W, wheat/ wheat; N, N fertiliser; G, grazed; M, mown. *P < 0.05; ** P < 0.01; n.s., not significant

A new approach to soil organic matter in Australia

Perhaps there is a need to consider soil organic matter differently in the Australian context, by managing it more specifically for soil types by farming systems and also managing differently in high versus low rainfall periods. Sandy or granitic soils have very limited capacity to build soil organic matter as carbon is less protected from decomposition by microorganisms in these soil types. Clay soils generally have far higher potential to sequester carbon when rainfall is sufficient to maintain carbon inputs from stubble, roots or residual pasture biomass. The key to building soil carbon is to understand the capacity for the soil to store carbon in your specific environment (climate x soil type) and management system. This capacity varies considerably even within the same district. Therefore, we should not view the landscape with a single sequestration potential but target the areas that are low in carbon but high in sequestration potential, for example, the rehabilitation of degraded lands.

We should also be thinking of El Niño versus La Niña years quite differently, in that we have probably built more soil organic matter in eastern Australia during the recent La Niña, than in the previous three years put together. In higher rainfall years, we should focus on strategies that maximise the sequestration of carbon in our soils, and in low rainfall or drought periods, we focus on minimising the losses to provide a net positive result. Rather than focus on building soil carbon year by year, a longer-term approach would aim for a net increase in carbon over a 10-year period.

Selling soil carbon for short-term gain may mean long-term pain

Finally, whilst carbon neutrality is being strongly supported by the agricultural supply chain companies, there is an inevitable point where growers will need to demonstrate progress towards lower emissions farming systems. Any increase in soil organic carbon you which to bank as a credit will be negated by on-field emissions, for example CO₂ from fuel, N₂O from N fertilisers or CH₄ from grazing livestock. Selling soil or tree carbon means that asset value leaves your property, but you are left with the liability of maintaining the asset for the next 25 to 100 years (short term gain, long term pain). If the soil carbon is sold internationally, it also leaves the industry and the country, making any industry or national targets increasingly difficult to achieve as the carbon offset has left the industry and left Australia. Once the soil carbon is sold, the new buyer will be using it against their carbon footprint, which means that the farm will never again be able to use that soil carbon against their future liability, making their carbon neutral target increasingly impossible to achieve. In the end, we should encourage growers to build soil organic matter for the right reasons. Bank the inherent productivity benefit of improved soil health and don’t sell your soil carbon, as you will need this asset for the day when you might need to table it against the balance of your greenhouse gas emissions to meet supply chain demands.

References

Badgery WB, Mwendwa JM, Anwar MR, Simmons AT, Broadfoot KM, Rohan M, Singh BP (2020) Unexpected increases in soil carbon eventually fell in low rainfall farming systems. Journal of Environmental Management, 261, 110192.

Chan KY, Conyers MK, Li GD, Helyar KR, Poile G, Oates A, Barchia IM (2011) Soil carbon dynamics under different cropping and pasture management in temperate Australia: results of three long-term experiments. Soil Research 49(4), 320-328.

Love G (2005) Impacts of climate variability on regional Australia. In ‘Outlook 2005. Conference proceedings, climate session papers’. (Eds R. Nelson, G. Love) pp. 10–19. (Australian Bureau and Resource Economics: Canberra, ACT, Australia)

Meyer R, Cullen BR, Johnson IR, Eckard RJ (2015) Process modelling to assess the sequestration and productivity benefits of soil carbon for pasture. Agriculture, Ecosystems and Environment 213, 272-280.

Robertson F, Nash D (2013) Limited potential for soil carbon accumulation using current cropping practices in Victoria, Australia. Agriculture, Ecosystems and Environment 165, 130–140.

Contact details

Richard Eckard
University of Melbourne
rjeckard@unimelb.edu.au