- Soil carbon is a food source for the microorganisms, and the breakdown of organic matter can release nutrients for plants
- Soils have a limited capacity to store carbon, and increasing carbon storage is a slow process
- Growers can improve soil fertility by increasing the rate that carbon is cycled through soils
- Methods include use of rotations, stubble retention and growing more biomass
Dr Gupta Vadakattu explains all about soil carbon
Soil carbon plays a key role in crop growth, so a good understanding of the role of carbon in cropping systems can help improve crop productivity. CSIRO researcher Dr Gupta Vadakattu explains why carbon is important and how growers can make the most of their soil carbon.
What is soil carbon?
Carbon is transferred from carbon dioxide in the air into plant biomass through photosynthesis. As the crop residues break down, soil organic carbon (SOC) is generated. The amount of SOC that can be stored in the soil depends on soil physical and chemical properties, and is related to other environmental factors such as rainfall. Since land was first cleared for agricultural development, SOC levels have generally been declining, especially in cropping systems where pasture and livestock production are not a major part of the system.
Why is soil carbon important?
One of the key roles of SOC is in facilitating microbial activity. Soil microorganisms breakdown organic matter and consume its carbon as a food source. These microorganisms, in conjunction with other soil fauna, mineralise nutrients from organic matter, providing growing crops with the nutrition they need. This is particularly important for nitrogen supply.
In many Australian agricultural soils carbon availability is the most limiting soil property for microbial activity, so management of biologically available carbon is key to improving nutrient availability to growing crops which in-turn is key to maximising crop yields and profits.
Microbial activity and SOC also promote a favourable soil structure by holding particles together as stable aggregates which has a positive effect on plant available water capacity. Higher SOC also increases the cation exchange capacity of soils and their ability to retain nutrients.
What is soil carbon turnover?
Soil carbon turnover (SCT) is the rate of cycling of carbon through the soil biota to plants and back into organic matter in the soil. The cycle transfers carbon and nutrients through different pools of soil organic matter and a high SCT can deliver benefits through soil biology and fertility. Changes in turnover can be observed in a much shorter timeframe than absolute carbon levels which change very slowly.
Long-term adoption of management practices that improve SCT coupled with increased carbon inputs can also increase suppression of some diseases, reducing crop losses.
How does soil type and rainfall influence soil carbon?
Soil organic carbon (SOC) levels may be as low as 0.1 percent for a sand dune in a low rainfall area and up to about two per cent or more for well-structured clay soils in a high rainfall zone.
Coarse structured soils, such as sands, give micro-organisms easy access to carbon, resulting in faster decomposition and less carbon storage capacity.
By comparison, in well-structured soils such as red-brown earth or duplex soils the finer clay particles provide protection for carbon, slowing down the rate of decomposition, and providing a buffer capacity to store more SOC.
Rainfall also has a major effect on the amount of biomass produced and added to the soil, hence SOC levels tend to be greater in wet areas.
How can I improve soil carbon?
To increase SOC growers need to supply more carbon to the soil than is used by microorganisms. However, there is also a limit to how much SOC can be stored which is driven by soil physical and chemical properties, environmental conditions and nutrient availability.
Attempting to increase SOC by supplying more carbon than a soil can hold can lead to losses by microorganisms producing carbon dioxide or leaching.
Changes in absolute SOC can be achieved by improving the soil’s capability to store carbon, using the same methods that increase soil carbon turnover in the short term (see below).
However, these changes are slow even in well-managed soils. Trials on sandy soils in the low rainfall Mallee have shown a timeframe upwards of 20 years is required to measure significant changes. In soils with high clay content the timeframe will be shorter but still much longer than the time required to improve soil fertility through increasing SCT.
A soil with very low SOC can be improved in shorter timeframes until the SOC stabilises at a level consistent with well-managed soils. But the maximum SOC is largely determined by the amount of clay in the soil and rainfall.
Theoretically, increasing the clay content of a soil could increase the SOC storage capacity of sandy soils. However, this has not yet been confirmed in clay spreading trials.
While improving SOC is slow and unreliable, growers can instead focus on carbon turnover which can provide benefits to biological fertility and therefore, crop growth in the short-term. Long term adoption of practices that improve turnover can also promote improvements in SOC.
How can I improve carbon turnover?
The best methods for improving SCT are:
- Improve the quality and quantity of carbon inputs through crop rotation.
- Increase the quantity of carbon input through increasing biomass and stubble retention.
Using crop rotation to improve carbon
Diverse crop rotations will provide a variety of organic matter entering the soil and potentially increase microbial diversity and activity. To maximise carbon turnover, growers should include regular legume crops and consider pasture phases in mixed farming systems.
Legume residues are generally nitrogen rich, leading to lower carbon to nitrogen (C:N) ratios of about 25:1, compared to wheat, which is about 90-100:1.
Soil microorganisms need a ratio of about 25:1 for optimal mineralisation, meaning that legume residues are far better matched as a food source than cereal residues. The high C:N ratio of cereal residues will reduce the rate of SCT and can tie-up plant-available nitrogen (in a process known as immobilisation).
Brown or green manuring also increase the quantity and quantity of crop residues returned to the soil, improving SCT. It should be noted that intense cultivation of the soil will accelerate carbon breakdown and carbon dioxide losses. Therefore, the long-term SCT benefits from green manuring will be mitigated by excessive tillage, especially in sandy soils.
Integrating a pasture phase into a farming system, especially perennials can have significant SCT benefits because perennial plants grow more root mass than annual crops, grow for a higher proportion of the year and experience less frequent soil disturbance. Long-term trials by the University of Adelaide at the Waite Campus have shown a significant positive relationship between increases in pasture frequency and increased SOC in red-brown soils (read more on carbon in pastures).
Will retaining more cereal stubble increase soil carbon?
Cereal stubble consists of about 45 per cent carbon, and will support SCT by providing a significant input of carbon to soils.
Because of the high turnover of cereals, retaining more cereal stubble is unlikely to significantly improve absolute SOC in the short term, except in very degraded soils.
As mentioned above, increasing SOC is a long-term process, requiring high-quality carbon sources. Because cereal stubbles have a much higher C:N ratio (about 100:1) than is required by microorganisms (25:1), significant levels of added nutrition are required for optimal microbial activity and growth which are key for SOC accumulation.
Including non-cereal rotations, in combination with stubble retention, is the best way to optimise soil carbon turnover and gradually improve soil organic carbon.
Using biomass to improve carbon
The more high-quality (i.e. low C:N ratio) carbon entering the soil through crop residues, the more turnover will occur. Any methods of improving the biomass of crops will increase the quantity of crop residue remaining after harvest.
To maximise soil carbon turnover, growers should maximise biomass growth through optimum crop management, for example, early sowing, fertiliser applications and disease management.
There is a common perception that fertilisers are bad for soil carbon. However, this is not necessarily the case because appropriate use of fertilisers increases the biomass of crops, therefore increasing carbon inputs and facilitating SCT.
Dr Gupta Vadakattu, 08 8303 8579, gupta [dot] vadakattu [at] csiro [dot] au
GRDC Project Code CSE00043, MSF00003