All organic material in the soil will eventually convert to CO2.
Plants can achieve a steady state whereby they produce organic root material and litter, as fast as it decays. The difference is the amount of carbon in the soil.
Soil organic carbon
In the soil there are 3 separate pools of organic carbon:
1) short lived, average 3 yrs,
2) long lived, 10 - 100 years, average 30 yrs,
3) very long lived humous, 100 - 1,000s years.
All these, if left alone, will be oxidised to CO2 via biological processes, or fire. It is also called mineralisation.
If organic material is constantly added to the soil, then eventually there will be a steady state where the additions are matched by the decomposition. The faster the addition, the larger the store of organic material.
For example, native forest adds 10 T C /ha/yr. At steady state there is 95 T C/ha in the soil. All three pools are present.
A cultivated field may produce 6 T C /ha/yr and reach a steady state of 37 T C /ha.
When forest is cleared for cultivation, the soil carbon will be oxidised and escape as CO2 over the next hundred years or so. The new steady state under cultivation will have far less carbon.
If, during this reduction process, the land management is improved, for example stubble is retained, then the addition rate of organic soil carbon will rise from 6 to 7 T C /ha/yr. At first the soil carbon levels will improve. However if the soil carbon is still decreasing after clearing 50 years ago, then this continuing loss will probably wipe out the improvement.
If however, the land has been cleared long ago and reached a new low steady state, then the new practice will be an improvement. But nothing l ike the original forest.
To pay farmers to increase soil carbon is probably worth while for soil improvement, but it is not a valid way of removing CO2 from the atmosphere. Over time, it all converts to CO2.
CSIRO soil scientists estimate that under ideal conditions using every hectare of agricultural land in Australia, then 100 MT of carbon dioxide could be stored per year for a few years. In reality 10% of this may be achievable with a maximum effort. But it must be done every year for ever.
A government promise to store 85 GT of CO2 per year in form soils is an unrealistic election promise, that can make no permanent change.
A warming and drying climate will quickly undo this effort.
A recent University of Western Australia study estimated the costs at $80 per tonne of abatement.
Soil carbon can be increased if plants produce more roots. The obvious answer is trees. Converting grass to forests or orchards in one answer. Another is to graze animals more efficiently.
Cell grazing is a method of moving animals off pasture before they eat it enough to expose the earth. Bare earth captures no sunlight, so the plant becomes smaller and the root mass is reduced.
Similarly, once the grass is too high, the dead leaves shade the plant and slow growth.
Often electric fences are used and the cells designed to give access to a central water supply.
Detailed notes on soil carbon
Clearing of land for agriculture has reduced soil organic carbon (SOC) by 40 - 60%.
Globally, this has released at least 550 GT of carbon dioxide to the atmosphere.
(NB we say 285 GT later on. Does this include timber, or just soil carbon? To be checked).
Globally, the top metre of soil stores approximately 5,500 GT CO2 equivalent as organic C, and exchanges 220 GT CO2/yr with the atmosphere, which contains ~2750 GT CO2.
Currently, there is much political, (i.e. uninformed) debate, particularly within Australia, as to the total potential of agricultural soils to store additional carbon.
Carbon and carbon dioxide are often used interchangeably. This is confusing as they are so different. When carbon burns it combines with oxygen to form CO2 which is much heavier.
1 T of Carbon produces 3.67 T of CO2.
12 g of C + 32 g of O2 ----> 44 g of CO2
This is a summary of the following publication: Soil Carbon Sequestration Potential: A review for Australian agriculture - Jonathan Sanderman, Ryan Farquharson and Jeffrey Baldock - CSIRO Land and Water
1 To cause to withdraw into seclusion.
Residence times of organic matter
Once soil organic matter has been added to the soil, it can be divided into three groups, or pools, and the soil conditions dictate which pool the organic material joins:
Pool 1) Fresh plant residues with a mean residence time of, about 3 years;
Pool 2) Intermediate residence times, of the order of 10 to 100 years. If some of the organic material is buried without air or access by microorganisms, the mean residence time may be 30 years;
Pool 3) Some of the organic material becomes humous and is bonded to the face of minerals and become inaccessible to microorganisms. It has a residence times of 100-1,000 years. The mean residence time of the organic material is 120 years.
The graph shows what happens if the organic material being added to the soil becomes part of group 1,2, or 3.
In each case 2 T of C is added to the soil each year.
Case #1. Because it is decomposing within 3 years, the maximum stored is 6 T/ha. After the addition is stopped, it all decomposes and the soil carbon drops to almost zero within 10 years.
In case #2 the C levels will build up to 8 T C/ha. Again, stop adding C and the levels will drop within 40 years.
Case #3 After 60 years of adding new carbon, it reaches 11 T C/ha. Then if new carbon is no longer added, it too drops eventually to zero.
In each case, 2 T of carbon
Carbon saturation of soil
It is possible to saturate the humous that is bonded to mineral faces as there are a finite number of faces.
However the fast and slowly decomposing pools cannot be saturated. A farm could be piled 100 M high with compost, or have 100 M of compost buried under the clay. There is no limit.
If the material being added is matched by the rate of decomposition, then we have steady state.
The faster the material is added, the higher the organic material in the soil.
In the examples on the right we are looking at two examples:
Native bush cleared for cultivation
Pre 1950: Native bush produces 10 T C/ha/yr and buries it as organic material. At this rate of production, the organic carbon level is kept at 95 T C/ha in the soil.
1950: Bush cleared for cultivation. Crops now produce only 6 T C/ha/yr so the level of organic material is reducing to a new steady state level as the old matter decomposes. It will take a few hundred years.
2000: Stubble retention produces 7 T C/ha/yr so organic material rises. However the original organic material is still decomposing so the gain is only temporary.
Old cultivated soil with improved farming
With cultivated soil at the new steady state,
2000: stubble retention now increases the level of organic material by 5 T/ha over 50 years.
Mg = Mega grams = Tonne
Work by the CSIRO has found that, improved management of cropland, whether enhanced rotation, adoption of no-till or stubble retention, has resulted in a relative gain of stored carbon of (0.7-1.1 T CO2/ha/yr) compared to conventional management across a range of Australian soils.
However this reduced after 5-10 years and dropped to zero after 40 years. Overall the soil did not reach the original carbon level it had when vegetated with native bush.
Pasture improvements, including fertilisation, liming, irrigation and sowing of more productive grass varieties, may give gains of 0.1 – 0.3 Tonne C/ha/yr. (0.37 - 1.1 T CO2/ha/yr
Larger gains of 0.3 – 0.6 Tonne C/ha/yr (1.1-2.2 T CO2/ha/yr) have been produced by conversion of cultivated land to permanent pasture.
Grazing is expected to reduce the soil carbon by 100-200 MT over 5 years.
However, currently, there is much uncertainty and debate, particularly within Australia, as to the total potential of soils to store additional carbon, the rate at which soils can store carbon, the permanence of this carbon sink, and how best to monitor changes in soil carbon stocks.
It is very important to realize that this technical sequestration potential will likely never be fully realized due to a whole host of economic, social and political constraints.
Sequestration rates generally range between 0.05 and 0.8 T C /ha/yr the mean relative difference between paired
Carbon in Australian soil
Total soil organic carbon stocks in 1990 for the Australia continent were estimated at 19 GT for the top 30 cm with a natural flux of ~700 GT CO2 exchanged between the soil and atmosphere every year. This natural exchange is ~12% larger than Australia’s 2007 anthropogenic GHG emissions excluding land use and land-use change (available online at: http://www.climatechange.gov.au/inventory/index.html). At face value, a mere 0.8 % per annum increase in SOC stocks would effectively mitigate Australia’s annual GHG emissions.
The agricultural sector covers approximately 60% of the land area of Australia, with >90% of that area (419 Mha) being used for low-density grazing of natural vegetation (Table 1). This leaves ~50 Mha of land that is actively managed, primarily for grazing of modified pastures and various cropping systems.
Source; Soil Carbon Sequestration Potential by Jonathan Sanderman, Ryan Farquharson and Jeffrey Baldock. - CSIRO Land and Water
Loss of organic carbon in soil after cultivation
Can soil sequestration fix enough carbon?
Globally, it has been estimated that:
78 GT of soil carbon (285 GT CO2) have been lost due to agricultural practices, with
26 GT carried away by erosion (What happened to it?)
52 GT converted to CO2 by microorganisms and fire producing
192 GT of CO2 which would be 25 ppm in the atmosphere. However half the CO2 is absorbed by the ocean so contribution of lost soil carbon is about
13 ppm CO2 in the atmosphere which is 400 ppm in 2013
Australian maximum sequestration potential is theoretically
100 MT CO2-eq /yr which is about
15% of Australia’s current annual emissions.
This is of course a gross overestimation. There is no way all agricultural land could be improved. Who would pay? Also with global warming the climate will get drier with droughts and fires, and the losses will worsen, not improve. 10% of this would be a more realistic estimate.
Australian emissions in 2011 were 550 T of CO2 equiv ~150 T C.
If best land management practices, were adopted on all agricultural land, then 50-66% of historic losses could be sequestered over the next 50 years. That is 1% of 78 GT each year. or 2 GT CO2 per year.
In Australia this means 40-50 GT C or 150 - 185 GT CO2.
Globally, this equates to 30 to 60 GT C at a rate of 0.9 ± 0.3 GT C /yr over 25 to 50 years.
This sequestration rate is equivalent to approximately 10% of current total GHG emissions due to fossil fuel burning and land-use change or 25 to 33% of the annual increase in atmospheric CO2 levels. Using a more detailed inventory approach, Smith et al. (2008) estimated that the global technical mitigation potential, considering all greenhouse gases, from agriculture by 2030 is 5.5 to 6.0 GT CO2-eq yr-1 (or ~ 1.5 GT C yr-1).
It is extremely important to keep in mind that these types of estimates assume adoption of best management practices across every acre of land under production.
Extracting data for Australia from the analysis of Smith et al. (2008), it is found that the sequestration potential, including avoided emissions, for Australia’s agricultural land is approximately 100 MT CO2-eq /yr for the land area considered in this study, which would equate to nearly 15% of Australia’s current annual emissions. Given the differences that we found in average sequestration rates between Australian specific data (Table 3) and the global data sets, this value is likely an overestimation of the actual potential. Additionally,
this broad analysis does not consider the impact of drought years that are known to have severe impacts on productivity which should translate into SOC losses for the drought period.
In the absence of active management, most soils are believed to become net sources of CO2 to atmosphere primarily due to rising temperatures
Carbon Cycling in soil
Soil organic carbon is initially derived from photosynthetically captured carbon.
The amount of organic mater in the soil is a balance between how much is being produced by plants, less the amount escaping to the atmosphere.
On average, 1-2% of plant residues become stabilised as humified soil organic matter for significant periods of time. Yet this small net accrual over millennia has led to the large stores of C found in soils around the globe.
Some plant residue is easily decomposed so will not stay in the soil for long. Difficult to know if this should be counted as sequestration.
However, some plant material will become humous or humic substance and remain for millennia.
Recent research shows that humous is a joining together of organic molecules is such a way that enzymes cannot attack them. The process of producing humous can be quite quick if the right conditions are present.
Most stable organic matter has reacted with or been adsorbed onto the surfaces of mineral or metals.
If the soil is compacted, then oxygen and microbes cannot decompose the organic matter. Tilling the soil however changes everything. Decomposition after tilling is 20 times faster in sandy soil compared to clay soil.