Tropical Grasslands (2009) Volume 43, 227–231 227 Soil carbon sequestration – myths and mysteries MIKE BELL1 and DAVID LAWRENCE2 in soil, it is essential to understand the forms in 1Queensland Primary Industries and Fisheries, which soil C exists. Kingaroy, and Soil C is found in either inorganic (mineral) 2 Toowoomba, Queensland, Australia or organic forms. Inorganic soil C is generally found as carbonates of calcium (CaCO3—lime- stone) and magnesium (MgCO3). Excluding con- Introduction centrated deposits of these materials that arose from deposition of the shells of aquatic inver- The balance between carbon (C) sequestration (or tebrates, most of these inorganic forms of C are storage) in various sinks such as soils and veg- found in alkaline soils, with local examples being etation and that released to the atmosphere as the heavy clay soils of inland Queensland and carbon dioxide (CO2) is topical, given the global northern New South Wales. Calcium carbonate warming/climate change projections. However, is effectively insoluble in water at pH>8.3; thus, there is a considerable degree of uncertainty sur- when these extreme conditions are met (typically rounding what options are available, how realistic in subsoils of the black and grey cracking clays), they are and, more specifically (from a soil per- CaCO3 precipitates out of the soil solution to spective), what role does soil C play in mitigating form whitish nodules embedded in the clay soil. the effects of CO2 emissions. This paper tries to While these inorganic forms of C can represent place soil C in a global perspective and to assess a significant amount of the C stored in the pro- how realistic it is for significant soil C seques- file of these particular soils in some areas, much tration to occur in different land use systems. In more carbon is found in the organic form. order to do that, we need to provide some back- The organic forms of C in soil are a diverse ground on soil organic matter, the main soil C group of materials that can be defined as ‘eve- pool which can be influenced through land man- rything in or on the soil that is of biological agement. This will consider the different forms of origin—whether it’s alive or dead’. It therefore organic C in soil, how they can be measured and includes live plant roots and litter (not shoots), the role they play in determining soil functions humus, charcoal and other recalcitrant residues of and properties. organic matter decomposition. It also includes the organisms living in the soil that are collectively called the soil biota (including fungi, bacteria, Soil as a carbon sink mites, earthworms, ants and centipedes). The one factor that all these materials have in common Carbon stored in soils world-wide represents the is that they contain C, and so you will generally third largest sink in existence, after oceans and hear talk of the interchangability of soil organic geologic sinks, representing 2–4 times the C in matter and soil organic C. On average, organic the atmosphere, and about 4 times the C stored in matter in soil contains about 60% C; thus, if a vegetative material (plants). It is therefore under- test shows 1% organic C, the soil will contain standable that soil C is being viewed as a sink about 1.7% (by weight) organic matter. that could potentially have a significant impact on sequestering CO2 emissions. However, before we consider how feasible it is to store extra C Forms of organic matter and C in soil Correspondence: Mike Bell, QPI&F, Kingaroy, Qld 4610, Soil organic matter is ultimately derived from Australia. E-mail: [email protected] decomposing plant material, with that decom- 228 Mike Bell and David Lawrence position driven by the soil biota for which the there is generally more C released as CO2 than plant material and decomposition products are there are surplus nutrients released (i.e., a fungus the primary source of energy and nutrients. In the needs 8 C atoms for every N atom to grow more process of this decomposition, populations of dif- hyphal threads, but can digest poor-quality crop ferent components of the biota wax and wane in residue with a C:N ratio that starts at 100:1). As response to the abundance of their preferred food a result, surplus C is respired, while the N is con- source, and to their predation by other organ- served, and through the aging process in soil the isms. Similarly, the abundance and age of dif- organic materials become increasingly nutrient ferent components of soil organic matter fluctuate rich. This enrichment of nutrients in humus and in response to the quality and quantity of inputs more recalcitrant, charcoal-like materials occurs (i.e., residue type and frequency of addition) and particularly with N and sulphur (S). The humus the influence of moisture and temperature on the that eventually forms from decomposition of, decomposing organisms. say, cereal straw will have a C:N ratio of approx- imately 12:1, rather than the 100:1 in its orig- Crop residues on the soil inal form. After so many cycles of digestion and surface (weeks(weeks-months - months)) Extent of excretion these materials are less readily decom- Buried crop residues and decomposition posed than when they entered the soil as plant roots (months (months - -years) years) increases residue, but the nutrients they contain ensure that Particulate organic matter C/N/P ratio decreases they remain a valuable source of nutrients con- ((yearsyears-decades - decades)) (become nutrient rich)rich) tributing to soil fertility. Humus (decades(decades-centuries - centuries)) Legume residue Cereal straw DominatedDominated by charcoal C:N ratio 20:1 C:N ratio 100:1 Resistant organic matter –like- like materialmaterial withwith ((centuriescenturies -–millennia millennia)) variablevariable propertiesproperties Fungi Bacteria (C:N = 8) The principal components of the non-living (C:N = 4) soil organic matter pool are shown in the sche- matic above, along with an indication of the Bacterial-feeding Fungal-feeding ‘half-lives’ for material to progress from one nematodes (C:N = 6) nematodes (C:N = 9) pool to another. Obviously, this time varies with environmental conditions, such as temperature Excess N excreted and available moisture that influence microbial in mineral form for use by plants activity, with the decomposition process slowed Diagram courtesy enormously by cold temperatures (e.g. on North of Graham Stirling American prairies or Russian steppes) and low annual rainfall. This time-scale is useful for thinking about both the decline in soil organic A good example of biological nutrient cycling matter that occurs under exploitative land uses and release for plant utilisation is shown in rela- and also the time to replenish soil organic matter tion to N in the above diagram. Bacteria or fungi stocks (especially the more stable compounds growing on plant residues are preyed on by a like humus) under natural conditions. number of more complex organisms such as free- living nematodes, which typically have higher C:N ratios than the organisms on which they Fate of C and nutrients during feed. The excess N in this case will be released transformations in an inorganic form (either ammonium-N or nitrate-N) for use by plants or other organisms. As the microbial decomposition process occurs, This inorganic N can build up as decomposition C and some nutrients are liberated. The C is continues (if it is surplus to requirements of the released as carbon dioxide (or methane under microbial community) and will form the basis of certain conditions) from microbial respiration, the N supplied to the next crop. The lower the and surplus nutrients are released (mineral- C:N ratio of the material being decomposed (i.e., ised) in inorganic forms suitable for use by other humus vs fresh cereal straw) the more likely there microbes and plants. is to be net release of mineral N. However, soils are generally nutrient-poor The converse of this is that, if we want to raise environments so, as the decomposition process soil organic C levels by increasing soil organic occurs and the organic materials age in the soil, matter (rather than adding a relatively inert mate- Soil carbon sequestration 229 30 Clearing native Total soil organic C vegetation for Particulate organic C Conversion to 25 cropping Humus organic C permanent pasture Resistant organic C 20 Same total OC 15 8% particulate 26% particulate (g C/kg soil) 44% humus 10 63% humus 27% resistant Soil organic carbon 27% resistant 5 J. Baldock, CSIRO 0 10 20 30 40 50 60 70 Years rial like coal dust, for example), we will be tying portionally more char-like material in their native up nutrients. As an example, for an increase in condition (from regular natural fires) than similar soil organic C content of 1% (e.g. from soil C soil types under a rainforest, for example. Sim- of 1% to 2%) to occur, 900–1500 kg/ha N and ilarly, soils growing sugar cane in areas where 70–120 kg/ha P must be available to form that trash is burned regularly can also have a higher organic matter. If the soil has a low fertility status proportion (up to 50%) of soil C as charcoal (e.g. is run down in N), then soil organic matter -much higher than where there has been a history cannot increase unless the N is provided (as from of trash blanketing. a legume-based pasture). Similarly, increases These proportions are important. Consider the in soil organic matter levels in soils with low P soils shown in the following example (adapted status will be limited unless the P deficiency is from Baldock and Skjemstad 1999), where soil is overcome, just as legumes will struggle to persist examined on 2 occasions in its management his- in pastures and fix N.