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Biophysical Aspects of Carbon Sequestration in Drylands
University of Essex Feb 03
Biophysical Aspects of Carbon Sequestration in Drylands
Peter Farage, Jules Pretty, Andrew Ball
University of Essex, UK
The process of carbon sequestration, or flux of carbon, into soils forms part of the global
carbon cycle. Movement of carbon between the soil and the above ground environment is bidirectional and consequently carbon storage in soils reflects the balance between the
opposing processes of accumulation and loss. This reservoir of soil carbon is truly dynamic, not only is carbon continually entering and leaving the soil, the soil carbon itself is partitioned between several pools, the residence times of which, span several orders of magnitude. Neither is soil carbon an inert reservoir, the organic matter with which it is associated is vital for maintaining soil fertility and it plays a part in such varied phenomena as nutrient cycling and gaseous emissions. A detailed description and analysis of soil carbon and organic matter can be found elsewhere (Schnitzer, 1991; FAO, 2001).
The quantity of carbon stored in soils is highly significant; soils contain approximately three times more carbon than vegetation and twice as much as that which is present in the atmosphere (Batjes & Sombroek, 1997). Of concern here, is the fact that various environmental factors, not least human dependent land management, can significantly affect the dynamic equilibrium that controls the size of the soil carbon pool.
The quantity of carbon within any soil has been described by (Reicosky, 1997) as a simple
mass balance relationship:
carbon input – carbon output = net carbon accumulation Many of the factors determining carbon input and output are influenced, intentionally or unintentionally, by land management practices. There is consequently much scope for agroecological processes to influence carbon input and output from soils. To promote carbon sequestration human activity needs to maximise the inputs and minimise the outputs.
However, if carbon sequestration in soils is to be used to offset gaseous emissions of CO2, it is necessary to look beyond the fluxes of carbon directly into and out of the soil. The carbon budget for the whole land management system must be considered. Where this system boundary is drawn may be as contentious as it is difficult to define. From a purely carbon accounting point of view, the system should envelope all aspects that are involved in soil management.
Specific Features of Drylands Dryland environments are characterised by a unique set of features and these will impact on particular aspects of carbon sequestration. By definition, lack of water is the key aspect, and this is accompanied in many dryland regions by the occurrence of higher temperatures at some time in the diurnal or annual cycle. The deficiency of water in drylands severely constrains plant productivity that provides the ultimate source of soil carbon. Additionally, the size of soil organic matter pools in natural ecosystems decreases exponentially with Biophysical Aspects of Carbon Sequestration in Drylands University of Essex Feb 03 temperature (Lal, 2002a) and consequently most dryland soils contain less than 1%, and frequently less than 0.5%, carbon (Lal, 2002b). Furthermore, although agricultural practices have reduced soil carbon levels globally, dryland landscapes are particularly prone to degradation and desertification. The quantity of carbon in dryland soils has therefore been reduced from an initially low base. However, there are some positive aspects relating to carbon storage in dryland soils. Drier soil per se is less likely to lose carbon (Glenn et al, 1993) and consequently the residence time of carbon in dryland soils is much longer than, for example, forest soils (Gifford et al, 1992). The organic matter itself and the processes operating on it are not believed to be any different in dry or hot regions compared to temperate zones where carbon levels are generally higher and where most research has been conducted (Batjes & Sombroek, 1997; Ayanaba & Jenkinson, 1990; Syers, 1997).
In spite of the generally low carbon content of dryland soils, the fact that drylands cover approximately 40% of the global land area (FAO, 2000), together with the fact that many of these soils have been degraded, means that drylands may well have the greatest potential to sequester carbon (Scurlock & Hall, 1998; Rosenberg et al, 1999).
Factors Controlling Soil Carbon Sequestration
The carbon sequestration potential of a soil depends on its capacity to store resistant plant components together with protecting, and accumulating, humic substances. The quantity of soil carbon present is controlled by a complex interaction of processes determined by carbon inputs and decomposition rates. Factors controlling the quantity of organic matter in soil include temperature, moisture, oxygen, pH, nutrient supply, clay content and mineralogy.
Accumulation of carbon will be favoured by conditions that do not promote decomposition, i.e. low temperature, acid parent materials and anaerobic conditions. Himes (1998) estimates that it takes 833 kg N, 200 kg P and 143 kg S to sequester 10 tonnes of carbon in humus. Soil fertility is therefore an important aspect of carbon sequestration yet many dryland soils are now low in nutrients. Rasmussen & Parton (1994) suggest that carbon levels in dryland soils rise between 10 - 25% of the rate that carbon is added.
The ultimate source of soil carbon is atmospheric CO2 that is captured by plants in the process of photosynthesis. Primary production therefore sets the upper limit on the amount of carbon that can be stored in soil. The biomass produced by net primary production will ultimately be available for decomposition and incorporation into the soil either directly as dead plant material or as organic matter that has passed through the animal food chain.
The most efficient method of accumulating carbon in soils must be by direct decomposition of plant material. If carbon passes through the heterotrophe chain some will be lost directly to the atmosphere as CO2 as a consequence of respiratory activity and through digestive and assimilatory inefficiencies. This is illustrated in Figure 1 where the Rothamsted soil carbon model has been used to show the effect of adding plant residue and cattle manure to a vertisol soil in semi-arid Andhra Pradesh. The system is in steady state when, in 1989, 5 t C ha-1 in the form of plant residue or manure is added for 5 consecutive years. The soil organic carbon content rises continually during the period of application and then declines over subsequent years. Assuming a 50% carbon content of the plant material (Schlesinger, 2000), 10 t of plant residue would actually be required to provide each 5 t application of plant carbon. For the manure application, if the digestive efficiency of livestock is 60% (Schlesinger, 2000), 10 t of plant residue would only produce 4 t of manure. If cattle manure
contains 25% carbon, only 1 t of carbon would actually be available for incorporating into the soil, Figure 1. Clearly, much less carbon is available for sequestering into the soil when it passes through the heterotrophic chain.
Figure 1 Change in organic soil carbon for a vertisol in semi-arid Andhra Pradesh, India modelled with the RothC soil carbon model. Five tonnes of plant residue carbon and five tonnes of cattle manure carbon were applied in 1989 for five consecutive years. Application of one tonne of carbon in cattle manure represents the amount of manure carbon that would be available for incorporation into the soil if the plant residues had been directly fed to the cattle.
Long-term field experiments have shown that there is a direct linear relationship between the quantity of carbon added to soil as organic matter and the amount of carbon accumulated in the soil, other factors remaining constant (Cole et al, 1993; Duiker & Lal, 1999). However, the dynamics of soil organic matter are complex and the factors controlling the flux of carbon will interact uniquely at each site. Predictions of how future carbon pools will change must be made with caution. This is particularly so as effects of land management practice on soil carbon may not be measurable for twenty years (Rasmussen et al, 1998). Eventually, if conditions are maintained the system will reach a new steady state, commonly within 50-100 years (Swift, 2001). The crucial point of consideration with regard to soil carbon sequestration is the dynamic nature of the process; even if soils are acting as a sink, they will be simultaneously losing some carbon and many factors can act at any time to shift the balance. All carbon in soils has the ability to completely decompose.
Other than erosion, the major route for carbon loss from soil is through CO2 efflux, commonly referred to as soil respiration. This loss is enhanced in warmer environments because the flux of CO2 emanating from microbial activity increases with temperature, although the relationship is non-linear, and best described by an Arrhenius model (Fang & Moncrieff, 2001; Qi et al, 2002). This is a primary factor acting against carbon sequestration in Biophysical Aspects of Carbon Sequestration in Drylands University of Essex Feb 03 hot, arid climates. However, decomposition also requires moisture and in drylands the availability of water is, by definition, frequently limiting. The interaction between temperature and moisture in controlling decomposition is complex and dependent on site characteristics. For example, Mielick and Dugas (2000) found that CO2 efflux from the soil in a tall-grass prairie depended primarily on temperature and secondly on water. Conversely, in semi-arid Alberta, moisture was shown to be the main factor controlling soil respiration during the growing season (Akinremi et al, 1999). A more complex situation was discovered in Central Texas by Franzluebbers et al (1995). They discovered that depending on crop sequence, tillage regime and season, between 65-98% of the variation in CO2 flux could be accounted for by a combination of soil temperature, moisture and time of year.
Bursts of microbial activity following episodic rainfall are common in drylands. Such seasonality of climate can make the timing of some land management practices, such as plant residue and manure applications, critical in terms of their effectiveness, not only for soil fertility, but also for maximum carbon sequestration.
Plants – Capturing Carbon for Soils
Carbon in soils ultimately comes from CO2 in the atmosphere, which is captured by plants using the process of photosynthesis. Globally, productivity is strongly correlated with water availability. Consequently drylands are disadvantaged by the fact that carbon fixation is lower than in many other ecosystems because of the limitation imposed by water shortage.
From a purely carbon sequestration point of view, plants that are grown in drylands should be selected on the basis of having the best adapted features for these conditions. The growing of vegetation on badly degraded dryland soils not suited to conventional agriculture has been suggested as a way of increasing soil carbon stocks (Lal et al, 1999;
Pretty et al, 2002).
In the most arid areas true xerophytic species are required which are able to produce biomass with zero or minimal inputs from man. Plant species that are best adapted to restricted water supply use either the C4 or CAM (crassulacean acid metabolism) photosynthetic mechanism. The latter are able to open their stomatal pores at night when the transpiratory water loss is least. They fix CO2 into four carbon acids that can be used as an internal reservoir of CO2 to supply photosynthesis during daylight. C4 species have a CO2 concentrating mechanism, the net result of this is to reduce the flux of gases into and out of their leaves, and consequently enhance water-use-efficiency. This is accompanied by a higher nitrogen-use-efficiency, which reduces the need for fertiliser application. These species also lack photorespiration, a consequence of which is a higher optimum temperature for photosynthesis and an ability to increase CO2 uptake to maximum irradiance in the tropics. Desert plants were considered to have slow rates of growth but research with Agave has shown that this CAM utilising species, which is adapted to severe arid conditions, can exhibit very high rates of productivity (Nobel et al, 1992).
Whilst above ground biomass provides soils with organic matter following senescence, below ground biomass is an important source of carbon, not only through root death but also from root exudates. However, quantifying root exudation is a difficult task. Xerophytes typically have deep root systems that enable them to tap below ground water. These deep root systems are ideal for taking carbon deeper into the soil where it is less susceptible to oxidation. Fisher (1994) has emphasised the importance of introduced deep-rooted grasses for increasing carbon storage in South American savannas, while Gifford (1992), has
estimated that thirty percent of total primary production enters long-term carbon storage in Australian soils from roots.
Plants of even the most arid regions have, therefore, a good potential to assimilate carbon that is then available for sequestering into the soil. Not only is there a variety of plant species available for growing in dryland environments, a number of these can be harvested for useful products (Lal et al, 1999). Harvesting will of course decrease the amount of plant residues available for incorporation into the soil, but the potential for carbon sequestration does exist.