Land Theme Report
Australia State of the Environment Report 2001 (Theme Report)
Prepared by: Ann Hamblin, Bureau of Rural Sciences, Authors
Published by CSIRO on behalf of the Department of the Environment and Heritage, 2001
ISBN 0 643 06748 5
Nutrient and carbon cycling (continued)
Carbon - and its relationship to other nutrients [L Indicator 5.2]
On land, most carbon is stored in soil, and in living and dead vegetation. This forms the primary energy store and structural basis of all biological productivity. Without carbon in the form of organic matter, soil cannot develop the full range of biota, so necessary for the decomposition and transformation of debris, residues and detritus. Where decaying and dead vegetation is removed from the land surface (as it is in many agricultural systems, by harvesting, cultivation and burning) soil organic matter declines, even without the added problems of surface erosion. Where plant cover is thick and growing rapidly, more carbon is fixed through photosynthesis than where vegetation is thin, growing slowly, or not at all.
In semi-arid Australia plant growth rates and carbon accumulation is inevitably slow and lost carbon takes longer to replenish than in sub-humid and humid environments. In addition carbon retention is important for ecosystem renewal and survival.
Because the composition of most woody vegetation is chemically similar, the relationship between carbon, nitrogen and phosphorus is remarkably stable between different environments: most natural plant-soil systems have carbon to nitrogen ratios of about 20:1. Agricultural systems have ratios often approaching 10:1 (i.e. they have a larger proportion of protein and non-cellulose based components in the plant material). Domesticated plants have been selected and bred to enhance their harvestable (protein or oil-rich) components, whereas natural plants invest more energy in structural and protective components that will withstand climatic fluctuation and predation by herbivores. The ratio of carbon to phosphorus in such plants may be as high as 100:1.
How much carbon is being lost from our ecosystems? [L Indicator 5.2]
The pattern and rate of release of soil carbon is not known for the full range of Australian soil and vegetation types. In addition the net uptake or emission of carbon from certain management practices is not known (AGO 2001a; for updated information see http://www.greenhouse.gov.au ). There are good data for only a few of the parameters that have to be regularly reported to the United Nations Framework Convention on Climate Change. As a result, the National Carbon Accounting System (NCAS) was established in 1998 to overcome these uncertainties and provide improved data towards meeting Australia's obligations to the Kyoto Protocol.
An important starting point has been provided by the work of Barrett (2001), in establishing what the long-term carbon sequestration and residence time for continental Australia would be if the whole continent were still under a minimally disturbed vegetation cover. This work provides the values against which contemporary and projected future variations, both natural and anthropogenic, can then be assessed. Figure 68 shows the amount of carbon present in all parts of the system that are likely to have accumulated carbon, across the continent, with individual examples from three contrasting environments.
Figure 68: Continental estimate of net primary productivity extrapolated from 183 undisturbed natural habitats.
Source: Barrett (2001)
The important findings from this significant study are:
- Mean residence time of carbon in the terrestrial ecosystem (either in living or dead plant material, or in soil) ranges from 15-300 years, with the highest residence times in the interior southern desert environments, and the smallest residence times in the north-east wet tropics.
- The amount of carbon sequestered over the two-thirds of the continent that are arid and semi-arid regions is much less than international calculated amounts based on northern hemisphere modelling have assumed. The total annual net primary productivity (NPP) for the continent is just under 1 Gt (1000 Mt), much less than previous estimates. Kirschbaum (1993) estimated the total at 1.6 Gt C/year using a different model, while an estimate by Gifford et al. (1992) put the pre-industrial annual NPP at 2.4 Gt and recent annual NPP at 2.7 Gt, assuming a fertilisation effect from carbon dioxide had more than compensated for any loss of vegetation through clearing.
- The most recent estimate is likely to be the closest to actual amounts, both because it is based on a very much larger and more rigorously screened set of measured values, and because the model used has the capacity to account for a larger number of carbon pools and interactions.
- On the time-scale of climate change (centuries to millennia) most annual effects on vegetation and soil carbon cycling, such as grazing or burning have only minor effect when considered against the seasonal variability of climate at any one site. Permanent clearing of one type of vegetation structure and replacement with another is the only change in which the signal is larger than the 'noise' of statistical variation.
This last point is of particular interest. The Atmosphere Theme Report describes in detail the intra-annual and inter-annual variations and growth in carbon dioxide recorded at Cape Grim since 1976. There is large inter-annual variability in the rate of atmospheric carbon dioxide increase and very recent work suggests that most of this is due to variations in the exchange of carbon dioxide between the atmosphere and the terrestrial biosphere (Schimel et al. 2001).
Fossil fuel-derived carbon dioxide concentration is increasing at an overall linear rate of some 15 ppm per ten years, short-term concentration fluctuates substantially between 0.8 and 3.2 ppm per year. These oscillations, similar to the large variations in pressure cell strength and location, and resulting rainfall events, are now ascribed to fluctuations in the land-air flux, as the ocean-air flux appears remarkably constant (CSIRO Biosphere Working Group unpublished).
In the light of these large seasonal fluctuations, it may be difficult to determine the extent to which anthropogenically induced carbon changes are influencing the total atmospheric carbon dioxide concentration.
Net primary productivity (NPP) is the term for the amount of plant growth that accumulates in a time period minus the amount that has been lost through decomposition. However, because it is a fluctuating variable it is less easy to appreciate than the term 'biomass', or store of carbon. Consequently, in this report the term biomass is used in reporting carbon stores. Biomass tells us how much food and energy there is to support all the other biota (decomposers, herbivores and carnivores) in the food chain.
Baseline data for net primary productivity and standing biomass store has recently been calculated at continental scale (Raupach et al. 2001). A combination of land and climate information sources have been used, together with a modelling approach, to calculate the seasonal accumulation and decomposition of carbon over the past decade, and compare that with the continental baseline study. This baseline study has attempted to combine the above-ground store of plant material, including standing dead wood and litter, with estimates of below-ground soil pools. These include roots, below ground lignotubers and other woody plant structures, and the soil organic matter that is finely dispersed and adsorbed onto soil clays, and stored in micro-organisms.
The results show that regional differences are largely driven by effective rainfall (rainfall minus evaporation), with the major stores of carbon associated with high rainfall regions.
Most of the continent, being semi-arid and low-lying accumulates carbon very slowly, and can lose it fast when land is disturbed. While stored carbon amounts vary by three orders of magnitude across the full range, over 90% of the land area has less than 500 kg carbon per hectare. Forested regions store the most carbon, and desert environments the least.
The results show that:
- rainfall is the primary driver of seasonal and spatial variation across the continent,
- less plant biomass is produced than was anticipated in the northern half of the continent because of the strong evaporative demand, which reduces the plant-available water more than in southern regions, and
- variations in computed carbon, nitrogen and phosphorus stored in soil agree with logarithmic distributions of measured values of soil carbon, nitrogen and phosphorus drawn from literature collated by Barrett (1999) (Figure 69).
Figure 69: Logarithmic plot of carbon pools computed from BIOS model compared with measured values from Barrett (1999).
Source: Raupach et al. (2001)
This good relationship gives the modellers some confidence that their results are reasonably accurate and can be used where no actual data exist.
Regional differences in carbon stores with and without the presence of agriculture were also computed. Details of how this is calculated are given in Barrett (1999). Figure 70 shows where total biomass is highest and lowest per unit area by major drainage basins.
Figure 70: Biomass stores of above and below ground carbon (kg C/m2/year) for Australia's 12 drainage basins, with and without agriculture.
Source: Barrett (1999)
Agriculture has made little difference to the carbon stores in the arid regions of the rangelands, but has made a significant difference in those areas of higher rainfall where tall perennial vegetation has been replaced by short annuals and scattered perennials (Drainage Divisions 1, 2 and 3, and parts of 4, 5 and 6; see Figure 70). The below ground store of carbon, already larger than above ground prior to vegetation change, has become even more pronounced.