J.M. Adams, W.P. Post, & A. Stangenburger
(1.) MS 6335, Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
(2.) Dept. of Env. Sci., Policy, & Mgt., Univ. of California at Berkeley, 145 Mulford Hall # 3114, Berkeley, CA 94720-3114, USA.
In this brief review we examine the data on carbon storage characteristics of cool desert soils.
Cool desert soils are often thought of as being a major store of organic carbon; in the widely cited Zinke et al. (1985) database they contain almost as much carbon as temperate forest soils. A careful reassessment of the data sources suggest that this view is erroneous; many data points were missasigned during the compilation of the database, and are either from local moist pockets within deserts, or fall outside the desert biome altogether. The 'true' figures for semi-desert organic carbon storage are undoubtedly much lower than these estimates suggest.In addition, the substantial store of inorganic caliche carbon in soils must not be regarded as behaving like organic carbon in terms of the global carbon cycle; a decrease in caliche carbon (though weathering) acts at least temporarily as an atmospheric CO2 sink, while a decrease in organic carbon (through oxidation) acts as a net CO2 source.
Introduction
Discussion and numerical modelling of the global carbon cycle on the time timescales of decades to millennia requires a knowledge of the potential of the world's ecoregions to store soil carbon. A change in climate and vegetation over time, whether due to future greenhouse warming or past climate changes during the Quaternary Period, can also be expected to affect broad scale soil carbon storage.
For the purpose of estimating soil reservoirs and fluxes, the major source of data for carbon cycle modelling is still the global database compiled by Zinke et al. (1985). To make further progress, it is necessary to start reassessing this database in terms of detailed study of the data sources it contains using knowledge from the broad soil science and ecological community, and to bring in more recent additional sources of data, comparison and study of types of data sources.
Some paradoxical features emerge if one examines the Zinke et al. database in more detail. For example, the results suggest that there is as much carbon in temperate deserts as in temperate forests and woodlands. Such large amounts of carbon in desert or semi-desert soils would certainly seem paradoxical, considering the low rates of input of organic matter.
The paradoxically high soil carbon storage found for temperate desert soils has major implications for calculating carbon storage on the broader Quaternary time scale, because under the cooler and drier conditions of the last Glacial, cool deserts and semi-deserts were one of the world's most widespread ecosystems. The very high soil organic carbon values in the Zinke et al. database have been accepted and used in a number of attempts to reconstruct last Glacial-to-Holocene changes in global carbon storage (e.g. many of the papers cited by Peng et al. in press), in which it has a substantial effect in reducing the glacial-interglacial change in carbon storage. Desert soil inorganic carbon storage (in calcrete) has also been discussed as a factor tending to 'damp down' the change between glacial and interglacial carbon storage, because semi-arid soils are relatively rich in carbonates (Schlesinger 1992).
Desert soil carbon in organic form
(HISTOGRAM SHOWING THE ZINKE et al DESERT SOIL CARBON vs TEMPERATE FOREST)
9.7 tC/ha in cool desert vs 10.0 tC/ha in temperate forest.In the Zinke et al database, around 40 datapoints are assigned to the 'cool temperate desert' biome. Most of the points in the dataset come from from the SW USA, and the remainder is from central Asia. On the basis of these data points, Zinke et al. suggest global values for cool desert soil carbon of around 10 kg/m2, and these figures have been used in most of the paleobiome-based estimates of long-term global changes in the carbon cycle such as (LIST THE REFERENCES) and other studies cited by Peng. et al. (in press).
Detailed assessment of the databases suggests that in fact the soil carbon values for SW USA and central Asian desert regions are indeed inflated due to a number of factors.
Firstly, in the face of the task of assessing many thousands of soil data points from around the world, some data points were accidentally mis-assigned.In the central Asian dataset (co-ordinates given below in Table 1.), all 30-odd points fall within the dry steppe and steppe-marginal zones.
Table 1. Datapoints for 'cool desert scrub' soil organic carbon in Central Asia from the database of Zinke et al. (1985). In fact, all of these fall outside the 'true' desert or semi-desert regions.
Database no. ....Biome..Organic C........Grid co-ordinates....Life zone type
2050001 245.01 SB 15.3 1001. 53.0N 78.6E 200. 13 B 40
2050002 245.03 SB 6.3 0. 54.0N 78.7E 200. 13 B 40
2050003 245.04 SB 8.0 0. 53.3N 78.3E 200. 13 B 40
2050004 245.05 SB 5.5 0. 52.3N 79.1E 200. 13 B 40
2051001 248.01 SB 8.3 0. 53.5N 80.2E 200. 13 B 40
2051002 248.02 SB 6.6 0. 53.5N 80.2E 200. 13 B 40
2052001 251.01 SB 4.3 0. 53.4N 79.0E 200. 13 B 40
2052002 251.02 SB 11.3 0. 52.7N 83.0E 200. 13 B 40
2053001 254.01 SB 6.4 711. 53.8N 91.4E 375. 13 B 40
2053002 254.02 SB 3.5 0. 53.8N 91.4E 375. 13 B 40
2053003 254.03 SB 4.9 394. 52.2N 90.7E 750. 13 B 40
2053004 254.04 SB 6.7 377. 53.2N 90.5E 375. 13 B 40
2053005 254.05 SB 5.5 357.. 53.2N 90.5E 375. 13 B 40
2053006 254.06 SB 10.6 658. 52.1N 90.7E 750. 13 B 40
2053007 254.07 SB 6.3 473. 53.2N 90.5E 375. 13 B 40
2053008 254.08 SB 6.4 938. 56.1N 92.8E 0200 13 B 40
2054001 257.01 SB 17.4 948. 50.3N 87.6E 1600 13 B 40
2060001 VV14 SB 5.6 768. 50.1N 88.9E 2000 42 B 60
2060002 VV87 SB 3.3 225. 50.1N 88.3E 42 B 60
2060003 VV2 SB 8.7 869. 50.1N 88.9E 2000 42 B 60
2060004 VV9 SB 4.0 271. 50.1N 88.9E 42 B 60
2069001 SC01 RS 8.7 1180. 49.6N 57.2E 300 19 B
2069002 SC01A RS 6.0 980. 49.6N 57.2E 300 19 B
2069003 SC02 RS 7.8 1237. 49.6N 57.2E 300 19 B
We re-examined the source literature data for various potential problems which might have inflated the estimates of soil organic carbon for this biome. Inclusion of inorganic carbon with the organic carbon estimate was not found to have been a problem with any of the source data points. However, various other problems were prevalent within the data. We found that the paradoxically high values for soil carbon that are found in the soils database are due to a combination of problems;
1. Most samples in the Zinke et al database have been mis-assigned in the biome sense; they fall in steppe regions and not desert.
2. Of those samples which do fall within desert regions, many were origonally gathered with an eye to agricultural potential, and so the sampling often favours locally moist pockets within the landscape, no matter how unrepresentative these might be.
3. Even those areas in the western USA which do fall within 'desert' as generally described from that region are in fact from relatively dense sagebrush semi-desert, which is capable of sustaining commericial ranching. On a global scale this would not qualify as 'true' desert, yet the high carbon storage values are extrapolated to include all past and present cool desert zones, no matter how dry. Many other authors (e.g. most studies cited by Peng et al. in press) have accepted this extrapolation without distinguishing between the relatively densely vegetated semi-desert and extremely dry and barren desert that exists in some core areas of central Asia, and which occupied larger areas of the world during the last Glacial period.
On the basis of these considerations we suggest that the true general value for cool temperate semi-desert would fall in the region of X kg/m2 organic carbon. The values obtained for warm deserts (around 6.0 kgC/m2 in the Zinke et al. database may well be inflated by similar processes), and it might be necessary to consider a lower value such as that of Schlesinger of 2.2 kgC/m2 as being more representative of warm semi-deserts. True deserts in warm climates would presumably be even lower than this.
Desert soil carbon in inorganic form
As another important aspect of desert soil carbon storage, it is necessary to point out a common source of confusion in terms of understanding spatial heterogeneity and overall totals of desert soil carbon. This is the important and fundamental distinction between organic and inorganic carbon. Calcrete, carbonate precipitated into the soil, often as nodules or a crust near the surface, is a major reservoir of carbon in present-day soils in arid and semi-arid regions of both cool and warm climate desert. Inorganic carbon is excluded from the Zinke et al. database, but there is still possibiity of confusion from other sources. Many biogeochemists (e.g. Schlesinger 1992) discuss this soil reservoir in a purely empirical sense, pointing out its importance but not its actual behaviour as part of the carbon cycle. Yet its effect on the carbon cycle is opposite to that of organic carbon. A decrease in calcrete, by chemical weathering, takes up CO2 from the atmosphere, at least temporarily. A decrease in organic matter in a soil (due to oxidation) should increase atmospheric CO2. This is an obvious distinction, yet it is suprising how many good scientists fail to notice this when they discuss soil carbonate reservoirs.
Overall, it appears that a careful reassessment of the evidence of desert soil carbon storage results in very different perspective from that generally accepted in the biogeochemical literature at present. Cool desert soils (and probably all desert soils) contain much less organic matter than had been thought. There is a need for basic fieldwork to find out what the 'true' representative values for desert soils actually are, and a need for closer dialogue with the soil science community to prevent misunderstandings from recurring. In addition, inorganic carbon in desert soils should not be seen as an analogue of organic carbon in terms of its effects on atmospheric CO2 levels.
References