Soil Carbon in Global Perspective

•Two main anthropogenic C inputs are form fossil fuel and soil/plants

•Main C sinks are atmosphere, oceans and (???) ecosystems

Soil C Residence Times

•Calculate residence times of C pools in previous figure. Measure of response time to perturbations.

•Table below shows approximate average range vs climate:Table 1. Ecosystem-based distribution of global soil C pools and fluxes.

Life Zone1 Area (1012m2)

Mean Soil C Content (kg

m-2) MAP (mm) MAT (C)C Inputs (kg

m-2yr-1)2 k (yr-1)Residence

Time (yr) kf (yr-1)4

Tundra 8.8 21.8 500 2.25 0.102 0.0047 213 0.0094

Boreal desert 2 10.2 125 4.5 0.050 0.0049 204 0.0098

Cool desert 4.2 9.9 125 9 0.214 0.0216 46 0.0433

Warm desert 14 1.4 125 22 0.043 0.0306 33 0.0612

Tropical desert bush 1.2 2 188 25 0.083 0.0417 24 0.0833

Cool temperate steppe 9 13.3 375 9 0.300 0.0226 44 0.0451

Temperate thorn steppe 3.9 7.6 375 14.5 0.462 0.0607 16 0.1215

Tropical woodland and savanna 24 5.4 375 23.5 0.479 0.0887 11 0.1775

Boreal forest-moist 4.2 11.6 375 4.5 0.190 0.0164 61 0.0328

Boreal forest-wet 6.9 19.3 1250 4.5 0.681 0.0353 28 0.0706

Temperate forest -cool 3.4 12.7 2250 9 0.912 0.0718 14 0.1436

Temperate forest-warm 8.6 7.1 4250 14.5 0.826 0.1163 9 0.2326

Tropical forest -very dry 3.6 6.1 750 28 0.472 0.0774 13 0.1548

Tropical forest-dry 2.4 9.9 1250 23.5 0.458 0.0463 22 0.0926

Tropical forest-moist 5.3 11.4 2500 23.5 2.491 0.2185 5 0.4369

Tropical forest-wet 4.1 19.1 6000 23.5 3.732 0.1954 5 0.3908Globe3

129.6 10.8 0.585 0.0542 18.5 0.1084

1All data, except C inputs, from Table 2 in Post et al. (1982). 2C input data from Table 1 in Jenkinson et al. (1991).3Total global soil also includes the estimates for cultivated lands and wetlands. 4Decomposition constant calculated assuming 50% of soil C is reactive on decadal or less time scales (C inputs/(0.5xC))

Effect of Cultivation on Soil C Storage

Inputs:• can increase or decrease depending on cultural practices

Losses/loss rate:• almost always increase due to:

– disruption of aggregates which releases physically protected C to microbes

– increases in soil temperature (increasing surface albedo)

– increase physical erosion which may more rapidly oxide soil C

Soil C storage vs. population:

??

Paired plot (left) and time series (below) analyses of soil C vs. cultivation. Apparent gains in soils of low original C may be due to enhanced inputs by irrigation, etc.

Soil C Trends with Time (natural and after cultivation: models

€

C(t) =1

k(I − Ie−kt )

€

Css =I

k

At start of cultivation:

€

C(t) = (Iaka

−Css)e−ka t −

Iaka

Can Soil C be Restored Under Improved Ag Practices: major UN/Kyoto issue

Approaches:

•Reduce/eliminate tillage which allows C to be stored in soil structure and away from microbes

•Major possibility for USA in Great Plains

•Minimum tillage reduces ka in previous diagram back toward original value (and soil C increases)

• Return marginal lands to native vegetation

•Important also in Great Plains

•Again, k changes and soil C rebuilds (rate is dependent on many things, including climate, N availability, etc).

•Better manage grazing lands, establish agro-forestry in tropics etc.

Limitations to Agricultural Storage of CO2

•Rate is likely to be originally high, but should rapidly decline on decadal time scales.

•Total potential storage in all cultivated soils is about 70 Gt, which = about 12 years of fossil fuel burning

•Restored C can be easily and rapidly lost through changes in land management (back to plowing, etc.).