2. 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:

3. 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

4. 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:

6. ??

9. 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.

10. Soil C Trends with Time (natural and after cultivation: models At start of cultivation:

11. 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 k a 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.

12. Limitations to Agricultural Storage of CO 2 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.).