Soil C Residence Times Calculate residence times of

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:

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

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 At start of cultivation:

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 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. ).