Soil, carbon residence time

Note that each state factor can influence soil carbon storage in two ways by influencing the quantity and quality of plant inputs and by influencing the residence time of organic matter in the soil. Figures 6.2-6.4 show the influence of three state factors on soil C storage. Many of the descriptions of state factors below touch upon the importance of controls on SOM stabilization, and these controls are discussed further in a following section. 

Gaudinski, J. B.,Trumbore, S. E., Davidson, E. A., and Zheng, S. H. (2000). Soil carbon cycling in a temperate forest Radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes. Bio geochemistry 51(1), 33-69. 

The rainwater of Bermuda is in near equilibrium with atmospheric Pc02 = 10-3.5 atm., and contains small amounts of sea salt (0.07 wt. % seawater). The rainfall of 147 cm y1 is seasonally distributed. The rain enters the saturated zone by two main paths direct rainfall on marshes and ponds, and percolation downward from the vadose zone as vadose seepage and flow through rocks during times of soil water excess (Vacher, 1978). Total annual recharge of the saturated zone is about 40 cm y-1 (Vacher and Ayers, 1980). The residence time of the groundwater has been calculated as 6.5 years, and the average age of the sampled water as 4 years (Vacher et al., 1989). Such estimates are necessary for calculations of carbonate mineral stabilization rates, as shown in a later section. 

Based on the discussion in the previous section it is apparent that plant photosynthesis produces organic matter from today s atmosphere, but respiration releases carbon that has been stored for different periods in plant tissues and soil organic matter. It was also noted already that over the industrial era, the of the atmosphere has been declining at a mean rate of 0.02%cyr The carbon respired today is, therefore, enriched in compared to current photosynthate, to an extent that depends on the residence time of the carbon in the biosphere (Figure 4). For example, adopting the estimated global mean t of 27 yr mentioned above, soil respired CO2 could be 27 X 0.02 = 0.5%o more enriched in with respect to current atmospheric CO2. Under such circumstances, an isotopic flux between the land biosphere and the atmosphere will exist even if there is no net CO2 flux. Similar to the case for the ocean, this represents the land disequilibrium, Dl. Note that Dl will tend to enrich the atmosphere in... 

Elzein A. and Balesdent J. (1995) Mechanistic simulation of vertical distribution of carbon concentrations and residence times in soils. Soil Sci. Soc. Am. J. 59, 1328-1335. 

The evolution of the isotopic compositions of carbon-bearing substances in uncontaminated systems where carbon is derived from carbonate minerals and soil CO2 is bounded between two limiting cases (i) open systems, where carbonate reacts with water in contact with a gas phase having a constant Pco, and (ii) closed systems, where the water is isolated from the CO2 reservoir before carbonate dissolution (Deines et al., 1974 Clark and Fritz, 1997). Both of the extremes assume water residence times long enough for significant isotope exchange between the gas and the aqueous phase to take place. 

Soil carbon cycling in a temperate forest radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes. Biogeochemistry (Dordrecht) 51(1), 33-69. 

Volatilization of selenium from volcanoes, soils, sediments, the oceans, microorganisms, plants, animals, and industrial activity all contribute to selenium in the atmosphere. Natural background concentrations of selenium in nonvolcanic areas are only around 0.01-1 ngm , but the short residence time, usually a matter of weeks, makes the atmosphere a rapid transport route for selenium. Volatilization of selenium into the atmosphere results from microbial methylation of selenium from soil, plant, and water, and is affected by the availability of selenium, the presence of an adequate carbon source, oxygen availability, and temperature (Frankenberger and Benson, 1994 Jacobs, 1989). 

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