Soil Mean Residence Time

An 02 flow must be maintained through the contaminated zone at a level sufficient for the aerobic biodegradation of contaminants. Note that during bioventing the main aim is the maximum utilization of 02 by the microbial cultures. For this reason, air flow rate is usually an order of magnitude lower than that applied in simple SVE systems. A simple empirical rule is that the mean residence time of air in the contaminated soil pore volume should be between 1 and 2 days. 

In soils, lead concentrates in organic-rich surface horizons (NRCC 1973). In one instance, only 17 mg of soluble Pb/kg was found in soils 3 days after the addition of 2784 mg of lead (as lead nitrate)/kg (NRCC 1973). The estimated residence time of lead in soils is about 20 years complete turnover in topsoil is expected every few decades (Nriagu 1978a). In forest litter, however, the mean residence time of lead is lengthy estimates range from 220 years (Turner et al. 1985) to more than 500 years (Friedland and Johnson 1985). 

Campbell et al. (1967) applied sequential extraction to characterize FA, HA, and humin from gray podzolic and chemozemic soils. The fractions of FA and HA extracted by 0.5 M NaOH without acid pretreatment, which they called mobile humates (since the researchers assumed that they are not bound to minerals), had a lower mean residence time (ranging from 85 to 785 for HA, respectively, in the chernozemic and gray podzolic soils) as compared to Ca-humates extracted from humin (1410 years in the chemozemic soil) and to the total FA and HA extracted after acid pretreatment (195-1235 years for HA). This study showed that in the chernozemic soil, Ca-humates and clays play an equally important role in the stabilization of HS, whereas in the podzolic soil the oldest fraction was associated with clays. 

Laboratory Incubations. Laboratory incubations provide a controlled environment for characterizing and comparing C and nutrient dynamics in isolated soils. While subject to artifacts, they do provide one way to quantify the amount of fast-turnover C in soils (Paul et al., 2001). Most often, soils are incubated in jars (Stotzky, 1965 Hart et al., 1994) or microlysimeters (Nadelhoffer, 1990). Incubations have been used to estimate C mean residence times (Torn et al., 2005 Paul et al., 2006) and stability (Whalen et al., 2000 Swanston et al., 2002), interactions of SOM... 

By considering the mass balance of HT in the southern hemisphere, where there are no artificial sources other than through inter-hemispheric transfer, Mason Ostlund estimated that HT is removed from the atmosphere with a rate constant 0.155 a-1, giving a mean residence time of 6 a. The main sink for H2 or HT is oxidation by soil bacteria (Schmidt, 1974 Garland Cox, 1980 Sweet Murphy, 1981). Land comprises 29% of the earth s surface. The effective depth of the atmosphere (mass per unit ground area divided by density at ground level) is 8000 m and HT is well mixed. It follows that a deposition velocity to land of 0.135 mm s-1 would give a removal constant of 0.155 a-1. 

These changes in soil C stocks are relatively rapid, due to rapid turnover of soil organic matter (SOM) in these tropical soils. Using radiocarbon derived from atmospheric testing of nuclear weapons in the 1960s as a tracer, Trumbore et al. (1995) estimated that the mean residence time of C in the top 10 cm of soil is about 3 years for 30% of the SOM and 10-30 years for 60% of the SOM. Only about 10% of the C in the top 10 cm of soil cycles on a millennial time scale. This very old C fraction increases to 40-80% in the... 

Another approach to determining the residence time of °Pb in the atmosphere is to divide the mean air column inventory of °Pb by the flux of °Pb to the surface at a given location. This quotient yields a climatological average for the removal processes at that particular site. Graustein and Turekian (1986) used the atmospheric profiles of °Pb from Moore et al. (1973) and their own measured °Pb fluxes from soil profiles and bucket collection to obtain a value of 6 d over the central and eastern United States. As the source of Rn and thus Pb is from the ground and the major removal by precipitation is in the lower troposphere, the mean residence time is dominated by the processes of the lower troposphere. Modeling by Balkanski et al. (1993)... 

Alexandre et al. (1997) found that the biogenic sihca input into the biogeochemical silica cycle from the dissolution of phytoliths is twice as large as silica input from primary silicate mineral weathering in the tropical Congo rainforest. Biogenic (opaline) silica dissolves faster than sihcate minerals. While most of the phytoliths dissolve rapidly with a mean residence time of 6 months (Alexandre et al., 1994), and the sihca is recycled by the forest, a small part (7.5%) does not dissolve and is preserved in the soil. 

Sanderman J., Amundson R., and Baldocchi D. D. (2003) Application of eddy covariance measurements to the temperature dependence of soil organic matter mean residence times. Global Biogeochem. Cycles, 17(2), 1061. 

Absolute ages cannot be determined for the humus of present-day soils because of continued decomposition of old humus and resynthesis of new humus by microorganisms. The term mean residence time (MRT) has been used to express the results of C measurements for the average age of modern humus. The resistance of humus to biological decomposition has long been known but not until the advent of C dating was it possible to express average age on a quantitative basis. 

TABLE 7. Mean Residence Times of Organic Matter in Some Typical Soils... 

TABLE 8. Mean Residence Times (MRTs) for Different Organic Matter Fractions of a Chernozemic Black Soil (Melfort Silt Loam) as Reported by Campbell et al. (1967)... 

The calculated transit time - which was under the conditions described taken as identical to the mean residence time - had a value of t = 5-6 years for a 5 cm thick layer of partially saturated natural loan y soil. 

Ma, L., and H.M. Selim. 1994. Tortuosity, mean residence time, and deformation of tritium breakthrough from soil columns. Soil Sci. Soc. Am. J. 58 1076-1085. 

The quantification of the mean residence time of vegetation and soil compartments and an understanding of the parameters that control this time-scale is necessary for process-based predictions of carbon storage. 

Lead is known to be fairly immobile in organic soils, with a mean residence time... 

Campbell (1965) made an extensive study of the carbon-dating method as applied to the study of problems in soil organic matter investigations and found it to be very satisfactory. The mean residence times of humus ftom the Ap-horizon of 5 Saskatchewan soils ranged from 250 years for a gray-wooded podzolic soil to 1000 years for black chernozemic soils the humus from chernozems from Alberta was even older. The calcium humates and non-hydrolyzable humic acids of the chernozems had an age of 1,400 years, whereas the age of the humic acid hydrolysate was only 25 years. This hydrolysate, which was a small fraction of the total humus, accounted for 80% of the nitrogen released per annum the major portion of the soil humus was found to be inert. In general, the humus of podzolic soils was much more labile than that of chernozems. 

Activity concentrations of Be, °Pb, and °Po in precipitation (rain, throughfall, and stemflow), wood, and soil were determined to estimate the migration behavior of these radionuclides in the forest canopy (Sugihara et al. 1999). Output/input ratios of precipitation for the forest canopy were 0.53 for Be, 0.79 for °Pb, and 1.5 for °Po in Pasania edulis forest. Mean residence times of the radionuclides were estimated to be 56 days for Be, 765 days for Pb, and 653 days for °Po for Pasania edulis forest. 

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