Radiocarbon Dating of Soil Organic Matter

Abstract Radiocarbon ages of soil organic matter are evaluated with a model which incorporates the dynamics of the 14C content of soil organic matter. Measured 14C ages of soil organic matter or its fractions are always younger than the true ages of soils due to continuous input of organic matter into soils. Differences in soil C dynamics due to climate or soil depth will result in significantly different 14C signatures of soil organic matter for soils of the same age. As a result, the deviation of the measured 14C age from the true age of soil formation could differ significantly among different soils or soil horizons. Our model calculations also suggest that 14C ages of soil organic matter will eventually reach a steady state provided that no climatic or ecological perturbations occur. Once a soil or a soil horizon has reached a steady state, 14C dating of soil organic matter will provide no useful information regarding the age of the soil. However, for soils in which steady state has not been reached, it is possible to estimate the age of soil formation by modeling the measured 14C contents of soil organic matter. Radiocarbon dating of buried soils could, in general, overestimate the true age of the burial by as much as the steady-state age of the soil or soil horizon.


INTRODUCTION
''mean residence time'' (MRT) of the steady-state soil organic matter. However, there has been no clear definition of Radiocarbon dating of organic matter in soils has been what MRT implies and its relationship to steady state (Paul used to study the chronology of soil development (Paul et et al., 1964;Campbell et al., 1967a,b;Goh and Stout, 1972;Jenkinson, 1969;Herrera and Tamers, 1971;Mar-Scharpenseel, 1972;Sheppard et al., 1976). tel and Paul, 1974;Goh et al., 1977;Hammond et al., 1991).
Recently, a series of models has been developed to study However, the significance of the measured dates or ages is the uptake of bomb 14 C in soil organic matter pools with the a perplexing problem (Perrin et al., 1964;Campbell et al., purpose of understanding the dynamics of soil organic matter 1967a,b;Gerasimov and Chichagova, 1971;Scharpenseel, (O'Brien and Stout, 1977;Trumbore et al., 1989Trumbore et al., , 1990Har-1971aHar- ,b, 1972Har- , 1976Grant-Taylor, 1972;Gerasimov, 1974;rison et al., 1993;Trumbore, 1993). The difference in 14 C Sheppard et al., 1976;Goh et al., 1976Goh et al., , 1977Gilet-Blein et in soil organic carbon between prebomb and postbomb soils al., 1980;Hammond et al., 1991;Scharpenseel and Beckerhas demonstrated differences in the amount, character, and Heidmann, 1991) because soil organic matter is the product steady-state turnover rate of carbon in organic matter (Trumbore, 1993). Here we modify the approach to use it to evaluate the evolution of 14 C age of soil organic matter

SAMPLE SELECTION AND ANALYTICAL METHODS
Berkeley. We deliberately selected soil samples collected before the peak of nuclear weapons testing for this study to We selected three soils from different climatic zones (Taavoid any ''bomb 14 C'' contamination. ble 1) to reveal the effect of climate on soil C dynamics and Samples of the forest and desert soils were hand-picked on the 14 C ages of soil organic matter: a forest soil from of visible roots and plant material and ground into powder. central California, a prairie soil from Iowa, and a desert soil The powder was treated with 1 M HCl overnight to remove from the Mojave Desert. The prairie soil was obtained from carbonate, rinsed with deionized water six times, and dried. the archives at the U.S. National Soil Survey Laboratory. CO 2 was produced by combustion of the treated sample with This soil, a Mollisol, was collected in 1959 from Tama CuO and silver foil under vacuum at 875ЊC for 2 hr. The County, Iowa. The 14 C contents of three different organic resulting CO 2 was purified cryogenically and its stable carfractions separated by physical and chemical means were bon isotope ratio was measured on a mass spectrometer. For analyzed and reported by Trumbore et al. (1990). In this 14 C analysis, purified CO 2 was reduced to graphite with H 2 paper, we calculated the bulk C and 14 C contents of soil over Co and its 14 C/ 13 C ratio was measured on an accelerator organic matter (Table 2) from the C and 14 C measurements mass spectrometer (AMS) at Lawrence Livermore National on different fractions (Trumbore et al., 1990) based on mass Laboratory. balance relationships. The calculated bulk C and 14 C contents Stable C isotope data are reported in the standard permil of soil organic matter were then used in the following model notation relative to the PDB standard as evaluation. The forest soil, a Musick loam, classified as Ultisol, was collected in 1958 from the Sierra Nevada (Fresno County) in central California. The desert soil, an Aridisol, was collected from Barstow area in the Mojave Desert in 1951. Both the forest soil and the desert soil were obtained from the archives at the Department of Environmental Science, Policy, and Management, University of California at 14 C data are expressed as (Stuiver and Polach, 1977)  nates the C input processes. For the simplicity of the model- ing, we will combine the in situ production and diffusional transport of carbon into one ''net production of organic carbon'' term, F (moles/cm 3 /yr). Our purpose here is to demonwhere d 14 C is the permil value for 14 C content (Stuiver and strate, at least semi-quantitatively, the effect of soil genesis Polach, 1977), R sample is the 14 C/ 12 C ratio in the sample, and on the 14 C ages of soil organic matter. R std is the absolute 14 C/ 12 C ratio in the isotopic standard (NBS Equation (1) can then be reduced to oxalic acid), or where A SN is the 14 C activity in the sample normalized to d 13 C Å 025%, and A abs is the absolute 14 C activity in the Similarly, the 12 C and 14 C content of soil organic matter can international isotopic standard (NBS oxalic acid). be expressed by The 14 C age is calculated from the equation (Stuiver and Polach, 1977) ÌC 12 Ìt Å F 12 0 kC 12 (3) Radiocarbon age Å 08033 ln ͩ pMC 100 ͪ 0 y 0 1950 where y is the year of 14 C measurement.
where C 12 and C 14 are the soil organic 12 C and 14 C contents, respectively, and (5) One of the criteria for accurate radiocarbon dating of a sample is that the system has to be closed with respect to 14 C. Soils, which form over long periods of time, represent

A MODEL FOR INTERPRETING THE 14 C AGE OF SOIL ORGANIC MATTER
open systems with respect to carbon and are in apparent violation of this criterion. Obviously, the standard 14 C dating models are not applicable to soils. However, if we assume that organic matter decomposition is the only mechanism dO Å for carbon loss in soils, the variation in organic carbon with time for a soil or any of its horizons can be described by and l is the decay constant of 14 C (0.0001245/yr), and d 13 C input o.m. and d 14 C input o.m. are the 13 C and 14 C contents of input organic matter in permil notation, respectively. If we assume that C Å 0 at t Å 0, and f and k are constant with where C is the organic carbon content (moles/cm 3 ). The first time but vary with depth, the solutions to Eqs. (2), (3), and term on the right side of the equation represents the transport (4) are of C by biodiffusion and the D B is the biodiffusion coefficient (cm 2 /sec). I is the in situ production of C by root growth and decay (moles/cm 3 /yr). The third term represents loss of organic carbon by microbial decomposition and k is the decay constant (yr 01 ). Diffusional transport of carbon is not well understood from a quantitative standpoint in any soil.
C 12 Å F 12 k (1 0 e 0kt ) (9) In humid, coarse-textured soils downward transport of particulate C has been recognized (Chapelle, 1993). However, in arid and semi-arid soils (those examined here), downward (10) movement of organic C is far less understood but is likely not as important. For example, the concentration of organic C in these soils is commonly strongly correlated with root When a soil is at steady state, the steady-state input rate and decay rate can be calculated from Eqs. (8) to (10): distribution, suggesting that direct input from roots domi-matter input is from surface litter and root turnover at shallow depths, the mechanisms by which ''new'' C can be transported to deeper depths are vertical translocation of C k steady-state Å

1000
/ 1 ͪ (R std ) by biodiffusion or solution and deep roots turnover. Minimum influence of bomb 14 C in subsoils may therefore suggest that biodiffusional transport of C is less important in subsoils. Goh and Stout (1972) suggested that the subsoil (11) organic matter is subjected to a slow but continuous incorpof steady-state Å kC.
(12) ration of 14 C from the atmosphere via plant residues, especially plant roots, and translocation of soluble organic com-If the true age of a soil (i.e., the age since the initiation pounds from the surface horizon. These processes, together of a soil's development) is known (or if a soil is at steady with the slow but continuous decomposition of organic matstate), and d 14 C input o.m. , the organic carbon content, and 14 C ter already formed in the subsoil, lead to interchanges becontent of organic matter at different depth intervals are tween the various carbon isotopes not only within the differmeasured, Eqs. (8) to (10) can be solved for f and k for the ent organic fractions but also between the organic matter in soil depth intervals of interest. The f and k can then be used the different soil horizons. Further work is needed to study to calculate the organic carbon, 14 C content, and 14 C ages of the biodiffusional transport and solution translocation of carorganic matter vs soil depth at different times of the soil's bon in different soils and how they affect the 14 C content of development. We have made such calculations for the three soil organic matter. prebomb soils described in the previous section (Table 2) using Eqs. (5) to (12). The evolution of the 14 C content of DISCUSSION organic matter in these soils is discussed in relation to its implication for 14 C dating. Our model calculations simulate the way in which 14 C content or 14 C ages of soil organic The carbon inventory, 14 C content of bulk soil organic matter, and calculated steady-state input and decay rates of matter might have evolved from time t Å 0 to various ''true ages.'' the three prebomb soils are shown in Table 2. Using the data in Table 2, we calculated the 14 C ages of soil organic In our calculations, we assume that organic C input (f) and decay (k) are constant with time. This assumption is matter vs depth at different times of the soils development, as shown in Figure 1. These diagrams show that (1) the 14 C unlikely to be valid for real soils because soil organic matter is a heterogeneous mixture of compounds with differing de-age of soil organic matter decreases with soil depth, (2) the 14 C age of soil organic matter is always younger than the cay rates (Trumbore et al., 1990;Trumbore, 1993) that are affected by seasonal/annual soil temperature and moisture true age of the soil due to a continuous input of fresh organic matter, (3) the 14 C content/age of soil organic matter will variations. However, we have no ability to predict their behavior with time and have no data regarding the decay rates eventually reach a steady state provided that no climatic or ecological perturbations occur, and (4) ''steady-state 14 C of various C pools. We, therefore, emphasize that the f and k in our model should be considered as representing long-age,'' which is defined as the minimum time required for a soil horizon or a soil to reach steady state, increases with term average behaviors of these soil C properties. Second, we assume that the 14 C content of input organic carbon, depth.
Once a soil or a soil horizon is at steady state, the 14 C d 14 C input o.m. , is the same as the atmospheric CO 2 . This implies that most of the carbon input is from root growth and decay, age of organic matter becomes constant with respect to time and therefore cannot give any indication of the age of the and diffusional transport of carbon is not important except for recently added material. At present, we have no quantita-soil. The time for this to occur varies greatly with climate (from less than 5000 yr in the temperate climate of California tive knowledge of the amount and 14 C content of C associated with biodiffusion. Our calculations, therefore, may not to more than 300,000 yr in the arid environment of the Mojave Desert) and also with depth in a soil profile (Fig. be entirely quantitative for soils in which biodiffusional transport of organic fractions, with lower 14 C contents than 1). It is evident from these diagrams that the measured 14 C ages at any time can be significantly different from the true the atmosphere, is important. Nuclear weapons testing in the late 1950s and 1960s injected considerable 14 C into the ages of the soils. For example, soil samples taken from the 25-cm depth in a 40,000-year-old forest soil (similar to the atmosphere; this ''bomb'' 14 C is an ideal tracer for studying C cycling in soils. Plant materials added to a soil in the late Sierra Nevada soil) will have a measured 14 C age of about 5000 yr B.P. (Fig. 1a). 1950s to early 1990s had much higher than ''normal'' 14 C content. Studies (Goh and Stout, 1972;Stout and O'Brien, The most important factor affecting the measured 14 C ages of soil organic matter is the rate of organic carbon cycling 1972) have shown that bomb 14 C enrichment of soil organic matter occurred mainly in the top soils. Since most organic in soils. The rate of C cycling in soils is affected by a number rates, has the oldest steady-state 14 C age. The steady-state 14 C ages for the three soils are ca. 15,000 yr below and õ10,000 yr above 40-cm depth for the forest soil, ú40,000 yr below and õ40,000 yr above 50-cm depth for the prairie soil, and much greater than 50,000 yr (the current 14 C dating limit) at 4-22-cm depth interval for the desert soil (Table  3). Clearly, it is not prudent to compare the ages of different soils by simply looking at the measured 14 C dates of soil organic matter or its fractions. For the same type of soils, the measured 14 C ages of soil organic matter from the same depth interval can give some indication of the relative ages of the soils. However, for different types of soils in different climates, differences in the dynamics of soil carbon can result in completely different 14 C ages even when they do have the same true age.
Our modeling exercise demonstrates that 14 C method of dating soil organic matter will be limited by the steady-state 14 C age of a soil or a soil horizon. However, it is theoretically possible to estimate the age of a soil provided that the input and decay rates are known and the steady state has not yet established. In practice, this will involve a considerable amount of work: the availability of a prebomb soil similar to the soils to be dated from a nearby locality, 14 C analysis of this soil, and modeling.
One possible way of applying this concept is the following. In areas where a series of terraces or geomorphic surfaces of different ages can be identified based on geomorphological and soil development evidence, the soil on the oldest terrace can be assumed to be at steady state, and the steadystate input and decay rate can be determined from its 14 C and C content. Because of bomb 14 C contamination, archived samples best serve this purpose. However, in environments such as deserts where C inputs are low, deep soil horizons may be effectively utilized. The calculated steady-state input and decay rate can be used to calculate the 14 C ages of soil organic matter at time t Å 0 to various true ages using the  In contrast, the desert soil, with the lowest input and decay provided by the National Institute of Global Change (WESTGEC) and the method has been applied to date Holocene alluvial fan sur-University of California Agricultural Experiment Station.
faces in the Providence Mountains area of the Mojave Desert. The model ages of soil formation based on measured