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Soil Organic Matter (SOM) Dynamics Determined by Stable Isotope Techniques

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Soil Organic Matter (SOM) Dynamics Determined by Stable Isotope Techniques AT9900043 Soil organic matter (SOM) dynamics determined by stable isotope techniques Martin H. Gerzabek September 1998 OEFZS—4826 SEIBERSDORF 30-20 OEFZS—4826 September 1998 Soil organic matter (SOM) dynamics determined by stable isotope techniques Martin H. Gerzabek Abteilung Umweltforschung Bereich Lebenswissenschaften Mitteilungen der Deutschen Bodenkundlichen Gesellschaft, 87, 161-174 ______ (1998) Soil organic matter (SOM) dynamics determined by stable isotope techniques von GERZABEK, M.H. Abstract Being aware of limitations and possible bias the 13C natural abundance technique using the different 13C enrichments in plants with differing photosynthetic pathways is a powerful tool to quantify turnover processes, both in long-term field studies and short-term laboratory experiments. Special care is needed in choosing reference plots and the proper number of replicate samples. The combination of 13C and 14C measurements has a high potential for a further improvement of isotope techniques in SOM studies. Natural abundance of 15N is less powerful with respect to quantification of SOM processes than the isotope dilution technique. However, its usefulness could be distinctly improved by introducing other stable isotopes into the studies. Introduction Soil organic matter has a large number of functions within the biosphere. It is e.g. the basis of a favourable soil structure, hydrological properties and biological activity, offers sorption sites for plant nutrients and pollutants, is itself an important source for nutrients and acts as a sink for atmospheric C02. All these functions may be influenced by changes in climate (Tinker and Ingram, 1994), land use and the overall situation of the environment. Thus, the questions we ask in soil science concerning soil organic matter are much more complex than a few decades ago, when soil productivity was the main matter of concern. The major difficulty is the quantification of the different processes involved in the previously mentioned functions of soil organic matter. Especially the determination of turnover rates, both of labile and of stable or resistant or recalcitrant or refractory organic matter pools are the basis for modelling the overall system. The use of isotopes in this respect is not new. Already in the sixties and seventies quite a number of experiments focused on the determination of organic matter pools of different stability by using 14C-labelling or radiocarbon dating (Fuhr and Sauerbeck, 1968, Goh, 1991, Jenkinson, 1977, Oberlander and Roth, 1972, 1974, 1975) and basic models were derived from these data (Jenkinson and Rayner, 1977). The use of the radioactive 14C with a physical half-life of 5730 years implies considerable radiation protection measures, which is a great disadvantage. Field experiments with 14C-labelled material are not longer admitted in most countries. Stable isotopes of carbon, nitrogen and sulphur allow in principle similar experimental designs and the rapid development of mass spectrometry and its applications overcame the disadvantage of complicated and time consuming analytical procedures. The following pages attempt to sum up the present use of stable isotopes in organic matter studies and to present some examples from typical experiments using tracer or isotopic dilution methods. Determination of stable isotopes The main method determining stable isotopes abundances is mass spectrometry. Since the fifties dual inlet gas isotope ratio mass spectrometers were used, especially for 13C/I2C ratio analysis (Boutton, 1991). Due to technical and electronical improvements the precision of such instruments is now a factor of ten better than in the early days. The great problem was that the element of interest had to be isolated from the sample and the conversion into the measured gas (e.g. C02, N2, S02) must not cause any isotopic fractionation. In the case of carbon the conversion to C02 is accomplished by dry combustion in an excess of oxygen (Boutton, 1991). The sample preparation of an off-line method has a precision of app. 0.1 %o (1 SD), the mass spectrometric precision is much better (1 SD = 0.01%o). For nitrogen isotope measurements the standard Austrian Research Centers Seibersdorf, Division of Life Sciences, A-2444 Seibersdorf; e-mail: [email protected] 162 procedures were the methods of Kjeldahl-Rittenberg (wet chemistry) and Dumas (dry combustion) (Banie, 1991). In the eighties a considerable step forward was undertaken by coupling an elemental analyser with an isotope ratio mass spectrometer (Preston and Owens, 1985, Pichlmayer and Blochberger, 1988). This combination results in a similar overall precision compared to the off-line methods and an app. fivefold increase of the number of analyses per day (app. 100 instead of 20 per day). For the determination of N-isotopic composition at higher 15N enrichments optical emission spectroscopy offers a cheap and simpler alternative (Preston, 1991). For this technique samples are prepared by direct Dumas conversion in sealed sample tubes containing app. 10 ug total N. The 15N enrichment is determined from the relative intensities of the three N2 lines of masses 28, 29 and 30 after exciting the sample nitrogen gas in a discharge tube by an external radioffequency source. Precision is about + 0.01 - 0.02 at.%15N, which is satisfactory for most tracer studies, but not suitable for natural abundance measurements. The ratio of the peaks of the masses 28, 29, 30 and 44, 45, 46 are used to determine the ratio of 15N/14N and !3C/12C, respectively. Normally the quantity reported is not the isotope ratio of a sample itself, but the relative difference between the isotope ratio of the sample and that of a standard (5-notation). The 5 values are calculated as: 5 (%o) — [(Rsample ~ Rstondaid)/Rstondard] X 1000 (1) where R is the ratio of the heavy to the light isotope of sample or standard. For N, the standard is air, for C it is the Cretaceous carbonate fossil Bellemnitella americana from the PeeDee Formation in South Carolina, USA (PDB), and for S the mineral troilite from the Canon Diablo meteorite is used as standard (Barrie and Prosser, 1996). Especially for C the accepted standard has limited availability. In practice the samples (SAM) are measured against a laboratory reference material Table 1. Primary isotope standards (5 = 0%o) (according to Barrie and Prosser, 1996) PeeDee Belemnite (PDB) 13C/12C = 0.0112372 18 0/160 = 0.0020671 Atmospheric nitrogen (air) 15N/14N = 0.0036765 Canon Diablo meteorite troilite (CDT) 34S/32S = 0.0450045 which has been calibrated against the international standard (Boutton, 1991). Table 1 shows the isotope ratios of important primary standards. Values obtained by using the laboratory reference material (REF) can be converted to values relative to a standard (STD) by (Barrie and Prosser, 1996): 5 SAM - STD — SsaM-REF + 5ref-std + (§SAM-REf8 rEF-STd)/1 000 (2) Another way to report results, especially for tracer studies, is using the abundance: ,3C abundance (atom% 13C) = [(13C) / (12C + 13C)] x 100 (3) Atom% excess is a similar index, but gives the enrichment level of the sample in excess of the background. It should be mentioned that due to minor 17 0 contributions to the mass 45 signal the calculation of 13C/12C ratios (13R) from mass 45/44 (45R) and mass 46/44 (^R) has to be corrected. There is no way in which an equation exactly relating 17 0/160 and 18 0/160 ratios in all pools of oxygen can be deduced (Santrock et al, 1985). However, the mentioned authors describe one possible way using three basic equations, which partly contain empirical values. A simpler method was already proposed 1957 by Craig, which yields quite similar results (Equation 4): 513Csam-std = [(45Rsam - 45Rsto)/(45Rstd - 1/I333.7)]xl0 3 - 0.0336x[{( 46R5AM - ^Rsm^RsmjxlO 3] The use of the natural abundance of isotopes in SOM dynamics studies SOM turnover - changes in plant input Theory Atmospheric C02 is the important link between inorganic and organic compounds involved in the global C- cycle. Atmospheric C02 has a mean 513C value of approximately -7.8%o (Boutton, 1991). Plants differ 163 distinctly in their C-isotopic composition from atmospheric C02 due to fractionation processes during assimilation. The largest difference occur between plant species with different photosynthetic pathways. Trees and gramineae of temperate regions use the Calvin-cycle (C3) and incorporate less 13C than plants with a Hatch-Slack (C4) cycle. The first group exhibit a S13C value of app. -27%o, whereas the second group is significantly higher (-13%o513C). Thus, C3- and C4-plants are differentiated by approximately 14%o on the 8- scale. As already mentioned, modem elemental analyser - mass spectrometer combinations reach acurracies of at least 0.1 %o S13C. Therefore, the difference between C3 and C4 plants is sufficient to be used for tracer studies (Baldesdent et al., 1987). A third photosynthetic pathway, the Crassulacean acid metabolism is used by 10% of all plant species, by some of them obligate, by some of them facultative. The latter can switch to C3 photosynthesis, which causes a wide range of S13C values from -28 %o to -10%o (Boutton, 1996) and make them less suitable for SOM turnover determinations. Several investigations of SOM turnover under field conditions were undertaken using the fact that the vegetation cover changed between C3 and C4 plants (Baldesdent and Mariotti, 1996). Cerri et al. (1991) described the discrimination between SOM originating from indigenous vegetation (forest, C3)
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