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United States Department of Agriculture

Agricultural Research Service

Title: Natural Abundance of 13c in Soils and Water

Authors
item Clapp, Charles
item Layese, Milegua - UNIVERSITY OF MINNESOTA
item Hayes, Michael - UNIV. OF BIRMINGHAM
item Huggins, David - UNIVERSITY OF MINNESOTA
item Allmaras, Raymond

Submitted to: Royal Society of Chemistry Meetings
Publication Type: Book / Chapter
Publication Acceptance Date: November 1, 1995
Publication Date: N/A

Interpretive Summary: Organic materials in soils and waters are made up of a combination of components in close association. The major organic fraction, the "humic substances", provides a network for water, air and nutrient interaction. Formation of the humic substances--humic and fulvic acids--can be traced to the type of original plant residue by use of a new stable isotope method based on a heavy form of carbon (C-13). Some plants such as corn (a C4 type) and soybean (a C3 type) are so called because of the number of carbon atoms in their building blocks for energy. Measuring the amounts of C-13 in these C4 and C3 plants, and in soils where these plants and residues are found, allows us to predict the fate of carbon into soil organic storage and losses as gas into the atmosphere. The organic materials in corresponding soil drainage waters and larger sources such as lakes and rivers also can be analyzed for C-13 to measure their contribution to the overall carbon cycle. These research results can provide valuable information as to the quantities of carbon divided into components of the soil-water-air ecosystem.

Technical Abstract: The isotopic composition of C is governed by the process of photosynthesis. C4 plants such as corn have average delta 13C values of about -12/mil; C3 plants such as soybean and trees have average values of about -26/mil. Analyses of 13C can provide information as to type of plant material that contributes to soil organic matter (SOM) or humics in soil and water. Differences in 13C of C3 and C4 plants have been used for labelling of SOM for organic C turnover studies. This paper presents C and delta 13C analyses of soils and humics, e.g., humic acid (HA), fulvic acid (FA), and XAD-4 acids (XA) derived from a variety of soils, peats, coals and different sources of water, using an elemental C/N analyzer- isotope ratio mass spectrometer system. We found a wide spread of delta 13C values for mineral soil humics indicative of plant type. Peats and coals had a limited range of delta 13C values. About 1/mil difference existed between continuous corn and continuous soybean after 11 years, with other corn/soybean sequences having intermediate values. The fine fraction was the least depleted of the different size fractions. The C of the clay fraction of soil cropped to corn with residues returned vs. no residues was found to be younger. Water humics from different sources had delta 13C values of about -27 to -29/mil. The FAs were either the same or slightly less depleted than HAs; XAs were consistently the least depleted of the water humics. Four soils and their waters gave delta values of humics under similar conditions. Soil water humics are always more depleted. This observation suggests that humics dissolved in soil solution have origins which are not identical to those in the solid or gel states in soil.

Last Modified: 8/29/2014