HR: 16:35h
AN: B44A-02    [Abstracts]
TI: Short-term d13C changes in cultivated soils from Mexico
AU: * Lounejeva, E
EM: elenal@servidor.unam.mx
AF: Institute of Geology, Av. Universidad, 3000 Ciudad Universitaria Copilco Coyoacan, Mexico, D.F 04510, Mexico
AU: Etchevers, J
EM: jetchev@colpos.mx
AF: Colegio de Postgraduados, Campus Montecillo, Texcoco 56230, Mexico, 56230, Mexico
AU: Morales Puente, P
EM: mopuente@servidor.unam.mx
AF: Institute of Geology, Av. Universidad, 3000 Ciudad Universitaria Copilco Coyoacan, Mexico, D.F 04510, Mexico
AU: Cienfuegos Alvarado, E
EM: edithca@servidor.unam.mx
AF: Institute of Geology, Av. Universidad, 3000 Ciudad Universitaria Copilco Coyoacan, Mexico, D.F 04510, Mexico
AU: Sedov, S
EM: sergey@geologia.unam.mx
AF: Institute of Geology, Av. Universidad, 3000 Ciudad Universitaria Copilco Coyoacan, Mexico, D.F 04510, Mexico
AU: Solleiro, E
EM: solleiro@geologia.unam.mx
AF: Institute of Geology, Av. Universidad, 3000 Ciudad Universitaria Copilco Coyoacan, Mexico, D.F 04510, Mexico
AU: Hidalgo, C
EM: jetchev@colpos.mx
AF: Colegio de Postgraduados, Campus Montecillo, Texcoco 56230, Mexico, 56230, Mexico
AB: The soils of the Mexican Volcanic Belt are part of ecosystems subjected to strong human impact during the last six centuries. One measurable characteristic of the soil is the stable carbon isotopic relation of the soil organic matter (SOM) or d13C. The d13C SOM parameter is a genetic characteristic of soil reflecting the relative proportion of C3 and C4 that comes from colonizing plants having different photosynthetic C pathway and is used as a high-spatial resolution tool to infer paleoenvironmental changes.The d13C mean signatures of C3 and C4 plants are -27 and -13 %o, respectively. This work focuses on short-term changes in d13C on soils subjected to controlled agricultural practices during 2002-2005 in two sites of Mexico with similar annual precipitation and temperature. The tepetate was broken up 20y ago and ameliorated with fertilizers and organic matter. In both sites three experimental treatments consisting of traditional soil management and two variations of this one were evaluated. Traditional treatment implies low fertilizer and any chemical input, sowing annual crops during the rainy season and, in general, using low energy input. The crops planted were: legumes C3, oat C3, and a mixture of maizeC4 and beanC3, and wheatC3. The Improved and Organic treatments, had higher input of N and P as chemical fertilizers, and of organic manure (manure or compost), respectively. Soil samples were collected from the plow layer in Tlaxcala and in Michoacán, before C4 maize was planted. An Andisol from a pine-oak (C3 species) forest close to the Atecuaro site was also sampled up to 40 cm. This soil was considered a reference site not recently influenced by human activity. To analyze the d13C ratios of the SOM carbonate free samples, a routine combustion method and mass spectrometry (Finnigan MAT250) were used. In both agricultural sites a general excess of C3 species over C4 was evidenced through a mass balance equation derived from experimental d13C values (generally less than -20%o). The common feature for the soils under the Traditional and Traditional improved treatments was a C3 enrichment of the superficial SOM component compared to the underlying layers as a consequence of the dominance of the cultivated C3 species. A similar but more accentuated negative shift is also observed in the SOM from the forest soil (non-cultivated soil d13C -25.2), so the interpretation is uncertain. In the Traditional Organic treatment a clear and perceptible increment of d13C in the SOM carbon signature was observed. This was attributed mainly to the fact that cows manure may contain a lot of C4 coming from feedstuff rich in corn grain that is provided to the animals during grass shortage periods . However, the maize crop introduced in the rotation during the 3rd year had no major effect on the tepetates carbon isotopic signature. The stable isotopic carbon data corresponding to a short period (4 years) of observation in uniformly managed soil ecosystems showed that d13C changed due to the quality of the residues (relative abundance of C3/C4 species) incorporated to the SOC, but this memory is susceptible to undergo changes in the short term and could be rapidly reversed as a consequence of crop management.
DE: 0428 Carbon cycling (4806)
DE: 0439 Ecosystems, structure and dynamics (4815)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0473 Paleoclimatology and paleoceanography (3344, 4900)
DE: 0486 Soils/pedology (1865)
SC: Biogeosciences [B]
MN: 2007 Joint Assembly