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TECHNICAL REPORTS
Landscape and Watershed Processes

Stable Carbon Isotope Ratio and Composition of Microbial Fatty Acids in Tropical Soils

^a USEPA, National Exposure Research Lab, 960 College Station Rd., Athens, GA 30605
^b Dep. of Biological Sciences, Univ. of Arkansas, Fayetteville, AR 72701
^c Dep. of Plant, Soil, and Environmental Sciences, Univ. of Maine, Orono, ME 04469
^d Centro de Energia Nuclear na Agricultura (CENA-USP), Laboratorio de Biogeoquimica Ambiental, Av. Centenario 303 cep 13416.000, Piracicaba, SP Brasil

* Corresponding author (burke.roger{at}epa.gov)

Received for publication February 15, 2002. The soil microbial community plays a critical part in tropical ecosystem functioning through its role in the soil organic matter (SOM) cycle. This study evaluates the relative effects of soil type and land use on (i) soil microbial community structure and (ii) the contribution of SOM derived from the original forest vegetation to the functioning of pasture and sugarcane (Saccharum spp.) ecosystems. We used principal components analysis (PCA) of soil phospholipid fatty acid (PLFA) profiles to evaluate microbial community structure and PLFA stable carbon isotope ratios (¹³C) as indicators of the ¹³C of microbial substrates. Soil type mainly determined the relative proportions of Gram positive versus Gram negative bacteria whereas land use primarily determined the relative proportion of fungi, protozoa, and actinomycetes versus other types of microorganisms. Comparison of a simple model to our PLFA ¹³C data from land use chronosequences indicates that forest-derived SOM is actively cycled for appreciably longer times in sugarcane ecosystems developed on Andisols (mean turnover time = 50 yr) than in sugarcane ecosystems developed on an Oxisol (mean turnonver time = 13 yr). Our analyses indicate that soil chronosequence PLFA ¹³C measurements can be useful indicators of the contribution that SOM derived from the original vegetation makes to continued ecosystem function under the new land use.

Abbreviations: G-, Gram negative • G+, Gram positive • GC, gas chromatography • FAME, fatty acid methyl ester • MS, mass spectrometry • PC, principal component • PCA, principal components analysis • PLFA, phospholipid fatty acid • SOM, soil organic matter