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Intelligence and CIS Se( tion )(iiiil POVV/'MIA Accounting Command Hickam AFB. HI 9í)í\".->553o Li SA Debate over agricultural methods and productivity during the Preclassic and Classic Maya period (1000 BCE to 900 CE) focuses on the agronomic utility of both upland and lowland soil resources of the karst topography characteristic of northern Guatemala and much of the Yucatan peninsula. In settings where direct evidence of agriculture is sparse, stable carbon (C) isotope evidence from soil organic matter (SOM) provides information on past vegetation changes related to ancient maize agriculture. Areas of ancient sustained maize cultivation can be identified in an ecosystem dominated by Cj forest vegetation because of the unique C4 photosynthetic pathway of maize. The decomposition of plant materials with divergent photosynthetic mechanisms (C3 versus C4) results in SOM with correspondingly distinct stable C isotope ratios (ô^^C). Such differences are preserved and detectable within the refractory humin fraction of the SOM. This study analyzes the physical and chemical soil properties including stable C isotopes of soils collected from upland and lowland locations near Tikal, Guatemala. Toeslope soils contained strong isotopic evidence of C4 vegetation likely the result of ancient maize agriculture, while the evidence from shallow soils of the summit and backslopes was less conclusive. In both upland and lowland contexts, however, the stronger evidence of ancient vegetation shifts associated with maize agriculture was found in the deeper finetextured soils in the footslopes and toeslopes. Examination of soils on toeslopes revealed evidence of deposition from erosional processes upslope that may have been related to the spread of agriculture to less productive soils over time.Abbreviations: COLE, coefficient of linear extensibility; DEM, digital elevation model; SOM, soil organic matter.