The pre-Colombian footprint on terrestrial nutrient cycling in Costa Rica: insights from phosphorus in a lake sediment record |
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| Authors: | Gabriel M Filippelli Catherine Souch Sally P Horn Derrick Newkirk |
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| Institution: | (1) Department of Earth Sciences, Indiana University Purdue University Indianapolis (IUPUI), Indianapolis, IN 46202, USA;(2) Department of Research and Education, Royal Geographical Society, London, UK;(3) Department of Geography, University of Tennessee, Knoxville, TN 37996, USA |
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| Abstract: | The impact of pre-Columbian subsistence agriculture on soil nutrient cycling in the American tropics is poorly quantified.
Paleolimnological research can address this deficit by documenting the temporal evolution of nutrient cycling in lake watersheds
over different time scales. Here we describe our use of a chemical sequential extraction technique adapted from soil fertility
research to discern geochemical fractions of phosphorus (P) in lake sediments that serve as proxies for landscape-scale soil
nutrient status. These P fractions are mineral P (the original lithic source of bioavailable P), occluded P (mainly bound
to soil oxides), and organic P (remains of organic matter production by plants). We applied the P fractionation technique
to a lake sediment core from a small lake in southern Costa Rica, Laguna Zoncho. Prior analyses of microfossils and stable
carbon isotopes in this core documented an approximately 3,000 year history of human occupation and agricultural activity
in the Zoncho watershed, and shifts in diatom communities in the lake associated both with human impacts and with climate-driven
changes in lake level. Our P analyses revealed relatively constant P geochemistry during the first approximately 2,500 years
of the record, when other sedimentary proxies reveal forest clearance and maize agriculture of varying intensity. However,
the period from approximately 500 to 100 years BP is marked by a drastic shift toward a P geochemistry dominated by occluded
forms, with a concomitant decrease in the relative content of both the organic and mineral P forms. This interval coincides
with post-Conquest depopulation and forest regeneration at the site, and with an apparent deepening of the lake caused by
a shift toward a wetter climate. The dominance of the occluded P fraction during this interval is the opposite of the trend
expected with such a climate shift, implicating human dynamics as the principal driver of the changes in soil nutrient status
indicated by the P fractions in the Zoncho core. We propose that the entire P geochemical record is dominated by human-induced
alteration of the soil nutrient cycles via agriculture and occupation, and that the only interval that reveals the “natural”
nutrient status in the region is the short interval when the site is abandoned and surrounding forests regrow. These results
for Laguna Zoncho reveal the close connection between even relatively low-technology human activities and soil nutrient status. |
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