Patterns of nutrient circulation were examined to provide insight into the controlling mechanisms behind differences in aboveground net primary production (ANPP) among floodplain forest types. Geochemical and biogeochemical contrasts typical of many riverine forests are exemplified by the floodplains of the Satilla and Altamaha Rivers, blackwater and redwater river systems, respectively, located in Georgia, USA. Since floodplains possess high microtopographic variation, measurement plots were established to cover a range of topographic positions across both floodplains. Given the low nutrient and sediment loads that are characteristic of blackwater rivers, we hypothesized that the Satilla floodplain (SAT) would have lower ANPP and more efficient nutrient use efficiency (NUE). While both floodplains had similar amounts of litterfall production, the Altamaha floodplain (ALT) had greater stem production and, therefore, higher rates of ANPP. On the SAT, the relationship between litterfall mass and litterfall phosphorus (P) content, high P NUE, and high P resorption proficiency suggested that P was the primary limiting nutrient. While (N) circulation patterns were similar for the floodplains, ALT floodplain litterfall N/P ratios and N NUE suggested that N limitation was paramount there. Patterns of base cation circulation on the SAT indicated low calcium and potassium circulation and higher NUE of these elements compared to the ALT. In contrast, the Altamaha floodplain displayed low magnesium (Mg) circulation and high Mg NUE. Differences in P circulation and P NUE appeared to have the largest influence on ANPP between these eutrophic (ALT) and oligotrophic (SAT) floodplain forests. Furthermore, differences in base cation circulation and NUE suggested that biogeochemical distinctions between these two floodplain types could be made on the basis of base cation availability.
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