Contribution of humic substances as a sink and source of carbon in tropical floodplain lagoons
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We evaluated the decay of humic (HA) and fulvic acids (FA) in order to discuss the contribution of these substances as a sink and source of carbon in a tropical lagoon.
Materials and methods
Experiments were conducted under aerobic and anaerobic conditions using FA and HA isolated from decomposition of Oxycaryum cubense submitted to 10 and 60 days of degradation. HA and FA were added to water samples from a tropical floodplain oxbow system, the Infernão Lagoon. The mineralization chambers were incubated in the dark at 21.0 °C. The carbon balance, electrical conductivity, pH, and optical density were measured over 95 days.
Results and discussion
The results from the carbon budget were fitted with a first-order kinetics model. The mineralization of refractory fractions predominated for both FA and HA. Overall, although the mineralization pathway yields varied according to the type of resource and oxygen availability, the mineralization half-lives were quite similar (49 to 64 days), suggesting a similar microbial catabolism efficiency during the decay of humic substances. The short-term routes are represented by biochemical oxidations, and the immobilization and labile fractions (varying from 0 to 30%) of FA and HA supported these processes. A yield varying from 61.0 to 91.3% represents a carbon source degradation in the middle term (ca. 2 months) considering the ecosystem.
In tropical floodplain lagoons, there are three carbon routes: (i) the IN1, representing a short-term pathway (hours to days) in the carbon transformation and (ii) IN3, a middle-term carbon source from HA and FA mineralization to the water column and subsequently to the atmosphere. A third route (IN2) supported the heterotrophic metabolism of the lagoon acting as a transitory sink of carbon.
KeywordsAquatic plants Floodplain lagoon Fulvic acid Humic acid Mathematical models Oxycaryum cubense
The authors would like to thank the São Paulo Research Foundation (FAPESP processes n°: 95/0119-8; 2007/08602-9) and the Brazilian National Council for Scientific and Technological Development for the scholarship (CNPq process number 301765/2010-3). We are also indebted to Dr. Osvaldo N. Oliveira Jr. (IFSC-USP) for his critical proofreading of the manuscript.
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