The effect of nutrient enrichment on mangrove sediment accretion and carbon accumulation rates is poorly understood. Here we quantify sediment accretion through radionuclide tracers to determine organic carbon (OC), total nitrogen (TN), and total phosphorus (TP) accumulation rates during the previous 60 years in both a nutrient-enriched and a pristine mangrove forest within the same geomorphological region of southeastern Brazil. The forest receiving high nutrient loads has accumulated OC, TN, and TP at rates that are fourfold, twofold, and eightfold respectively, higher than those from the undisturbed mangrove. Organic carbon and TN stable isotopes (δ 13 C and δ 15 N) reflect an increased presence of organic matter (OM) originating with either phytoplankton, benthic algae, or another allochthonous source within the more rapidly accumulated sediments of the impacted mangrove. This suggests that the accumulation rate of OM in eutrophic mangrove systems may be enhanced through the addition of autochthonous and allochthonous nonmangrove material.

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The Amazon floodplains cover approximately 10% of the Amazon Basin and are composed of predominantly anoxic sediments that may store large amounts of carbon. Our study combines 210 Pb derived sedimentation rates from four recently analyzed sediment cores (n 5 4) with previously published organic carbon (OC) burial estimates (n 5 18) to provide a broad, first order estimate of carbon accumulation in Amazon floodplain lakes. The OC burial rates were 266 6 57 g C m 22 yr 21. This rate is several folds greater than those reported for lakes in arctic, boreal, temperate, and tropical regions. The large amount and spatial variation of OC burial rates in these floodplain lakes highlights the need for increased sampling efforts to better measure these potentially important components of the Amazon Basin carbon budget. This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. Scientific Significance StatementIn the Amazon Basin, floodplains and the lakes within them are major sources of greenhouse gases. However, it is not known whether carbon burial in the floodplain lakes is large enough to offset their greenhouse gas emissions. Here, we show that Amazonian floodplain lakes can partly offset greenhouse gas emissions through significant carbon burial rates (266 6 57 g C m 22 yr 21) that are greater than any other measured lake type. By extrapolating our observations, we show that burial in Amazon floodplain lakes may play an important role in the carbon budget of the Amazon Basin.

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