Low carbon accumulation in a macro‐tidal mangrove forest on the Amazon coast

Abstract

The important role mangrove forests play in sequestering organic carbon is well known, yet rates of organic carbon accumulation in macro‐tidal mangrove ecosystems are poorly resolved. Here we use 210Pb dating to present a 125‐yr record of carbon, nutrient and trace metal accumulation in sediments from an Amazon macro‐tidal mangrove forest. We find that the rate of organic carbon accumulation ranged from 23.7 to 74.7 g m−2 yr−1 (average 38 ± 13.5 g m−2 yr−1), significantly lower than global averages for mangrove forests. These low rates may be associated with sediment grain‐size and sediment–water interface processes that drive organic matter oxidation and reduce carbon stocks in these highly dynamic macro‐tidal forests. Total nitrogen accumulation ranged from 1.4 to 5.1 g m−2 yr−1 (average 2.7 ± 0.9 g m−2 yr−1) and phosphorus from 1.5 to 8.4 g m−2 yr−1(average 4.3 ± 1.9 g m−2 yr−1). Trace metal accumulation rates (As, Pb, Cr, Cu, Mn, Ni, Zn, Hg, Bo, V, Co, Mo, S, and Ba) were also lower than other tropical mangrove forests globally, but trace metal in more recent sediments for Mn, As, Cu, and Hg were elevated, likely reflecting human footprint in the region since early the 20th century. The ability to accurately quantify carbon accumulation rates in mangrove ecosystems is critical for climate change mitigation strategies and the implementation of carbon offset schemes globally.

The inclusion of Amazon mangroves in Brazil’s REDD+ program

“…The high regional variability in mangrove TECS within estuaries and other hydro-geomorphological settings suggest localized effects on carbon sequestration, which could result from the interplay of soil mineralogy, sea level rise, and coastal dynamics processes 29 – 32 . Organic carbon accumulation rates in estuarine mangroves to the east of the Amazon Delta are markedly lower (30–59 g m −2 yr −1 33 ) when compared to other Brazilian mangroves 17 , 18 and to the global average of 138 g m −2 yr −1 34 . Carbon accumulation rates and the relative contribution of allochthonous sources of carbon are of additional interest in areas with large intact forests such as the Brazilian Amazon coast as preserved mangroves may contribute to climate mitigation by removing carbon from the atmosphere 9 , 17 .…”

“…Carbon accumulation rates and the relative contribution of allochthonous sources of carbon are of additional interest in areas with large intact forests such as the Brazilian Amazon coast as preserved mangroves may contribute to climate mitigation by removing carbon from the atmosphere 9 , 17 . Further assessment of C accumulation rates in mangroves of the Amazon Delta will be key to understand the potential effects of high riverine (allochthonous) mineral input on carbon stabilization at those sites as vertical accretion rates are positively correlated with sediment supply 30 , 33 . It is also possible that mechanisms that accelerate organic matter decomposition or mechanisms behind SOC interactions with soil minerals within Amazon soils may be a key factor to soil C stocks in the region 32 , 35 , 36 .…”