Recent Carbon Storage and Burial Exceed Historic Rates in the San Juan Bay Estuary Peri-Urban Mangrove Forests (Puerto Rico, United States)

Abstract

Mangroves sequester significant quantities of organic carbon (C) because of high rates of burial in the soil and storage in biomass. We estimated mangrove forest C storage and accumulation rates in aboveground and belowground components among five sites along an urbanization gradient in the San Juan Bay Estuary, Puerto Rico. Sites included the highly urbanized and clogged Caño Martin Peña in the western half of the estuary, a series of lagoons in the center of the estuary, and a tropical forest reserve (Piñones) in the easternmost part. Radiometrically dated cores were used to determine sediment accretion and soil C storage and burial rates. Measurements of tree dendrometers coupled with allometric equations were used to estimate aboveground biomass. Estuary-wide mangrove forest C storage and accumulation rates were estimated using interpolation methods and coastal vegetation cover data. In recent decades (1970–2016), the highly urbanized Martin Peña East (MPE) site with low flushing had the highest C storage and burial rates among sites. The MPE soil carbon burial rate was over twice as great as global estimates. Mangrove forest C burial rates in recent decades were significantly greater than historic decades (1930–1970) at Caño Martin Peña and Piñones. Although MPE and Piñones had similarly low flushing, the landscape settings (clogged canal vs forest reserve) and urbanization (high vs low) were different. Apparently, not only urbanization, but site-specific flushing patterns, landscape setting, and soil fertility affected soil C storage and burial rates. There was no difference in C burial rates between historic and recent decades at the San José and La Torrecilla lagoons. Mangrove forests had soil C burial rates ranging from 88 g m–2 y–1 at the San José lagoon to 469 g m–2 y–1 at the MPE in recent decades. Watershed anthropogenic CO2 emissions (1.56 million Mg C y–1) far exceeded the annual mangrove forest C storage rates (aboveground biomass plus soils: 17,713 Mg C y–1). A combination of maintaining healthy mangrove forests and reducing anthropogenic emissions might be necessary to mitigate greenhouse gas emissions in urban, tropical areas.

Recent Nitrogen Storage and Accumulation Rates in Mangrove Soils Exceed Historic Rates in the Urbanized San Juan Bay Estuary (Puerto Rico, United States)

“…It may be argued that in mature, isolated mangrove systems, nutrient recycling may be of greater significance than allochthonous inputs (Twilley and Day, 1999;Alongi et al, 2004;Marchand et al, 2006). Some of the highest measures of N storage and accumulation (this study) as well as C storage were observed in the poorly flushed, least urbanized, Piñones Forest (Wigand et al, 2021). Elevated porewater NH 4…”

“…; δ 15 N = −4.52 ) and USGS 41 (δ 13 C = 37.63 ; δ 15 N = 47.57 ) were used to normalize isotopic values of working standards (blue mussel homogenate) to the air (δ15N) and Vienna Pee Dee Belemnite (δ 13 C) scales (Paul et al, 2007). Analysis of carbon storage and dominant carbon sources to the SJBE mangrove soils determined from these same cores were reported earlier (Eagle et al, 2021;Wigand et al, 2021).…”

“…National Wetland Inventory data were used to depict wetlands, which included estuarine and marine, freshwater emergent, and freshwater forested/shrub wetlands (U.S. Fish and Wildlife Service, 2018). Tidal flushing and mangrove soil characteristics were based on previously published data (Branoff, 2020a;Eagle et al, 2021;Wigand et al, 2021); see Table 1 for site abbreviations.…”

Impacts of Climate Change on Blue Carbon Stocks and Fluxes in Mangrove Forests

“…The two main inputs were mangrove forest primary production and net DIC production in mangrove soils to a depth of 1 m. The latter is not usually considered in carbon mass balance estimates, but evidence suggests that centuries-old, sequestered carbon is still susceptible to remineralization and tidal DOC and DIC export [47], supporting the idea that organic carbon deposited prior to mangrove colonization continues to be decomposed as all mangroves colonize intertidal flats that have considerable stores of C ORG [1]. This idea is supported by the fact that radiocarbon dating of mangrove soil profiles of the upper 1-3 m [32,34,[38][39][40] indicate subsurface mangrove deposits that are centuries-old, and certainly older that the existing mangrove vegetation. Another unqualified pathway is groundwater which can in many locations provide significant quantities of carbon-and nutrient-rich subsurface water [6].…”

“…Rates of carbon sequestration in mangrove forests average 183.6 g C ORG m −2 a −1 (1 SE = 14.7 g C ORG m −2 a −1 ) with a median of 114.8 g C ORG m −2 a −1 and a very wide range from 0.001 to 1722.2 g C ORG m −2 a −1 (data in Alongi [8] updated with additional refs. [9,28,[32][33][34][35][36][37][38][39][40]). More than half of all measurements were <100 g C ORG m −2 a −1 with the highest rates observed in degraded or heavily impacted, disturbed forests and catchments such as in China, South and Southeast Asia, and Africa.…”