Citation:

Wigand, C., A. Oczkowski, B. Branoff, M. Eagle, Alana Hanson, R. Martin, S. Balogh, K. Miller, E. Huertas, J. Loffredo, AND E. Watson. Recent Nitrogen Storage and Accumulation Rates in Mangrove Soils Exceed Historic Rates in the Urbanized San Juan Bay Estuary (Puerto Rico, United States). Frontiers in Forests and Global Change. Frontiers, Lausanne, Switzerland, 4:765896, (2021). https://doi.org/10.3389/ffgc.2021.765896

Impact/Purpose:

Tropical mangrove forests have been described as coastal kidneys, because they can filter wastewater inputs from people’s homes and help clean coastal waters. Coastal areas throughout the world are experiencing increases in people living along the coastline and land development, which can cause increased wastewater inputs into coastal waters. To effectively manage coastal areas, it is important to understand wastewater nitrogen (N) storage in mangrove systems. We examined N storage in recent (1970 – 2016) and historic (1930 – 1970) decades in mangrove stands in the urban San Juan Bay Estuary (SJBE; Puerto Rico). The N storage in recent decades was significantly greater than in historic decades at Martin Peña East, the site with the greatest land development and population density, and also at the Piñones forest reserve, the site with the least development. Mangrove forest at Martin Peña East fringed a clogged canal and often received raw sewage from people’s homes. The Piñones mangrove was a healthy, old forest, but isolated and had low tidal exchange with the bay. In addition to increases in N storage in the soils due to inputs from people, N can increase naturally in old mangroves with high growth. Averaging over the total bay, the N storage rates were over twice as great in recent than in historic decades. Mangrove N storage rates in the San Juan Bay Estuary in recent times were twice as great as the rate of food N that is exported as human wastewater, suggesting the possibility for mangroves to intercept and store human wastewater. Conservation and effective management of mangrove forests are important for providing clean water in tropical, coastal communities throughout the world.

Description:

Tropical mangrove forests have been described as “coastal kidneys,” promoting sediment deposition and filtering contaminants, including excess nutrients. Coastal areas throughout the world are experiencing increased human activities, resulting in altered geomorphology, hydrology, and nutrient inputs. To effectively manage and sustain coastal mangroves, it is important to understand nitrogen (N) storage and accumulation in systems where human activities are causing rapid changes in N inputs and cycling. We examined N storage and accumulation rates in recent (1970 – 2016) and historic (1930 – 1970) decades in the context of urbanization in the San Juan Bay Estuary (SJBE, Puerto Rico), using mangrove soil cores that were radiometrically dated. Local anthropogenic stressors can alter N storage rates in peri-urban mangrove systems either directly by increasing N soil fertility or indirectly by altering hydrology (e.g., dredging, filling, and canalization). Nitrogen accumulation rates were greater in recent decades than historic decades at Piñones Forest and Martin Peña East. Martin Peña East was characterized by high urbanization, and Piñones, by the least urbanization in the SJBE. The mangrove forest at Martin Peña East fringed a poorly drained canal and often received raw sewage inputs, with N accumulation rates ranging from 17.7 to 37.9 g m–2 y–1 in recent decades. The Piñones Forest was isolated and had low flushing, possibly exacerbated by river damming, with N accumulation rates ranging from 18.6 to 24.2 g m–2 y–1 in recent decades. Nearly all (96.3%) of the estuary-wide mangrove N (9.4 Mg ha–1) was stored in the soils with 7.1 Mg ha–1 sequestered during 1970–2017 (0–18 cm) and 2.3 Mg ha–1 during 1930–1970 (19–28 cm). Estuary-wide mangrove soil N accumulation rates were over twice as great in recent decades (0.18 ± 0.002 Mg ha–1y–1) than historically (0.08 ± 0.001 Mg ha–1y–1). Nitrogen accumulation rates in SJBE mangrove soils in recent times were twofold larger than the rate of human-consumed food N that is exported as wastewater (0.08 Mg ha–1 y–1), suggesting the potential for mangroves to sequester human-derived N. Conservation and effective management of mangrove forests and their surrounding watersheds in the Anthropocene are important for maintaining water quality in coastal communities throughout tropical regions.  

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