Buried hurricane legacies: increased nutrient limitation and decreased root biomass in coastal wetlands

Abstract

Plant identity and cover in coastal wetlands is changing in worldwide, and many subtropical salt marshes dominated by low-stature herbaceous species are becoming woody mangroves. Yet, how changes affect coastal soil biogeochemical processes and belowground biomass before and after storms is uncertain. We experimentally manipulated the percent mangrove cover (Avicennia germinans) in 3 × 3 m cells embedded in 10 plots (24 × 42 m) comprising a gradient of marsh (e.g., Spartina alterniflora, Batis maritima) and mangrove cover in Texas, USA. Hurricane Harvey made direct landfall over our site on 25 August 2017, providing a unique opportunity to test how plant composition mitigates hurricane effects on surface sediment accretion, soil chemistry (carbon, C; nitrogen, N; phosphorus, P; and sulfur, S), and root biomass. Data were collected before (2013 and 2016), one-month after (2017), and one-year after (2018) Hurricane Harvey crossed the area, allowing us to measure stocks before and after the hurricane. The accretion depth was higher in fringe compared with interior cells of plots, more variable in cells dominated by marsh than mangrove, and declined with increasing plot-scale mangrove cover. The concentrations of P and δ 34 S in storm-driven accreted surface sediments, and the concentrations of N, P, S, and δ 34 S in underlying soils (0-30 cm), decreased post-hurricane, whereas the C concentrations in both compartments were unchanged. Root biomass in both marsh and mangrove cells was reduced by 80% in 2017 compared with previous dates and remained reduced in 2018. Post-hurricane loss of root biomass in plots correlated with enhanced nutrient limitation. Total sulfide accumulation as indicated by δ 34 S, increased nutrient limitation, and decreased root biomass of both marshes and mangroves after hurricanes may affect ecosystem function and increase vulnerability in coastal wetlands to subsequent disturbances. Understanding how changes in plant composition in coastal ecosystems affects responses to hurricane disturbances is needed to assess coastal vulnerability.

Coastal carbon processing rates increase with mangrove cover following a hurricane in Texas, USA

“…In our prior research, we measured higher rates of black mangrove (Avicennia germinans) litter breakdown in mangrove than marsh cells and these rates increased with increasing plot-level mangrove cover (Table 1; Charles et al, 2020). However, we also reported declines in nutrient (nitrogen and phosphorus) concentrations and increases in total sulfide accumulation as indicated by δ 34 S in accreted surface sediments and soils in both marsh and mangrove cells, especially in plot fringes, following Hurricane Harvey (Table 1; Kuhn et al, 2021). As post-hurricane soil nutrient concentrations uniformly decreased in marsh and mangrove cells (Kuhn et al, 2021), we predicted litter breakdown rates would be lower than pre-hurricane (Charles et al, 2020).…”

“…However, we also reported declines in nutrient (nitrogen and phosphorus) concentrations and increases in total sulfide accumulation as indicated by δ 34 S in accreted surface sediments and soils in both marsh and mangrove cells, especially in plot fringes, following Hurricane Harvey (Table 1; Kuhn et al, 2021). As post-hurricane soil nutrient concentrations uniformly decreased in marsh and mangrove cells (Kuhn et al, 2021), we predicted litter breakdown rates would be lower than pre-hurricane (Charles et al, 2020). As total soil sulfide accumulation was highest in fringe cells post-hurricane (Kuhn et al, 2021), we predicted lower litter breakdown and microbial respiration rates in fringe than interior cells (both marsh and mangrove).…”

“…As post‐hurricane soil nutrient concentrations uniformly decreased in marsh and mangrove cells (Kuhn et al, 2021), we predicted litter breakdown rates would be lower than pre‐hurricane (Charles et al, 2020). As total soil sulfide accumulation was highest in fringe cells post‐hurricane (Kuhn et al, 2021), we predicted lower litter breakdown and microbial respiration rates in fringe than interior cells (both marsh and mangrove). We expected labile litter to be less sensitive than recalcitrant litter to mangrove cover and post‐hurricane changes in soil chemistry.…”

“…Nonsignificant (NS) differences in categorical comparisons are noted. Trends are derived from actual data analyses in Charles et al (2020) and Kuhn et al (2021).…”

“…Hurricanes transform vegetation and soil composition in coastal ecosystems and can both increase and decrease nutrient availability (Castañeda‐Moya et al, 2010, 2020; Kuhn et al, 2021). However, it is uncertain how hurricane‐induced changes in soil chemistry interact with changes in marsh–mangrove composition to affect soil organic matter processes that are fundamentally linked to carbon storage.…”