Surface and SubSurface Soil Organic Matter Processing following Hurricane Harvey, Texas, USA

Coastal wetland plant identity and cover is changing, as many subtropical salt marshes dominated by low-stature herbaceous species transition to woody mangroves. How changes in dominant plant species affect carbon processing in coastal wetlands during storms is uncertain. We experimentally manipulated patch-scale (3 × 3 m) cover of black mangroves (Avicennia germinans) and saltmarsh plants (e.g., Spartina alterniflora, Batis maritima) in fringe and interior locations of ten plots (24 × 42 m) to create a gradient in mangrove cover in coastal Texas, USA. Hurricane Harvey made direct landfall over our site on 25 August 2017. To test how mangrove cover affected carbon retention after the storm, we measured litter breakdown rates (k) of A. germinans and S. alterniflora in surface soils and fast- and slow-decomposing standard litter substrates (green and red tea, respectively) in subsurface soils (15 cm depth). Soil temperatures were lower in mangrove than marsh patches, and prior microclimate measurements showed non-linear increases in air and soil temperatures with increasing mangrove cover (highest temperatures at intermediate % cover). Litter breakdown rates (k) were 2 higher in surface than in subsurface soils. Avicennia germinans litter k increased linearly in surface soils with plot-level mangrove cover, whereas slow-decomposing red tea had similar k in subsurface soils of all plots. Litter k of S. alterniflora in surface soils and fast-decomposing green tea in subsurface soils increased non-linearly with mangrove cover (highest k at intermediate % cover), explained largely by temperature. Microbial respiration rates (R) were highest in interior marsh patches for S. alterniflora litter and increased with plot-level mangrove cover, whereas R associated with A. germinans litter was similar among fringe and interior patches and highest at higher mangrove cover. Despite widespread declines in soil nutrient concentrations throughout marsh and mangrove patches in all plots, most surface and subsurface rates of carbon processing increased with mangrove cover in coastal wetlands following a hurricane.

滨海湿地植物种类与盖度正发生显著变化，诸多以低矮草本物种为优势的亚热带盐沼（salt marsh）正逐步向木本红树（mangrove）群落演替。目前学界尚不清楚，优势植物物种的更替会如何影响风暴事件期间滨海湿地的碳循环过程。我们于美国德克萨斯州沿海的10个（24×42 m）样地的边缘与内部斑块中，对黑红树（Avicennia germinans）与盐沼植物（如互花米草Spartina alterniflora、海滨碱蓬Batis maritima等）的斑块尺度（3×3 m）盖度进行人工调控，以此构建红树林盖度梯度。2017年8月25日，飓风哈维（Hurricane Harvey）直接登陆该研究区域。为探究红树林盖度对风暴后碳留存的影响，我们分别测定了表层土壤中黑红树与互花米草的枯落物分解速率（k值），以及地下15 cm土层内快、慢分解标准枯落物基质（分别对应绿茶与红茶）的分解速率。相较于盐沼斑块，红树林斑块的土壤温度更低；前期微气候监测表明，空气与土壤温度随红树林盖度增加呈非线性升高趋势，中等盖度下温度达到峰值。表层土壤的枯落物分解速率（k值）较地下土层高出2倍。黑红树枯落物的k值随样地尺度红树林盖度线性升高，而慢分解的红茶在所有样地的地下土层中k值无显著差异。互花米草表层土壤枯落物与地下土层快分解绿茶的k值随红树林盖度呈非线性升高，中等盖度下分解速率最高，该规律主要由温度驱动。对于互花米草枯落物，微生物呼吸速率（R）在内部盐沼斑块中最高，且随样地红树林盖度升高而增加；而与黑红树枯落物相关的呼吸速率在边缘与内部斑块间无显著差异，且在高红树林盖度下达到峰值。尽管所有样地的盐沼与红树林斑块的土壤养分浓度普遍下降，但飓风过后的滨海湿地中，大部分表层与地下碳循环速率均随红树林盖度升高而增加。