Ishtiaq Khandker S.

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Ishtiaq
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Khandker S.
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  • Article
    Carbon dioxide fluxes reflect plant zonation and belowground biomass in a coastal marsh
    (John Wiley & Sons, 2016-11-15) Moseman-Valtierra, Serena M. ; Abdul-Aziz, Omar I. ; Tang, Jianwu ; Ishtiaq, Khandker S. ; Morkeski, Kate ; Mora, Jordan ; Quinn, Ryan K. ; Martin, Rose M. ; Egan, Katherine E. ; Brannon, Elizabeth Q. ; Carey, Joanna C. ; Kroeger, Kevin D.
    Coastal wetlands are major global carbon sinks; however, they are heterogeneous and dynamic ecosystems. To characterize spatial and temporal variability in a New England salt marsh, greenhouse gas (GHG) fluxes were compared among major plant-defined zones during growing seasons. Carbon dioxide (CO2) and methane (CH4) fluxes were compared in two mensurative experiments during summer months (2012–2014) that included low marsh (Spartina alterniflora), high marsh (Distichlis spicata and Juncus gerardii-dominated), invasive Phragmites australis zones, and unvegetated ponds. Day- and nighttime fluxes were also contrasted in the native marsh zones. N2O fluxes were measured in parallel with CO2 and CH4 fluxes, but were not found to be significant. To test the relationships of CO2 and CH4 fluxes with several native plant metrics, a multivariate nonlinear model was used. Invasive P. australis zones (−7 to −15 μmol CO2·m−2·s−1) and S. alterniflora low marsh zones (up to −14 μmol CO2·m−2·s−1) displayed highest average CO2 uptake rates, while those in the native high marsh zone (less than −2 μmol CO2·m−2·s−1) were much lower. Unvegetated ponds were typically small sources of CO2 to the atmosphere (<0.5 μmol CO2·m−2·s−1). Nighttime emissions of CO2 averaged only 35% of daytime uptake in the low marsh zone, but they exceeded daytime CO2 uptake by up to threefold in the native high marsh zone. Based on modeling, belowground biomass was the plant metric most strongly correlated with CO2 fluxes in native marsh zones, while none of the plant variables correlated significantly with CH4 fluxes. Methane fluxes did not vary between day and night and did not significantly offset CO2 uptake in any vegetated marsh zones based on sustained global warming potential calculations. These findings suggest that attention to spatial zonation as well as expanded measurements and modeling of GHG emissions across greater temporal scales will help to improve accuracy of carbon accounting in coastal marshes.
  • Article
    Soil carbon consequences of historic hydrologic impairment and recent restoration in coastal wetlands
    (Association for the Sciences of Limnology and Oceanography, 2022-08-06) Eagle, Meagan ; Kroeger, Kevin D. ; Spivak, Amanda C. ; Wang, Faming ; Tang, Jianwu ; Abdul-Aziz, Omar I. ; Ishtiaq, Khandker S. ; O'Keefe Suttles, Jennifer A. ; Mann, Adrian G.
    Coastal wetlands provide key ecosystem services, including substantial long-term storage of atmospheric CO2 in soil organic carbon pools. This accumulation of soil organic matter is a vital component of elevation gain in coastal wetlands responding to sea-level rise. Anthropogenic activities that alter coastal wetland function through disruption of tidal exchange and wetland water levels are ubiquitous. This study assesses soil vertical accretion and organic carbon accretion across five coastal wetlands that experienced over a century of impounded hydrology, followed by restoration of tidal exchange 5 to 14 years prior to sampling. Nearby marshes that never experienced tidal impoundment served as controls with natural hydrology to assess the impact of impoundment and restoration. Dated soil cores indicate that elevation gain and carbon storage were suppressed 30–70 % during impoundment, accounting for the majority of elevation deficit between impacted and natural sites. Only one site had substantial subsidence, likely due to oxidation of soil organic matter. Vertical and carbon accretion gains were achieved at all restored sites, with carbon burial increasing from 96 ± 33 to 197 ± 64 g C m−2 y−1. The site with subsidence was able to accrete at double the rate (13 ± 5.6 mm y−1) of the natural complement, due predominantly to organic matter accumulation rather than mineral deposition, indicating these ecosystems are capable of large dynamic responses to restoration when conditions are optimized for vegetation growth. Hydrologic restoration enhanced elevation resilience and climate benefits of these coastal wetlands.
  • Article
    Environmental controls, emergent scaling, and predictions of greenhouse gas (GHG) fluxes in coastal salt marshes
    (John Wiley & Sons, 2018-07-28) Abdul-Aziz, Omar I. ; Ishtiaq, Khandker S. ; Tang, Jianwu ; Moseman-Valtierra, Serena M. ; Kroeger, Kevin D. ; Gonneea, Meagan E. ; Mora, Jordan ; Morkeski, Kate
    Coastal salt marshes play an important role in mitigating global warming by removing atmospheric carbon at a high rate. We investigated the environmental controls and emergent scaling of major greenhouse gas (GHG) fluxes such as carbon dioxide (CO2) and methane (CH4) in coastal salt marshes by conducting data analytics and empirical modeling. The underlying hypothesis is that the salt marsh GHG fluxes follow emergent scaling relationships with their environmental drivers, leading to parsimonious predictive models. CO2 and CH4 fluxes, photosynthetically active radiation (PAR), air and soil temperatures, well water level, soil moisture, and porewater pH and salinity were measured during May–October 2013 from four marshes in Waquoit Bay and adjacent estuaries, MA, USA. The salt marshes exhibited high CO2 uptake and low CH4 emission, which did not significantly vary with the nitrogen loading gradient (5–126 kg · ha−1 · year−1) among the salt marshes. Soil temperature was the strongest driver of both fluxes, representing 2 and 4–5 times higher influence than PAR and salinity, respectively. Well water level, soil moisture, and pH did not have a predictive control on the GHG fluxes, although both fluxes were significantly higher during high tides than low tides. The results were leveraged to develop emergent power law‐based parsimonious scaling models to accurately predict the salt marsh GHG fluxes from PAR, soil temperature, and salinity (Nash‐Sutcliffe Efficiency = 0.80–0.91). The scaling models are available as a user‐friendly Excel spreadsheet named Coastal Wetland GHG Model to explore scenarios of GHG fluxes in tidal marshes under a changing climate and environment.