Patrick Megonigal - Academia.edu (original) (raw)

Papers by Patrick Megonigal

Forest and Rangeland Soils of the United States Under Changing Conditions, 2020

Biogeochemistry, 2021

Direct measurement of methane emissions is cost-prohibitive for greenhouse gas offset projects, n... more Direct measurement of methane emissions is cost-prohibitive for greenhouse gas offset projects, necessitating the development of alternative accounting methods such as proxies. Salinity is a useful proxy for tidal marsh CH4 emissions when comparing across a wide range of salinity regimes but does not adequately explain variation in brackish and freshwater regimes, where variation in emissions is large. We sought to improve upon the salinity proxy in a marsh complex on Deal Island Peninsula, Maryland, USA by comparing emissions from four strata differing in hydrology and plant community composition. Mean CH4 chamber-collected emissions measured as mg CH4 m−2 h−1 ranked as S. alterniflora (1.2 ± 0.3) ≫ High-elevation J. roemerianus (0.4 ± 0.06) > Low-elevation J. roemerianus (0.3 ± 0.07) = S. patens (0.1 ± 0.01). Sulfate depletion generally reflected the same pattern with significantly greater depletion in the S. alterniflora stratum (61 ± 4%) than in the S. patens stratum (1 ± 9%)...

Abiotic global change factors such as rising atmospheric CO2, and biotic factors such as exotic p... more Abiotic global change factors such as rising atmospheric CO2, and biotic factors such as exotic plant invasion, interact to alter the function of terrestrial ecosystems. An invasive lineage of the common reed, Phragmites australis, was introduced to the North America over a century ago, but the belowground mechanisms underlying Phragmites invasion and persistence in natural systems remain poorly studied. For instance, Phragmites has a nitrogen (N) demand higher than native plant communities in many of the ecosystems it invades, but the source of the additional N is not clear. We exposed introduced Phragmites and native plant assemblages, containing Spartina patens and Schoenoplectus americanus, to factorial treatments of CO2 (ambient or +300 ppm), N (0 or 25 g m-2 y-1), and hydroperiod (4 levels), and focused our analysis on changes in root productivity as a function of depth and evaluated the effects of introduced Phragmites on soil organic matter mineralization We report that non-native invasive Phragmites exhibits a deeper rooting profile than native marsh species under all experimental treatments, and also enhanced soil organic matter decomposition. Moreover, exposure to elevated atmospheric CO2 induces a sharp increase in deep root production in the invasive plant. We propose that niche separation accomplished through deeper rooting profiles circumvents nutrient competition where native species have relatively shallow root depth distributions; deep roots provide access to nutrient-rich porewater; and deep roots further increase nutrient availability by enhancing soil organic matter decomposition. We expect that rising CO2 will magnify these effects in deeprooting invasive plants that compete using a tree-like strategy against native herbaceous plants, promoting establishment and invasion through niche separation.

Marine Ecology Progress Series, 2021

Elevation is a major driver of plant ecology and sediment dynamics in tidal wetlands, so accurate... more Elevation is a major driver of plant ecology and sediment dynamics in tidal wetlands, so accurate and precise spatial data are essential for assessing wetland vulnerability to sea-level rise and making forecasts. We performed survey-grade elevation and vegetation surveys of the Global Change Research Wetland, a brackish microtidal wetland in the Chesapeake Bay estuary, Maryland (USA), to both intercompare unbiased digital elevation model (DEM) creation techniques and to describe niche partitioning of several common tidal wetland plant species. We identified a tradeoff between scalability and performance in creating unbiased DEMs, with more data-intensive methods such as kriging performing better than 3 more scalable methods involving post-processing of light detection and ranging (LiDAR)-based DEMs. The LiDAR Elevation Correction with Normalized Difference Vegetation Index (LEAN) method provided a compromise between scalability and performance, although it underpredicted variability...

Nature Communications, 2020

Blue carbon (C) ecosystems are among the most effective C sinks of the biosphere, but methane (CH... more Blue carbon (C) ecosystems are among the most effective C sinks of the biosphere, but methane (CH4) emissions can offset their climate cooling effect. Drivers of CH4 emissions from blue C ecosystems and effects of global change are poorly understood. Here we test for the effects of sea level rise (SLR) and its interactions with elevated atmospheric CO2, eutrophication, and plant community composition on CH4 emissions from an estuarine tidal wetland. Changes in CH4 emissions with SLR are primarily mediated by shifts in plant community composition and associated plant traits that determine both the direction and magnitude of SLR effects on CH4 emissions. We furthermore show strong stimulation of CH4 emissions by elevated atmospheric CO2, whereas effects of eutrophication are not significant. Overall, our findings demonstrate a high sensitivity of CH4 emissions to global change with important implications for modeling greenhouse-gas dynamics of blue C ecosystems.

Proceedings of the National Academy of Sciences, 2019

Significance Interactions between nutrient supply and plant demand dictate key terrestrial ecosys... more Significance Interactions between nutrient supply and plant demand dictate key terrestrial ecosystem feedbacks to global climate change. We investigated these responses in a marsh ecosystem and found that plants and soils respond to warming at different temperatures. Modest warming caused plant demand for nitrogen (N) to outpace the soil N supply, while more extreme warming caused the N supply to outpace plant N demand. These responses changed when elevated CO 2 was added to the warming treatments. Globally, terrestrial plant N demand has exceeded soil N supply in unfertilized ecosystems over the past century; our results reveal that this imbalance is driven primarily by elevated CO 2 and forecast that N supply will likely increase over plant demand as warming exceeds 2 °C.

Nature Climate Change, 2019

Coastal wetlands provide valuable ecosystem services that are increasingly threatened by anthropo... more Coastal wetlands provide valuable ecosystem services that are increasingly threatened by anthropogenic activities 1. Atmospheric carbon dioxide (CO2) concentration has increased from 280 to 404 ppm since the industrial revolution, and is projected to exceed 900 ppm by 2100 2. In terrestrial ecosystems, elevated CO2 typically stimulates C3 plant photosynthesis and primary productivity leading to an increase in plant size 3. However, compared to woody plants or crops 4 , the morphological responses of clonal non-woody plants to elevated CO2 have been less well-studied. We show that 30 years of experimental CO2 enrichment in a brackish marsh increased primary productivity and stem density, but decreased the stem diameter and height of the dominant clonal species Schoenoplectus americanus. Smaller, denser stems were associated with the expansion of roots and rhizomes to alleviate nitrogen (N) limitation as evidenced by high N immobilization in live tissue and litter, high tissue C:N, and low available porewater N. Changes in morphology and tissue chemistry induced by elevated CO2 were reversed by N addition. We demonstrate that morphological responses to CO2 and N supply in a clonal plant species influences the capacity of marshes to gain elevation at rates that keep pace with rising sea levels. Terrestrial plants are experiencing the highest atmospheric CO2 concentration in the past 800,000 years 5. The stimulation of carbon (C) fixation by elevated CO2-the CO2 fertilization effect-is well documented 6. Increases in leaf-level C uptake rates are often accompanied by changes in plant morphology or morphometric sizes, such as increased height, stem diameter, leaf area index, leaf number, and root volume 7, 8, 9, 10. These morphometric changes are widely observed to influence competitive dynamics 11, 12 , with implications for ecosystem structure and function. Resource availability regulates the effect of elevated CO2 on total biomass production but can also influence changes in plant size and patterns of biomass allocation. Elevated CO2induced changes in plant morphology are poorly understood compared to changes in total biomass, and morphological changes are highly variable along environmental gradients and between plant functional groups, reflecting species-specific strategies for resource acquisition 7, 9. For instance, plant species can overcome N limitation caused by CO2 fertilization in low-N

Environmental Research Letters, 2015

The primary productivity of coastal wetlands is changing dramatically in response to rising atmos... more The primary productivity of coastal wetlands is changing dramatically in response to rising atmospheric carbon dioxide (CO 2) concentrations, nitrogen (N) enrichment, and invasions by novel species, potentially altering their ecosystem services and resilience to sea level rise. In order to determine how these interacting global change factors will affect coastal wetland productivity, we quantified growing-season carbon assimilation (≈gross primary productivity, or GPP) and carbon retained in living plant biomass (≈net primary productivity, or NPP) of North American mid-Atlantic saltmarshes invaded by Phragmites australis (common reed) under four treatment conditions: two levels of CO 2 (ambient and +300 ppm) crossed with two levels of N (0 and 25 g N added m −2 yr −1). For GPP, we combined descriptions of canopy structure and leaf-level photosynthesis in a simulation model, using empirical data from an open-top chamber field study. Under ambient CO 2 and low N loading (i.e., the Control), we determined GPP to be 1.66±0.05 kg C m −2 yr −1 at a typical Phragmites stand density. Individually, elevated CO 2 and N enrichment increased GPP by 44 and 60%, respectively. Changes under N enrichment came largely from stimulation to carbon assimilation early and late in the growing season, while changes from CO 2 came from stimulation during the early and mid-growing season. In combination, elevated CO 2 and N enrichment increased GPP by 95% over the Control, yielding 3.24±0.08 kg C m −2 yr −1. We used biomass data to calculate NPP, and determined that it represented 44%-60% of GPP, with global change conditions decreasing carbon retention compared to the Control. Our results indicate that Phragmites invasions in eutrophied saltmarshes are driven, in part, by extended phenology yielding 3.1× greater NPP than native marsh. Further, we can expect elevated CO 2 to amplify Phragmites productivity throughout the growing season, with potential implications including accelerated spread and greater carbon storage belowground.

Treatise on Geochemistry, 2003

Wetlands, 2006

We examine the carbon balance of North American wetlands by reviewing and synthesizing the publis... more We examine the carbon balance of North American wetlands by reviewing and synthesizing the published literature and soil databases. North American wetlands contain about 220 Pg C, most of which is in peat. They are a small to moderate carbon sink of about 49 Tg C yr-l, although the uncertainty around this estimate is greater than 100%, with the largest unknown being the role of carbon sequestration by sedimentation in freshwater mineral-soil wetlands. We estimate that North American wetlands emit 9 Tg methane (C h) yr-l; however, the uncertainty of this estimate is also greater than 100%. With the exception of estuarine wetlands, C& emissions from wetlands may largely offset any positive benefits of carbon sequestration in soils and plants in terms of climate forcing. Historically, the destruction of wetlands through land-use changes has had the largest effects on the carbon fluxes and consequent radiative forcing of North American wetlands. The primary effects have been a reduction in their ability to sequester carbon (a small to moderate increase in radiative forcing), oxidation of their soil carbon reserves upon drainage (a small increase in radiative forcing), and reduction in C& emissions (a small to large decrease in radiative forcing). It is uncertain how global changes will affect the carbon pools and fluxes of North American wetlands. We will not be able to predict accurately the role of wetlands as potential positive or negative feedbacks to anthropogenic global change without knowing the integrative effects of changes in temperature, precipitation, atmospheric carbon dioxide concentrations, and atmospheric deposition of nitrogen and sulfur on the carbon balance of North American wetlands.

Wetlands, 2005

Elevated CO 2 generally stimulates C 3-type photosynthesis, but it is unclear how an increase in ... more Elevated CO 2 generally stimulates C 3-type photosynthesis, but it is unclear how an increase in CO 2 assimilation will interact with other factors that influence plant growth. In wetlands, the response of plants to elevated CO 2 will interact with soil saturation, particularly in forested wetlands where soil saturation is a strong regulator of plant productivity. We performed a four-month experiment to determine whether elevated CO 2 and flooding interact to influence the growth of a flood-tolerant tree (Taxodium distichum) and a flood-tolerant herbaceous emergent macrophyte (Orontium aquaticum). Seedlings were grown in glasshouses at two CO 2 levels (350 and 700 L L Ϫ1) crossed with two water depths (5 cm above and Ն5 cm below the soil surface). We hypothesized that elevated CO 2 would increase photosynthesis regardless of water depth and species; however, we also expected flooding to prevent elevated CO 2 from increasing the growth of the tree species due to O 2 limitation or other physiological stresses associated with reduced soil environments. We found that elevated CO 2 increased whole-plant photosynthesis in both species regardless of the flooding treatment. For T. distichum, this higher photosynthetic rate resulted in greater biomass only in the non-flooded treatment. This result suggests that some factor related to flooding constrained the biomass response of the flooded woody plants to elevated CO 2. In contrast, elevated CO 2 increased O. aquaticum biomass regardless of the flooding regime, perhaps because it occurs in wetter landscape positions than T. distichum and is less sensitive to flooding. We conclude that flooding may limit plant growth responses to elevated CO 2 , particularly in woody plant species.

Tidal wetlands experiencing increased rates of sea-level rise (SLR) must increase rates of soil e... more Tidal wetlands experiencing increased rates of sea-level rise (SLR) must increase rates of soil elevation gain to avoid permanent conversion to open water. The maximal rate of SLR that these ecosystems can tolerate depends partly on mineral sediment deposition, but the accumulation of organic matter is equally important for many wetlands. Plant productivity drives organic matter dynamics and is sensitive to global change factors, such as rising atmospheric CO2 concentration. It remains unknown how global change will influence organic mechanisms that determine future tidal wetland viability. Here, we present experimental evidence that plant response to elevated atmospheric [CO2] stimulates biogenic mechanisms of elevation gain in a brackish marsh. Elevated CO2 (ambient ؉ 340 ppm) accelerated soil elevation gain by 3.9 mm yr ؊1 in this 2-year field study, an effect mediated by stimulation of below-ground plant productivity. Further, a companion greenhouse experiment revealed that the CO2 effect was enhanced under salinity and flooding conditions likely to accompany future SLR. Our results indicate that by stimulating biogenic contributions to marsh elevation, increases in the greenhouse gas, CO2, may paradoxically aid some coastal wetlands in counterbalancing rising seas. coastal wetlands ͉ nitrogen pollution ͉ tidal marsh loss ͉ root productivity ͉ salinity

Smithsonian Contributions to the Marine Sciences, 2009

Coastal marshes must accumulate soil to keep up with rising sea levels. It is unknown how the res... more Coastal marshes must accumulate soil to keep up with rising sea levels. It is unknown how the response of these ecosystems to global change will infl uence their ability to continue to keep up with sea-level rise. Here, we describe an in situ experimental chamber approach for manipulating key environmental variables, such as atmospheric CO 2 and soil N availability, in a brackish marsh. We outfi tted each chamber with surface elevation tables (SETs) to closely monitor soil elevation change, a sensitive indicator of marsh vulnerability to sea-level rise. Further, the design facilitates measurements of ecosystem exchange of CO 2 , plant productivity, porewater chemistry, and other environmental parameters.

Wetlands, 2011

The relationship between methane emissions and salinity is not well understood in tidal marshes, ... more The relationship between methane emissions and salinity is not well understood in tidal marshes, leading to uncertainty about the net effect of marsh conservation and restoration on greenhouse gas balance. We used published and unpublished field data to investigate the relationships between tidal marsh methane emissions, salinity, and porewater concentrations of methane and sulfate, then used these relationships to consider the balance between methane emissions and soil carbon sequestration. Polyhaline tidal marshes (salinity >18) had significantly lower methane emissions (mean ± sd=1±2 gm −2 yr −1) than other marshes, and can be expected to decrease radiative forcing when created or restored. There was no significant difference in methane emissions from fresh (salinity=0-0.5) and mesohaline (5-18) marshes (42±76 and 16±11 gm −2 yr −1 , respectively), while oligohaline (0.5-5) marshes had the highest and most variable methane emissions (150±221 gm −2 yr −1). Annual methane emissions were modeled using a linear fit of salinity against log-transformed methane flux (logðCH 4 Þ ¼ À0:056 Â salinity þ 1:38; r 2 = 0. 5 2 ; p < 0.0001). Managers interested in using marshes as greenhouse gas sinks can assume negligible methane emissions in polyhaline systems, but need to estimate or monitor methane emissions in lower-salinity marshes.

Wetlands, 2012

Your article is protected by copyright and all rights are held exclusively by Society of Wetland ... more Your article is protected by copyright and all rights are held exclusively by Society of Wetland Scientists. This e-offprint is for personal use only and shall not be selfarchived in electronic repositories. If you wish to self-archive your work, please use the accepted author's version for posting to your own website or your institution's repository. You may further deposit the accepted author's version on a funder's repository at a funder's request, provided it is not made publicly available until 12 months after publication.

Soil Science Society of America Journal, 2010

In estuarine systems, naturally occurring soluble S2− is an indicator of anaerobic decomposition ... more In estuarine systems, naturally occurring soluble S2− is an indicator of anaerobic decomposition by the SO42−reduction pathway and can, at high concentrations, be detrimental to plant communities. Depth distributions of soluble S2− in marsh pore water are typically measured using either equilibrium dialysis samplers (peepers) or pore water extractors (sippers). The former technique provides concentrations equilibrated over one or more weeks at centimeter‐scale resolution, while the latter allows rapid sampling and analysis but with a coarser vertical resolution (5–10 cm). We report on a novel technology for documenting marsh pore water S2− concentrations based on reactive synthetic Fe oxides and image analysis, which allows rapid sampling but still captures small‐scale spatial resolution. During the last few years, this new technology associated with synthetic Fe oxides known as IRIS (Indicator of Reduction In Soils) has been developed to aid in documenting reducing conditions in we...

Soil Science Society of America Journal, 2013

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or b... more All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher. Nutrient Availability and Soil Organic Matter Decomposition Response to Prescribed Burns in Mid-Atlantic Brackish Tidal Marshes Wetland Soils F ire is a natural component of many wetland ecosystems (Lynch, 1941; Kirby et al., 1988; Nyman and Chabreck, 1995). Historically, prescribed fire has been used to remove vegetation to facilitate seasonal hunting and trapping in tidal marsh ecosystems (O'Neil, 1949), for example to facilitate the trapping of muskrat (Ondatra zibethicus) by making their lodges more visible to hunters (Lay, 1945). Prescribed fire has become an integral part of resource management in coastal wetlands and is widely used as a technique to promote the growth of favorable wetland vegetation on the U.

Plant and Soil, 2006

Belowground biomass is a critical factor regulating ecosystem functions of coastal marshes, inclu... more Belowground biomass is a critical factor regulating ecosystem functions of coastal marshes, including soil organic matter (SOM) accumulation and the ability of these systems to keep pace with sea-level rise. Nevertheless, belowground biomass responses to environmental and vegetation changes have been given little emphasis marsh studies. Here we present a method using stable carbon isotopes and color to identify root and rhizomes of Schoenoplectus americanus (Pers.) Volk. ex Schinz and R. Keller (C 3) and Spartina patens (Ait.) Muhl. (C 4) occurring in C 3) and C 4-dominated communities in a Chesapeake Bay brackish marsh. The functional significance of the biomass classes we identified is underscored by differences in their chemistry, depth profiles, and variation in biomass and profiles relative to abiotic and biotic factors. C 3 rhizomes had the lowest concentrations of cellulose (29.19%) and lignin (14.43%) and the lowest C:N (46.97) and lignin:N (0.16) ratios. We distinguished two types of C 3 roots, and of these, the dark red C 3 roots had anomalously high C:N (195.35) and lignin:N (1.14) ratios, compared with other root and rhizome classes examined here and with previously published values. The C 4-dominated community had significantly greater belowground biomass (4119.1 g m)2) than the C 3-dominated community (3256.9 g m)2), due to greater total root biomass and a 3.6-fold higher C 3-root:rhizome ratio in the C 4-dominated community. C 3 rhizomes were distributed significantly shallower in the C 4-dominated community, while C 3 roots were significantly deeper. Variability in C 3 rhizome depth distributions was explained primarily by C 4 biomass, and C 3 roots were explained primarily by water table height. Our results suggest that belowground biomass in this system is sensitive to slight variations in water table height (across an 8 cm range), and that the reduced overlap between C 3 and C 4 root profiles in the C 4-dominated community may account for the greater total root biomass observed in that community. Given that future elevated atmospheric CO 2 and accelerated sea-level rise are likely to increase C 3 abundance in Atlantic and Gulf coast marshes, investigations that quantify how patterns of C 3 and C 4 belowground biomass respond to environmental and biological factors stand to improve our understanding of ecosystem-wide impacts of global changes on coastal wetlands. Abbreviations: SOM-soil organic matter; C-carbon; CO 2-carbon dioxide; N-nitrogen; d 13 C-13 C abundance relative to standard Peedee Belemnite; SERC-Smithsonian Environmental Research Center