Julie Shahan

All Publications

• Fate of methane in canals draining tropical peatlands. Nature communications Perryman, C. R., Bowen, J. C., Shahan, J., Silviani P A B, D., Dayanti, E., Andriyani, Y., Asyhari, A., Gangga, A., Novita, N., Anshari, G. Z., Hoyt, A. M. 2024; 15 (1): 9766

  Abstract

  Tropical wetlands and freshwaters are major contributors to the growing atmospheric methane (CH4) burden. Extensive peatland drainage has lowered CH4 emissions from peat soils in Southeast Asia, but the canals draining these peatlands may be hotspots of CH4 emissions. Alternatively, CH4 oxidation (consumption) by methanotrophic microorganisms may attenuate emissions. Here, we used laboratory experiments and a synoptic survey of the isotopic composition of CH4 in 34 canals across West Kalimantan, Indonesia to quantify the proportion of CH4 that is consumed and therefore not emitted to the atmosphere. We find that CH4 oxidation mitigates 76.4 ± 12.0% of potential canal emissions, reducing emissions by ~70 mg CH4 m-2 d-1. Methane consumption also significantly impacts the stable isotopic fingerprint of canal CH4 emissions. As canals drain over 65% of peatlands in Southeast Asia, our results suggest that CH4 oxidation significantly influences landscape-scale CH4 emissions from these ecosystems.

  View details for DOI 10.1038/s41467-024-54063-x

  View details for PubMedID 39528506

  View details for PubMedCentralID 9540782

• Controls on spatial variation in porewater methane concentrations across United States tidal wetlands. The Science of the total environment Koontz, E. L., Parker, S. M., Stearns, A. E., Roberts, B. J., Young, C. M., Windham-Myers, L., Oikawa, P. Y., Megonigal, J. P., Noyce, G. L., Buskey, E. J., Derby, R. K., Dunn, R. P., Ferner, M. C., Krask, J. L., Marconi, C. M., Savage, K. B., Shahan, J., Spivak, A. C., St Laurent, K., Argueta, J. M., Baird, S. J., Beheshti, K. M., Crane, L. C., Cressman, K. A., Crooks, J. A., Fernald, S. H., Garwood, J. A., Goldstein, J. S., Grothues, T. M., Habeck, A., Lerberg, S. B., Lucas, S. B., Marcum, P., Peter, C. R., Phipps, S. W., Raposa, K. B., Rovai, A. S., Schooler, S. S., Twilley, R. R., Tyrrell, M. C., Uyeda, K. A., Wulfing, S. H., Aman, J. T., Giacchetti, A., Cross-Johnson, S. N., Holmquist, J. R. 2024: 177290

  Abstract

  Tidal wetlands can be a substantial sink of greenhouse gases, which can be offset by variable methane (CH4) emissions under certain environmental conditions and anthropogenic interventions. Land managers and policymakers need maps of tidal wetland CH4 properties to make restoration decisions and inventory greenhouse gases. However, there is a mismatch in spatial scale between point-based sampling of porewater CH4 concentration and its predictors, and the coarser resolution mapping products used to upscale these data. We sampled porewater CH4 concentrations, salinity, sulfate (SO42-), ammonium (NH4+), and total Fe using a spatially stratified sampling at 27 tidal wetlands in the United States. We measured porewater CH4 concentrations across four orders of magnitude (0.05 to 852.9 μM). The relative contribution of spatial scale to variance in CH4 was highest between- and within-sites. Porewater CH4 concentration was best explained by SO42- concentration with segmented linear regression (p < 0.01, R2 = 0.54) indicating lesser sensitivity of CH4 to SO42- below 0.62 mM SO42-. Salinity was a significant proxy for CH4 concentration, because it was highly correlated with SO42- (p < 0.01, R2 = 0.909). However, salinity was less predictive of CH4 with segmented linear regression (p < 0.01, R2 = 0.319) relative to SO42-. Neither NH4+, total Fe, nor relative tidal elevation correlated significantly with porewater CH4; however, NH4+ was positively and significantly correlated with SO42- after detrending CH4 for its relationship with SO42- (p < 0.01, R2 = 0.194). Future sampling should focus on within- and between-site environmental gradients to accurately map CH4 variation. Mapping salinity at sub-watershed scales has some potential for mapping SO42-, and by proxy, constraining spatial variation in porewater CH4 concentrations. Additional work is needed to explain site-level deviations from the salinity-sulfate relationship and elucidate other predictors of methanogenesis. This work demonstrates a unique approach to remote team science and the potential to strengthen collaborative research networks.

  View details for DOI 10.1016/j.scitotenv.2024.177290

  View details for PubMedID 39491559

• A New Coupled Biogeochemical Modeling Approach Provides Accurate Predictions of Methane and Carbon Dioxide Fluxes Across Diverse Tidal Wetlands JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES Oikawa, P. Y., Sihi, D., Forbrich, I., Fluet-Chouinard, E., Najarro, M., Thomas, O., Shahan, J., Arias-Ortiz, A., Russell, S., Knox, S. H., Mcnicol, G., Wolfe, J., Windham-Myers, L., Stuart-Haentjens, E., Bridgham, S. D., Needelman, B., Vargas, R., Schafer, K., Ward, E. J., Megonigal, P., Holmquist, J. 2024; 129 (10)

  View details for DOI 10.1029/2023JG007943

  View details for Web of Science ID 001336482300001

• Methane fluxes in tidal marshes of the conterminous United States. Global change biology Arias-Ortiz, A., Wolfe, J., Bridgham, S. D., Knox, S., McNicol, G., Needelman, B. A., Shahan, J., Stuart-Haëntjens, E. J., Windham-Myers, L., Oikawa, P. Y., Baldocchi, D. D., Caplan, J. S., Capooci, M., Czapla, K. M., Derby, R. K., Diefenderfer, H. L., Forbrich, I., Groseclose, G., Keller, J. K., Kelley, C., Keshta, A. E., Kleiner, H. S., Krauss, K. W., Lane, R. R., Mack, S., Moseman-Valtierra, S., Mozdzer, T. J., Mueller, P., Neubauer, S. C., Noyce, G., Schäfer, K. V., Sanders-DeMott, R., Schutte, C. A., Vargas, R., Weston, N. B., Wilson, B., Megonigal, J. P., Holmquist, J. R. 2024; 30 (9): e17462

  Abstract

  Methane (CH4) is a potent greenhouse gas (GHG) with atmospheric concentrations that have nearly tripled since pre-industrial times. Wetlands account for a large share of global CH4 emissions, yet the magnitude and factors controlling CH4 fluxes in tidal wetlands remain uncertain. We synthesized CH4 flux data from 100 chamber and 9 eddy covariance (EC) sites across tidal marshes in the conterminous United States to assess controlling factors and improve predictions of CH4 emissions. This effort included creating an open-source database of chamber-based GHG fluxes (https://doi.org/10.25573/serc.14227085). Annual fluxes across chamber and EC sites averaged 26 ± 53 g CH4 m-2 year-1, with a median of 3.9 g CH4 m-2 year-1, and only 25% of sites exceeding 18 g CH4 m-2 year-1. The highest fluxes were observed at fresh-oligohaline sites with daily maximum temperature normals (MATmax) above 25.6°C. These were followed by frequently inundated low and mid-fresh-oligohaline marshes with MATmax ≤25.6°C, and mesohaline sites with MATmax >19°C. Quantile regressions of paired chamber CH4 flux and porewater biogeochemistry revealed that the 90th percentile of fluxes fell below 5 ± 3 nmol m-2 s-1 at sulfate concentrations >4.7 ± 0.6 mM, porewater salinity >21 ± 2 psu, or surface water salinity >15 ± 3 psu. Across sites, salinity was the dominant predictor of annual CH4 fluxes, while within sites, temperature, gross primary productivity (GPP), and tidal height controlled variability at diel and seasonal scales. At the diel scale, GPP preceded temperature in importance for predicting CH4 flux changes, while the opposite was observed at the seasonal scale. Water levels influenced the timing and pathway of diel CH4 fluxes, with pulsed releases of stored CH4 at low to rising tide. This study provides data and methods to improve tidal marsh CH4 emission estimates, support blue carbon assessments, and refine national and global GHG inventories.

  View details for DOI 10.1111/gcb.17462

  View details for PubMedID 39234688

• On the Relationship Between Aquatic CO₂ Concentration and Ecosystem Fluxes in Some of the World's Key Wetland Types WETLANDS Richardson, J. L., Desai, A. R., Thom, J., Lindgren, K., Laudon, H., Peichl, M., Nilsson, M., Campeau, A., Jarveoja, J., Hawman, P., Mishra, D. R., Smith, D., D'Acunha, B., Knox, S. H., Ng, D., Johnson, M. S., Blackstock, J., Malone, S. L., Oberbauer, S. F., Detto, M., Wickland, K. P., Forbrich, I., Weston, N., Hung, J. Y., Edgar, C., Euskirchen, E. S., Bret-Harte, S., Dobkowski, J., Kling, G., Kane, E. S., Badiou, P., Bogard, M., Bohrer, G., O'Halloran, T., Ritson, J., Arias-Ortiz, A., Baldocchi, D., Oikawa, P., Shahan, J., Matsumura, M. 2024; 44 (1)

  View details for DOI 10.1007/s13157-023-01751-x

  View details for Web of Science ID 001118749200001

• Combining Eddy Covariance and Chamber Methods to Better Constrain CO₂ and CH₄ Fluxes Across a Heterogeneous Restored Tidal Wetland JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES Shahan, J., Chu, H., Windham-Myers, L., Matsumura, M., Carlin, J., Eichelmann, E., Stuart-Haentjens, E., Bergamaschi, B., Nakatsuka, K., Sturtevant, C., Oikawa, P. 2022; 127 (9)

  View details for DOI 10.1029/2022JG007112

  View details for Web of Science ID 000859015400001

• Tidal and Nontidal Marsh Restoration: A Trade-Off Between Carbon Sequestration, Methane Emissions, and Soil Accretion JOURNAL OF GEOPHYSICAL RESEARCH-BIOGEOSCIENCES Arias-Ortiz, A., Oikawa, P. Y., Carlin, J., Masque, P., Shahan, J., Kanneg, S., Paytan, A., Baldocchi, D. D. 2021; 126 (12)

  View details for DOI 10.1029/2021JG006573

  View details for Web of Science ID 000734472100017

• The Potential of Satellite Remote Sensing Time Series to Uncover Wetland Phenology under Unique Challenges of Tidal Setting REMOTE SENSING Miller, G., Dronova, I., Oikawa, P. Y., Knox, S., Windham-Myers, L., Shahan, J., Stuart-Haentjens, E. 2021; 13 (18)

  View details for DOI 10.3390/rs13183589

  View details for Web of Science ID 000701274300001