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Methane dynamics within salt marshes are complex because vegetation types, temperature, oscillating water levels, and changes in salinity and redox conditions influence CH4 production and emission. These non-linear and complex interactions among variables affect the traditionally expected functional relationships and present challenges for interpretation and developing process-based models. We employ empirical dynamic modeling (EDM) and convergent cross mapping (CCM) as a novel approach for characterizing seasonal/multiday and diurnal CH4 dynamics by identifying causal variables, lags, and interconnections among multiple biophysical variables within a temperate salt marsh using five years of eddy covariance data. EDM/CCM is a nonparametric approach capable of quantifying the coupling between variables while determining time scales where variable interactions are most relevant. We found that gross primary productivity, tidal creek dissolved oxygen, and temperature were important for seasonal/multiday dynamics (rho=0.73-0.80), while water level was most important for diurnal dynamics during both the growing and dormancy phenoperiods (rho=0.72 and 0.56, respectively). Lags for top causal variables (gross primary productivity, tidal creek dissolved oxygen, temperature, water level) occurred between 1-5 weeks at the seasonal scale and 1-24 hours at the diurnal scale. The EDM had high prediction capabilities for intra-/inter-seasonal patterns and annual CH4 sums but with limitations to represent large infrequent fluxes. Results highlight the importance of non-linearity, causal drivers, lag times, and interconnections among multiple biophysical variables that regulate CH4 fluxes in tidal wetlands. This study presents a new dimension for analyzing CH4 fluxes, which will prove helpful to test current paradigms in wetlands and other ecosystems.

Process-based land surface models are important tools for estimating global wetland methane (CH4) emissions and projecting their behavior across space and time. So far there are no performance assessments of model responses to drivers at multiple time scales. In this study, we apply wavelet analysis to identify the dominant time scales contributing to model uncertainty in the frequency domain. We evaluate seven wetland models at 23 eddy covariance tower sites. Our study first characterizes site-level patterns of freshwater wetland CH4 fluxes (FCH4) at different time scales. A Monte Carlo approach has been developed to incorporate flux observation error to avoid misidentification of the time scales that dominate model error. Our results suggest that 1) significant model-observation disagreements are mainly at short- to intermediate time scales (< 15 days); 2) most of the models can capture the CH4 variability at long time scales (> 32 days) for the boreal and Arctic tundra wetland sites but have limited performance for temperate and tropical/subtropical sites; 3) model error approximates pink noise patterns, indicating that biases at short time scales (< 5 days) could contribute to persistent systematic biases on longer time scales; and 4) differences in error pattern are related to model structure (e.g. proxy of CH4 production). Our evaluation suggests the need to accurately replicate FCH4 variability in future wetland CH4 model developments.

This article is composed of three independent commentaries about the state of ICON principles (Goldman et al. 2021) in the AGU Biogeosciences section and discussion on the opportunities and challenges of adopting them. Each commentary focuses on a different topic: Global collaboration, technology transfer and application (Section 2), Community engagement, citizen science, education, and stakeholder involvement (Section 3), and Field, experimental, remote sensing, and real-time data research and application (Section 4). We discuss needs and strategies for implementing ICON and outline short- and long-term goals. The inclusion of global data and international community engagement are key to tackle grand challenges in biogeosciences. Although recent technological advances and growing open-access information across the world have enabled global collaborations to some extent, several barriers ranging from technical to organizational to cultural have remained in advancing interoperability and tangible scientific progress in biogeosciences. Overcoming these hurdles is necessary to address pressing large-scale research questions and applications in the biogeosciences, where ICON principles are essential. Here, we list several opportunities for ICON, including coordinated experimentation and field observations across global sites, that are ripe for implementation in biogeosciences as a means to scientific advancements and social progress.

The development of several large-, ‘continental’-scale ecosystem research infrastructures over recent decades has provided a unique opportunity in the history of ecological science. The Global Ecosystem Research Infrastructure (GERI) is an integrated network of analogous, but independent, site-based ecosystem research infrastructures (ERI) dedicated to better understand the function and change of indicator ecosystems across global biomes. Bringing together these ERIs, harmonizing their respective data and reducing uncertainties enables broader cross-continental ecological research. It will also enhances the research community capabilities to anticipate and address future global scale ecological challenges to the planet. Moreover, increasing the international capabilities of these ERIs goes beyond their original design intent, and is an unexpected added value of these large national investments. Here, we identify specific global grand challenge areas and research trends to advance the ecological frontiers across continents that can be addressed through the federation of these cross-continental-scale ERIs.

Terrestrial-aquatic interfaces such as salt marshes, mangroves, and similar wetlands provide an optimum natural environment for the sequestration and long-term storage of carbon (C) from the atmosphere, commonly known as coastal blue carbon. There are over 4 million acres of salt marsh in the US and over half of these are along the east coast of the US. Due to anthropogenic activities, this area presents the greatest nitrogen (N) pollution problem in coastal ecosystems in the U.S. as part of atmospheric N deposition, runoff, and riverine export. Ammonia (NH3) is the most abundant alkaline gas in the atmosphere. Agricultural intensification is the primary anthropogenic source of NH3 leading to a doubling of reactive nitrogen (Nr) entering the biosphere. Despite this, there are limited atmospheric measurements of NH3 concentrations in coastal areas along the east coast. The objective of this study is to advance our process-level understanding of NH3 air-surface exchange over a tidal salt marsh at the Saint Jones Reserve (DE), which is part of the National Estuarine Research Reserve System (NERRs). Continuous and high temporal resolution measurements of atmospheric NH3 concentrations were measured using a cavity ring-down spectrometer, reporting 30 min concentration averages. These high temporal resolution measurements allowed the estimation of the average diurnal cycle of NH3 fluxes using a new analytical methodology. Micrometeorological measurements were also measured using the eddy covariance system operated concurrently above the tidal marsh at the research site, which is part of the AmeriFlux network (US-StJ). This pilot study represents one of the few atmospheric measurements of NH3 over a tidal salt marsh in the eastern U.S. Such measurements are important to characterize the processes that influence the exchanges of NH3 between the atmosphere and the aquatic surface and provide baseline data to form more reliable parameterizations to simulate NH3 deposition and emissions in tidal salt marshes using surface-atmosphere transfer models.

Environmental observatory networks (EONs) provide information to better understand, model and forecast the spatial and temporal dynamics of Earth’s biophysical process. Consequently, representativeness analyses of EONs are important to provide insights for improving EONs’ management, design, and interpretation of their value-added products (e.g., datasets, model predictions). We assessed the representativeness of registered FLUXNET sites (n=41, revised on September 2018) across Latin America (LA), a region of great importance for the global carbon and water cycles, which represents nearly 13% of the world’s land surface area. Representativeness analyses were performed using a 0.05o spatial grid for multiple environmental variables, gross primary productivity (GPP) and evapotranspiration (ET) across LA. Our results showed a potential spatial representativeness of 34% of the surface area for climate properties, 36% for terrain parameters, 34% for soil resources, and 45% when all aforementioned environmental variables were summarized into a principal component analysis. Furthermore, there was a 48% potential representativeness for GPP and 34% for ET. Unfortunately, data from these 41 sites is not all readily available for the scientific community, limiting synthesis studies and model benchmarking/parametrization. We discussed the need to enhance interoperability, promote the participation of active/inactive sites to share information with local, regional and international networks, and promote monitoring efforts across this region of the world to increase the accuracy of regional-to-global data-driven products.

Coastal salt marshes store large amounts of carbon but the magnitude and patterns of greenhouse gas (GHG; i.e., carbon dioxide (CO) and methane (CH)) fluxes are unclear. Information about GHG fluxes from these ecosystems comes from studies of sediments or at the ecosystem-scale (eddy covariance) but fluxes from tidal creeks are unknown. We measured GHG concentrations in water, water quality, meteorology, sediment CO efflux, ecosystem-scale GHG fluxes, and plant phenology; all at half-hour time-steps over one year. Manual creek GHG flux measurements were used to calculate gas transfer velocity () and parameterize a model of water-to-atmosphere GHG fluxes. The creek was a source of GHGs to the atmosphere where tidal patterns controlled diel variability. Dissolved oxygen and wind speed were negatively correlated with creek CH efflux. Despite lacking a seasonal pattern, creek CO efflux was correlated with drivers such as turbidity across phenological phases. Overall, night-time creek CO efflux (3.6 ± 0.63 µmol/m/s) was over two times higher than night-time marsh sediment CO efflux (1.5 ± 1.23 µmol/m/s). Creek CH efflux (17.5 ± 6.9 nmol/m/s) was four times lower than ecosystem-scale CH fluxes (68.1 ± 52.3 nmol/m/s) across the year. These results suggest that tidal creeks are potential hotspots for CO emissions and could contribute to lateral transport of CH to the coastal ocean due to supersaturation of CH (>6000 µmol/mol) in water This study provides insights for modelling GHG efflux from tidal creeks and suggests that changes in tide stage overshadow water temperature in determining magnitudes of fluxes.

Forests are under major pressures due to contemporary land-use, which creates mosaics of stand-stage development that follow different successional paths, that imply ecosystem complexity. The interplay of carbon and water dynamics across succession involves physical and biological interactions that shape net ecosystem production (NEP) and water use efficiency. Here we present 13 years-site of eddy covariance data (2016-2020) from a seasonally dry tropical forest in Northwestern Mexico to elucidate the environmental controls on ecosystem fluxes and explore the interactions with changes in resource availability. Across a successional gradient, an early (9 years since abandonment) and a mid-successional (about 45 years with natural recruitment and regrowth) sites were net carbon sinks (in the order of 100 to 500 g C m-2 y-1) while an old- growth forest was a chronic net source over the 5 years studied (losing between 100 and 300 g C m-2 y-1). In contrast evapotranspiration was alike at sites and close to the precipitation input. Ecosystem water use efficiency tended to be higher at the old-growth forest site (ca. 3.0 g C m-2 /mm H2O vs. ca. 2.0 g C m-2 / mm H2O at the secondary sites). Water availability and radiation where clearly dominant environmental controls across sites, but notably vapor pressure deficit was not a controlling factor for gas exchange at the old-growth forest. Surface characteristics, canopy structure and species composition may explain differences in NEP across succession in TDF at its northernmost extent.