Alfalfa is the most widely grown forage crop worldwide and is thought to be a significant carbon sink due to high productivity, extensive root systems, and nitrogen-fixation. However, these ...conditions may increase nitrous oxide (N
O) emissions thus lowering the climate change mitigation potential. We used a suite of long-term automated instrumentation and satellite imagery to quantify patterns and drivers of greenhouse gas fluxes in a continuous alfalfa agroecosystem in California. We show that this continuous alfalfa system was a large N
O source (624 ± 28 mg N
O m
y
), offsetting the ecosystem carbon (carbon dioxide (CO
) and methane (CH
)) sink by up to 14% annually. Short-term N
O emissions events (i.e., hot moments) accounted for ≤1% of measurements but up to 57% of annual emissions. Seasonal and daily trends in rainfall and irrigation were the primary drivers of hot moments of N
O emissions. Significant coherence between satellite-derived photosynthetic activity and N
O fluxes suggested plant activity was an important driver of background emissions. Combined data show annual N
O emissions can significantly lower the carbon-sink potential of continuous alfalfa agriculture.
•Restored, managed wetlands sequestered carbon by inhibiting ecosystem respiration.•Agricultural land uses on degraded peat soils lost soil carbon to the atmosphere.•Large CH4 emissions result in ...restored wetlands being short-term greenhouse gas sources.•Restoration often resulted in greenhouse gas switchover times of more than a century.
Restoring degraded peat soils presents an attractive, but largely untested, climate change mitigation approach. Drained peat soils used for agriculture can be large greenhouse gas sources. By restoring subsided peat soils to managed, impounded wetlands, significant agricultural emissions are avoided, and soil carbon can be sequestered and protected. Here, we synthesize 36 site-years of continuous carbon dioxide and methane flux data from a mesonetwork of eddy covariance towers in the Sacramento-San Joaquin Delta in California, USA to compute carbon and greenhouse gas budgets for drained agricultural land uses and compare these to restored deltaic wetlands. We found that restored wetlands effectively sequestered carbon and halted soil carbon loss associated with drained agricultural land uses. Depending on the age and disturbance regime of the restored wetland, many land use conversions from agriculture to restored wetland resulted in emission reductions over a 100-year timescale. With a simple model of radiative forcing and atmospheric lifetimes, we showed that restored wetlands do not begin to accrue greenhouse gas benefits until nearly a half century, and become net sinks from the atmosphere after a century. Due to substantial interannual variability and uncertainty about the multi-decadal successional trajectory of managed, restored wetlands, ongoing ecosystem flux measurements are critical for understanding the long-term impacts of wetland restoration for climate change mitigation.
Wetlands are the largest source of methane (CH4) globally, yet our understanding of how process‐level controls scale to ecosystem fluxes remains limited. It is particularly uncertain how variable ...soil properties influence ecosystem CH4 emissions on annual time scales. We measured ecosystem carbon dioxide (CO2) and CH4 fluxes by eddy covariance from two wetlands recently restored on peat and alluvium soils within the Sacramento–San Joaquin Delta of California. Annual CH4 fluxes from the alluvium wetland were significantly lower than the peat site for multiple years following restoration, but these differences were not explained by variation in dominant climate drivers or productivity across wetlands. Soil iron (Fe) concentrations were significantly higher in alluvium soils, and alluvium CH4 fluxes were decoupled from plant processes compared with the peat site, as expected when Fe reduction inhibits CH4 production in the rhizosphere. Soil carbon content and CO2 uptake rates did not vary across wetlands and, thus, could also be ruled out as drivers of initial CH4 flux differences. Differences in wetland CH4 fluxes across soil types were transient; alluvium wetland fluxes were similar to peat wetland fluxes 3 years after restoration. Changing alluvium CH4 emissions with time could not be explained by an empirical model based on dominant CH4 flux biophysical drivers, suggesting that other factors, not measured by our eddy covariance towers, were responsible for these changes. Recently accreted alluvium soils were less acidic and contained more reduced Fe compared with the pre‐restoration parent soils, suggesting that CH4 emissions increased as conditions became more favorable to methanogenesis within wetland sediments. This study suggests that alluvium soil properties, likely Fe content, are capable of inhibiting ecosystem‐scale wetland CH4 flux, but these effects appear to be transient without continued input of alluvium to wetland sediments.
Legacy soil properties may influence current greenhouse gas fluxes from restored wetlands, but soil interactions with ecosystem‐scale methane emissions are often uncertain. Using an eddy covariance tower network, we observed substantially reduced methane emissions from wetlands restored on high iron, alluvium soils compared with nearby sites restored on peat soils; however, these differences disappeared multiple years postrestoration as new wetland soils were accreted. Our results demonstrate how soil properties can regulate methane fluxes from restored wetlands, though these effects are short in duration compared with the lifetime of wetland restoration projects.
Salinity gradients across estuaries influence wetland carbon storage, methane (CH₄) biogeochemistry, and plant productivity. Estuarine freshwater wetlands may experience increases in salinity during ...drought; however, the impact of salinization on greenhouse gas (GHG) emissions is uncertain. We measured ecosystem-scale GHG emissions from a wetland experiencing salinization during the 2011–2017 California drought and used information theory analyses to quantify couplings and causal interactions between CH₄ fluxes and two dominant environmental drivers; gross ecosystem photosynthesis and temperature. Machine learning models were then used to estimate salinizationinduced changes in CH₄ fluxes and plant productivity. We observed dynamic CH₄ flux-driver relationships across the salinization disturbance event, where temperature connections strengthened, and productivity connections dampened during salinization. Annual gross ecosystem productivity reduced by 64% during peak salinization, whereas annual CH₄ emissions only reduced by 10%, suggesting that other CH₄ substrate sources compensated for reductions in recent photosynthate. Our results demonstrate the value of applying information theory and machine learning approaches to ecological analyses and suggest that drought-induced salinization may increase GHG emissions from estuarine freshwater wetlands.
Inundated wetlands can potentially sequester substantial amounts of soil carbon (C) over the long-term because of slow decomposition and high primary productivity, particularly in climates with long ...growing seasons. Restoring such wetlands may provide one of several effective negative emission technologies to remove atmospheric CO2 and mitigate climate change. However, there remains considerable uncertainty whether these heterogeneous ecotones are consistent net C sinks and to what degree restoration and management methods affect C sequestration. Since wetland C dynamics are largely driven by climate, it is difficult to draw comparisons across regions. With many restored wetlands having different functional outcomes, we need to better understand the importance of site-specific conditions and how they change over time. We report on 21 site-years of C fluxes using eddy covariance measurements from five restored fresh to brackish wetlands in a Mediterranean climate. The wetlands ranged from 3 to 23 years after restoration and showed that several factors related to restoration methods and site conditions altered the magnitude of C sequestration by affecting vegetation cover and structure. Vegetation established within two years of re-flooding but followed different trajectories depending on design aspects, such as bathymetry-determined water levels, planting methods, and soil nutrients. A minimum of 55% vegetation cover was needed to become a net C sink, which most wetlands achieved once vegetation was established. Established wetlands had a high C sequestration efficiency (i.e. the ratio of net to gross ecosystem productivity) comparable to upland ecosystems but varied between years undergoing boom-bust growth cycles and C uptake strength was susceptible to disturbance events. We highlight the large C sequestration potential of productive inundated marshes, aided by restoration design and management targeted to maximise vegetation extent and minimise disturbance. These findings have important implications for wetland restoration, policy, and management practitioners.
As changes in precipitation dynamics continue to alter the water availability in dryland ecosystems, understanding the feedbacks between the vegetation and the hydrologic cycle and their influence on ...the climate system is critically important. We designed a field campaign to examine the influence of two‐layer soil moisture control on bare and canopy albedo dynamics in a semiarid shrubland ecosystem. We conducted this campaign during 2011 and 2012 within the tower footprint of the Santa Rita Creosote Ameriflux site. Albedo field measurements fell into one of four Cases within a two‐layer soil moisture framework based on permutations of whether the shallow and deep soil layers were wet or dry. Using these Cases, we identified differences in how shallow and deep soil moisture influence canopy and bare albedo. Then, by varying the number of canopy and bare patches within a gridded framework, we explore the influence of vegetation and soil moisture on ecosystem albedo. Our results highlight the importance of deep soil moisture in land surface‐atmosphere interactions through its influence on aboveground vegetation characteristics. For instance, we show how green‐up of the vegetation is triggered by deep soil moisture, and link deep soil moisture to a decrease in canopy albedo. Understanding relationships between vegetation and deep soil moisture will provide important insights into feedbacks between the hydrologic cycle and the climate system.
Key Points
Semiarid ecosystem greenness is triggered by deep soil moisture
Deep soil moisture decreases canopy albedo
Deep soil moisture is important in semiarid land‐atmosphere interactions
•Eddy covariance and surface renewal measure fluxes accurately, yet are costly and complex.•The variance-Bowen ratio (VB) approach is a novel alternative that merits inspection.•VB is a simple, yet ...efficient approach uses low-cost sensors and simple data processing.•VB-based sensible and latent heat flux agree strongly with eddy covariance data.
Eddy covariance has been the gold standard used to quantify on-farm crop water use and validate evapotranspiration and remote sensing models, but it comes with a high cost. Development of surface renewal allowed cheaper implementations, but it still requires complex data processing. Additionally, the thermocouple used in this method is vulnerable to breakage. With the depleting water resources in the western U.S., there needs to be a method that is cost-effective and simple to quantify water use across wide range of crops. Here, we employed the variance-Bowen ratio approach at an alfalfa site to measure sensible and latent heat flux (λE) using 1) a sonic anemometer and 2) an air temperature and relative humidity sensor (T-RH). The variance-based sensible heat flux, using a sonic anemometer and a T-RH sensor, agreed strongly (slope = 0.98, R2 = 0.96, n = 3726) with eddy covariance. The modified-Bowen-based latent heat flux (λEβ) also showed very good results (slope = 0.89, R2 = 0.91, n = 3773) with some uncertainty. We then assessed cases where λEβ performed lesser based on heat advection measurements and remote sensing observations from ECOSTRESS. The problematic λEβ was attributed as a product of the advection of heat and moisture from upwind fields because the assumed equal diffusion of heat and water no longer stands under advection. Overall, our results suggest that the variance-Bowen ratio is a robust, inexpensive, yet user-friendly approach to measure sensible and latent heat flux. Given its reduced cost, this approach also has the potential for measuring horizontal heat advection and validating remote sensing models across many different crop types and ecosystems. Furthermore, the inclusion of advection measurement could be crucial in addressing certain remote sensing models (i.e. DisALEXI and PT-JPL) as they systematically omit any advection components.
Ecohydrological processes in semiarid shrublands and other dryland ecosystems are sensitive to discrete pulses of precipitation. Anticipated changes in the frequency and magnitude of precipitation ...events are expected to impact the spatial and temporal distribution of soil moisture in these drylands, thereby impacting their ecohydrological processes. Recent field studies have shown that in dryland ecosystems, transpiration dynamics and plant productivity are largely a function of deep soil moisture available after large precipitation events, regardless of where the majority of plant roots occur. However, the strength of this relationship and how and why it varies throughout the year remains unclear. We present eddy covariance, soil moisture, and sap flow measurements taken over an 18‐month period in conjunction with an analysis of biweekly precipitation, shallow soil, deep soil, and stem stable water isotope samples from a creosotebush‐dominated shrubland ecosystem at the Santa Rita Experimental Range in southern Arizona. Within the context of a hydrologically defined two‐layer conceptual framework, our results support that transpiration is associated with the availability of deep soil moisture and that the source of this moisture varies seasonally. Therefore, changes in precipitation pulses that alter the timing and magnitude of the availability of deep soil moisture are expected to have major consequences for dryland ecosystems. Our findings offer insights that can improve the representation of drylands within regional and global models of land surface atmosphere exchange and their linkages to the hydrologic cycle.
Key Points
Shallow and deep soil moisture are isotopically distinct at this warm semiarid shrubland
Deep moisture tends to be isotopically similar to precipitation from previous season
Shrubs use deep moisture all year round
•Generated CubeSats derived NIRvP maps at daily, 3 m resolution.•Matching NIRvP maps to flux tower footprint improved the NIRvP-GPP relationship.•The effects of matching the footprints were largest ...for wetland sites.
Mapping canopy photosynthesis in both high spatial and temporal resolution is essential for carbon cycle monitoring in heterogeneous areas. However, well established satellites in sun-synchronous orbits such as Sentinel-2, Landsat and MODIS can only provide either high spatial or high temporal resolution but not both. Recently established CubeSat satellite constellations have created an opportunity to overcome this resolution trade-off. In particular, Planet Fusion allows full utilization of the CubeSat data resolution and coverage while maintaining high radiometric quality. In this study, we used the Planet Fusion surface reflectance product to calculate daily, 3-m resolution, gap-free maps of the near-infrared radiation reflected from vegetation (NIRvP). We then evaluated the performance of these NIRvP maps for estimating canopy photosynthesis by comparing with data from a flux tower network in Sacramento-San Joaquin Delta, California, USA. Overall, NIRvP maps captured temporal variations in canopy photosynthesis of individual sites, despite changes in water extent in the wetlands and frequent mowing in the crop fields. When combining data from all sites, however, we found that robust agreement between NIRvP maps and canopy photosynthesis could only be achieved when matching NIRvP maps to the flux tower footprints. In this case of matched footprints, NIRvP maps showed considerably better performance than in situ NIRvP in estimating canopy photosynthesis both for daily sum and data around the time of satellite overpass (R2 = 0.78 vs. 0.60, for maps vs. in situ for the satellite overpass time case). This difference in performance was mostly due to the higher degree of consistency in slopes of NIRvP-canopy photosynthesis relationships across the study sites for flux tower footprint-matched maps. Our results show the importance of matching satellite observations to the flux tower footprint and demonstrate the potential of CubeSat constellation imagery to monitor canopy photosynthesis remotely at high spatio-temporal resolution.
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A significant challenge in global change research is understanding how vegetation interacts with the environment to influence ecosystem gross primary productivity (GPP) through carbon assimilation. ...One emerging objective is to consistently predict GPP fluctuations worldwide by establishing a robust scaling relationship between GPP measured at flux towers and satellite spectral reflectance data. However, a major hurdle in achieving this goal is the discrepancy in spatial resolution between early satellite measurements and eddy flux measurements. By using a large set of growing season data covering 100 site-years in North and Central America, we explored the potential of transforming incident and reflected shortwave (Rg) and photosynthetically active radiation (PAR) measurements into a broadband normalized difference vegetation index (NDVI) and near-infrared (NIR) reflectance of vegetation (NIRv) which simultaneously explains the GPP variability. We found that the broadband NDVI and NIRv derived from Rg and PAR measurements at the daily time scale were highly correlated with Planet Fusion, Landsat-8/9, and Sentinel-2 narrowband NDVI and NIRv across a wide range of climate and ecological gradients. The differences between satellite and broadband NDVI and NIRv were found to be significantly associated with soil background variations, phenological stages, water stress and signal saturation of broadband NIR reflectance at high biomass. The seasonal variability of broadband NDVI and NIRv remarkably captured the seasonality of vegetation phenology, evaporative fraction, GPP and rainfall in different ecosystems. Although saturation of GPP at high NDVI was evident, a linear relationship between broadband NIRv times incident PAR versus GPP indicated the effectiveness of NIRv-based approach to capture the hidden light use efficiency impacts on GPP. Our study concludes that inexpensive measurement of Rg and PAR components can provide reliable information on NDVI, NIRv, and GPP uninterruptedly. This enhances the sensing capability of flux tower sites without requiring additional spectrometer measurements. The proposed in-situ vegetation indices make a compelling case on using radiation signals for handshaking between ecosystem-scale measurements and remote sensing observables relevant to carbon uptake.
•Shortwave radiation and PAR measurements transformed into a broadband NDVI and NIRv.•Broadband NDVI highly correlated with NDVI from Planet, Sentinel-2, Landsat8/9.•PAR*NIRv linearly explained GPP variability across ecosystems of varying water stress.•Broadband NDVI from proximal sensing is a proxy of satellite NDVI at flux tower sites.
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