Housen Chu

Net ecosystem methane and carbon dioxide exchanges in a Lake Erie coastal marsh and a nearby cropland

Net ecosystem carbon dioxide (FCO2) and methane (FCH4) exchanges were measured by using the eddy covariance method to quantify the atmospheric carbon budget at a Typha- and Nymphaea-dominated freshwater marsh (March 2011 to March 2013) and a soybean cropland (May 2011 to May 2012) in northwestern Ohio, USA. Two year average annual FCH4 (49.7 g C-CH4 m−2 yr−1) from the marsh was high and compatible with its net annual CO2 uptake (FCO2: −21.0 g C-CO2 m−2 yr−1). In contrast, FCH4 was small (2.3 g C-CH4 m−2 yr−1) and accounted for a minor portion of the atmospheric carbon budget (FCO2: −151.8 g C-CO2 m−2 yr−1) at the cropland. At the seasonal scale, soil temperature associated with methane (CH4) production provided the dominant regulator of FCH4 at the marsh (R2 = 0.86). At the diurnal scale, plant-modulated gas flow was the major pathway for CH4 outgassing in the growing season at the marsh. Diffusion and ebullition became the major pathways in the nongrowing season and were regulated by friction velocity. Our findings highlight the importance of freshwater marshes for their efficiency in turning over and releasing newly fixed carbon as CH4. Despite marshes accounting for only ~4% of area in the agriculture-dominated landscape, their high FCH4 should be carefully addressed in the regional carbon budget.

Evapotranspiration of annual and perennial biofuel crops in a variable climate

Eddy covariance measurements were made in seven fields in the Midwest USA over 4 years (including the 2012 drought year) to estimate evapotranspiration (ET) of newly established rain‐fed cellulosic and grain biofuel crops. Four of the converted fields had been managed as grasslands under the USDAs Conservation Reserve Program (CRP) for 22 years, and three had been in conventional agriculture (AGR) soybean/corn rotation prior to conversion. In 2009, all sites were planted to no‐till soybean except one CRP grassland that was left unchanged as a reference site; in 2010, three of the former CRP sites and the three former AGR sites were planted to annual (corn) and perennial (switchgrass and mixed‐prairie) grasslands. The annual ET over the 4 years ranged from 45% to 77% (mean = 60%) of the annual precipitation (848–1063 mm; November–October), with the unconverted CRP grassland having the highest ET (622–706 mm). In the fields converted to annual and perennial crops, the annual ET ranged between 480 and 639 mm despite the large variations in growing‐season precipitation and in soil water contents, which had strong effects on regional crop yields. Results suggest that in this humid temperate climate, which represents the US Corn Belt, water use by annual and perennial crops is not greatly different across years with highly variable precipitation and soil water availability. Therefore, large‐scale conversion of row crops to perennial biofuel cropping systems may not strongly alter terrestrial water balances.

Fluxes all of the time? A primer on the temporal representativeness of FLUXNET

FLUXNET, the global network of eddy covariance flux towers, provides the largest synthesized data set of CO2, H2O, and energy fluxes. To achieve the ultimate goal of providing flux information “everywhere and all of the time,” studies have attempted to address the representativeness issue, i.e., whether measurements taken in a set of given locations and measurement periods can be extrapolated to a space- and time-explicit extent (e.g., terrestrial globe, 1982–2013 climatological baseline). This study focuses on the temporal representativeness of FLUXNET and tests whether site-specific measurement periods are sufficient to capture the natural variability of climatological and biological conditions. FLUXNET is unevenly representative across sites in terms of the measurement lengths and potentials of extrapolation in time. Similarity of driver conditions among years generally enables the extrapolation of flux information beyond measurement periods. Yet such extrapolation potentials are further constrained by site-specific variability of driver conditions. Several driver variables such as air temperature, diurnal temperature range, potential evapotranspiration, and normalized difference vegetation index had detectable trends and/or breakpoints within the baseline period, and flux measurements generally covered similar and biased conditions in those drivers. About 38% and 60% of FLUXNET sites adequately sampled the mean conditions and interannual variability of all driver conditions, respectively. For long-record sites (≥15 years) the percentages increased to 59% and 69%, respectively. However, the justification of temporal representativeness should not rely solely on the lengths of measurements. Whenever possible, site-specific consideration (e.g., trend, breakpoint, and interannual variability in drivers) should be taken into account.

A New Data Set to Keep a Sharper Eye on Land-Air Exchanges

Author(s): Pastorello, GZ; Papale, D; Chu, H; Trotta, C; Agarwal, DA; Canfora, E; Baldocchi, DD; Torn, MS | Abstract: FLUXNET15, the latest update of the longest global record of ecosystem carbon, water, and energy fluxes, features improved data quality, new data products, and more open data sharing policies.

The effects of grazing and watering on ecosystem CO2 fluxes vary by community phenology

Grazing profoundly influences vegetation and the subsequent carbon fluxes in various ecosystems. However, little effort has been made to explore the underlying mechanisms for phenological changes and their consequences on carbon fluxes at ecosystem level, especially under the coupled influences of human disturbances and climate change. Here, a manipulative experiment (2012-2013) was conducted to examine both the independent and interactive effects of grazing and watering on carbon fluxes across phenological phases in a desert steppe. Grazing advanced or delayed phenological timing, leading to a shortened green-up phase (GrP: 23.60 days) in 2013 and browning phase (BrP: 12.48 days) in 2012 from high grazing, and insignificant effects on the reproductive phase (ReP) in either year. High grazing significantly enhance carbon uptake, while light grazing reduce carbon uptake in ReP. Watering only delayed the browning time by 5.01 days in 2013, producing no significant effects on any phenophase. Watering promoted the net ecosystem exchange (NEE), ecosystem respiration (ER), and gross ecosystem productivity (GEP) only in the GrP. When calculating the yearly differences in phenophases and the corresponding carbon fluxes, we found that an extended GrP greatly enhanced NEE, but a prolonged ReP distinctly reduced it. The extended GrP also significantly promote GEP. Increases in growing season length appeared promoting ER, regardless of any phenophase. Additionally, the shifts in NEE appeared dependent of the variations in leaf area index (LAI).

The effect of algal blooms on carbon emissions in western lake erie: An integration of remote sensing and eddy covariance measurements

Lakes are important components for regulating carbon cycling within landscapes. Most lakes are regarded as CO2 sources to the atmosphere, except for a few eutrophic ones. Algal blooms are common phenomena in many eutrophic lakes and can cause many environmental stresses, yet their effects on the net exchange of CO2 (FCO2) at large spatial scales have not been adequately addressed. We integrated remote sensing and Eddy Covariance (EC) technologies to investigate the effects that algal blooms have on FCO2 in the western basin of Lake Erie—a large lake infamous for these blooms. Three years of long-term EC data (2012–2014) at two sites were analyzed. We found that at both sites: (1) daily FCO2 significantly correlated with daily temperature, light, and wind speed during the algal bloom periods; (2) monthly FCO2 was negatively correlated with chlorophyll-a concentration; and (3) the year with larger algal blooms was always associated with lower carbon emissions. We concluded that large algal blooms could reduce carbon emissions in the western basin of Lake Erie. However, considering the complexity of processes within large lakes, the weak relationship we found, and the potential uncertainties that remain in our estimations of FCO2 and chlorophyll-a, we argue that additional data and analyses are needed to validate our conclusion and examine the underlying regulatory mechanisms.

Grassland productivity and carbon sequestration in Mongolian grasslands: The underlying mechanisms and nomadic implications

Background: Quantifying carbon (C) dioxide exchanges between ecosystems and the atmosphere and the underlying mechanism of biophysical regulations under similar environmental conditions is critical for an accurate understanding of C budgets and ecosystem functions. Methods: For the first time, a cluster of four eddy covariance towers were set up to answer how C fluxes shift among four dominant ecosystems in Mongolia – meadow steppe (MDW), typical steppe (TPL), dry typical steppe (DRT) and shrubland (SHB) during two growing seasons (2014 and 2015). Results: Large variations were observed for the annual net ecosystem exchange (NEE) from 59 to 193 g C m−2, though all four sites acted as a C source. During the two growing seasons, MDW acted as a C sink, TPL and DRT were C neutral, while SHB acted as a C source. MDW to SHB and TPL conversions resulted in a 2.6‐ and 2.2‐fold increase in C release, respectively, whereas the TPL to SHB conversion resulted in a 1.1‐fold increase at the annual scale. C assimilation was higher at MDW than those at the other three ecosystems due to its greater C assimilation ability and longer C assimilation times during the day and growing period. On the other hand, C release was highest at SHB due to significantly lower photosynthetic production and relatively higher ecosystem respiration (ER). A stepwise multiple regression analysis showed that the seasonal variations in NEE, ER and gross ecosystem production (GEP) were controlled by air temperature at MDW, while they were controlled mainly by soil moisture at TPL, DRT and SHB. When air temperature increased, the NEE at MDW and TPL changed more dramatically than at DRT and SHB, suggesting not only a stronger C release ability but also a higher temperature sensitivity at MDW and TPL. Conclusions: The ongoing and predicted global changes in Mongolia likely impact the C exchange at MDW and TPL more than at DRT and SHB in Mongolia. Our results suggest that, with increasing drought and vegetation type succession, a clear trend for greater CO2 emissions may result in further global warming in the future. This study implies that diverse grassland ecosystems will respond differently to climate change in the future and can be seen as nature‐based solutions (NBS) supporting climate change adaptation and mitigation strategies. HighlightsAll the four grassland sites acted as C source at a yearly scale.Growing season meadow, typical, and shrub acted as C sink, neutral and source.Meadow to shrub conversion would result in a 2.6 times increase in C release.Predicted future drought may cause more CO2 emission.

Field Observation of Lateral Detritus Carbon Flux in a Coastal Wetland

The lateral transport of carbon has been increasingly recognized as an important component of carbon budget in wetlands. We studied a typical coastal salt marsh located at the estuary of the Yangtze River by measuring lateral transfer of macro-detritus carbon within a creek during each month’s largest spring tide period, and simultaneously, we measured the gross primary production (GPP) by the eddy covariance (EC) method. The results showed a bimodal seasonal pattern of net detritus carbon export, with one peak associated with mainly green/fresh carbon materials and the other peak associated with mainly yellow-dark/senescent carbon materials. We also found that the export of green detritus carbon was highly correlated with plant phenology and the height of tides, suggesting influences from both the standing stock of living biomass and the force of tides. GPP measured by the EC technique (GPPEC) and by remote sensing (GPPRS) differed substantially. We found this difference was correlated well with the net export of green macro-detritus. In general, we concluded that the lateral flux is an important component of the carbon budget in the marsh and that to cross validate between GPPEC and GPPRS, it must be included as a calibration term for computing GPPEC.

Hunting Data Rogues at Scale: Data Quality Control for Observational Data in Research Infrastructures

Data quality control is one of the most time consuming activities within Research Infrastructures (RIs), especially when involving observational data and multiple data providers. In this work we report on our ongoing development of data rogues, a scalable approach to manage data quality issues for observational data within RIs. The motivation for this work started with the creation of the FLUXNET2015 dataset, which includes carbon, water, and energy fluxes plus micrometeorological and ancillary data measured in over 200 sites around the world. To create an uniform dataset, including derived data products, extensive work on data quality control was needed. The unpredictable nature of observational data quality issues makes the automation of data quality control inherently difficult. Developed based on this experience, the data rogues methodology allows for increased automation of quality control activities by systematically identifying, cataloging, and documenting implementations of solutions to data issues. We believe this methodology can be extended and applied to others domains and types of data, making the automation of data quality control a more tractable problem.