Environmental Control of Inter-annual Variability of Net Ecosystem Exchange in Rice-rice Cropping System

Consecutive five-year long eddy covariance measurements in a lowland tropical rice-rice system were used to investigate the impacts of gross primary productivity (GPP), climate drivers and ecosystem responses (i.e. ecosystem respiration, RE) on the inter-annual variability (IAV) of the net ecosystem exchange (NEE), which is directly related to the agricultural productivity and climate change. The IAV of carbon dioxide fluxes in two crop growing phases i.e. dry and wet season along with fallow period were analysed. The respiratory fluxes build up during the non-growing season were lower by net uptake in growing season. Annual cumulative value of NEE was negative (sink) in both the crop growing season. The variability of climate drivers and changes in the ecosystem responses to drivers revealed a large intra-annual as well as inter-annual variability of net ecosystem fluxes. NEE was found to be strongly correlated with GPP and RE and also with other metrological variables such as photosynthetically active radiation (PAR), precipitation, air temperature and soil temperature. The anomalies of NEE, GPP and RE were observed to be less in 2017 and 2018 which may be due to lower temperature anomalies recorded in these years. Further understanding of biological mechanisms is needed which is involved in the variation of climatological variables to improve our ability to predict future IAV of NEE.

Airborne quantification of net methane and carbon dioxide fluxes from European Arctic wetlands in Summer 2019

Arctic wetlands and surrounding ecosystems are both a significant source of methane (CH 4 ) and a sink of carbon dioxide (CO 2 ) during summer months. However, precise quantification of this regional CH 4 source and CO 2 sink remains poorly characterized. A research flight using the UK Facility for Airborne Atmospheric Measurement was conducted in July 2019 over an area (approx. 78 000 km 2 ) of mixed peatland and forest in northern Sweden and Finland. Area-averaged fluxes of CH 4 and carbon dioxide were calculated using an aircraft mass balance approach. Net CH 4 fluxes normalized to wetland area ranged between 5.93 ± 1.87 mg m −2 h −1 and 4.44 ± 0.64 mg m −2 h −1 (largest to smallest) over the region with a meridional gradient across three discrete areas enclosed by the flight survey. From largest to smallest, net CO 2 sinks ranged between −513 ± 74 mg m −2 h −1 and −284 ± 89 mg m −2 h −1 and result from net uptake of CO 2 by vegetation and soils in the biosphere. A clear gradient of decreasing bulk and area-averaged CH 4 flux was identified from north to south across the study region, correlated with decreasing peat bog land area from north to south identified from CORINE land cover classifications. While N 2 O mole fraction was measured, no discernible gradient was measured over the flight track, but a minimum flux threshold using this mass balance method was calculated. Bulk (total area) CH 4 fluxes determined via mass balance were compared with area-weighted upscaled chamber fluxes from the same study area and were found to agree well within measurement uncertainty. The mass balance CH 4 fluxes were found to be significantly higher than the CH 4 fluxes reported by many land-surface process models compiled as part of the Global Carbon Project. There was high variability in both flux distribution and magnitude between the individual models. This further supports previous studies that suggest that land-surface models are currently ill-equipped to accurately capture carbon fluxes inthe region. This article is part of a discussion meeting issue 'Rising methane: is warming feeding warming? (part 2)'.

Importance of the Webb, Pearman, and Leuning (WPL) correction for the measurement of small CO₂ fluxes

The WPL (Webb, Pearman, and Leuning) correction is fully accepted to correct trace gas fluxes like CO2 for density fluctuations due to water vapour and temperature fluctuations for open-path gas analysers. It is known that this additive correction can be on the order of magnitude of the actual flux. However, this is hardly ever included in the analysis of data quality. An example from the Arctic shows the problems, because the size of the correction is a multiple of the actual flux. As a general result, we examined and tabulated the magnitude of the WPL correction for carbon dioxide flux as a function of sensible and latent heat flux. Furthermore, we propose a parameter to better estimate possible deficits in data quality and recommend integrating the quality flag derived with this parameter into the general study of small carbon dioxide fluxes.

The diurnal cycle of <i>p</i>CO₂ in the coastal region of the Baltic Sea

The direction and magnitude of carbon dioxide fluxes between the atmosphere and the sea are regulated by the gradient in the partial pressure of carbon dioxide (pCO2) across the air–sea interface. Typically, observations of pCO2 at the sea surface are carried out by using research vessels and ships of opportunity, which usually do not resolve the diurnal cycle of pCO2 at a given location. This study evaluates the magnitude and driving processes of the diurnal cycle of pCO2 in a coastal region of the Baltic Sea. We present pCO2 data from July 2018 to June 2019 measured in the vicinity of the island of Utö at the outer edge of the Archipelago Sea, and quantify the relevant physical, biological, and chemical processes controlling pCO2. The highest monthly median of diurnal pCO2 variability (31 µatm) was observed in August and predominantly driven by biological processes. Biological fixation and mineralization of carbon led to sinusoidal diurnal pCO2 variations, with a maximum in the morning and a minimum in the afternoon. Compared with the biological carbon transformations, the impacts of air–sea fluxes and temperature changes on pCO2 were small, with their contributions to the monthly medians of diurnal pCO2 variability being up to 12 and 5 µatm, respectively. During upwelling events, short-term pCO2 variability (up to 500 µatm within a day) largely exceeded the usual diurnal cycle. If the net annual air–sea flux of carbon dioxide at our study site and for the sampled period is calculated based on a data subset that consists of only one regular measurement per day, the bias in the net exchange depends on the sampling time and can amount up to ±12 %. This finding highlights the importance of continuous surface pCO2 measurements at fixed locations for the assessment of the short-term variability of the carbonate system and the correct determination of air–sea CO2 fluxes.

Importance of the WPL correction for the measurement of small CO₂ fluxes

The WPL (Webb, Pearman, and Leuning) correction is fully accepted to correct trace gas fluxes like CO2 for density fluctuations due to water vapor and temperature fluctuations for open-path gas analysers. It is known that this additive correction can be in the order of magnitude of the actual flux. However, this is hardly ever included in the analysis of data quality. An example from the Arctic shows the problems, because the size of the correction is a multiple of the actual flux. As a general result, we examined and tabulated the magnitude of the WPL correction for carbon dioxide flux as a function of sensible and latent heat flux. Furthermore, we propose a parameter to better estimate possible deficits in data quality and recommend integrating the quality flag derived with this parameter into the general study of small carbon dioxide fluxes.