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The recent development and improvement of commercially available laser-based spectrometers have expanded onsite continuous water vapor (H2O) stable isotope composition (e.g. δ18O, δ17O and δ2H) measurements in a variety of sites across the world in the last decade. However, we still lack continuous observations in the Amazon basin region, a region that significantly influences atmospheric and hydrological cycles on local to global scales.The Amazon Tall Tower Observatory (ATTO) site is located in well-preserved central Amazon upland rainforest. In August 2022, a commercial cavity-ring down (CRDS) analyzer (L2140-i model, Picarro, Inc., USA) was installed to continuously measure water vapor isotope compositions at four levels (79, 38, 24, and 4 m above ground) of the 80 m walk-up tower. Also, deuterium excess (hereinafter called d-excess; d-excess = δ2H–8δ18O) was assessed to trace processes that contribute to diel variation in atmospheric moisture inside and above canopy.During the dry season, d-excess generally decreased during the nighttime, reaching minimum values at 6 am to 8 am local time (LT), followed by an increase to maximum values at 12 pm to 4 pm. The diel d-excess variation indicates that atmospheric entrainment occurred in the early morning and evapotranspiration was a dominant moisture source in the afternoon. Further results will be analyzed and discussed in the presentation.

The ATTO (Amazon Tall Tower Observatory) tower is a 325m tall tower located in the middle of the pristine Amazon rainforest. Since 2022, continuous greenhouse gas concentrations at different heights (4m, 42m, 81m, 150m, 273m, 321m) are monitored by use of a Spectronus FTIR-spectrometer, measuring CO2, CH4, CO, N2O and del13CO2 hourly, compliant to WMO/GAW standards. This unique measurement system is the first set up which measures greenhouse gases continuously until 325m above a tropical rainforest, which fill an important gap in the global continous observation network. The measurements can be used for regional and global modelling, and can be used for biosphere-atmosphere exchange flux estimates. In this presentation, we will show the main observations of the first year of data collection, and will present the typical daily cycles observed for the different gases at different heights.

Methane (CH4) is one of the most important anthropogenic greenhouse gases. Despite its importance, natural sources of methane, such as tropical wetlands and termites, are still not well understood and a large source of uncertainty in the tropical CH4 budget. The Amazon rainforest is a key region for the (global) CH4 budget but, due to its remote location, local CH4 concentration and flux measurements are still rare.Fieldsite ZF2 (60 km NW of Manaus, Brazil) is located in pristine tropical rain forest. At this fieldsite, a Spectronus FTIR-analyzer (measuring CO2, CO, CH4, N2O & δ13CO2) was installed at the foot of the K34 tower, set up to measure different heights above and below the canopy continuously. In addition, by use of a Los Gatos portable analyzer (measuring CO2 & CH4), additional semi-continuous concentration measurements were performed at the valley tower (studying the nighttime build up of valley CH4), above the igarapé (capturing the CH4 ebullition bubbles leaving the water surface), and on the plateau (studying the spatial horizontal heterogeneity of CH4 concentrations within the canopy). Furthermore, the portable analyzer was used for soil, water, termite mound, and termites flux measurements.By combining tower and flux chamber measurements, the role and magnitude of different ecosystem sources could be assessed. We observed that, while soils in the valley are a small source of CH4 (0.1 to 0.2 nmol CH4 m-2 s-1), overall the soils of this ecosystem are expected to be a net CH4 sink (-0.3 to -0.5 nmol m-2 s-1 ). Estimated total ecosystem CH4 flux, based on nighttime concentration analyses of the tower data, indicate that the ecosystem is a net CH4 source (~1 to 2 nmol CH4 m-2 s-1). We propose that the net CH4 emission of the ecosystem is driven by local emitting hotspots, such as the valley stream and standing water, termites and termite mounds (~1 nmol CH4 m-2 s-1), anoxic soil spots and decaying dead wood.

Soil stores over 2500 Pg carbon (C), with the majority of C stored in deep soil layers (> 30 cm). Soil C can be lost to the atmosphere when organic compounds are mineralized to carbon dioxide (CO2, via oxidative decay or respiration) and moved upward through the soil profile (via diffusion). Soil moisture status can influence the balance between respiration and diffusion, thereby altering the soil CO2 concentration and flux. However, it is unclear how wetting and drying influence soil CO2 dynamics in surface vs. deep soil layers. Thus, we irrigated three soil profiles in Mediterranean arable land and continuously monitored soil CO2 concentration at 15, 30, 50, 70 and 90 cm during wetting and drying phases under ambient temperature conditions. We estimated gas diffusivity, CO2 flux, and temperature responses of soil CO2 concentration during the experiment. Decreases in gas diffusivity during the wetting period coincided with increases in soil CO2 concentrations. However, the negative gas diffusivity-soil CO2 concentration relationship did not hold true all the time, implying that CO2 production was the driving factor for the apparent soil CO2 concentration. We observed hysteretic responses of soil CO2 concentration to temperature as soil moisture varied, with deeper soil CO2 concentration being more sensitive to temperature than surface soil CO2 concentration, especially during the drying phase. The movement of CO2 was upward at all depths during the ambient phase, but the direction and the magnitude of CO2 fluxes varied across the depth gradient during the wetting and drying phases. This study highlights that the relative contribution of gas diffusivity vs. CO2 production to soil CO2 concentration changes with wetting and drying, and that the responses of soil CO2 concentration to temperature are dependent on the antecedent environmental conditions. Also, the downward movement of CO2 during the wetting and drying phases suggests that quantifying surface soil CO2 efflux may underestimate dynamic C processes in deeper soils.

Methane (CH4) is one of the most important anthropogenic greenhouse gases. Despite its importance, natural sources of methane, such as tropical wetlands and termites, are still not well understood and a large source of uncertainty in the tropical CH4 budget. The Amazon rainforest is a key region for the (global) CH4 budget but, due to its remote location, continous CH4 concentration and flux measurements are still rare. The 50 m high K34 tower (field site ZF2) is located in a pristine ‘Terra Firme’ tropical forest region 60 km northwest of Manaus (Brazil), and is located next to a waterlogged valley, a possible location for anaerobic CH4 production. In October 2018, in addition to the existing EC CO2 system, an in-situ FTIR-analyzer (measuring CO2, CO, CH4, N2O and δ13CO2) was set up to measure tower profile concentrations, above and below the canopy, continuously. By analyses of vertical and temporal nighttime concentrations patterns, an emission estimate for all gases could be made, and an ecosystem emission of ~1 nmol CH4 m-2 s-1 was estimated. In addition, by use of different types of flux chambers, possible CH4 sinks and sources such as soils, trees, water and termite mounds were measured. By combining tower and flux chamber measurements, the role and magnitude of different ecosystem sources could be assessed. In this presentation, an overview of the measured CH4 forest concentrations and fluxes will be given.

The magnitude of termite methane (CH4) emissions is still an uncertain part of the global CH4 budget and current emission estimates are based on limited field studies. We present in-situ CH4 emission measurements of termite mounds and termite mound sub samples, performed in the Amazon rain forest. Emissions of five termite mounds of the species Neocapritermes brasiliensis were measured by use of a large flux chamber connected to a portable gas analyser, measuring CH4 and CO2. In addition, the emission of mound sub samples was measured, after which termites were counted, so that a termite CH4 and CO2 emission factor could be determined. Mound emissions were found to range between 17.0–34.8 nmol mound−1 s−1 for CH4 and between 1.6–13.5 μmol mound−1 s−1 for CO2. A termite emission factor of 0.32 μmol CH4 gtermite−1 h−1 was found, which is twice as high as the only other reported average value for the Amazon. By combining mound emission measurements with the termite emission factor, colony sizes could be estimated, which were found to range between 50–120 thousand individuals. Estimates were similar to literature values, and we therefore propose that this method can be used as a quick non-intrusive method to estimate termite colony size in the field. The role of termites in the ecosystems CH4 budget was evaluated by use of two approaches. Termite mound emission values were combined with local termite mound density numbers, leading to an estimate of 0.15–0.71 nmol CH4 m−2 s−1 on average emitted by termite mounds. In addition, the termite CH4 emission factor from this study was combined with termite density numbers, resulting in an estimate of termite emitted CH4 of ~1.0 nmol m−2 s−1. Considering the relatively low net CH4 emissions previously measured at this ecosystem, we expect that termites play an important role in the CH4 budget of this Terra Firme ecosystem.

Methane (CH4) is the second most important long-lived anthropogenic atmospheric greenhouse gas. Despite its importance, natural sources of methane, such as tropical wetlands, are still not well understood and a large source of uncertainty to the global CH4 budget. The Amazonian rain forest is estimated to hold 90-120 Pg of carbon, which is approximately 14-27% of the carbon stored in vegetation worldwide. The region is characterized by high precipitation rates and large wetlands, and it has been estimated that the Amazon basin emits 7% of the annual total CH4 emissions. Due to its remote location, micro-meteorological measurements are rare and absent for other gases than CO2.The 50 m high K34 tower (field site ZF2) is located in a pristine tropical forest region 60 km northwest of Manaus (Brazil), and is located next to a waterlogged valley, a possible location for anaerobic CH4 production. In October 2018, in addition to the existing EC CO2 system, a Relaxed Eddy Accumulation (REA) system was set up at this tower, connected to an in-situ FTIR-analyzer. This set up continually measures fluxes and concentration profiles of CO2, CO, CH4, N2O and δ13CO2. In addition, CH4, CO2, and N2O uptake and emission processes were studied by flux chamber measurements in the footprint of the REA tower, focusing on different possible sources (soil, stream, trees and termites). In this presentation, an overview of the measured CH4 and N2O forest concentrations and fluxes will be shown.

Amazon rainforests and soils contain large amounts of carbon, which is under pressure from ongoing climate and land use change in the Amazon basin. It is estimated that methane (CH4), an important greenhouse gas, is largely released from the flooded wetlands of the Amazon, but the trends and balances of CH4 in the Amazon rainforest are not yet well understood. In addition, the change in atmospheric CH4 concentration is strongly associated with a change in carbon monoxide (CO) concentration, often caused by the human-induced combustion of biomass that usually peaks during dry season. Understanding the long-term fluctuations in the fluxes of greenhouse gases in the Amazon rainforest is essential for improving our understanding of the carbon balance of the Amazon rainforest.Since March 2012, we have continuously measured atmospheric CO2/CH4/CO concentrations at five levels (79, 53, 38, 24, and 4 m a.g.l.) using two wavelength-scanned cavity ring-down spectroscopy analyzers (G1301 and G1302, Picarro Inc., USA), which are automatically calibrated on site every day. In addition, we measured the CO2 flux by the eddy covariance method at the same tower. We estimated the CO2/CH4/CO fluxes by combining the vertical profile of the CO2/CH4/CO concentrations with the flux gradient method. Our results generally show no major difference in CO2 flux between the wet and dry seasons except for year 2017, when an elevated CO2 uptake was documented during the dry season despite the lowest precipitation between 2014 and 2018. The CH4 flux showed the largest CH4 emission during the dry season in year 2016. Further results will be analyzed and discussed in the presentation.

The measurement of soil CO 2 fluxes and their isotopic composition is increasingly used to study and distinguish respiration sources in the soil profile. The influence of non-steady state processes on soil respiratory δ 13 CO 2 values, such as diffusive fractionation, remains unclear but is suggested to be strong, especially in arid ecosystems. In this study, an in situ Fourier transform infrared spectrometer was coupled to automated soil flux chambers and installed in an arid grassland in Italy. Hourly flux and concentration measurements of CO 2 and its isotopic composition were taken over bare soil and senescent grass material. CO 2 fluxes were stable at night (∼1 μmol m −2 s −1 ), and showed peak emissions during the day (up to 10 μmol m −2 s −1 ). Respiratory δ 13 CO 2 values showed a diel pattern. Daytime respiratory δ 13 CO 2 values were on average −24.8‰ and nighttime values were on average −26.9‰, but showed significantly more depleted values during nights with strong increase in atmospheric CO 2 . We hypothesize that the observed diel variation in respiratory δ 13 CO 2 values was a consequence of the non-steady state process “diffusive fractionation” induced by the increasing atmospheric CO 2 concentrations at night.

Qualitative biosphere-atmosphere flux measurements are of great importance for future climate predictions. Multiple flux measurement techniques exist, all having its own advantages and disadvantages. We present a method wherein FTIR-spectrometry is connected to a flux gradient system and to flux chambers, which enables us to study different (greenhouse) gases and different ecosystem scales simultaneously. The method was tested in a peatland in Northern Germany (Himmelmoor, Quickborn). The application of the method, detection limits and first results are presented and the application of the method is evaluated. The results showed that the different techniques capture different ecosystem processes, therefore complementing each other. We believe that the developed method could be valuable for field campaigns and can help further improve our understanding of biosphere-atmosphere gas exchange dynamics.