Cold air drainage flows subsidize montane valley ecosystem productivity.

Abstract. The eddy covariance (EC) technique is currently the most widely used method for measuring carbon exchange between terrestrial ecosystems and the atmosphere at the ecosystem scale. Using this technique, a regional carbon flux network comprising a total of 34 sites has been established in the Heihe River basin (HRB) in northwest China. This network has been measuring the net ecosystem exchange (NEE) of CO2 for a variety of vegetation types. In this study, we have compiled and post-processed half-hourly flux data from these 34 EC flux sites in the HRB to create a continuous, homogenized time series dataset. We employ standardized processing procedures to fill data gaps in meteorological and NEE measurements at half-hourly intervals. NEE measurements are also partitioned into gross primary production (GPP) and ecosystem respiration (Reco). Furthermore, half-hourly meteorological and NEE data are aggregated into daily, weekly, monthly, and yearly timescales. As a result, we produced a continuous carbon flux and auxiliary meteorological dataset, which includes 18 sites with continuous multi-year observations during 2008–2022 and 16 sites observed only during the 2012 growing season, amounting to a total of 1513 site months. Evapotranspiration and energy flux measurements are also included. Using the post-processed dataset, we explored the temporal and spatial characteristics of carbon exchange in the HRB. In the diurnal variation curve, GPP, net carbon uptake, and Reco peak later for ecosystems in the artificial oasis (cropland and wetlands) compared to those outside the artificial oasis (grassland, forest, woodland, and Gobi/desert). Seasonal net carbon uptake, GPP, and Reco peak in early July for grassland, forest, woodland, and cropland but remain close to zero throughout the year for Gobi/desert. In the last decade, net carbon uptake of wetlands has significantly increased, while NEE for other ecosystems has not exhibited significant trends. Annual net carbon uptake, GPP, and Reco are significantly higher for sites inside the artificial/natural oasis compared to those outside the oasis. This post-processed carbon flux dataset has numerous applications, including exploring the carbon exchange characteristics of alpine and arid ecosystems, analyzing ecosystem responses to climate extremes, conducting cross-site synthesis from regional to global scales, supporting regional and global upscaling studies, interpreting and calibrating remote sensing products, and evaluating and calibrating carbon cycle models. The dataset can be accessed at https://doi.org/10.11888/Terre.tpdc.301321 (Wang et al., 2024).

A combination of theory and experiments predicts that increasing soil nutrients will modify herbivore and microbial impacts on ecosystem carbon cycling. However, few studies of herbivores and soil nutrients have measured both ecosystem carbon fluxes and carbon pools. Even more rare are studies manipulating microbes and nutrients that look at ecosystem carbon cycling responses. We added nutrients to a long‐term, experiment manipulating foliar fungi, soil fungi, mammalian herbivores and arthropods in a low fertility grassland. We measured gross primary production (GPP), ecosystem respiration (ER), net ecosystem exchange (NEE) and plant biomass throughout the growing season to determine how nutrients modify consumer impacts on ecosystem carbon cycling. Nutrient addition increased above‐ground biomass and GPP, but not ER, resulting in an increase in ecosystem carbon uptake rate. Reducing foliar fungi and arthropods increased plant biomass. Nutrients amplified consumer effects on plant biomass, such that arthropods and foliar fungi had a threefold larger impact on above‐ground biomass in fertilized plots. Synthesis . Our work demonstrates that throughout the growing season soil resources modify carbon uptake rates as well as animal and fungal impacts on plant biomass production. Taken together, ongoing nutrient pollution may increase ecosystem carbon uptake and drive fungi and herbivores to have larger impacts on plant biomass production.

Some effects of corn and oak forest canopies on cold air drainage

Agriculture can improve the global carbon budget by reducing emissions and increasing soil carbon sequestration. Woody crops like vineyards present biological, structural, and management peculiarities, such as perennial structure, abundant pruning debris, and interrow cover cropping, which could potentially sequester a significant amount of CO2. However, such ecosystems are still poorly understood. Here, we present results from a long-term study from northeastern Italy on the carbon budget of a vineyard (cv. Sauvignon blanc, Guyot pruned, vertical shoot-positioned trellised, with grass-covered alleys). We used the eddy covariance technique to continuously monitor the net ecosystem CO2 exchange (NEE), and here we present the first three years of data (May 2014 to April 2017). The vineyard acted as a moderate carbon sink, with an average annual net carbon uptake of 134 g C/ m2. However, the inter-annual variability of NEE was high, and environmental conditions during the growing season had a strong impact. The annual carbon uptake was greatest in 2014 (207 g C/m2), when there was abundant rain in summer. In contrast, in 2015, several heat waves decreased the annual carbon uptake to 69 g C/m2. Elevated air temperature and low soil moisture in summer significantly reduced net carbon flux during times when the vineyard activity was at maximum. Nevertheless, we show that a vineyard can behave as a carbon sink in the medium-to-long term, which should be factored into agricultural and environmental policies, complementing the standard life cycle assessment of the wine production chain.

<p>Reversing the European trend toward a declining land carbon sink has become the new mantra. <span>The decision to increase net carbon uptake in the forest, however, is made without adequately considering the consequences for the circular bioeconomy and climate change mitigation more generally. LULUCF (Land Use, Land Use Change and Forestry) accounting fails to balance net carbon uptake with the positive substitution effects from forest use recorded in the energy sector. The singular focus on the net forest carbon sink distracts attention from the fact that, over the past century, many EU Member states have managed to harvest ever larger amounts of forest, and, simultaneously, to continuously increase both forest carbon stocks, as well as the size of the annual forest increment. In the language of the European Commission’s proposed LULUCF policy framework, however, high harvesting rates and high forest use intensity are seen as anathema to climate change mitigation because they are thought to threaten the reliability of the EU UNFCCC emission reduction commitment framework, as well as the longevity of Europe’s forests, more generally. This, at least, is the unabashed message of one recent Joint Research Centre (JRC) assessment of EU forestry, which argues that </span>rapid increases in harvest rates challenge the environmental integrity of EU emission-reduction commitments. According to the authors, ‘the loss of carbon in standing forests will require additional efforts in other sectors to reach the EU climate neutrality target by 2050’.<sup>1</sup> We explore the parameters of this assessment, highlighting the fact that preferences for more protected forests obscure our understanding of the climate benefits forestry and forest resource use can provide. We argue the continuous net annual contributions to the global carbon budget provided by the circular bioeconomy (avoided emissions, net removals and renewed forest growth) should be more carefully considered. Increasing net forest carbon uptake by reducing forest use intensity (i.e., annual harvest) is not the only, nor is it even the most beneficial, pathway to increasing LULUCF-based climate change mitigation potential. Policymakers should pay greater attention both to the mix of micro-level mechanisms and incentives created by the EU LULUCF policy framework and the mix of public and private sector interests and investment goals intended to undergird future forest growth and LULUCF-based mitigation potential. As we demonstrate, promoting reforestation efforts beyond Managed Forest Lands (MFL) has contributed only minimally to the global carbon budget and has failed to garner the kind of investment momentum required to make it a meaningful mitigation solution. MFL, on the other hand, have witnessed far greater contributions to the global carbon budget but are generally not fully mobilized in the EU climate policy framework. Striking an appropriate balance between the many climate change mitigation, human livelihood and consumption benefits forests and forest-based resources can provide, on the one hand, and protected, biodiverse-rich forest environments, on the other, requires a more balanced approach.</p>

It has been well established by field experiments that warming stimulates either net ecosystem carbon uptake or release, leading to negative or positive carbon cycle-climate change feedback, respectively. This variation in carbon-climate feedback has been partially attributed to water availability. However, it remains unclear under what conditions water availability enhances or weakens carbon-climate feedback or even changes its direction. Combining a field experiment with a global synthesis, we show that warming stimulates net carbon uptake (negative feedback) under wet conditions, but depresses it (positive feedback) under very dry conditions. This switch in carbon-climate feedback direction arises mainly from scaling effects of warming-induced decreases in soil water content on net ecosystem productivity. This water scaling of warming effects offers generalizable mechanisms not only to help explain varying magnitudes and directions of observed carbon-climate feedback but also to improve model prediction of ecosystem carbon dynamics in response to climate change.

Climatic warming influences ecosystem-scale carbon fluxes directly via effects on photosynthesis and respiration, and indirectly via effects on the plant community. Here, we report on a 10-year factorial warming and dominant plant species removal experiment established in both a high- and in a low-elevation montane meadow to explore how dominant plants modify the effect of warming on the carbon cycle across space and over time. At low-elevation, warming increased peak-season net carbon uptake in most years, driven by higher primary productivity, but only in plots where the dominant was left intact. Here, net carbon uptake tended to be positive, but was more likely to be negative when the dominant plant was removed, and in dry years. Surprisingly, the high-elevation site was unaffected by the warming and plant removal treatments, suggesting these sites are resistant to these disturbances. Taken together, these results demonstrate that dominant plant species can modify the impacts of warming on carbon fluxes, but show how this influence can vary spatially and temporally. These findings provide insight into when and how abiotic and biotic factors influence ecosystem carbon source/sink dynamics.

Nitrogen (N) deposition has been well documented to cause substantial impacts on ecosystem carbon cycling. However, the majority studies of stimulating N deposition by direct N addition to forest floor have neglected some key ecological processes in forest canopy (e.g., N retention and absorption) and might not fully represent realistic atmospheric N deposition and its effects on ecosystem carbon cycling. In this study, we stimulated both canopy and understory N deposition (50 and 100 kg N ha−1 year−1) with a local atmospheric NHx:NOy ratio of 2.08:1, aiming to assess whether canopy and understory N deposition had similar effects on soil respiration (RS) and net ecosystem production (NEP) in Moso bamboo forests. Results showed that RS, soil autotrophic (RA), and heterotrophic respiration (RH) were 2971 ± 597, 1472 ± 579, and 1499 ± 56 g CO2 m−2 year−1 for sites without N deposition (CN0), respectively. Canopy and understory N deposition did not significantly affect RS, RA, and RH, and the effects of canopy and understory N deposition on these soil fluxes were similar. NEP was 1940 ± 826 g CO2 m−2 year−1 for CN0, which was a carbon sink, indicating that Moso bamboo forest the potential to play an important role alleviating global climate change. Meanwhile, the effects of canopy and understory N deposition on NEP were similar. These findings did not support the previous predictions postulating that understory N deposition would overestimate the effects of N deposition on carbon cycling. However, due to the limitation of short duration of N deposition, an increase in the duration of N deposition manipulation is urgent and essential to enhance our understanding of the role of canopy processes in ecosystem carbon fluxes in the future.

During the growing season, nighttime ecosystem respiration emits 30–100% of the daytime net photosynthetic uptake of carbon, and therefore measurements of rates and understanding of its control by the environment are important for understanding net ecosystem exchange. Ecosystem respiration can be measured at night by eddy covariance methods, but the data may not be reliable because of low turbulence or other methodological problems. We used relationships between woody tissue, foliage, and soil respiration rates and temperature, with temperature records collected on site to estimate ecosystem respiration rates at six coniferous BOREAS sites at half‐hour or 1‐hour intervals, and then compared these estimates to nocturnal measurements of CO2 exchange by eddy covariance. Soil surface respiration was the largest source of CO2 at all sites (48–71%), and foliar respiration made a large contribution to ecosystem respiration at all sites (25–43%). Woody tissue respiration contributed only 5–15% to ecosystem respiration. We estimated error for the scaled chamber predictions of ecosystem respiration by using the uncertainty associated with each respiration parameter and respiring biomass value. There was substantial uncertainty in estimates of foliar and soil respiration because of the spatial variability of specific respiration rates. In addition, more attention needs to be paid to estimating foliar respiration during the early part of the growing season, when new foliage is growing, and to determining seasonal trends of soil surface respiration. Nocturnal eddy covariance measurements were poorly correlated to scaled chamber estimates of ecosystem respiration (r2=0.06–0.27) and were consistently lower than scaled chamber predictions (by 27% on average for the six sites). The bias in eddy covariance estimates of ecosystem respiration will alter estimates of gross assimilation in the light and of net ecosystem exchange rates over extended periods.

The Amazon rainforest stores ~150–200 Pg of carbon and plays a central role in the global carbon cycle, yet it is highly vulnerable to land-use change, fire, and climate change, leaving its future carbon balance uncertain. Net Ecosystem Exchange (NEE) quantifies the balance between ecosystem carbon uptake and release, but long-term NEE assessments in Central Amazonia remain scarce due to limited observational coverage from eddy covariance towers and CO₂ profile measurements. The Amazon Tall Tower Observatory (ATTO; https://www.attoproject.org) helps overcome this limitation by providing more than a decade of continuous measurements and by capturing two extreme drought events (2015/2016 and 2023/2024). In this work, we analyzed 11 years of NEE estimates at ATTO (2014-2024) and we propose a methodology for selecting the friction velocity threshold (u*), based on the identification of a plateau in the NEE-u* relationship, where NEE becomes independent of increasing turbulence, and all periods with u below the threshold are filtered due to insufficient turbulence*. We estimate monthly u* thresholds ranging from 0.18 to 0.21 m s⁻¹. We found that, on average, the forest in the ATTO flux footprint generally acted as a net carbon sink. However, in years following severe droughts, such as 2016 and 2024, we detect a temporary reversal, with the ecosystem becoming a CO₂ source during the wet season. We also quantified the effect of environmental drivers modulating NEE across seasons. We find that higher air temperature reduces carbon uptake during the wet season. In contrast, soil moisture shows opposite relationships depending on season: during the dry season, increasing soil moisture (10 cm depth) reduces net carbon uptake, whereas during the wet season, increasing it enhances net carbon uptake. Our findings deliver critical observational evidence to refine model parameterizations of tropical carbon-water interactions and to reduce uncertainty in predictions of the Amazon carbon balance under future climate scenarios.

Warming during late winter and spring in recent decades has been credited with increasing high northern latitude CO 2 uptake, but it is unclear how different species and plant functional types contribute to this response. To address this, we measured net ecosystem exchange (NEE) at a deciduous broadleaf (aspen and willow) forest and an evergreen conifer (black spruce) forest in interior Alaska over a 3-year period. We partitioned NEE into gross primary production (GPP) and ecosystem respiration ( R e) components, assessing the impact of interannual climate variability on these fluxes during spring and summer. We found that interannual variability in both spring and summer NEE was greatest at the deciduous forest. Increases in spring air temperatures between 2002 and 2004 caused GPP to increase during the early part of the growing season (April, May, and June), with a 74% increase at the deciduous forest and a 16% increase at the evergreen forest. R e increased in parallel, by 61% and 15%, respectively. In contrast, a summer drought during 2004 caused GPP during August to decrease by 12% at the deciduous forest and by 9% at the evergreen forest. Concurrent increases in R e, by 21% and 2% for the two forests, further contributed to a reduction in net carbon uptake during the drought. Over the growing season (April–September) net carbon uptake increased by 40% at the deciduous forest and 3% at the evergreen forest in 2004 as compared with 2002. These results suggest that deciduous forests may contribute disproportionately to variability in atmospheric CO 2 concentrations within the northern hemisphere and that the carbon balance of deciduous forests may have a greater sensitivity to future changes in climate.

According to contemporary research, mass greening of the northern latitudes is likely to take place as global temperatures continue to rise. While this could support an overall increase in autotrophic uptake of CO2, rising temperatures could also expose a higher CO2 emission potential as reparation rates respond to our changing climate. Thus, the future carbon balance in high-latitude ecosystems remains uncertain. Utilizing high-frequency measurements of ecosystem-level carbon exchange in these regions could unearth a valuable understanding of just how rising temperatures will affect the soil-plant continuum under varying future climate scenarios.Over the course of a two year study period, we measured in-situ carbon exchanges using four ECO2flux automated chambers at one of the geothermal grasslands sites within the FutureArctic network. The chambers were placed at different locations along a soil temperature gradient with treatments covering an average of 0, 2.5, 8.5, and 15.0 degree warming. The major aim was to investigate the underlying carbon exchange processes in order to garner better insight into how future climate change induced temperature increases could affect comparable ecosystems under long-term warming. Following a detailed analysis of carbon uptake (gross primary production, GPP) and carbon release (ecosystem respiration, RE) along the temperature gradient would likely expose a positive net plant carbon uptake with increasing temperature as a direct response to the greening effect while respiration could remain lower than GPP, follow a similar trajectory, or offset this increase in uptake entirely depending on length of exposure to soil warming.Preliminary analysis from a subset within the two-year study period was conducted. The fluxes of CO2 showed evident heterogeneity between our four treatments with increasing totals of GPP moving up the temperature gradient. However, during this period, GPP was highest in the treatment with warming around 8.5 degrees above ambient, which suggests that there is likely a temperature threshold for increased uptake with greening between 8.5 to 15 degrees soil warming for this ecosystem. The observed temperature response appears non-linear where both GPP and RE start to decline after reaching this temperature threshold. Knowledge of these non-linear temperature responses for GPP and RE will be of great importance when trying to predict future changes to the carbon balance in Arctic and Sub-Arctic ecosystems.

An over-arching goal of the DOE TCP program is to understand the mechanistic controls over the fate, transport, and residence time of carbon in the terrestrial biosphere. Many of the modern process and modeling studies focus on seasonal to interannual variability. However, much of the carbon on the landscape and in soils is in separate reservoirs with turnover times that are multi-decadal to millennial. It is the controls on these longer term pools or reservoirs that is a critical unknown in the face of rising GHGs and climate change and uncertainties of the terrestrial biosphere as a future global sink or source of atmospheric CO{sub 2} [eg., Friedlingstein et al., 2006; Govindasamy et al., 2005; Thompson et al., 2004]. Radiocarbon measurements, in combination with other data, can provide insight into, and constraints on, terrestrial carbon cycling. Radiocarbon (t{sub 1/2} 5730yrs) is produced naturally in the stratosphere when secondary neutrons generated by cosmic rays collide with {sup 14}N atoms [Libby 1946; Arnold and Libby, 1949]. Upon formation, {sup 14}C is rapidly oxidized to CO and then to CO{sub 2}, and is incorporated into the carbon cycle. Due to anthropogenic activities, the amount of {sup 14}C in the atmosphere doubled in the mid/late 1950s and early 1960s from its preindustrial value of {sup 14}C/{sup 12}C ratio of 1.18 x 10{sup -12} [eg., Nydal and Lovseth, 1983]. Following the atmospheric weapons test ban in 1963, the {sup 14}C/{sup 12}C ratio, has decreased due to the net isotopic exchange between the ocean and terrestrial biosphere [eg., Levin and Hessheimer, 2000] and a dilution effect due to the burning of {sup 14}C-free fossil fuel carbon, the 'Suess Effect' [Suess, 1955]. In the carbon cycle literature, radiocarbon measurements are generally reported as {Delta}{sup 14}C, which includes a correction for mass dependent fractionation [Stuiver and Polach, 1977]. In the context of carbon cycle studies radiocarbon measurements can be used to determine the 'age' and rate of change of carbon stocks or as a biogeochemical tracer to elucidate processes and pathways. It is this dual nature that can be exploited across scales in space (individual plant, plot or research site, ecosystem, regional, and global) and time (days to millennia). For example, across regional scales, {Delta}{sup 14}C measurements of atmosphere CO{sub 2} can be used to attribute carbon dioxide to sources (e.g., respiration vs. fossil fuel emissions) or sinks ( e.g,. photosynthesis), which cannot be readily inferred from concentration, net flux measurements, or {delta}{sup 13}CO{sub 2} [eg. Graven et al., 2009; Levin and Hessheimer, 2000; Turnbull et al., 2007]. At smaller scales, similar analyses can be used to elucidate the source, and 'age' of the below ground component undergoing heterotrophic respiration. Net (biome or ecosystem) uptake of carbon is the difference of two large fluxes: photosynthesis and respiration. Carbon fixation by photosynthesis is, to a large extent, a single process with theoretical underpinnings. On the other-hand, net ecosystem or biome respiration integrates microbial (heterotrophic) and plant (autotrophic) respiration. Eddy covariance methods can be used to estimate bulk CO{sub 2} fluxes but they cannot discriminate the process nor the source of the respired CO{sub 2}. It is these processes that are parameterized in predictive models and contribute to the uncertainty in the climate forcing effect of the carbon cycle in the future [Friedlingstein et al., 2006; Heimann and Reichstein, 2008].

Using root form to improve our understanding of root function.

Landscape trees sequester carbon during their growth processes, but they emit carbon through production in nurseries, which may offset carbon uptake. This study quantified the carbon footprint of landscape tree production. After determining the scope of life cycle for landscape tree production, the energy and material used to produce trees of a target size were analyzed by conducting a field survey of 35 nurseries. This energy consumption and input material were converted to an estimate of carbon emitted using data on carbon emission coefficients. The net carbon uptake was 4.6, 12.2, and 24.3 kg/tree for trees with a DBH of 7, 10, and 13 cm, respectively. Thus, even though carbon is emitted during the production process, landscape trees can act as a source of carbon uptake in cities that have high energy consumption levels. This study broke new ground for quantifying the carbon footprint of landscape tree production by overcoming limitations of the past studies that only considered carbon uptake due to absence of data on energy consumption and difficulty of field survey. These study results are expected to provide information on the carbon footprint of landscape trees and to be useful in determining optimal greenhouse gas emissions reduction goal through urban greenspaces.