Forest Flux Research Articles

Unveiling dark matter free streaming at the smallest scales with the high redshift Lyman-alpha forest

This study introduces novel constraints on the free streaming of thermal relic warm dark matter (WDM) from Lyman-α forest flux power spectra. Our analysis utilizes a high resolution, high redshift sample of quasar spectra observed using the HIRES and UVES spectrographs (z=4.2–5.0). We employ a Bayesian inference framework and a simulation-based likelihood that encompasses various parameters including the free streaming of dark matter, cosmological parameters, the thermal history of the intergalactic medium, and inhomogeneous reionization to establish lower limits on the mass of a thermal relic WDM particle of 5.7 keV (at 95% CL). This result surpasses previous limits from the Lyman-α forest through reduction of the measured uncertainties due to a larger statistical sample and by measuring clustering to smaller scales (kmax=0.2 km−1 s). The approximately two-fold improvement due to the expanded statistical sample suggests that the effectiveness of Lyman-α forest constraints on WDM models at high redshifts are limited by the availability of high quality quasar spectra. Restricting the analysis to comparable scales and thermal history priors as in prior studies (kmax<0.1 km−1 s) lowers the bound on the WDM mass to 4.1 keV. As the precision of the measurements increases, it becomes crucial to examine the instrumental and modeling systematics. On the modeling front, we argue that the impact of the thermal history uncertainty on the WDM particle mass constraint has diminished due to improved independent observations. At the smallest scales, the primary source of modeling systematic arises from the structure in the peculiar velocity of the intergalactic medium and inhomogeneous reionization. Published by the American Physical Society 2024

Substantial contribution of tree canopy nitrifiers to nitrogen fluxes in European forests

Human activities have greatly increased the reactive nitrogen in the biosphere, thus profoundly altering global nitrogen cycling. The large increase in nitrogen deposition over the past few decades has led to eutrophication in natural ecosystems, with negative effects on forest health and biodiversity. Recent studies, however, have reported oligotrophication in forest ecosystems, constraining their capacity as carbon sinks. Here we demonstrate the widespread biological transformation of atmospheric reactive nitrogen in the canopies of European forests by combining nitrogen deposition quantification with measurements of the stable isotopes in nitrate and molecular analyses across ten forests through August–October 2016. We estimate that up to 80% of the nitrate reaching the soil via throughfall was derived from canopy nitrification, equivalent to a flux of up to 5.76 kg N ha−1 yr−1. We also document the presence of autotrophic nitrifiers on foliar surfaces throughout European forests. Canopy nitrification thus consumes deposited ammonium and increases nitrate inputs to the soil. The results of this study highlight widespread canopy nitrification in European forests and its important contribution to forest nitrogen cycling.

Phosphorus limitation on CO2 fertilization effect in tropical forests informed by a coupled biogeochemical model

Tropical forests store more than half of the world's terrestrial carbon (C) pool and account for one-third of global net primary productivity (NPP). Many terrestrial biosphere models (TBMs) estimate increased productivity in tropical forests throughout the 21st century due to CO2 fertilization. However, phosphorus (P) limitations on vegetation photosynthesis and productivity could significantly reduce the CO2 fertilization effect. Here, we used a carbon-nitrogen-phosphorus coupled model (Dynamic Land Ecosystem Model; DLEM-CNP) with heterogeneous maximum carboxylation rates to examine how P limitation has affected C fluxes in tropical forests during 1860–2018. Our model results showed that the inclusion of the P processes enhanced model performance in simulating ecosystem productivity. We further compared the simulations from DLEM-CNP, DLEM-CN, and DLEM-C and the results showed that the inclusion of P processes reduced the CO2 fertilization effect on gross primary production (GPP) by 25% and 45%, and net ecosystem production (NEP) by 28% and 41%, respectively, relative to CN-only and C-only models. From the 1860s to the 2010s, the DLEM-CNP estimated that in tropical forests GPP increased by 17%, plant respiration (Ra) increased by 18%, ecosystem respiration (Rh) increased by 13%, NEP increased by 121% per unit area, respectively. Additionally, factorial experiments with DLEM-CNP showed that the enhanced NPP benefiting from the CO2 fertilization effect had been offset by 135% due to deforestation from the 1860s to the 2010s. Our study highlights the importance of P limitation on the C cycle and the weakened CO2 fertilization effect resulting from P limitation in tropical forests.

Combined effect of burrowing mangrove crabs and tides on carbon fluxes

Sesarmid crabs act as mangrove ecosystem engineers due to their burrowing behavior in the sediment. The burial of leaves inside the sediment suggests a positive relationship between crab activity and carbon storage in mangrove forests. However, crab burrows increase the sediment-air interface and, thus, might amplify CO2 fluxes from the sediment. Additionally, the tidal export of carbon from burrows acting as preferential flow paths may offset the enhancing effect of crab burrows on carbon storage. In this study, we investigated the interactive effect of burrowing crabs and tidal flows on mangrove carbon storage in a laboratory experiment. Significantly higher dissolved organic carbon (DOC) concentrations in the porewater were found in microtidal compared to mesotidal treatments, while the total amount of outflowing DOC was similar across tidal treatments. No significant effect of burrowing crabs on the DOC content of the porewater was found. Significantly lower CO2 fluxes into the atmosphere were found in treatments with crabs present which is contrary to previous studies. We suggest that lower CO2 flux values were a result of collapsed burrows that preserved the particulate organic carbon (POC) in deeper sediment layers. Previous studies, showing enhanced CO2 fluxes from crab burrows, have been carried out in the field and did not take the potential of burrow collapse into account. We stress the importance of considering temporal variability in crab burrow stability and spatial variability in tidal dynamics when evaluating their interactive effect on carbon fluxes in mangrove forests.

Contrasting Land and Atmospheric Controls on the Generalized Complementary Relationship of Evaporation Over Grasslands and Forests

AbstractThe generalized complementary relationship (CR) rooted on land–atmosphere coupling has been widely used to study evaporation over various ecosystems. Evaporation estimation models based on CR usually contain two parameters. The first one controls the shape of CR, and the second one is related to potential evaporation (Epo). Previous studies have found substantial variations in these parameters, but how they are affected by land surface and/or the atmosphere under conditions with varying land–atmosphere coupling remains unclear. In this study, the land and atmospheric controls for the generalized CR were comparatively investigated focusing on the two parameters of the sigmoid type function by using the data of 24 grassland and 19 forest flux sites and by considering the contrasting land–atmosphere coupling strength. The strong coupling to the outer atmosphere at the forest sites resulted in a large variation in the Epo‐related parameter and its significant correlation with the mean relative humidity. The shape parameter had a large variation at the grassland sites due to the weak coupling with the outer atmosphere and was significantly correlated with the mean soil water content. The simple parameterizations for the two parameters based on the correlations improved the predictions of monthly actual evaporation compared with the parameterizations for the fixed parameters corresponding to ecosystem types. The effects of land surface wetness and local advections on the generalized CR over grasslands and forests were also discussed respectively.

Elevated soil moisture amplified the effects of freeze–thaw cycles on soil CO2 and CH4 fluxes in subalpine forests

Soil carbon emissions from subalpine ecosystems have been demonstrated to be influenced by freeze–thaw cycles (FTCs). Under climate change, moisture and number of FTCs have been altered significantly in subalpine regions. Thus, we selected a typical subalpine forest in Northwest China and conducted an incubation study to explore the effects of various soil moisture (SM) levels and numbers of FTCs on soil CO2 and CH4 fluxes during nine FTCs. Our results revealed that the soil CO2 emissions and CH4 uptakes had significant responses to changes in SM (FCO2 = 2327.32, p < 0.001; FCH4 = 353.51, p < 0.001) and the number of FTCs (FCO2 = 2506.45, p < 0.001; FCH4 = 60.85, p < 0.001). Specifically, CO2 emissions in the thawing phases and CH4 uptakes in the freezing phases were largest in the first FTC and then gradually decreased and stabilized with an increase in the number of FTCs. Regarding SM, the soil CO2 emissions and CH4 uptakes at 60 and 90 % water-filled pore space (WFPS) at the same number of FTCs were significantly higher than those at 30 % WFPS. Moreover, the interactive effects of the number of FTCs and SM on soil CO2 emissions and CH4 uptakes were significant (FCO2 = 279.70, p < 0.001; FCH4 = 17.76, p < 0.001). Especially in the first FTC, SM dramatically amplified the effects of FTCs on CO2 emissions and CH4 uptakes. A partial least squares path model further confirmed that the number of FTCs had significant negative effects while SM had significant positive effects on CO2 emissions and CH4 uptakes. In addition, SM explained more of the variation than the number of FTCs in CO2 emissions and CH4 uptakes. CO2 emissions were modulated primarily by substrate accessibility and nutrient availability, and for CH4 uptakes, microbial properties also played a substantial role in addition to substrate accessibility and nutrient availability. We conclude that the increases in the number of FTCs and SM due to climate warming and humidification may trigger the potentially substantial increases in soil CO2 emissions and CH4 uptakes.

Canonical Hubble-Tension-Resolving Early Dark Energy Cosmologies Are Inconsistent with the Lyman-α Forest.

Current cosmological data exhibit discordance between indirect and some direct inferences of the present-day expansion rate H_{0}. Early dark energy (EDE), which briefly increases the cosmic expansion rate prior to recombination, is a leading scenario for resolving this "Hubble tension" while preserving a good fit to cosmic microwave background (CMB) data. However, this comes at the cost of changes in parameters that affect structure formation in the late-time universe, including the spectral index of scalar perturbations n_{s}. Here, we present the first constraints on axionlike EDE using data from the Lyman-α forest, i.e., absorption lines imprinted in background quasar spectra by neutral hydrogen gas along the line of sight. We consider two independent measurements of the one-dimensional Lyα forest flux power spectrum from the Sloan Digital Sky Survey (SDSS eBOSS) and from the MIKE/HIRES and X-Shooter spectrographs. We combine these with a baseline dataset comprised of Planck CMB data and baryon acoustic oscillation (BAO) measurements. Combining the eBOSS Lyα data with the CMB and BAO dataset reduces the 95% confidence level (C.L.) upper bound on the maximum fractional contribution of EDE to the cosmic energy budget f_{EDE} from 0.07 to 0.03 and constrains H_{0}=67.9_{-0.4}^{+0.4} km/s/Mpc (68% C.L.), with maximum aposteriori value H_{0}=67.9 km/s/Mpc. Similar results are obtained for the MIKE/HIRES and X-Shooter Lyα data. Our Lyα-based EDE constraints yield H_{0} values that are in >4σ tension with the SH0ES distance-ladder measurement and are driven by the preference of the Lyα forest data for n_{s} values lower than those required by EDE cosmologies that fit Planck CMB data. Taken at face value, the Lyα forest severely constrains canonical EDE models that could resolve the Hubble tension.

Interannual precipitation controls on soil CO2 fluxes in high elevation conifer and aspen forests

Long-term soil CO2 emission measurements are necessary for detecting trends and interannual variability in the terrestrial carbon cycle. Such records are becoming increasingly valuable as ecosystems experience altered environmental conditions associated with climate change. From 2013 to 2021, we continuously measured soil CO2 concentrations in the two dominant high elevation forest types, mixed conifer and aspen, in the upper Colorado River basin. We quantified the soil CO2 flux during the summer months, and found that the mean and total CO2 flux in both forests was related to the prior winter’s snowfall and current summer’s rainfall, with greater sensitivity to rainfall. We observed a decline in surface soil CO2 production, which we attributed to warming and a decrease in amount and frequency of summer rains. Our results demonstrate strong precipitation control on the soil CO2 flux in mountainous regions, a finding which has important implications for carbon cycling under future environmental change.

Litterfall and element fluxes in secondary successional forests of South Korea

Determining litterfall and corresponding element fluxes is valuable in understanding productivity and biogeochemical cycling in floristically distinct secondary successional zones. Secondary-growth forests form a major portion of the cool-temperate forests of South Korea following decades of severe disturbance and intensive rehabilitation efforts. This study monitored the total and fraction-specific litterfall mass, element flux, and nutrient use efficiency patterns of a secondary-growth broadleaf deciduous forest (hereafter, BDFA) and compared these values in forests with contrasting sub-canopy compositions (BDFB) and with coniferous evergreen forests (CEF). Mean annual total litterfall mass was uniform across stands at a range of 871.5 g m−2 yr−1 to 990.2 g m−2 yr−1. Litterfall mass of seed, bark, and miscellaneous fractions varied per stand (p < 0.05 for seed; p < 0.01 for bark and miscellaneous). A pronounced unimodal litterfall peak was detected for all stands, with total litterfall responding positively to relative humidity but negatively to windspeed and solar radiation. Coniferous evergreen litter contained higher C but lower macronutrients relative to broadleaf deciduous litter. Macronutrient fluxes ranged from 156.2 to 183.9 kg ha−1 yr−1, with broadleaf stands having significantly higher K and Ca rates. Macronutrients were returned in all stands in the order of N > Ca > K > Mg > P, whereas NUE followed a reverse sequence. NUE in CEF was higher than BDFA and BDFB for all macronutrients, reflecting the distinct biomass allocation and nutrient utilization strategies between conifers and broadleaves. In temperate secondary-growth forests, nutrient cycling via litterfall is substantiated primarily by litterfall quantity and secondarily by litter element concentration. Our findings have important implications for elucidating forest litter ecology and improving sustainable management practices in secondary successional forests.

Post-Fire Changes in Canopy Solute Leaching in Pinus densiflora Forests

Fires can burn canopy fuel and replace leafy crowns with charred branches and trunks, thereby affecting hydrological flow and water chemistry. However, little is known about the effects of fire on throughfall volumes and chemical fluxes in temperate forests. Therefore, we aimed to monitor the volume and chemistry of throughfall in pine trees (Pinus densiflora) damaged by the 2022 Gangneung-Donghae Forest fire in the Republic of Korea. Immediately after the forest fire, funnel-type measurements were performed to collect the throughfall beneath five trees at foliage necrosis and crown consumption sites. The amount of water that penetrated in a specified period was continually measured and analyzed in terms of the water quality components. Crown consumption resulted in the passage of more water due to the removal of the tree canopy; however, the ratio of throughfall to total rainfall remained constant as the rainfall amount increased. The throughfall volume was not significantly different owing to the fire damage. The solute concentrations of Ca and TOC at the crown consumption site were higher than those at the foliage necrosis site after the fire; however, no significant difference was observed three months after the fire. In this study, the changes in the amount and water quality of throughfall due to fire were examined over a relatively short period, providing fundamental data for nutrient cycling management of wildfire-damaged soil.

Decaying Dark Matter and Lyman-α forest constraints

Decaying Cold Dark Matter (DCDM) is a model that is currently under investigation regarding primarily the S 8 tension between cosmic microwave background (CMB) and certain large-scale structure measurements. The decay into one massive and one (or more) massless daughter particle(s) leads to a suppression of the power spectrum in the late universe that depends on the relative mass splitting ϵ = (1 - m 2/M 2)/2 between the mother and massive daughter particle as well as the lifetime τ. In this work we investigate the impact of the BOSS DR14 one-dimensional Lyman-α forest flux power spectrum on the DCDM model using a conservative effective model approach to account for astrophysical uncertainties. Since the suppression of the power spectrum due to decay builds up at low redshift, we find that regions in parameter space that address the S 8 tension can be well compatible with the Lyman-α forest. Nevertheless, for values of the degeneracy parameter ϵ ∼ 0.1-0.5%, for which the power suppression occurs within the scales probed by BOSS Lyman-α data, we find improved constraints compared to previous CMB and galaxy clustering analyses, obtaining τ ≳ 18 Gyrs for small mass splitting. Furthermore, our analysis of the BOSS Lyman-α flux power spectrum allows for values τ ∼ 102 Gyrs, ϵ ∼ 1%, that have been found to be preferred by a combination of Planck and galaxy clustering data with a KiDS prior on S 8, and we even find a hint for a marginal preference within this regime.

A dataset of carbon and water fluxes observed in Xishuangbanna tropical seasonal rain forest from 2011 to 2015

The proposal for China's dual-carbon target is based on the foundation of addressing climate change. It is imperative to enhance carbon sinks and reduce carbon sources to implement climate governance. The terrestrial ecosystem is an important atmospheric carbon sink, while the tropical seasonal rain forest, as a component of the terrestrial ecosystem in China characterized by rich biomass, and strong productivity, plays an essential role in carbon sequestration. As one of the basic observation and research stations of China Flux Research Network (ChinaFLUX), the Xishuangbanna Tropical Seasonal Rainforest Ecological Station built in 2002 has accumulated 20 years of carbon and water flux observation data. Based on the ChinaFLUX data processing system, this dataset processes and collates the tropical seasonal rain forest flux and conventional meteorological data of Xishuangbanna from January 2011 to December 2015. The result is a collection of data products at half-hourly, daily, monthly, and yearly intervals. This dataset can be applied to research fields such as the analysis of the carbon cycle response mechanism of ecosystems and the assessment of carbon sequestration benefits under climate change, and it can also serve as a valuable reference for the optimization of ecosystem model parameters and the verification of results. Through multi-platform architecture, multi-scale integration, and multi-method coupling, the dataset is expected to further provide basic support for analyzing the mechanisms of regional and even global carbon-water-heat cycles in ecosystems, spatiotemporal dynamics and predicting future carbon sink potential, thus providing the scientific basis for achieving China's carbon neutrality goal.

Effects of tree pollen on throughfall element fluxes in European forests

Effects of tree pollen on throughfall element fluxes in European forests

Phenology-Based Maximum Light Use Efficiency for Modeling Gross Primary Production across Typical Terrestrial Ecosystems

The maximum light use efficiency (LUE) (ε0) is a key essential parameter of the LUE model, and its accurate estimation is crucial for quantifying gross primary production (GPP) and better understanding the global carbon budget. Currently, a comprehensive understanding of the potential of seasonal variations of ε0 in GPP estimation across different plant functional types (PFTs) is still lacking. In this study, we used a phenology-based strategy for the estimation of ε0 to find the optimal photosynthetic responses of the parameter in different phenological stages. The start and end of growing season (SOS and EOS) from time series vegetation indices and the camera-derived greenness index were extracted across seven PFT flux sites using the methods of the hybrid generalized additive model (HGAM) and double logistic function (DLF). Optimal extractions of SOS and EOS were evaluated, and the ε0 was estimated from flux site observations during the optimal phenological stages with the light response equation. Coupled with other obligatory parameters of the LUE model, phenology-based GPP (GPPphe-based) was estimated over 21 site-years and compared with vegetation photosynthesis model (VPM)-based GPP (GPPVPM) and eddy covariance-measured GPP (GPPEC). Generally, GPPphe-based basically tracked both the seasonal dynamics and inter-annual variation of GPPEC well, especially at forest, cropland, and wetland flux sites. The R2 between GPPphe-based and GPPEC was stable between 0.85 and 0.95 in forest ecosystems, between 0.75 and 0.85 in cropland ecosystems, and around 0.9 in wetland ecosystems. Furthermore, we found that GPPphe-based was significantly improved compared to GPPVPM in cropland, grassland, and wetland ecosystems, implying that phenology-based ε0 is more appropriate in the GPP estimation of herbaceous plants. In addition, we found that GPPphe-based was significantly improved over GPPVPM in cropland, grassland, and wetland ecosystems, and the R2 between GPPphe-based and GPPEC was improved by up to 0.11 in cropland ecosystems and 0.05 in wetland ecosystems compared to GPPVPM, and RMSE was reduced by up to 5.90 and 2.11 g C m−2 8 day−1, respectively, implying that phenology-based ε0 in herbaceous plants is more appropriate for GPP estimation. This work highlights the potential of phenology-based ε0 in understanding the seasonal variation of vegetation photosynthesis and production.

Carbon and nutrient pools and fluxes in unmanaged mountain Norway spruce forests, and losses after natural tree dieback

Forest areas infected by insects are increasing in Europe and North America due to accelerating climate change. A 2000–2020 mass budget study on major elements (C, N, P, Ca, Mg, K) in the atmosphere-plant-soil-water systems of two unmanaged catchments enabled us to evaluate changes in pools and fluxes related to tree dieback and long-term accumulation/losses during the post-glacial period. A bark-beetle outbreak killed >75 % of all trees in a mature mountain spruce forest in one catchment and all dead biomass was left on site. A similar forest in a nearby catchment was only marginally affected. We observed that: (1) the long-term (millennial) C and N accumulation in soils averaged 10–22 and 0.5–1.1 kg ha−1 yr−1, respectively, while losses of Ca, Mg, and K from soils ranged from 0.1 to 2.6 kg ha−1 yr−1. (2) Only <0.8 % and <1.5 % of the respective total C and N fluxes entering the soil annually from vegetation were permanently stored in soils. (3) The post-disturbance decomposition of dead tree biomass reduced vegetation element pools from 27 % (C) to 73 % (P) between 2004 and 2019. (4) Tree dieback decreased net atmospheric element inputs to the impacted catchment, and increased the leaching of all elements and gaseous losses of C (∼2.3 t ha−1 yr−1) and N (∼14 kg ha−1 yr−1). The disturbed catchment became a net C source, but ∼50 % of the N released from dead biomass accumulated in soils. (5) Despite the severe forest disturbance, the dissolved losses of Ca and Mg represented 52–58 % of their leaching from intact stands during the peaking atmospheric acidification from 1970 to 1990. (6) Disturbance-related net leaching of P, Ca, Mg, and K were 4, 69, 16, and 114 kg ha−1, respectively, which represented 7–38 % of the losses potentially related to sanitary logging and subsequent removal of the aboveground tree biomass.

Leaf-level metabolic changes in response to drought affect daytime CO2 emission and isoprenoid synthesis pathways.

In the near future, climate change will cause enhanced frequency and/or severity of droughts in terrestrial ecosystems, including tropical forests. Drought responses by tropical trees may affect their carbon use, including production of volatile organic compounds (VOCs), with implications for carbon cycling and atmospheric chemistry that are challenging to predict. It remains unclear how metabolic adjustments by mature tropical trees in response to drought will affect their carbon fluxes associated with daytime CO2 production and VOC emission. To address this gap, we used position-specific 13C-pyruvate labeling to investigate leaf CO2 and VOC fluxes from four tropical species before and during a controlled drought in the enclosed rainforest of Biosphere 2 (B2). Overall, plants that were more drought-sensitive had greater reductions in daytime CO2 production. Although daytime CO2 production was always dominated by non-mitochondrial processes, the relative contribution of CO2 from the tricarboxylic acid cycle tended to increase under drought. A notable exception was the legume tree Clitoria fairchildiana R.A. Howard, which had less anabolic CO2 production than the other species even under pre-drought conditions, perhaps due to more efficient refixation of CO2 and anaplerotic use for amino acid synthesis. The C. fairchildiana was also the only species to allocate detectable amounts of 13C label to VOCs and was a major source of VOCs in B2. In C. fairchildiana leaves, our data indicate that intermediates from the mevalonic acid (MVA) pathway are used to produce the volatile monoterpene trans-β-ocimene, but not isoprene. This apparent crosstalk between the MVA and methylerythritol phosphate pathways for monoterpene synthesis declined with drought. Finally, although trans-β-ocimene emissions increased under drought, it was increasingly sourced from stored intermediates and not de novo synthesis. Unique metabolic responses of legumes may play a disproportionate role in the overall changes in daytime CO2 and VOC fluxes in tropical forests experiencing drought.

Effect of near-surface winds on the measurement of forest soil CO2 fluxes using closed air chambers

Forest soil CO2 flux measurements are important for studying global climate change. Current monitoring methods are based on closed gas chambers, which block the wind pumping effect of near-surface winds in the measurements, resulting in biased values. Therefore, in this study, the effects of near-surface winds on chamber-monitored fluxes were investigated. The CO2 flux was quantified using a designed flux reference system with different CO2 concentrations, and the monitoring performance of the closed chamber was studied. Wavelet coherence was used to analyze the response relationship between near-surface winds and soil gas, and was combined with a flux calculation model to explore the relevant factors influencing gas chamber measurement-produced bias. The data indicate that at near-surface wind speeds greater than 0.8 m·s−1, gas transport enhancement was significant and further increased the deviation of the gas chamber-monitored CO2 fluxes. The monitoring error of the flow chamber (NSF) increased from 7% to 30% in soils with low carbon content, but did not vary significantly (3–7%) in soils with high CO2 concentrations. The flux measurement bias of the non-flow chamber (NSNF) was positively correlated with the soil carbon content, with the measurement error expanding by 16–24% with increasing soil CO2 concentrations. The measurement errors of the exponential and linear models in a windless environment were 9.8% (Exp) and 18.7% (Lin), respectively. The estimation errors of both models were positively correlated with both the time of a single monitoring event and the wind-induced coefficient Dw. Therefore, flux calculation models should be improved by considering environments with wind disturbances to reduce the effect of wind on measured values, which will help improve the accuracy of ecosystem carbon budgets.

Effect of In Situ Large Soil Column Translocation on CO2 and CH4 Fluxes under Two Temperate Forests of Northeastern China

Global warming has a significant impact on soil carbon dioxide (CO2) and methane (CH4) fluxes in temperate forests. However, due to a lack of field observations, limited information is available about the responses of soil CO2 and CH4 fluxes to changes in temperature during the non-growing season and throughout the year in temperate forests. The broadleaf and Korean pine mixed mature forest (MF) and adjacent secondary white birch forest (BF) at different succession stages in the Changbai mountain region in northeastern China were selected, to study the effect of in situ soil column translocation on CO2 and CH4 fluxes in temperate forests. On average, the air temperature and soil temperature at 5 cm depth under BF stands from October 2018 to October 2022 increased by 0.64 and 0.42 °C during the non-growing season and by 0.49 and 0.43 °C throughout the year, respectively, compared with those under MF stands. Based on multi-year measurements in field experiments, it was shown that during the non-growing season, fluxes of CO2 and CH4 from soil columns under MF and BF stands ranged from 0.004 to 1.175 and from 0.015 to 1.401 (averages of 0.321 and 0.387) μmol CO2 m−2 s−1, and from −1.003 to 0.048 and from −1.037 to −0.013 (averages of −0.179 and −0.250) nmol CH4 m−2 s−1, respectively, accounting for approximately 20.8% and 25.3%, and 48.8% and 69.1% of the corresponding average fluxes during the growing season. When undisturbed soil columns of MF were transferred to a BF stand, to simulate warming, the cumulative soil CO2 emissions and CH4 uptake increased by 23.5% and 15.3% during the non-growing season, and by 9.5% and 16.3% across the year, respectively. However, when soil columns of BF were transferred to a MF stand, to simulate cooling, the cumulative soil CO2 emissions decreased by 16.9% and 0.1% during the non-growing season and across the year, respectively. Upon cooling, the cumulative soil CH4 uptake decreased by 21.8% during the non-growing season but increased by 15.4% across the year. The soil temperature and moisture at 5 cm depth in soil columns could explain 84–86% of the variability in CO2 fluxes and 16–51% of the variability in CH4 fluxes under the two forest stands throughout the field measurement period. The results of the in situ soil column translocation experiments highlight that a small climate warming in nature can increase soil CO2 emissions and CH4 uptake in the temperate forests of northeastern China, particularly during the non-growing season, which should be considered when predicting soil C fluxes in the temperate forests of northeastern China under global warming scenarios.

Enhanced CO2 uptake is marginally offset by altered fluxes of non-CO2 greenhouse gases in global forests and grasslands under N deposition.

Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO2 exchange and the emission or uptake of non-CO2 GHGs (CH4 and N2 O), few studies have assessed the climatic impact of forests and grasslands under N deposition globally based on different bottom-up approaches. Here, we quantify the effects of N deposition on biomass C increment, soil organic C (SOC), CH4 and N2 O fluxes and, ultimately, the net ecosystem GHG balance of forests and grasslands using a global comprehensive dataset. We showed that N addition significantly increased plant C uptake (net primary production) in forests and grasslands, to a larger extent for the aboveground C (aboveground net primary production), whereas it only caused a small or insignificant enhancement of SOC pool in both upland systems. Nitrogen addition had no significant effect on soil heterotrophic respiration (RH ) in both forests and grasslands, while a significant N-induced increase in soil CO2 fluxes (RS , soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N2 O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH4 , suggesting a declined sink capacity of forests and grasslands for atmospheric CH4 under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production-based method (forest, 1.28 Pg CO2 -eq year-1 vs. grassland, 0.58 Pg CO2 -eq year-1 ) was greater than those estimated using the SOC-based method (forest, 0.32 Pg CO2 -eq year-1 vs. grassland, 0.18 Pg CO2 -eq year-1 ) caused by N addition. Our findings revealed that the enhanced soil C sequestration by N addition in global forests and grasslands could be only marginally offset (1.5%-4.8%) by the combined effects of its stimulation of N2 O emissions together with the reduced soil uptake of CH4 .

Экологическая оценка пространственной неоднородности и временной изменчивости почвенных потоков СО2 в экосистемах различного типа (на примере Курской области)

The article presents data on the rate of carbon dioxide fluxes from the soil surface of forest, meadow communities and agroecosystems of the Kursk region. The rate of CO2 soil fluxes was estimated using the chamber method. It was established that in June CO2 emission from the soil surface of forest ecosystems varied from 10.2 to 41.7 g CO2 m-2 day-1, from the soil surface of agroecosystems – from 8.2 to 10.6 g CO2 m-2 day-1, and from the soils of meadow ecosystems it reached 37.7 g CO2 m-2 day-1. It was revealed that the rate of soil CO2 fluxes in oak forests was higher than in the community of Robinia pseudoacacia L. and birch forest by 25.6% and 14.9%, respectively. Keywords: CO2 EMISSION, PINE FOREST, BEREZNYAK, OAK, ROBINIA PSEUDOACACIA L., PURE STEAM