Net Ecosystem Exchange

Recent CO 2 flux measurements from towers and aircraft (net ecosystem exchange by eddy covariance) and remote sensing/modeling (net primary productivity-NPP) following fire show that the regenerating boreal forest in western Canada has a low initial flux that increases with time since fire. Daytime CO 2 fluxes are downward, even after 2 years following fire, although fluxes were upward during the first year after the fire. In summer, the forest is a net carbon sink a few years following fire. A regression of all data gives a relationship where the CO 2 flux relative to 10 years following fire = 0.11 + 0.92 log 10 (years since fire) (r 2 = 0.5). The CO 2 flux reaches the same rate as that of a mature site between 10 and 30 years following fire, depending on the site and the data set. Many studies in the literature indicate that soil respiration decreases following fire, although several models assume that heterotrophic respiration increases. If fire reduces respiration and our growing season measurements showing a net sink in early years are widely applicable, it is likely that some models may have overestimated the impact of fire on the carbon balance of the boreal landscape.

Estimation of the above-below ground plant biomass amount in terrestrial ecosystems is one of the most important components of climatic change programs and global carbon cycle modelling. Biomass of plant is one of the significant subjects in the studies carried out on climatic changes. For that reason, plant biomass samples were taken from different areas and different plant species in Southeastern Anatolian Region (SAR). In this study, the net plant biomass production amounts were tried to be determined with geo-statistical modelling by evaluating the results of 2293 plant biomass samples taken from the region. For that purpose, Ordinary Kriging in ArcGIS software was used and mapped. In the study, when available land uses were evaluated, the most dominant product was noticed to be wheat. According to this, samples were taken from wheat, pasture, barley, lentil, cotton and corn areas. In this study, samples were taken from 2293 points. The most sampling was performed from the areas of wheat agriculture. Average surface ground NPP amount varied between 154.66 and 1237.74 g m-2 , and the highest amount was determined in corn plant. Average under-soil NPP amounts were determined in wheat with 149.36, in barley with 63.31, in lentil with 55.90, in pastures with 41.41, in cotton with 172.73, and in corn with 437.66 g m-2. For the analysis of the data, SPSS 10.0 statistical software was used, and the data were analysed at the level of p < 0.01-0.05.

Large spatial‐scale effects of climate extremes on gross primary production (GPP), the largest terrestrial carbon flux, are highly uncertain even as these extremes increase in frequency and extent. Here we report the impacts of spring warming and summer drought in 2012 on GPP across the contiguous United States (CONUS) using estimates from four GPP models: Vegetation Photosynthesis Model (VPM), MOD17A2H V006, Carnegie‐Ames‐Stanford Approach, and Simple Biosphere/Carnegie‐Ames‐Stanford Approach. VPM simulations are driven by Moderate Resolution Imaging Spectroradiometer, North American Regional Reanalysis climate data, and C3 and C4 cropland maps from the United States Department of Agriculture Cropland Data Layer data set. Across 25 eddy covariance flux tower sites, GPP estimates from VPM (GPPVPM) showed better accuracy in terms of cross‐site variability and interannual variability (R2 = 0.84 and 0.46, respectively) when compared to MOD17 GPP. We further assessed the spatial and tem...

Carbon dioxide fluxes were examined over the growing seasons of 2002 and 2003 from 14 different sites in Upper Midwest (USA) to assess spatial variability of ecosystem-atmosphere CO 2 exchange. These sites were exposed to similar temperature/precipitation regimes and spanned a range of vegetation types typical of the region (northern hardwood, mixed forest, red pine, jack pine, pine barrens and shrub wetland). The hardwood and red pine sites also spanned a range of stand ages (young, intermediate, mature). While seasonal changes in net ecosystem exchange (NEE) and photosynthetic parameters were coherent across the 2 years at most sites, changes in ecosystem respiration (ER) and gross ecosystem production (GEP) were not. Canopy height and vegetation type were important variables for explaining spatial variability of CO 2 fluxes across the region. Light-use efficiency (LUE) was not as strongly correlated to GEP as maximum assimilation capacity (Amax). A bottom-up multi-tower land cover aggregated scaling of CO 2 flux to a 2000 km 2 regional flux estimate found June to August 2003 NEE, ER and GEP to be À290 AE 89, 408 AE 48, and 698 AE 73 gC m À2 , respectively. Aggregated NEE, ER and GEP were 280% larger, 32% smaller and 3% larger, respectively, than that observed from a regionally integrating 447 m tall flux tower. However, when the tall tower fluxes were decomposed using a footprint-weighted influence function and then reaggregated to a regional estimate, the resulting NEE, ER and GEP were within 11% of the multi-tower aggregation. Excluding wetland and young stand age sites from the aggregation

Aims Climate change largely impacts ecosystem carbon and water cycles by changing plant gas exchange, which may further cause positive or negative feedback to global climate change. However, long-term global change manipulative experiments are seldom conducted to reveal plant ecophysiological responses to climatic warming and altered precipitation regimes. Methods An 8-year field experiment with both warming and increased precipitation was conducted in a temperate grassland in northern China. We measured leaf gas exchange rates (including plant photosynthesis, transpiration and instantaneous water use efficiency [WUE]) of two dominant plant species (Stipa sareptana var. krylovii and Agropyron cristatum) from 2005 to 2012 (except 2006 and 2010) and those of other six species from 2011 to 2012. Important Findings Increased precipitation significantly stimulated plant photosynthetic rates (A) by 29.5% and 19.9% and transpiration rates (E) by 42.2% and 51.2% for both dominant species S. sareptana var. krylovii

Fine root production and turnover play important roles in regulating carbon (C) cycling in terrestrial ecosystems. In order to examine effects of climate change on root production and turnover, a field experiment with increased temperature and precipitation had been conducted in a semiarid temperate steppe in northern China since April 2005. Experimental warming decreased annual root production, mortality, and mean standing crop by 10.3%, 12.1%, 7.0%, respectively, while root turnover was not affected in 2006 and 2007 by the warming. Annual root production and turnover was 5.9% and 10.3% greater in the elevated than ambient precipitation plots. Changes in root production and mortality in response to increased temperature and precipitation could be largely attributed to the changes in gross ecosystem productivity (GEP) and belowground/aboveground C allocation. There were significant interactive effects of warming and increased precipitation on root productivity, mortality, and standing crop. Experimental warming had positive and negative effects on the three root variables (root production, mortality, standing crop) under ambient and increased precipitation, respectively. Increased precipitation stimulated and suppressed the three root variables in the unwarmed and warmed subplots, respectively. The positive dependence of soil respiration and ecosystem respiration upon root productivity and mortality highlights the important role of root dynamics in ecosystem C cycling. The nonadditive effects of increased temperature and precipitation on root productivity, mortality, and standing crop observed in this study are critical for model projections of climate-ecosystem feedbacks. These findings indicate that carbon allocation is a focal point for future research and that results from single factor experiments should be treated with caution because of factor interactions.

Model and satellite data used to study GPP trends in India. • Positive trend in GPP is observed at all study locations. • North East Region has seen the highest annual GPP, and Central India lowest in the last two decades. • LAI is better correlated with GPP than NDVI and EVI. Annual GPP trends during 2001-2019 at different locations in India.

Global carbon (C) cycling can be greatly affected by the increasing nitrogen (N) deposition and changing precipitation regimes in terrestrial ecosystems. Nevertheless, whether N enrichment and precipitation variability interactively or additively influence soil C release during a long-term period remain largely elusive. Here, a seven-year (2016-2022) field experiment was conducted to explore the responses of soil respiration to N addition, decreased and increased precipitation in a subtropical-warm temperate forest in Central China. The results showed soil respiration was elevated by 11.0%, 12.2%, and 12.3% under N addition, decreased precipitation, and increased precipitation, respectively, primarily originating from the enhancements in plant root respiration. Increased precipitation led to a temporal increase in soil respiration over the experimental period, mostly due to the elevating understory richness and reducing soil C/N ratio. There were additive effects between N addition and changing precipitation on soil respiration observed in our study, suggesting that the impacts of concurrent global change drivers can be predicted from their respective influences. These findings indicated that global change may intensify soil C release from climate transition forests, thus helping to accurately assess the climate change-C feedback in natural ecosystems.

1. Nitrogen (N) deposition and land-use practice (e.g. mowing) could have profound effects on soil respiration. However, the changes in other ecosystem components, such as plant functional groups (PFGs), may control soil carbon (C) efflux response

Secondary succession, a directional and predictable process of ecological community development following natural or anthropogenic perturbations (Odum, 1969), is primarily driven by disturbance legacy and inherent soil and vegetation characteristics (Estrada-Villegas

Drought events could have profound influence on plant community structure and ecosystem function, and have subsequent impacts on community stability, but we know little about how different climate warming scenarios affect community resistance and resilience to drought. Combining a daytime and nighttime warming experiment in the temperate steppe of north China with a natural drought event during the study period, we tested how daytime and nighttime warming influences drought resistance and resilience. Our results showed that the semi-arid steppe in north China was resistant to both daytime and nighttime warming, but vulnerable to drought. Nighttime warming, but not daytime warming, enhanced community resistance to drought via stimulating carbon sequestration, whereas neither daytime nor nighttime warming affected community resilience to drought. Large decline in plant community cover, primarily caused by the reduction in the cover of dominant and rare species rather than subordinate species during drought, did not preclude rapid ecosystem recovery. These findings suggest that nighttime warming may facilitate ecosystem sustainability and highlight the need to assess the effects of climate extremes on ecosystem functions at finer temporal resolutions than based on diurnal mean temperature. Climate warming and changes in precipitation patterns are two major components of climate change, carrying potentially large influences on plant community dynamics and terrestrial carbon cycling 1-4. Climate change scenarios, however, are often more complicated than originally thought. For example, nighttime temperature has been found to rise faster than daytime temperature 5,6 , rather than a symmetric increase in daytime and nighttime temperature. Precipitation patterns are also expected to be more variable, in addition to predicted changes in average rainfall, resulting in more frequent drought and flooding events 7,8. However, a good understanding of the effects of more realistic scenarios of warming (e.g., asymmetric daytime and nighttime warming) and extreme precipitation events on community and ecosystem properties is still lacking. Daytime and nighttime warming could influence plant photosynthesis and respiration differently, with potential effects on vegetation productivity and ecosystem carbon processes 9. For example, Myneni et al. 10 found that increased minimum temperature under nighttime warming enhanced net primary productivity as the result of the prolonged growing season. Consistent with Myneni et al. 10 , Wan et al. 2 reported a positive effect of nighttime warming and negative effect of daytime warming on gross ecosystem productivity and net ecosystem productivity. In contrast, Peng et al. 11 reported that vegetation productivity is positively related to maximum temperature in wet and cool ecosystems, and negatively related to maximum temperature in dry temperate regions. Water availability is also one of main drivers of community composition and ecosystem carbon flux. Changes in precipitation may shift species dominance, species distribution 12,13 , and grassland ecosystem carbon budgets 14,15. Recent studies have shown that the occurrence of droughts or heatwaves can partially offset carbon sinks or even cause net carbon losses, thereby releasing CO 2 to the atmosphere 16-19. In contrast Griffis et al. 20 found that drought can increase net ecosystem productivity by suppressing ecosystem respiration more than gross ecosystem productivity. The change in plant community composition and carbon cycles induced by both asymmetrically diurnal warming and extreme drought may have subsequent effect on the resistance of plant production to drought and

It has widely been documented that nitrogen (N) enrichment stimulates plant growth and net primary production. However, there is still dispute on how N addition affects net ecosystem CO 2 exchange (NEE), which represents the balance between ecosystem carbon (C) uptake and release. We conducted an experimental study to examine effects of N addition on NEE in a temperate steppe in northern China from 2005 to 2008. N was added at a rate of 10 g N m À2 yr À1 with NH 4 NO 3 alone or in combination with phosphorous (P, 5 g P 2 O 5 m À2 yr À1) in both clipped and unclipped plots. Over the 4 years, N addition significantly stimulated growing-season NEE, on average, by 27%. Neither the main effects of P addition or clipping nor their interactions with N addition were statistically significant on NEE in any of the 4 years. However, the magnitude of N stimulation on NEE declined over time. N addition significantly increased NEE by 60% in 2005 and 21% in 2006, but its effect was not significant in 2007 and 2008. N-induced shift in species composition was primarily responsible for the declined N stimulation over time. The gradually increasing coverage of the upper canopy species (Stipa krylovii) and standing litter accumulation induced light limitation on the lower canopy species (Artemisia frigida). Thus, N-induced shifts in plant species composition strongly regulated the direct effects of N addition on C sequestration in the temperate steppe.

Changes in precipitation and nitrogen (N) deposition can influence ecosystem carbon (C) cycling and budget in terrestrial biomes, with consequent feedbacks to climate change. However, little is known about the main and interactive effects of water and N additions on net ecosystem C exchange (NEE). In a temperate steppe of northern China, a field-manipulated experiment was conducted to evaluate the responses of NEE and its components to improve N and water availability from 2005 to 2008. The results showed that both water and N additions stimulated gross ecosystem productivity (GEP), ecosystem respiration (ER), and NEE. Water addition increased GEP by 17%, ER by 24%, and NEE by 11% during the experimental period, whereas N addition increased GEP by 17%, ER by 16%, and NEE by 19%. The main effects of both water and N additions changed with time, with the strongest water stimulation in the dry year and a diminishing N stimulation over time. When water and N were added in combination, there were nonadditive effects of water and N on ecosystem C fluxes, which could be explained by the changes in species composition and the shifts of limiting resources from belowground (water or N) to aboveground (light). The positive water and N additions effects indicate that increasing precipitation and N deposition in the future will favor C sequestration in the temperate steppe. The non-additive effects of water and N on ecosystem C fluxes suggest that multifactor experiments are better able to capture complex interactive processes, thus improving model simulations and projections.

• Global warming and a changing precipitation regime could have a profound impact on ecosystem carbon fluxes, especially in arid and semiarid grasslands where water is limited. A field experiment manipulating temperature and precipitation has been conducted in a temperate steppe in northern China since 2005. • A paired, nested experimental design was used, with increased precipitation as the primary factor and warming simulated by infrared radiators as the secondary factor. • The results for the first 2 yr showed that gross ecosystem productivity (GEP) was higher than ecosystem respiration, leading to net C sink (measured by net ecosystem CO 2 exchange, NEE) over the growing season in the study site. The interannual variation of NEE resulted from the difference in mean annual precipitation. Experimental warming reduced GEP and NEE, whereas increased precipitation stimulated ecosystem C and water fluxes in both years. Increased precipitation also alleviated the negative effect of experimental warming on NEE. • The results demonstrate that water availability plays a dominant role in regulating ecosystem C and water fluxes and their responses to climatic change in the temperate steppe of northern China.

A large remaining source of uncertainty in global model predictions of future climate is how ecosystem carbon (C) cycle feedbacks to climate change. We conducted a field manipulative experiment of warming and nitrogen (N) addition in a temperate steppe in northern China during two contrasting hydrological growing seasons in 2006 [wet with total precipitation 11.2% above the long-term mean (348 mm)] and 2007 (dry with total precipitation 46.7% below the long-term mean). Irrespective of strong intraand interannual variations in ecosystem C fluxes, responses of ecosystem C fluxes to warming and N addition did not change between the two growing seasons, suggesting independence of warming and N responses of net ecosystem C exchange (NEE) upon hydrological variations in the temperate steppe. Warming had no effect on NEE or its two components, gross ecosystem productivity (GEP) and ecosystem respiration (ER), whereas N addition stimulated GEP but did not affect ER, leading to positive responses of NEE. Similar responses of NEE between the two growing seasons were due to changes in both biotic and abiotic factors and their impacts on ER and GEP. In the wet growing season, NEE was positively correlated with soil moisture and forb biomass. Negative effects of warming-induced water depletion could be ameliorated by higher forb biomass in the warmed plots. N addition increased forb biomass but did not affect soil moisture, leading to positive effect on NEE. In the dry growing season, NEE showed positive dependence on grass biomass but negative dependence on forb biomass. No changes in NEE in response to warming could result from water limitation on both GEP and ER as well as little responses of either grass or forb biomass. N addition stimulated grass biomass but reduced forb biomass, leading to the increase in NEE. Our findings highlight the importance of changes in abiotic (soil moisture, N availability) and biotic (growth of different plant functional types) in mediating the responses of NEE to climatic warming and N enrichment in the semiarid temperate steppe in northern China.

The magnitude of daily minimum temperature increase is greater than that of daily maximum temperature increase under climate warming. This study was conducted to examine whether changes in soil respiration under diurnal warming are equal to the summed changes under day and night warming in a temperate steppe in northern China. A full factorial design with day and night warming was used in this study, including control, day (06:00 a.m.-06:00 p.m., local time) warming, night (06:00 p.m.-06:00 a.m.) warming, and diurnal warming. Day warming showed no effect on soil respiration, whereas night warming significantly increased soil respiration by 7.1% over the 3 growing seasons in 2006-2008. The insignificant effect of day warming on soil respiration could be attributable to the offset of the direct positive effects of increased temperature by the indirect negative effects via aggravating water limitation and suppressing ecosystem C assimilation. The positive effects of night warming on soil respiration were largely due to the stimulation of ecosystem C uptake and substrate supply via overcompensation of plant photosynthesis. Changes in both soil respiration (+20.7 g C m-2 y-1) and GEP (-2.8 g C m-2 y-1) under diurnal warming are smaller than their summed changes (+40.0 and +24.6 g C m-2 y-1 , respectively) under day and night warming. Our findings that the effects of diurnal warming on soil respiration and gross ecosystem productivity are not equal to the summed effects of day and night warming are critical for model simulation and projection of climatecarbon feedback.

A mechanistic understanding of the carbon (C) cycle-climate change feedback is essential for projecting future states of climate and ecosystems. Here we report a novel field mechanism and evidence supporting the hypothesis that nocturnal warming in a temperate steppe ecosystem in northern China can result in a minor C sink instead of a C source as models have predicted. Nocturnal warming increased leaf respiration of two dominant grass species by 36.3%, enhanced consumption of carbohydrates in the leaves (72.2% and 60.5% for sugar and starch, respectively), and consequently stimulated plant photosynthesis by 19.8% in the subsequent days. Our experimental findings confirm previous observations of nocturnal warming stimulating plant photosynthesis through increased draw-down of leaf carbohydrates at night. The enhancement of plant photosynthesis overcompensated the increased C loss via plant respiration under nocturnal warming and shifted the steppe ecosystem from a minor C source (1.87 g CÁm À2 Áyr À1) to a C sink (21.72 g CÁm À2 Áyr À1) across the three growing seasons from 2006 to 2008. Given greater increases in daily minimum than maximum temperature in many regions, plant photosynthetic overcompensation may partially serve as a negative feedback mechanism for terrestrial biosphere to climate warming.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Changes in rainfall regime and grazing pressure affect vegetation composition and diversity with ecological implications for savannahs. The savannah in East Africa has experienced increased livestock grazing and rainfall variability but the impacts associated with those changes on the herbaceous layer have rarely been documented. We investigated the effect of livestock grazing, rainfall manipulation and their interaction on the composition and diversity of the herbaceous community in the savannah for two years in Lambwe, Kenya. Rainfall manipulation plots were set up for vegetation sampling; these plots received either 50% more or 50% less rainfall than control plots. Simpson's diversity and Berger-Parker indices were used to determine diversity changes and dominance respectively. The frequency of species was used to compute their abundance and their life forms as determined from the literature. Grazing significantly increased species diversity through suppression of dominant species. Rainfall manipulation had no significant impact on plant diversity in fenced plots, but rainfall reduction significantly reduced diversity in grazed plots. In contrast, rainfall manipulation had no impact on dominance in either fenced or grazed plots. The interaction of grazing and rainfall manipulation is complex and will require additional survey campaigns to create a complete picture of the implications for savannah structure and composition.

The influence of livestock grazing and changing rainfall patterns on soil respiration (Rs) in the tropical savannah in Lambwe is unclear. Measurements of Rs using chambers were conducted between June and November 2012 in a humid savannah in Western Kenya to determine how simulated rainfall amounts and cattle grazing affected soil respiration. The experimental layout was split plot comprising rainfall simulations which were laid on grazed and fenced blocks, respectively. The rainfall simulations split plots each measured 6 m × 3 m and were covered with rainout gutters above the canopy of the herbaceous layer to either increase or reduce ambient rainfall by 50%.

Predictive capacity of a numerical model is determined by model structure as well as model parameters. To make reliable model predictions we have to show that model can replicate observations or, in other words, model has to be validated. During the model validation the parameters of the model are usually adjusted to achieve a better fit of modelled and observed variables. Parameters can be adjusted either manually by user or automatically by a computer program. In this report we use observations of CO 2 fluxes measured with the eddy covariance technique first, to validate the terrestrial ecosystem model BIOME-BGC and then to optimize model parameters for several forest sites. In the first chapter we presents validation of net ecosystem exchange, gross primary production, and respiration simulated by BIOME-BGC for 10 sites located in four northernhemispheric forest biomes, where CO 2 flux measurements have been made under the FLUXNET (AMERIFLUX 9 and EUROFLUX 10 ) program, at the inter-annual, annual, intraannual, and daily levels. We show that model works better for temperate than for boreal forests. Inclusion of multiple soil layers in the model instead of one bucket algorithm could help to improve the model performance especially in the North. Modelled respiration rate was higher than observed at low temperatures, but in good agreement at high temperatures. In the second chapter we estimate model parameters using optimisation routine, which is based on the Metropolis algorithm, and observations from six forest sites. In addition to carbon flux measurements we use also observations of leaf area index. We demonstrate that a more accurate estimation of model parameters helps to reduce the uncertainty in the model output -the estimates of carbon fluxes. The retrieved values of model parameters provide a reduction of an average flux uncertainty by 60%-80% over the modelled time period (year 2001). The largest reduction of flux and parameter uncertainties was achieved with the largest number of constraints on the model output for five sites and with the single constraint on the ecosystem respiration for Hesse. The new parameter estimates provided also a reduction of the flux uncertainty at two additional sites that were not included into the optimisation.

Perennial grass biofuels may contribute to long-term carbon sequestration in soils, thereby providing a broad range of environmental benefits. To quantify those benefits, the carbon balance was investigated over three perennial grass biofuel crops -miscanthus (Miscanthus × giganteus), switchgrass (Panicum virgatum) and a mixture of native prairie plants -and a row crop control (maize-maize-soy) in Central Illinois, USA, during the establishment phase of the perennial grasses (2008)(2009)(2010)(2011). The eddy covariance technique was used to calculate fluxes of carbon dioxide and energy balance components, such as latent and sensible heat fluxes. Whereas maize attained the highest maximal carbon uptake rates, the perennial grasses had significantly extended growing seasons, such that their total carbon uptake rivaled that of corn in the second growing season and greatly exceeded that of soy in the third growing season. To account for the removal of carbon through harvest, net ecosystem exchange of carbon (NEE) was combined with estimates of yields, resulting in the net ecosystem carbon balance (NECB). After 2.5 years, NECB for the maize/soybean plot was positive (a source of carbon), while the grasses were a sink of carbon. Continuous measurements over the next years are required in order to confirm whether miscanthus, switchgrass and prairie can sustain a long-term sink of carbon if managed for biofuels, i.e., if harvested annually.

Forests play a crucial role in carbon cycling, contributing significantly to global carbon cycling and climate change mitigation, but their capture strength is sensitive to the climatic zone in which they operate and its adjoining environmental stressors. This research investigated the carbon dynamics of a typical deciduous forest, the Daniel Boone National Forest (DBNF), in the Mixed-Humid climate of Kentucky, USA, employing the Eddy Covariance technique to quantify temporal CO 2 exchanges from 2016 to 2020 and to assess its controlling biometeorological factors. The study revealed that the DBNF functioned as a carbon sink, sequestering -1515 g C m -2 in the study period, with a mean annual Net Ecosystem Exchange (NEE) of -303 g C m -2 yr -1 . It exhibited distinct seasonal and daily patterns influenced by ambient sunlight and air temperature. Winter months had the lowest rate of CO 2 uptake (0.0699 g C m -2 h -1 ), while summer was the most productive (-0.214 g C m -2 h -1 ). Diurnally, carbon uptake peaked past midday and remained a sink overnight, albeit negligibly so. Light and temperature response curves revealed their controlling effect on the DBNF trees' photosynthesis and respiration. Furthermore, clear seasonality patterns were observed in the control of environmental variables. The DBNF is a carbon sink consistent with other North American deciduous forests.

We analyzed 13 years (1992−2004) of CO2 flux data, biometry, and meteorology from a mixed deciduous forest in central Massachusetts. Annual net uptake of CO2 ranged from 1.0 to 4.7 Mg‐C ha−1yr−1, with an average of 2.5 Mg‐C ha−1yr−1. Uptake rates increased systematically, nearly doubling over the period despite forest age of 75–110 years; there were parallel increases in midsummer photosynthetic capacity at high light level (21.5−31.5 μmole m−2s−1), woody biomass (101−115 Mg‐C ha−1 from 1993−2005, mostly due to growth of one species, red oak), and peak leaf area index (4.5−5.5 from 1998–2005). The long‐term trends were interrupted in 1998 by sharp declines in photosynthetic capacity, net ecosystem exchange (NEE) of CO2, and other parameters, with recovery over the next 3 years. The observations were compared to empirical functions giving the mean responses to temperature and light, and to a terrestrial ecosystem model (IBIS2). Variations in gross ecosystem exchange of CO2 (GEE) and ...

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–...

A set-up with 161 photosynthetically active radiation (PAR) sensors was used to investigate spatio-temporal variations of irradiance for five horizontal arrays within a Scots pine canopy. The measured PAR was converted to CO 2 exchange using of a shoot-scale photosynthetic response curve and the vertical distribution of the needle area. The net ecosystem exchange was simultaneously measured by the eddy covariance technique. The effect of spatial and temporal averaging of the PAR values and the number of sensors were analysed under different conditions as regards cloudiness and the shading by the foliage. In 1/2 h CO 2 exchange values for the entire canopy, a maximum overestimation of 30% resulted from a spatial averaging over horizontal arrays of 2-5 m and occurred under clear-sky conditions and significant foliage shading. Under partly cloudy conditions, the largest overestimation occurred for a case of little shading and the inaccuracy resulting from 1/2 h temporal averaging exceeded that of spatial averaging.

Hydraulic redistribution (HR) of water via roots from moist to drier portions of the soil occurs in many ecosystems, potentially influencing both water use and carbon assimilation. • By measuring soil water content, sap flow and eddy covariance, we investigated the temporal variability of HR in a loblolly pine (Pinus taeda) plantation during months of normal and below-normal precipitation, and examined its effects on tree transpiration, ecosystem water use and carbon exchange. • The occurrence of HR was explained by courses of reverse flow through roots. As the drought progressed, HR maintained soil moisture above 0.15 cm 3 cm )3 and increased transpiration by 30-50%. HR accounted for 15-25% of measured total site water depletion seasonally, peaking at 1.05 mm d )1 . The understory species depended on water redistributed by the deep-rooted overstory pine trees for their early summer water supply. Modeling carbon flux showed that in the absence of HR, gross ecosystem productivity and net ecosystem exchange could be reduced by 750 and 400 g C m )2 yr )1 , respectively. • Hydraulic redistribution mitigated the effects of soil drying on understory and stand evapotranspiration and had important implications for net primary productivity by maintaining this whole ecosystem as a carbon sink.

The Soil Moisture Active Passive (SMAP) mission is one of the first Earth observation satellites being developed by NASA in response to the National Research Council's Decadal Survey. SMAP will make global measurements of the soil moisture present at the Earth's land surface and will distinguish frozen from thawed land surfaces. Direct observations of soil moisture and freeze/thaw state from space will allow significantly improved estimates of water, energy, and carbon transfers between the land and the atmosphere. The accuracy of numerical models of the atmosphere used in weather prediction and climate projections are critically dependent on the correct characterization of these transfers. Soil moisture measurements are also directly applicable to flood assessment and drought monitoring. SMAP observations can help monitor these natural hazards, resulting in potentially great economic and social benefits. SMAP observations of soil moisture and freeze/thaw timing will also reduce a major uncertainty in quantifying the global carbon balance by helping to resolve an apparent missing carbon sink on land over the boreal latitudes. The SMAP mission concept will utilize L-band radar and radiometer instruments sharing a rotating 6-m mesh reflector antenna to provide highresolution and high-accuracy global maps of soil moisture and freeze/thaw state every two to three days. In addition, the SMAP project will use these observations with advanced modeling and data assimilation to provide deeper root-zone soil moisture and net ecosystem exchange of carbon. SMAP is scheduled for launch in the 2014-2015 time frame.

This study aimed to assess the combined effects of long-term nitrogen (N) supply and nitrogen deposition (N dep ) on carbon (C) accumulation within Sitka spruce [Picea sitchensis (Bong.) Carr.] plantations in Scotland. Six study sites established from 1970 to 1982 were periodically N-fertilized, monitored over time and commonly surveyed in 2010. Soil, aboveground biomass, and ground vegetation C stock changes were analyzed; aboveground C stocks were correlated with total additional N experienced at each site, that is, the sum of experimental N supply (N add ) and site-specific accumulated N dep from 1900 to 2010. Results showed a positive N effect on aboveground tree C stock and no decline in tree growth was observed either during fertilization or after the latest N addition. The amount of C in litter was significantly higher in experimentally N-treated plots, whereas the amount of C in understory vegetation was higher in control plots. Pooling all the compartments (that is, understory vegetation, litter, soil, and tree biomass) the total ecosystem C content was estimated for each site, and at most sites a higher C stock was estimated for N-treated plots. Differences in aboveground C accumulation rates between treated and control plots were lower at sites with high levels of accumulated N dep . Our results indicate that site-specific accumulated N dep should be considered to understand tree growth responses to N fertilization.

All material supplied via Jukuri is protected by copyright and other intellectual property rights. Duplication or sale, in electronic or print form, of any part of the repository collections is prohibited. Making electronic or print copies of the material is permitted only for your own personal use or for educational purposes. For other purposes, this article may be used in accordance with the publisher's terms. There may be differences between this version and the publisher's version. You are advised to cite the publisher's version. This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail.

The use of plant traits to predict ecosystem functions has been gaining growing attention. Above‐ground plant traits, such as leaf nitrogen (N) content and specific leaf area (SLA), have been shown to strongly relate to ecosystem productivity, respiration and nutrient cycling. Furthermore, increasing plant functional trait diversity has been suggested as a possible mechanism to increase ecosystem carbon (C) storage. However, it is uncertain whether below‐ground plant traits can be predicted by above‐ground traits, and if both above‐ and below‐ground traits can be used to predict soil properties and ecosystem‐level functions. Here, we used two adjacent field experiments in temperate grassland to investigate if above‐ and below‐ground plant traits are related, and whether relationships between plant traits, soil properties and ecosystem C fluxes (i.e. ecosystem respiration and net ecosystem exchange) measured in potted monocultures could be detected in mixed field communities. We foun...

Peer reviewed publications were used as the data source. Original data collection in the publications was by (i) inventories integrating change in soil C-stock, and (ii) CO2 monitoring fluxes by (a) chamber technique and/or (b) by eddy covariance technique. All CH4 and N2O fluxes monitored between soil and the atmosphere were by the chamber technique. Basic criteria for CO2, CO2-C, CH4 and N2O data inclusion to the database was:  for soil inventory studies, estimate on soil C stock change averaging change over specified period of years was attainable,  for eddy covariance studies, at least one annual soil CO2 balance estimate was attainable,  for chamber studies, at least one annual soil CO2 balance estimate was attainable,  for chamber studies, at least one annual or warm season soil total or soil heterotrophic cumulative CO2 emission estimate with potential to form respective annual soil CO2 balance estimate by using existing supplementary site-specific or site-type specific data was attainable,  at least one annual soil CH4 balance or warm season cumulative soil CH4 flux estimate was attainable,  at least one annual soil N2O balance or warm season cumulative soil N2O flux estimate was attainable. Annual soil CO2 balance estimates based on soil inventories were added into the database 'as is' basis. CO2 flux monitoring data based on chamber techniques usually necessitated further processing (e.g., measures on litter production and decomposition rates, evaluation of flux contributions from root respiration and/or ground vegetation dark respiration) before the estimate quantified soil CO2 balance. Such site specific or site-type specific supplementary data were collated from the relevant literature, authors and other specialists, and incorporated to CO2 flux data (applied changes are listed in Table -1 and in Data repository). Forming annual soil CH4 and N2O balance estimates in the data was less complicated because annualization was the only supplementary measure implemented to the estimates reported as cumulative warm season flux. Annualization. For studies providing GHG flux estimate based on warm season measurements, we used coefficient to supplement relative cold season fluxes. As in IPCC (2014), this was be made by adding flux-proportion of colder period GHG flux estimate which was based on the cold season (winter) measurements and flux estimates in boreal and subarctic peatlands. The values based on cold season flux proportion provided in Dise (1992), Alm et al. (1999), Aurela et al. (2002), Kim et al. (2007) and Leppälä et al. (2011). As the annualization coefficients we topped seasonal GHG flux estimate by 15.9% for CO2, by 16.7% for CH4, and 21.4% for N2O. Applied changes are listed in Table -1 and Table -2. Root respiration subtraction from the total respiration. Likely the largest and most coherent database providing both site type specific soil total (Rtot) and proportioned heterotrophic (Rhet) CO2 emissions in drained organic soils is analysed in Ojanen et al. ( ), and the database was available for our analyses. We split flux data from Ojanen et al. ( ) into ombrotrophic (n= 12) and minerotrophic (n= 53) site types and tested linear regression between CO2tot that includes autotrophic respiration of tree root systems and CO2het from soil. The analysis resulted in following linear regressions (95% confidence limits) for heterotrophic CO2het emission: emission = 1.1 + 0.39 × Rtot (R 2 = 0.641, SEmodel = 139, SEcoefficient = 183) for ombrotrophic sites and emission = -1.55 + 0.52 × Rtot (R 2 = 0.723, SEmodel = 217, SEcoefficient = 124) for minerotrophic sites. These equations were applied to form CO2het from the provided CO2tot estimates in data where CO2het was not readily available, and these estimates are listed in Table -1. Litter production and decomposition rates. Applied changes and data sources are shown in Table -1. Relative data reliability weighting. We determined a weigh for each annual soil GHG balance estimate formed from closed chamber data on the basis of estimate relative reliability (i.e. 1= no major issues

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

At soil chamber scale, soil CO2 efflux measurements were performed fortnightly during two months with a portative chamber. The response of the soil respiration to the soil temperature and the soil water content was established. Integration of these results could give an estimate of total soil respiration. The comparison of these different terms would allow a first assessment of the crop carbon balance.

With abbreviated return times of extreme events, management of nitrogen (N) will become even more a priority issue. The decision-making process of N application is complex because the N optimal rate varies spatially and temporally and the impact of a decision is delayed in time from its application. The objective of this paper is to develop a decision support system (DSS) to optimise N management in an ecosystem perspective, including crop production and environmental objectives. Winter wheat response to N was analysed between 2008 and 2014 on Luvisols, in temperate climate (Belgium). The usual Belgian practice consists to apply 60-60-60kgN.ha -1 respectively at tiller, stem extension and flag-leaf stages. By using the STICS model, the effect of variable N rates on crop growth and N available for leaching (NAL) were computed. 300 stochastic weather time-series were derived from LARS-WG for actual and future climatic conditions. The farmer's marginal net revenues (MNR) were computed as function of N costs and wheat selling prices. For each climatic probability level, the ratio R=MNR/NAL was estimated in function of the N rates. To minimize the risks, the DSS considered the curve R=f(N) corresponding to the probability level of 75%. The best N rate was the maximum value of this curve. Results indicated that, under actual conditions, the N rates applied at flag-leaf could, at least 3 years out of 4, be decreased by 10 to 30kgN.ha -1 . Under A1B scenarios, the optimal N practice could be decreased between 60-60-0kgN.ha -1 and 60-60-30kgN.ha -1 .

We review 15 techniques for estimating missing values of net ecosystem CO 2 exchange (NEE) in eddy covariance time series and evaluate their performance for different artificial gap scenarios based on a set of 10 benchmark datasets from six forested sites in Europe. The goal of gap filling is the reproduction of the NEE time series and hence this present work focuses on estimating missing NEE values, not on editing or the removal of suspect values in these time series due to systematic errors in the measurements (e.g., nighttime flux, advection). The gap filling was examined by generating 50 secondary datasets with artificial gaps (ranging in length from single half-hours to 12 consecutive days) for each benchmark dataset and evaluating the performance with a variety of statistical metrics. The performance of the gap filling varied among sites and depended on the level of aggregation (native halfhourly time step versus daily), long gaps were more difficult to fill than short gaps, and differences among the techniques were more pronounced during the day than at night. The non-linear regression techniques (NLRs), the look-up table (LUT), marginal distribution sampling (MDS), and the semiparametric model (SPM) generally showed good overall performance. The artificial neural network based techniques (ANNs) were generally, if only slightly, superior to the other techniques. The simple interpolation technique of mean diurnal variation (MDV)

A B S T R A C T Biogenic NMVOCs are mainly formed by plants and microorganisms. They have strong impact on the local atmospheric chemistry when emitted to the atmosphere. The objective of this study was to determine if there are significant emissions of non-methane volatile organic compounds (NMVOCs) from a subarctic mire in northern Sweden. Subarctic peatlands in discontinuous permafrost regions are undergoing substantial environmental changes due to their high sensitivity to climate warming and there is need for including NMVOCs in the overall carbon budget. Automatic and manual chamber measurements were used to estimate NMVOC fluxes from three dominating subhabitats on the mire during three growing seasons. Emission rates varied and were related to plant species distribution and seasonal net ecosystem exchange of carbon dioxide. The highest fluxes were observed from wetter sites dominated by Eriophorum and Sphagnum spp. Total NMVOC emissions from the mire (∼17 ha) is estimated to consist of ∼150 kgC during a growing season with 150 d. NMVOC fluxes can account for ∼5% of total net carbon exchange (-3177 kgC) at the mire during the same period. NMVOC emissions are therefore a significant component in a local carbon budget for peatlands.

During the summer of 2018, a widespread drought developed over Northern and Central Europe. The increase in temperature and the reduction of soil moisture have influenced carbon dioxide (CO2) exchange between the atmosphere and terrestrial ecosystems in various ways, such as a reduction of photosynthesis, changes in ecosystem respiration, or allowing more frequent fires. In this study, we characterize the resulting perturbation of the atmospheric CO2seasonal cycles. 2018 has a good coverage of European regions affected by drought, allowing the investigation of how ecosystem flux anomalies impacted spatial CO2gradients between stations. This density of stations is unprecedented compared to previous drought events in 2003 and 2015, particularly thanks to the deployment of the Integrated Carbon Observation System (ICOS) network of atmospheric greenhouse gas monitoring stations in recent years. Seasonal CO2cycles from 48 European stations were available for 2017 and 2018. Earlier data w...

Intensive pasture management that aims at providing season-long forage while minimizing soil degradation is increasingly becoming an important grazing strategy in Kentucky. Typically, it involves the use of high-yielding warm and cool season forage species that are well suited to local soil and climate conditions, meeting the dual-purpose provision of high nutritional value while remaining resilient to grazing pressure and changing climate. Monitoring carbon exchange is a crucial component for effective pasture management to promote sustainable pastureland management practices. We hypothesized that pasturelands, when intensively managed, would exhibit a small but important CO 2 cumulative uptake year-round. We used the Eddy covariance method to measure the net ecosystem exchange of CO 2 (NEE) and productivity of an intensively managed pastureland at Kentucky State University Research and Demonstration station from 2015 to 2020. The study has two objectives: to quantify interannual variability in net ecosystem exchange, and examine the controlling environmental factors, in particular temperature, sunlight, and precipitation of NEE. Diurnal and seasonal fluctuations followed typical patterns of carbon uptake and release. Overall, the pasture site consistently was carbon sink except for 2016, in part due to a warmer winter season than usual, sequestering 1394 gCm-2 over the study period. Precipitation and temperature were critical environmental factors underpinning seasonal CO 2 uptake and release. Of critical importance was the net carbon uptake during the non-growing season.

Forests play a crucial role in carbon cycling, contributing significantly to global carbon cycling and climate change mitigation, but their capture strength is sensitive to the climatic zone in which they operate and its adjoining environmental stressors. This research investigated the carbon dynamics of a typical deciduous forest, the Daniel Boone National Forest (DBNF), in the Mixed-Humid climate of Kentucky, USA, employing the Eddy Covariance technique to quantify temporal CO 2 exchanges from 2016 to 2020 and to assess its controlling biometeorological factors. The study revealed that the DBNF functioned as a carbon sink, sequestering-1515 g C m-2 in the study period, with a mean annual Net Ecosystem Exchange (NEE) of-303 g C m-2 yr-1. It exhibited distinct seasonal and daily patterns influenced by ambient sunlight and air temperature. Winter months had the lowest rate of CO 2 uptake (0.0699 g C m-2 h-1), while summer was the most productive (-0.214 g C m-2 h-1). Diurnally, carbon uptake peaked past midday and remained a sink overnight, albeit negligibly so. Light and temperature response curves revealed their controlling effect on the DBNF trees' photosynthesis and respiration. Furthermore, clear seasonality patterns were observed in the control of environmental variables. The DBNF is a carbon sink consistent with other North American deciduous forests.

The Sahel region has been identified as a “hot spot” of global environmental change, but understanding of the roles of different climatic and anthropogenic forcing factors driving change in the region is incomplete. We show that a process‐based ecosystem model driven by climatic and atmospheric CO2 data alone closely reproduces the satellite‐observed greening trend of the Sahel vegetation and its interannual variability between 1982 and 1998. Changes in precipitation were identified as the primary driver of the aggregated simulated vegetation changes. According to the model, the increasing carbon uptake through vegetation was associated with an increasing relative carbon sink; but integrated over the whole period, the Sahel was predicted to be a net source of carbon.