Total CO2 Flux Research Articles

Evaluation of carbon dioxide emission factor from urea during rice cropping season: A case study in Korean paddy soil

Fertilization with urea can lead to a loss of carbon dioxide (CO2) that was fixed during the industrial production process. The extent of atmospheric CO2 removal from urea manufacturing was estimated by the Industrial Processes and Product Use sector (IPPU sector). On its basis, the Intergovernmental Panel on Climate Change (IPCC) has proposed a value of 0.2 Mg C per Mg urea (available in 2006 revised IPCC guidelines for greenhouse gas inventories), which is the mass fractions of C in urea, as the CO2 emission coefficient from urea for the agricultural sector. Notably, due to the possibility of bicarbonate leaching to waters, all C in urea might not get released as CO2 to the atmosphere. Hence, in order to provide an accurate value of the CO2 emission coefficient from applied urea in the rice ecosystem, the CO2 emission factors were characterized under different levels of 13C-urea applied paddy field in the current study. The total CO2 fluxes and rice grain yields increased significantly with increasing urea application (110–130 kg N ha−1) and thereafter, decreased. However, with increasing 13C-urea application, a significant and proportional increase of the 13CO2C emissions from 13C-urea was also observed. From the relationships between urea application levels and 13CO2C fluxes from 13C-urea, the CO2C emission factor from urea was estimated to range between 0.0143 and 0.0156 Mg C per Mg urea. Thus, the CO2C emission factor of this study is less than that of the value proposed by IPCC. Therefore, for the first time, we propose to revise the current IPCC guideline value of CO2C emission factor from urea as 0.0143–0.0156 Mg C per Mg urea for Korean paddy soils.

Assessing the utility of satellite-based whitecap fraction to estimate sea spray production and CO2 transfer velocity

The utility of a satellite-based whitecap database for estimates of surface sea spray production and bubble-mediated gas transfer on a global scale is presented. Existing formulations of sea spray production and bubble-mediated CO2 transfer velocity involve whitecap fraction parametrization as a function of wind speed at 10 m reference height W(U10) based on photographic measurements of whitecaps. Microwave radiometric measurements of whitecaps from satellites provide whitecap fraction data over the world oceans for all seasons. Parametrizations W(U10) based on such radiometric data are thus applicable for a wide range of conditions and can account for influences secondary to the primary forcing factor, the wind speed. Radiometric satellite-based W(U10) relationship was used as input to: (i) the Coupled Ocean-Atmosphere Response Experiment Gas transfer (COAREG) algorithm to obtain CO2 transfer velocity and total CO2 flux; and (ii) the sea spray source function (SSSF) recommended by Andreas in 2002 to obtain fluxes of sea spray number and mass. The outputs of COAREG and SSSF obtained with satellite-based W(U10) are compared with respective outputs obtained with the nominal W(U10) relationship based on photographic data. Good comparisons of the gas and sea spray fluxes with direct measurements and previous estimates imply that the satellite- based whitecap database can be useful to obtain surface fluxes of particles and gases in regions and conditions difficult to access and sample in situ. Satellite and in situ estimates of surface sea spray production and bubble-mediated gas transfer thus complement each other: accurate in situ observations can constrain radiometric whitecap fraction and mass flux estimates, while satellite observations can provide global coverage of whitecap fraction and mass flux estimates.

Dynamics of microbial community composition and soil organic carbon mineralization in soil following addition of pyrogenic and fresh organic matter.

Pyrogenic organic matter (PyOM) additions to soils can have large impacts on soil organic carbon (SOC) cycling. As the soil microbial community drives SOC fluxes, understanding how PyOM additions affect soil microbes is essential to understanding how PyOM affects SOC. We studied SOC dynamics and surveyed soil bacterial communities after OM additions in a field experiment. We produced and mixed in either 350 °C corn stover PyOM or an equivalent initial amount of dried corn stover to a Typic Fragiudept soil. Stover increased SOC-derived and total CO2 fluxes (up to 6x), and caused rapid and persistent changes in bacterial community composition over 82 days. In contrast, PyOM only temporarily increased total soil CO2 fluxes (up to 2x) and caused fewer changes in bacterial community composition. Of the operational taxonomic units (OTUs) that increased in response to PyOM additions, 70% also responded to stover additions. These OTUs likely thrive on easily mineralizable carbon (C) that is found both in stover and, to a lesser extent, in PyOM. In contrast, we also identified unique PyOM responders, which may respond to substrates such as polyaromatic C. In particular, members of Gemmatimonadetes tended to increase in relative abundance in response to PyOM but not to fresh organic matter. We identify taxa to target for future investigations of the mechanistic underpinnings of ecological phenomena associated with PyOM additions to soil.

Responses of peat carbon at different depths to simulated warming and oxidizing

Warming and water table drawdown greatly reshape peatland carbon cycle, especially when considering the old carbon stored under the peatland subsurface. However, little is known about the effects of warming, oxidizing by drying or their combination on carbon decomposition at different depths (0–100 cm) of peat. In this research, soil of different depths from Zoige Plateau was incubated in four scenarios (8 °C-anaerobic, 8 °C-aerobic, 18 °C-anaerobic and 18 °C-aerobic) to detect the exported carbon. Our result showed that soil respiration (Rs) increased obviously with enhanced temperature and oxygen. The total CO2 fluxes of 2400.22 ± 57.69 mg m-2 d-1 under 8 °C-anaerobic condition increased by 73.6%, 40.7% and 176.5% with warming, oxidizing and their combined effect, respectively. The average dissolved organic carbon (DOC) concentration was 74.90 ± 8.09 mg kg-1 under 8 °C-anaerobic condition, but increased by 53.5%, 44% and 159.4%, respectively under the condition of warming, oxidizing and their combination. Rs and its variation under warming and oxidization differed significantly among different depths, probably caused by the differences of soil substrate, especially the variation in distribution of soil microbes and enzymes among depths of peatlands. By classifying the source of Rs as young soil (YS: 0–20 cm) and old soil (OS: 21–100 cm), this reseaerch found that OS accounted for a huge part of total Rs under 8 °C-anaerobic condition (CO2: 74.2%; DOC: 60.7%). Such relative contribution of OS to total Rs did not change obviously with warming or oxidizing. Though YS and OS responded equally to warming and oxidizing, OS was responsible for a larger proportion of total increase in Rs. Compared with other studies, we concluded that peatlands soil in our field of mid-latitude and high altitude is less sensitive to warming and oxidizing than peatlands of higher latitude, but that OS of this peatlands is more critical in predicting regional carbon cycle.

Does urban vegetation enhance carbon sequestration?

Many cities are developing policies to promote greenery as a measure to reduce their net greenhouse gas emissions. Studies suggest that urban forests may represent an important carbon reservoir. However, the potential to directly remove carbon dioxide (CO2) from the atmosphere by urban vegetation is still poorly supported by scientific evidence. Current assessments consider only the carbon accumulated by trees and usually neglect the contribution from soil respiration and the emissions associated with greenery management. Studies in mid-latitude cities suggest that the carbon uptake by urban vegetation is small compared to the magnitude of the anthropogenic emissions. To investigate if the typically evergreen vegetation in (sub)tropical cities has a larger potential for carbon sequestration, the CO2 flux data from two residential neighborhoods of Singapore and Mexico City were analyzed. Results suggest that (sub)tropical vegetation may act as either an emission source or sink depending on the species and characteristics of the trees and the amount and conditions of pervious surfaces for soil respiration. The biogenic component (vegetation and soil) was found to be a sink of 1Mgkm−2day−1 of CO2 in Mexico City, but an emission source of 0.8Mgday−1km−2 of CO2 in Singapore. The biogenic contribution to the total CO2 flux represents −1.4% and 4.4% at both sites, respectively.

Estimation of CO2 fluxes in the “atmosphere–water” system in the south Lake Baikal littoral with the floating chamber method

In this work, we discuss the applicability of the chamber method for measurements of carbon dioxide fluxes between the atmosphere and water in the littoral of Lake Baikal. Advantages and disadvantages of the method are considered. Previously submitted data are compared with observations in 2011–2014, when dramatically different weather situations occurred. It is shown that interannual variations in the total CO2 fluxes are much weaker than their seasonal variations. The annual flux of carbon dioxide from atmosphere to the water surface in the littoral of Lake Baikal is assessed to be at least 6.5–7 g/m2 per year in an “open water” period.

Plateau pikas burrowing activity accelerates ecosystem carbon emission from alpine grassland on the Qinghai-Tibetan Plateau

Pikas (Ochotona curzoniae) are common on the Qinghai-Tibetan Plateau (QTP). However, little is known about the effect of their burrowing activity on carbon emissions from alpine grassland ecosystems. In this study, we investigated soil organic carbon (SOC), total nitrogen (TN) density and ecosystem CO2 flux from the original vegetation and pika piles of three typical alpine grasslands in two climate regions. SOC loss induced by pika activity varied among grassland types. Alpine meadow with high SOC was susceptible to pika activity. Ecosystem CO2 flux from pika piles with open holes was significantly higher than that from the original vegetation (P<0.05, n=4). When holes near pika piles were blocked, ecosystem CO2 flux from pika piles was not significantly different from that of the original vegetation (P>0.05, n=4). Our results suggested that CO2 dissipates into pika tunnels and is easily emitted into the atmosphere through pika holes. Due to the large amount of pika holes in alpine grasslands, their significant effect on ecosystem CO2 flux should not be ignored in future studies.

Carbon Isotope Composition of Nighttime Leaf-Respired CO2 in the Agricultural-Pastoral Zone of the Songnen Plain, Northeast China.

Variations in the carbon isotope signature of leaf dark-respired CO2 (δ13CR) within a single night is a widely observed phenomenon. However, it is unclear whether there are plant functional type differences with regard to the amplitude of the nighttime variation in δ13CR. These differences, if present, would be important for interpreting the short-term variations in the stable carbon signature of ecosystem respiration and the partitioning of carbon fluxes. To assess the plant functional type differences relating to the magnitude of the nighttime variation in δ13CR and the respiratory apparent fractionation, we measured the δ13CR, the leaf gas exchange, and the δ13C of the respiratory substrates of 22 species present in the agricultural-pastoral zone of the Songnen Plain, northeast China. The species studied were grouped into C3 and C4 plants, trees, grasses, and herbs. A significant nocturnal shift in δ13CR was detected in 20 of the studied species, with the magnitude of the shift ranging from 1‰ to 5.8‰. The magnitude of the nighttime variation in δ13CR was strongly correlated with the daytime cumulative carbon assimilation, which suggests that variation in δ13CR were influenced, to some extent, by changes in the contribution of malate decarboxylation to total respiratory CO2 flux. There were no differences in the magnitude of the nighttime variation in δ13CR between the C3 and C4 plants, as well as among the woody plants, herbs and graminoids. Leaf respired CO2 was enriched in 13C compared to biomass, soluble carbohydrates and lipids; however the magnitude of enrichment differed between 8 pm and 4 am, which were mainly caused by the changes in δ13CR. We also detected the plant functional type differences in respiratory apparent fractionation relative to biomass at 4 am, which suggests that caution should be exercised when using the δ13C of bulk leaf material as a proxy for the δ13C of leaf-respired CO2.

Contribution of abiotic factors to CO2 emission from soils in the freeze–thaw cycles

Three consecutive freeze–thaw cycles (FTCs) have been performed in a laboratory experiment with lessivated burozems (Haplic Luvisols) under forest and agricultural crops in order to assess the contribution of physicochemical factors to the emission of CO2 from soils in consecutive FTCs. The use of native soils and their sterilized analogues (to exclude the effect of biotic factors) has allowed a quantitative description of physicochemical processes during the FTCs. It has been revealed that the ratio between the biotic and physicochemical factors during the FTCs largely depends on soil solution pH. The proportion of physicochemical factors is low (1.3–9.4%) in the frozen forest soil with low pH (3.9–4.1), while it is no less than 52% in the neutral arable soils, which indicates the prevalent contribution of physicochemical processes to the emission of CO2 from these soils under freezing. CO2 sorption has been observed in the thawed forest soils at an insignificant rate, which we estimate at only 2.3–3.6% of the total CO2 flux from the soil, which confirms the biotic nature of increase in CO2 emission bursts in the acid forest soils under thawing. In the arable soils, along with the enhanced CO2 emission caused by the activation of microbial community, a significant participation of physicochemical processes has been observed under thawing conditions, which makes up 17–20% of the total CO2 flux released during thawing.

ENaC inhibition stimulates HCl secretion in the mouse cortical collecting duct. II. Bafilomycin-sensitive H+ secretion.

Epithelial Na(+) channel (ENaC) blockade stimulates stilbene-sensitive conductive Cl(-) secretion in the mouse cortical collecting duct (CCD). This study's purpose was to determine the co-ion that accompanies benzamil- and DIDS-sensitive Cl(-) flux. Thus transepithelial voltage, VT, as well as total CO2 (tCO2) and Cl(-) flux were measured in CCDs from aldosterone-treated mice consuming a NaCl-replete diet. We reasoned that if stilbene inhibitors (DIDS) reduce conductive anion secretion they should reduce the lumen-negative VT. However, during ENaC blockade (benzamil, 3 μM), DIDS (100 μM) application to the perfusate reduced net H(+) secretion, which increased the lumen-negative VT. Conversely, ENaC blockade alone stimulated H(+) secretion, which reduced the lumen-negative VT. Application of an ENaC inhibitor to the perfusate reduced the lumen-negative VT, increased intercalated cell intracellular pH, and reduced net tCO2 secretion. However, benzamil did not change tCO2 flux during apical H(+)-ATPase blockade (bafilomycin, 5 nM). The increment in H(+) secretion observed with benzamil application contributes to the fall in VT observed with application of this diuretic. As such, ENaC blockade reduces the lumen-negative VT by inhibiting conductive Na(+) absorption and by stimulating H(+) secretion by type A intercalated cells. In conclusion, 1) in CCDs from aldosterone-treated mice, benzamil application stimulates HCl secretion mediated by the apical H(+)-ATPase and a yet to be identified conductive Cl(-) transport pathway; 2) benzamil-induced HCl secretion is reversed with the application of stilbene inhibitors or H(+)-ATPase inhibitors to the perfusate; and 3) benzamil reduces VT not only by inhibiting conductive Na(+) absorption, but also by stimulating H(+) secretion.

Effects of experimental warming and nitrogen addition on soil respiration and CH4 fluxes from crop rotations of winter wheat–soybean/fallow

Soil respiration and CH4 emissions play a significant role in the global carbon balance. However, in situ studies in agricultural soils on responses of soil respiration and CH4 fluxes to climate warming are still sparse, especially from long-term studies with year-round heating. A warming experiment was conducted at Luancheng research station in the North China Plain from 2008 to 2013. Two levels of temperature (T: increase on average 1.5°C at 5cm soil depth by infrared heaters, C: ambient temperature) were combined with two levels of nitrogen (N) treatments (N1: with 315kgNha−1 y−1, N0: no nitrogen input) in the farmland.Soil was found to be a sink for CH4 with no marked seasonal variations. In the wheat-growing season, warming and N input both decreased cumulative CH4 uptake, probably because warming-induced soil drying in N1 treatment reduced (or limited) methanotroph activity by affecting soil NH4 concentration. Across years, CH4 emissions were negatively correlated with soil temperature in N1 treatment.Soil respiration showed clear seasonal fluctuations, with the largest emissions during summer and smallest in winter. Warming and nitrogen fertilization had no significant effects on total cumulative soil CO2 fluxes. Soil respiration was positively correlated with microbial biomass C, and microbial biomass C was not affected significantly by warming or nitrogen addition. The lack of significant effects of warming on soil respiration may have resulted from: (1) warming-induced soil drying offsetting the effects of soil temperature; or (2) adaption of soil respiration to increased temperature.

Degassing regime of Hekla volcano 2012–2013

Hekla is a frequently active volcano with an infamously short pre-eruptive warning period. Our project contributes to the ongoing work on improving Hekla’s monitoring and early warning systems. In 2012 we began monitoring gas release at Hekla. The dataset comprises semi-permanent near-real time measurements with a MultiGAS system, quantification of diffuse gas flux, and direct samples analysed for composition and isotopes (δ13C, δD and δ18O). In addition, we used reaction path modelling to derive information on the origin and reaction pathways of the gas emissions.Hekla’s quiescent gas composition was CO2-dominated (0.8mol fraction) and the δ13C signature was consistent with published values for Icelandic magmas. The gas is poor in H2O and S compared to hydrothermal manifestations and syn-eruptive emissions from other active volcanic systems in Iceland. The total CO2 flux from Hekla central volcano (diffuse soil emissions) is at least 44Td−1, thereof 14Td−1 are sourced from a small area at the volcano’s summit. There was no detectable gas flux at other craters, even though some of them had higher ground temperatures and had erupted more recently.Our measurements are consistent with a magma reservoir at depth coupled with a shallow dike beneath the summit. In the current quiescent state, the composition of the exsolved gas is substantially modified along its pathway to the surface through cooling and interaction with wall-rock and groundwater. The modification involves both significant H2O condensation and scrubbing of S-bearing species, leading to a CO2-dominated gas emitted at the summit. We conclude that a compositional shift towards more S- and H2O-rich gas compositions if measured in the future by the permanent MultiGAS station should be viewed as sign of imminent volcanic unrest on Hekla.

Diffuse volcanic gas emission and thermal energy release from the summit crater of Pico do Fogo, Cape Verde

We report the first detailed study of diffuse emission of carbon dioxide (CO2), hydrogen sulfide (H2S), helium (He), and hydrogen (H2) from the summit crater of Pico do Fogo volcano, Cape Verde. Diffuse CO2, H2S, He, and H2 gas fluxes were measured at 57 sampling sites and ranged up to 12,800, 13, 1, and 6 g m−2 day−1, respectively. Soil temperature measurements at each sampling site were used to evaluate the heat flux. Most of the summit crater shows relatively high CO2 efflux, with highest values close to the fumarolic area, suggesting a structural control of the degassing process. In contrast, H2S effluxes were negligible or very low at the summit crater, except close to the fumarolic area where anomalously high CO2 efflux and soil temperatures were also measured. We estimate total CO2, H2S, He, and H2 diffuse gas fluxes of 219 t day−1, 25, 4, and 33 kg day−1, respectively. Based on a H2O/CO2 mass ratio of 1.52 measured at the fumaroles, we estimate a diffuse steam flux from the summit crater of approximately 330 t day−1. The enthalpy of this steam is equivalent to a heat flux of about 10.3 MW. The diffuse gas emission and thermal energy released from the summit crater of Pico do Fogo volcano are comparable to those observed at other volcanoes. Sustained surveillance of Pico do Fogo using these methods will be valuable for monitoring the activity of one of the most active volcanoes in the Atlantic Ocean.

Process-based isotope partitioning of winter soil respiration in a subalpine ecosystem reveals importance of rhizospheric respiration

Deep snow in sub-alpine ecosystems may reduce or eliminate soil freezing, thus contributing to the potential for winter soil respiration to account for a significant fraction of annual CO2 efflux to the atmosphere. Quantification of carbon loss from soils requires separation of respiration produced by roots and rhizosphere organisms from that produced by heterotrophic, decomposer organisms because the former does not result in a net loss of stored carbon. Our objective was to quantify winter soil respiration rates in a sub-alpine forest and meadow, and to partition that flux into its rhizosphere and heterotrophic components. We were particularly interested in comparing early winter soil respiration to late winter/early spring soil respiration of each component because previous work has shown a consistent increase in soil respiration of subalpine systems from early winter to late winter/spring. Field data on the total soil CO2 flux and its carbon isotope composition were coupled with data from laboratory incubations using a novel process-based stable isotope mixing model implemented in a hierarchical Bayesian framework. We found that soil respiration generally increased from early to later winter and was greatest mid-summer. After correcting for the effect of wind on snowpack δ13C–CO2, the δ13C of soil-respired CO2 varied little over winter, and the contributions of rhizospheric (~35 %) and heterotrophic (~65 %) respiration were relatively constant. The significance of winter respiration from the rhizosphere and apparent coupling of increases in rhizospheric and heterotrophic respiration in late winter are likely to be important for predicting changes in soil carbon in sub-alpine ecosystems.

Soil CO2 and N2O Emissions in Maize Growing Season Under Different Fertilizer Regimes in an Upland Red Soil Region of South China

Upland red soils have been identified as major CO2 and N2O sources induced by human activities such as fertilization. To monitor characteristics of soil surface CO2 and N2O fluxes in cropland ecosystems after continuous fertilizer applications over decades and to separate the respective contributions of root and heterotrophic respiration to the total soil CO2 and N2O fluxes, the measurements of soil surface CO2 and N2O fluxes throughout the maize growing season in 2009 were carried out based on a fertilization experiment (from 1990) through of the maize (Zea mays L.) growing season in red soil in southern China. Five fertilization treatments were chosen from the experiment for study: zero-fertilizer application (CK), nitrogen-phosphorus-potassium (NPK) fertilizer application only, pig manure (M), NPK plus pig manure (NPKM) and NPK with straw (NPKS). Six chambers were installed in each plot. Three of them are in the inter-row soil (NR) and the others are in the soil within the row (R). Each fertilizer treatment received the same amount of N (300 kg ha−1 yr−1). Results showed that cumulative soil CO2 fluxes in NR or R were both following the order: NPKS>M, NPKM>NPK>CK. The contributions of root respiration to soil CO2 fluxes was 40, 44, 50, 47 and 35% in CK, NPK, NPKM, M and NPKS treatments, respectively, with the mean value of 43%. Cumulative soil N2O fluxes in NR or R were both following the order: NPKS, NPKM>M>NPK>CK, and soil N2O fluxes in R were 18, 20 and 30% higher than that in NR in NPKM, M and NPKS treatments, respectively, but with no difference between NR and R in NPK treatment. Furthermore, combine with soil temperature at −5 cm depth and soil moisture (0–20 cm) together could explain 55–70% and 42–59% of soil CO2 and N2O emissions with root interference and 62–78% and 44–63% of that without root interference, respectively. In addition, soil CO2 and N2O fluxes per unit yield in NPKM (0.55 and 0.10 kg C t−1) and M (0.65 and 0.13 g N t−1) treatments were lower than those in other treatments. Therefore, manure application could be a preferred fertilization strategy in red soils in South China.

Estimating the Relative Contributions of Root Respiration and Peat Decomposition to the Total CO2 Flux from Peat Soil at an Oil Palm Plantation in Sumatra, Indonesia

The decomposition of organic matter infl uences CO2 fl ux in peat soils. However, determination of the fl ux from the decomposition process is usually over-estimated because the total CO2 fl ux calculation includes root respiration. We clarified this issue addressing the following aims: (i) to study the relation of oil palm root distribution in peat soils to the fl ux and (ii) to estimate the relative contribution of root respiration and peat decomposition to the total CO2 fl ux. The study was conducted between January and June 2012. In the research area, three transects were established perpendicular to drainage channels, where nine14- year-old oil palm trees were selected and used as observation sites. Eight closed chambers were established as observation points at each site. We measured CO2 fl ux at each point using an Infra Red Gas Analyzer (IRGA). Root and peat samples were collected from each observation point to measure root density and analyze peat chemical properties. Our results showed that the pHH2O and nutrient content of P, K, Ca and Mg in the peat soils significantly increased of CO2 fl ux. Oil palm roots at depths of 0-15 and 15-30 cm nearest the tree showed the highest density, while root densities gradually decreased with increasing distance from the tree. CO2 fl ux in the peat soils nearest to the tree were highest at 0.44 ± 0.23 mg CO2 m- 2 sec- 1 (or 137.7 ± 73.4 t CO2 ha- 1 yr - 1 ). CO2 fl ux significantly decreased with increasing distance from the tree, showing the lowest value of 0.10 ± 0.04 mg CO2 m- 2 sec- 1 (or 30.67 ± 12.4 t CO2 ha - 1 yr - 1 ). We conclude that the CO2 fl ux was derived from peat decomposition and root respiration. Using the integral equation approach, the relative contribution of root respiration and peat decomposition was 74 and 26%, respectively to the total CO2 fl ux.

The role of vegetation in the CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; flux from a tropical urban neighbourhood

Abstract. Urban surfaces are usually net sources of CO2. Vegetation can potentially have an important role in reducing the CO2 emitted by anthropogenic activities in cities, particularly when vegetation is extensive and/or evergreen. A direct and accurate estimation of carbon uptake by urban vegetation is difficult due to the particular characteristics of the urban ecosystem and high variability in tree distribution and species. Here, we investigate the role of urban vegetation in the CO2 flux from a residential neighbourhood in Singapore using two different approaches. CO2 fluxes measured directly by eddy covariance are compared with emissions estimated from emissions factors and activity data. The latter includes contributions from vehicular traffic, household combustion, soil respiration and human breathing. The difference between estimated emissions and measured fluxes should approximate the flux associated with the aboveground vegetation. In addition, a tree survey was conducted to estimate the annual CO2 sequestration using allometric equations and an alternative model of the metabolic theory of ecology for tropical forests. Palm trees, banana plants and turfgrass were also included in the survey with their annual CO2 uptake obtained from published growth rates. Both approaches agree within 2% and suggest that vegetation sequesters 8% of the total emitted CO2 in the residential neighbourhood studied. An uptake of 1.4 ton km−2 day−1 (510 ton km−2 yr−1) was estimated as the difference between assimilation by photosynthesis minus the aboveground biomass respiration during daytime (4.0 ton km−2 day−1) and release by plant respiration at night (2.6 ton km−2 day−1). However, when soil respiration is added to the daily aboveground flux, the biogenic component becomes a net source amounting to 4% of the total CO2 flux and represents the total contribution of urban vegetation to the carbon flux to the atmosphere.

Global air-sea surface carbon dioxide transfer velocity and flux estimated using 17 a altimeter data and a new algorithm

The global distributions of the air-seaCO(2) transfer velocity and flux are retrieved from TOPEX/Poseidon and Jason altimeter data from October 1992 to December 2009 using a combined algorithm. The 17 a average global, area-weighted, Schmidt number-corrected mean gas transfer velocity is 21.26 cm/h, and the full exploration of the uncertainty of this estimate awaits further data. The average total CO2 flux (calculated by carbon) from atmosphere to ocean during the 17 a was 2.58 Pg/a. The highest transfer velocity is in the circumpolar current area, because of constant high wind speeds and currents there. This results in strongCO(2) fluxes. CO2 fluxes are strong but opposite direction in the equatorial east Pacific Ocean, because the air-sea CO2 partial pressure difference is the largest in the global oceans. The results differ from the previous studies calculated using the wind speed. It is demonstrated that the air-sea transfer velocity is very important for estimating air-sea CO2 flux. It is critical to have an accurate estimation for improving calculation of CO2 flux within climate change studies.

Can soil respiration estimate neglect the contribution of abiotic exchange?

This study examines the hypothesis that soil respiration can always be interpreted purely in terms of biotic processes, neglecting the contribution of abiotic exchange to CO2 fluxes in alkaline soils of arid areas that characterize 5% of the Earth’s total land surface. Analyses on flux data collected from previous studies suggested reconciling soil respiration as organic (root/microbial respiration) and inorganic (abiotic CO2 exchange) respiration, whose contributions in the total CO2 flux were determined by soil alkaline content. On the basis of utilizing meteorological and soil data collected from the Xinjiang and Central Asia Scientific Data Sharing Platform, an incorporated model indicated that inorganic respiration represents almost half of the total CO2 flux. Neglecting the abiotic module may result in overestimates of soil respiration in arid alkaline lands, which partly explains the long-sought “missing carbon sink”.

Differences in carbon and nitrogen mineralization in soils of differing initial pH induced by electrokinesis and receiving crop residue amendments

Initial soil pH has been reported to have a great impact on the decomposition of added organic materials and hence to determine the direction and magnitude of subsequent soil pH changes. However, most previous investigations have been conducted on different soil types differing in initial pH and other soil properties. Here, we investigated the effects of initial soil pH on the subsequent soil pH and N mineralization changes caused by addition of crop residues to two soils (a Paleudalf and a Plinthudult) with different pHs induced by treatment with direct electric current. This produced pH gradients of 6.50–3.20 and 6.74–3.81, respectively. Three typical field crop residues differing in C/N ratio, i.e. rice straw C/N = 42, canola residue C/N = 36 and Chinese milk vetch C/N = 14 (vetch), were incubated with the soils for 102 days. With both soils, total CO2 fluxes differed between the type of added crop residues, with vetch > canola residue > rice straw, and decreased with decreasing initial soil pH. The incorporation of crop residues into the two soils at all pHs increased soil pH except for the Paleudalf at pH 3.81 and 4.25 amended with rice straw and canola residue, where pH slightly decreased. As expected, vetch incorporation caused a greater pH increase than the two non-legume residues (rice straw and canola residue), but this effect was transient in soils of higher initial pH. The pH declined rapidly with time in soils of initial pH ≥ 4.40 treated with vetch, while addition of rice straw and canola residue maintained stable pHs. Irrespective of crop residue addition, soil pH continuously increased or remained steady over the 102-day incubation period in the highly acidic soils (pH ≤ 4.25). While NH4+ accumulated in highly acidic soils, NO3− accumulated in higher pH soils. Contrasting effects on nitrification and subsequent pH changes occurred between soil pH ≤ 4.25 and pH ≥ 4.40. However, no clear relationship between net N mineralization and soil pH was found. This study demonstrated that low soil pH greatly inhibited nitrification, while net N mineralization was generally less affected. This, in turn, affected the direction and extent of soil pH changes after addition of crop residues.