Elevated CO2 Emissions Research Articles

Soil warming and nitrogen deposition alter soil respiration, microbial community structure and organic carbon composition in a coniferous forest on eastern Tibetan Plateau

Understanding the response of soil respiration (Rs) to future climate projections is critical for understanding feedbacks and developing effective mitigation strategies. We conducted a warming and N deposition manipulation experiment using a 2 × 2 full-factorial design to evaluate the interactive effects of soil warming and N deposition on Rs, microbial community composition and soil organic carbon (SOC) composition in a subalpine coniferous forest on the eastern slope of the Qinghai-Tibetan Plateau. Soil warming and N addition independently increased Rs and changed microbial community composition, resulting in a net increase in bacteria with N addition and an increase in protozoa with warming; but the interaction of these two factors proved to be antagonistic, resulting in a lower Rs and protozoa concentration in the combined warming and nitrogen addition (WN) treatment relative to the warming (W) treatment. The chemical structure of SOC changed with the treatments, with alkyl C increased with warming, aromatic C decreased with both warming and N addition, and the interaction of warming and N induced a lower O-alkyl C and a higher contribution of alkyl C to SOC. The negative correlation between Rs and aromatic C and the positive correlation between Rs and alkyl C and MBC indicates that the elevated CO2 emission under warming and N addition was related to the decomposition of aromatic C and the following relative accumulation of alkyl C. Our observations suggest that climate induced soil warming will likely increase soil CO2 emissions, but this effect could be largely offset by a simultaneous increase in N deposition in subalpine coniferous forests.

Seasonal variability of CO2 fluxes at different interfaces and vertical CO2 concentration profiles within a Rhizophora mangrove forest (Can Gio, Viet Nam)

Despite being organic-rich, mangrove soils are usually characterized by low rates of CO2 emissions due to waterlogged conditions, mangroves being thus considered as valuable blue carbon sinks. However, these emissions are highly variable, notably depending on climatic parameters. Can Gio mangrove is a tropical one, being the largest in Vietnam, having being replanted after the war in the late 70's. The main objectives of this study were to quantify the seasonal variability of CO2 emissions at different interfaces (soil-air, water-air, and trunk-air) within a mature Rhizophora stand, and to assess the influence of these emissions on the vertical CO2 concentration profiles in the canopy. CO2 emissions from the soil in the dark were the highest of the different interfaces studied, and were among the highest measured for a mangrove soil, with a mean yearly value reaching more than 270 mmol CO2 m−2 d−1. We suggest that the very high temperatures characterizing Southern Vietnam all year round were responsible for these elevated CO2 emissions. In addition to temperature, the first rainfall pulse of the monsoon resulted in the highest CO2 fluxes both from the soil, reaching up to 500 mmol CO2 m−2 d−1, and from the trunks, reaching almost 230 mmol CO2 m−2 d−1. Because rainfall pulses are usually followed by a period of increasing rates of ecosystem photosynthesis, and a decrease of ecosystem respiration, we measured lower CO2 fluxes during the middle of the wet season. Additionally, it is possible that, during the rainy season, part of the CO2 produced within the soil was exported trough pore-water seepage, which was enhanced because of the higher level of the river. Concerning CO2 emissions at high tide within the Rhizophora stand, the seasonal trend followed the one of precipitation. During the rainy season, estuarine waters are enriched in organic matter and CO2 due to increased runoff in watersheds, and consequently CO2 fluxes from the water column were the highest. However, these emissions were much lower than those from the soil at low tide or from the trunks. Eventually, CO2 concentrations profiles in the canopy varied with the season, the tide, and the night-day alternation. Elevated CO2 concentrations at the root level may imply that prop roots can produce high amount of CO2 but also that they may trap gas emitted from the soil or the water column, possibly acting as physical barriers to air movement. Further studies using eddy-covariance technique should be developed to confirm the hypothesis suggested in this preliminary study.

Rates and spatial variability of peat subsidence in Acacia plantation and forest landscapes in Sumatra, Indonesia

Many peatlands in Europe and North America have been developed for agriculture for over a century, whilst in Southeast Asia development has largely occurred since 1990. Cultivation of drained peatlands now supports the livelihoods of large numbers of people, and the ongoing economic development of countries such as Indonesia and Malaysia. However, peat subsidence linked to plantation drainage represents both an environmental and a socio-economic challenge, associated with elevated CO2 emissions, impacts on adjacent forest habitat, and long-term changes in plantation drainability. Whilst the fundamental challenges presented by peat subsidence are broadly recognised, the long-term rates and the potential for mitigation or avoidance of subsidence remain uncertain. We analysed over 2000 site-years of subsidence measurements from 312 sites in Sumatra, Indonesia, collected under Acacia pulpwood plantation and adjacent native forest, representing the largest peat subsidence dataset published to date. Subsidence averaged 4.3 cm yr−1 in the Acacia plantations, and extended at least 300 m into adjacent forest. Mean water table depth (WTD) was the best predictor of subsidence rate in both plantation and forest areas. We did not find conclusive evidence that subsidence was intrinsically faster under Acacia plantation than under native forest or (by comparison with previous studies) oil palm plantations for the same level of drainage. Our results suggest that raising average WTDs to the Indonesian Government's 40 cm target could – if practically and economically viable means of achieving this can be developed – reduce current plantation subsidence rates by 25–30%. Whilst some degree of peat subsidence under any form of plantation management may be unavoidable, these reductions would – if achieved at scale – both increase the economic lifetime of the plantations, and simultaneously deliver reductions in CO2 emissions of national and global significance.

Optimisation of cropping pattern considering stomatal response to elevated CO<sub align="right">2 emission and climate change

The current article seeks to explore the function of elevated CO2 emission on the variations of reference evapotranspiration (ET0) for optimising cropping pattern in 18 Iranian regions. Accordingly, meteorological data and agricultural information were collected since 1961 to 2010 and 2002 to 2013, respectively. A trend-free pre-whitening (TFPW) approach was applied to modify Mann-Kendall test to determine the ET0 trend based on the Food and Agriculture Organization of the United Nation (FAO)-Penman-Monteith (FPM) method. Then, the function of stomatal response was investigated as an assimilation coefficient to adjust the ET0 since 2050 to 2100. In the next step, the anomalies of the ET0 were calculated in both monthly and annual scales with respect to the significant trends of the meteorological parameters and stomatal response via three different schemes. Finally, to optimise cropping pattern, two indicators namely total virtual water (TVW) and unit blue water value (UBWV) were employed.

Optimisation of cropping pattern considering stomatal response to elevated CO<sub align="right">2 emission and climate change

The current article seeks to explore the function of elevated CO2 emission on the variations of reference evapotranspiration (ET0) for optimising cropping pattern in 18 Iranian regions. Accordingly, meteorological data and agricultural information were collected since 1961 to 2010 and 2002 to 2013, respectively. A trend-free pre-whitening (TFPW) approach was applied to modify Mann-Kendall test to determine the ET0 trend based on the Food and Agriculture Organization of the United Nation (FAO)-Penman-Monteith (FPM) method. Then, the function of stomatal response was investigated as an assimilation coefficient to adjust the ET0 since 2050 to 2100. In the next step, the anomalies of the ET0 were calculated in both monthly and annual scales with respect to the significant trends of the meteorological parameters and stomatal response via three different schemes. Finally, to optimise cropping pattern, two indicators namely total virtual water (TVW) and unit blue water value (UBWV) were employed.

The influence of high pCO2 on otolith shape, chemical and carbon isotope composition of six coastal fish species in a Mediterranean shallow CO2 vent

Naturally acidified environments, such as CO2 vents, are important sites to evaluate the potential effects of increased ocean acidification on marine ecosystems and biota. Here we assessed the effect of high CO2/low pH on otolith shape and chemical composition of six coastal fish species (Chromis chromis, Coris julis, Diplodus vulgaris, Gobius bucchichi, Sarpa salpa, Symphodus ocellatus) in a Mediterranean shallow CO2 vent. Taking into consideration the major and trace elements found near the vent and the gradient of dissolved inorganic carbon, we compared the otolith chemical signatures of fish exposed long-term to elevated CO2 emissions and reduced pH (mean pH 7.8) against fish living in two control sites (mean pH 8.2). A number of element:Ca ratios (Na:Ca, Mg:Ca, Mn:Ca, Cu:Ca, Zn:Ca, Sr:Ca, Ba:Ca and Pb:Ca), along with isotope ratios, were measured in otoliths (δ13C and δ18O) and water (δ13CDIC) samples. Additionally, we performed otolith outline shape and morphometric analysis to evaluate the effect of high CO2/low pH. We observed species-specific responses with regards to both shape and chemical signatures. Significant differences among sites were found in otolith shape (elliptical Fourier descriptors) of G. bucchichi and D. vulgaris. Elemental and isotopic signatures were also significantly different in these site attached species, though not for the other four. Overall, the carbon isotopic composition seems a good proxy to follow pH gradient in naturally acidified area. Ultimately, besides improving our knowledge of the effects of high CO2/low pH on otoliths, the present results contribute to our understanding on their use as natural tags.

Potential short-term effects of yak and Tibetan sheep dung on greenhouse gas emissions in two alpine grassland soils under laboratory conditions

Yak and Tibetan sheep graze extensively on natural grasslands in the Qinghai-Tibetan Plateau, and large amounts of excrement are directly deposited onto alpine grasslands. However, information on greenhouse gas (GHG) emissions from this excrement is limited. This study evaluated the short-term effects of yak and Tibetan sheep dung on nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) emissions from alpine steppe soil at a water holding capacity (WHC) of 40 or 60 % and from alpine meadow soil at a WHC of 60 or 80 % under laboratory conditions. Cumulative N2O emissions over a 15-day incubation period at low soil moisture conditions ranged from 111 to 232 mu g N2O-N kg soil(-1) in the yak dung treatments, significantly (P < 0.01) higher than that of sheep dung treatments (28.7 to 33.7 mu g N2O-N kg soil(-1)) and untreated soils (1.04-6.94 mu g N2O-N kg soil(-1)). At high soil moisture conditions, N2O emissions were higher from sheep dung than yak dung and non-treated soils. No significant difference was found between the yak dung and non-treated alpine meadow soil at 80 % WHC. Low N2O emission in the yak dung treatment from relatively wet soil was probably due to complete denitrification to N-2. Yak dung markedly (P < 0.001) increased CH4 and CO2 emissions, likely being the main source of these two gases. The addition of sheep dung markedly (P < 0.001) elevated CO2 emissions. Dung application significantly (P < 0.01) increased global warming potential, particularly for alpine steppe soil. In conclusion, our findings suggest that yak and Tibetan sheep dung deposited on alpine grassland soils may increase GHG emissions.

Implication of erosion on the assessment of decomposition and humification of soil organic carbon after land use change in tropical agricultural systems

Soil organic carbon decline after land use change from forest to maize usually leads to soil degradation and elevated CO2 emissions. However, limited knowledge is available on the interactions between rates of SOC change and soil erosion and how SOC dynamics vary with soil depth and clay contents. The 13C isotope based CIDE approach (Carbon Input, Decomposition and Erosion) was developed to determine SOC dynamics on erosion prone slopes. The aims of the present study were: (1) to test the applicability of the CIDE approach to determine rates of decomposition and SOC input under particular considerations of concurrent erosion events on three soil types (Alisol, Luvisol, Vertisol), (2) to adapt the CIDE approach to deeper soil layers (10–20 and 20–30 cm) and (3) to determine the variation of decomposition and SOC input with soil depth and soil texture. SOC dynamics were determined for bulk soil and physically separated SOC fractions along three chronosequences after land use change from forest to maize (up to 21 years). Consideration of the effects of soil erosion on SOC dynamics by the CIDE approach yielded a higher total SOC loss (6–32%), a lower decomposition (13–40%) and a lower SOC input (14–31%) relative to the values derived from a commonly applied 13C isotope based mass balance approach. Comparison of decomposition between depth layers revealed that tillage accelerated decomposition in the plough layer (0–10 cm), accounting for 3–34% of total decomposition. With increasing clay contents SOC input and mass of sand sized stable aggregates increased. In addition, decomposition increased with increasing clay contents, too, being attributed to decomposition of labile SOC which was attached to clay particles in the sand sized stable aggregate fraction. In conclusion, this study suggests that in situ SOC dynamics on erosion prone slopes are commonly misrepresented by erosion unadjusted approaches leading to an overestimation of SOC input and underestimation of SOC loss.

Role of non-mantle CO2 in the dynamics of volcano degassing: The Mount Vesuvius example

Mount Vesuvius, Italy, quiescent since A.D. 1944, is a dangerous volcano currently characterized by elevated CO2 emissions of debated origin. We show that such emissions are most likely the surface manifestation of the deep intrusion of alkalic-basaltic magma into the sedimentary carbonate basement, accompanied by sidewall assimilation and CO2 volatilization. During the last eruptive period (1631–1944), the carbonate-sourced CO2 made up 4.7–5.3 wt% of the vented magma. On a yearly basis, the resulting CO2 production rate is comparable to CO2 emissions currently measured in the volcanic area. The chemical and isotopic composition of the fumaroles supports the predominance of this crust-derived CO2 in volatile emissions at Mount Vesuvius.

Microbial eco-physiology of degrading aral sea wetlands: Consequences for C-cycling

The effects of elevated salinity level on the physiology of microbial communities and C-cycling were evaluated in soils located in areas near the degraded Aral Sea. One site on a freshwater wetland (FW) and two sites on a salt water wetland on the eastern (EW) and northern shores (NW) with salt contents of 1.1, 34.3, and 78.9 g L-1, respectively, were selected. The total microbial biomass estimated by fumigation-extraction and the respiration rate were lower at the two sites with elevated salinity level (EW and NW) than in FW while the amount of extractable organic carbon (EOC) was higher. The EOC/biomass-C ratio as an index of carbon supply was highest in NW where the lowest microbial activity was detected, indicating the restriction of microbial metabolism. Thus elevated salinity level appeared to restrict microbial colonization of organic substrates and C transformation rate but to increase the amount of soil available C. Difference in precipitation (rain events) between 2002 and 2003 did not affect the respiration rate and surface CO2 flux in FW. In NW, the respiration rate increased by more than 5 times, suggesting that in the ecosystems with a high content of available C, elevated CO2 emissions may occur under changing environmental conditions.