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The feedbacks of climate variability on CO2 consumption fluxes and carbon dynamics are thought to play an important role in moderating the global carbon cycle. High-frequency sampling campaigns and analyses were conducted in this study to investigate temporal variations of river water chemistry and the impacts of climate variability on CO2 consumption fluxes and carbon dynamics for the Xijiang River, Southwest China. Physical processes modify biogeochemical processes, so major ions display different responses to changing discharge. The annual CO2 consumption rate is (6.8 +/- 0.2) x 10(6) ton/year by carbonate weathering and (2.4 +/- 0.3) x 10(6) ton/year by silicate weathering. The annual CO2 consumption flux is much higher than most world rivers, and strong CO2 consumption capacities are observed in catchments in Southwest China. Lower negative delta C-13(DIC) values are found in the high-flow season which corresponds with high temperatures compared to those in the low-flow season. High discharge will accelerate material transport, and high temperatures will increase primary production in the catchment, both of which can be responsible for the shift of delta C-13(DIC) values in the high-flow season. Increased mineral weathering and biological carbon influx in the catchment are the main factors controlling carbon dynamics. Overall, these findings highlight the sensitivity of CO2 consumption fluxes and carbon dynamics in response to climate variability in the riverine systems. Plain Language Summary There are feedbacks between climate variability and CO2 consumption fluxes by chemical weathering and carbon dynamics. Significant temporal variations of major ions and delta C-13(DIC) are observed in the Xijiang River. Multiple biogeochemical processes occur under various hydrological conditions, shifting major ions concentrations, and delta C-13(DIC). The annual CO2 consumption rate is (6.8 +/- 0.2) x 10(6) ton/year by carbonate weathering and (2.4 +/- 0.3) x 10(6) ton/year by silicate weathering. The annual CO2 consumption rates in the Xijiang River only account for a small fraction in the global CO2 consumption rates; the CO2 consumption capacity is much higher than the global average, while much lower than its source tributaries (Beipan and Nanpan Rivers). Slow subsurface flow paths with longer transit times switch to rapid near-surface flow paths with shorter transit times, as discharge increases. High temperatures increase reaction rates, and high discharge rates remove transport limitation, both of which would accelerate the chemical weathering rates. In the high-flow season, high discharge accompanying with high temperatures, large amounts of delta C-13-depleted biological carbon, flushes into the river, affecting the carbon dynamics.

A new technique to control oxygen fugacity in situ under high-temperature and high-pressure conditions has been developed. In this method a special sample assembly constructed of a reference buffer, yttrium-stabilized zirconia (YSZ) disks, a sample, and an oxygen reservoir was used. Through a driving voltage e0 between the sample and the oxygen reservoir, oxygen was moved from the reservoir into the sample, or from the sample into the reservoir. The rate of oxygen transport was adjusted by regulating e0 and the thickness of the YSZ disk d0. The oxygen fugacity in the sample and its variation during the process were monitored in situ by measuring the voltage between the sample and reference buffer. In this way the oxygen fugacity in the sample was controlled independently of temperature, pressure, and composition of the sample. We made experiments at 773–994 K and 1.0–4.0 GPa in the systems Cu-O, C-O, and Ni-O, and the results show that the technique is successful.

[1] To assess the utility of epilithic mosses as bio-monitors of atmospheric N deposition in Southern China, single and multiple moss species were collected in forested, agricultural, suburban, urban, and sewage landfill habitats, and then analyzed for N tissue content andδ15N values. About 83% of all the moss samples showed an N content of >2.0% and significant differences were only found between forests and the other four kinds of areas. But no significant differences were found between single and multiple moss species at each area. Six previously reported relationships between N deposition and N contents of single moss species and four of multiple moss species were assessed to be used for estimation of N deposition. In the selected areas of South China, the average of N deposition estimated using the equations of single and multiple moss species was 30.8 to 41.6 kg N ha−1 yr−1 and 26.0 to 34.9 kg N ha−1 yr−1, respectively. And the former showed much larger standard deviation of N deposition than the latter, suggesting that the equations of multiple moss species might be preferred to be used for estimation of N deposition. There were significant differences in moss δ15N values in different kinds of areas except between urban areas and sewage landfills, where mosses were most 15N-depleted (−10‰ to −2‰, reflecting sewage-derived NHy). For both single and multiple moss species, those in suburban areas were between −5‰ to −1‰, reflecting the mixing of sewage-derived and soil-derived NHy. The δ15N values of mosses in forested (−3‰ to +3‰) and agricultural areas (−1‰ to +1‰) indicated soil-derived NHy and chemical fertilizer.

[1] Measurement of dry N deposition by physical methods is time-consuming because it is usually difficult to measure directly. In this study, an alternative approach has successfully been proposed by coupling isotopic ratios with tissue N contents of epilithic mosses. The method is to use moss N contents to quantitatively estimate total N (TN) deposition and then to use 15N natural abundance in mosses to discriminate dry and wet deposition in atmospheric NHy-dominated areas (NHy/TN > 0.75). On the basis of the isotopic balance between atmospheric NHy and moss tissue N and the correlation between atmospheric NHy concentrations and their isotopes, both wet and dry N deposition can be estimated. By the approach, we have estimated rainwater ammonium concentrations and contribution percentage of wet deposition to total N deposition (fwet) in some areas of southern China. The results indicated that rainwater ammonium concentrations increased relative to those reported previously in most cities, owing to stronger anthropogenic activity. The fwet values estimated in most sites were found to be slightly higher than those reported, because faster transformation rates due to higher SO2 emission later in acid rain areas of southern China favored deposition in the form of wet deposition instead of dry deposition. The largest uncertainty of the approach comes from the influence of NOx in the atmosphere, and thus it cannot be used in areas where NOx deposition is high. The presented isotopic approach represents a new application of moss biomonitoring for estimating atmospheric N deposition in NHy-dominated areas.

[1] Characterizing the level and sources of atmospheric N deposition in a large-scale area is not easy when using physical monitoring. In this study, we attempted to use epilithic mosses (Haplocladium microphyllum (Hedw.)) as a bioindicator. A gradient of atmospheric N deposition from 13.8 kg N ha−1 yr−1 to 47.7 kg N ha−1 yr−1 was estimated on the basis of moss tissue N concentrations and the linear equation between them. The estimated results are reliable because the highest atmospheric N deposition occurred in the middle parts of the Yangtze River, where the highest TN concentrations were also observed. Moss δ15N values in cities and forests were found in distinctly different ranges of approximately −10‰ to −6‰ and approximately −2‰ to 2‰, respectively, indicating that the main N sources in most of these cities were excretory wastes and those in forests were soil emissions. A negative correlation between moss δ15N values and the ratios of NH4-N/NO3-N in deposition (y = −1.53 x + 1.78) has been established when the ratio increased from 1.6 to 6.5. On the basis of the source information, the negative moss δ15N values in this study strongly indicate that NHy-N is the dominant N form in N deposition in the whole drainage basin. These findings are supported by the existing data of chemical composition of local N deposition.

[1] A considerable number of studies on rainwater sulfur isotopic ratios (δ34Srain) have been conducted to trace sulfur sources at a large number of sites in the past. If longitudinal studies on the isotope composition of precipitation sulfate were conducted, it is possible to relate that to changes in sulfur emissions. But direct measurement needs considerable labor and time. So, in this study, sulfur isotopic ratios in rainwater and mosses were analyzed at Guiyang and Nanchang to evaluate the possibility of using mosses as a substitute for rainwater. We found that present moss sulfur isotopic ratios were comparable to those of present rainwater. Additionally, we investigated the changes of atmospheric sulfur sources and sulfur concentrations using an isotopic graphic analysis at five industrial cities, two forested areas, and two remote areas in China. Mosses in industrial cities show a wide range of δ34S values, with the highest occurring at Chongqing (+3.9‰) and the lowest at Guiyang (−3.1‰). But as compared to those in forested and remote areas, δ34S values of mosses in all the five industrial cities are lower. On the basis of isotopic comparisons between past rainwater (reported in the literature) and present mosses, in the plot of δ34Smoss versus δ34Srain, six zones indicating different atmospheric sulfur change are separated by the 1:1 line and δ34S values of potential sulfur sources. Our results indicate that atmospheric sulfur pollution in most of the industrial cities decreased, while at the two forested areas, no significant changes were observed, and a new anxiousness coming from new energy sources (e.g., oil) appeared in some cities. Studies on the change of ambient SO2 concentrations support these results.

[1] Many mountainous regions in South China have been confronted with the consequences of acidic deposition, but studies on atmospheric S sources are still very limited. In this study, isotopic ratios in mosses were used to discriminate atmospheric S sources. A continuous increase in S isotopic ratios was observed from the south to the north in mountainous mosses and in accord with the previously reported changing trends in urban mosses, indicating a contribution of local anthropogenic S from urban cities. Based on comparisons of S isotopic ratios in mountainous mosses with those in nearby urban mosses, we found that mountainous mosses had significantly higher 34S contents than urban mosses, especially in West China, reflecting an introduction of 34S-enriched sulphur. In conjunction with cloud water data in the literature, we concluded that 34S-enriched sulphur in northerly air masses contributed much to atmospheric S in southern Chinese mountainous areas.

[1] Because of the significance of the first pressure derivative of the isothermal bulk modulus (K′T,0) at temperature, T, and zero pressure, a thermodynamic model was developed to calculate K′T,0 and its temperature derivative ∂K′T,0/∂T. This model constructs a differential equation of K′T,0 with respect to temperature based on the well-known empirical linear relationship of the Anderson-Gruneisen parameter, δT, and the compression ratio, η. By solving this differential equation, we derived both K′T,0 and ∂K′T,0/∂T at a temperature, T, and zero pressure in terms of the known parameters of thermal expansivity, α, isobaric heat capacity, CP, and isothermal bulk modulus, KT at ambient pressure, and K′T at a reference temperature, T0, and ambient pressure. The reliability of this model was confirmed by comparing the modeled MgO results with previous data. This model will provide a powerful tool for determining the equation of state and the thermoelastic properties of solid materials at high temperatures and high pressures.