Long-term drainage induces divergent changes of soil organic carbon contents but enhances microbial carbon accumulation in fen and bog

Drainage-induced changes in wetland soil organic carbon (SOC) composition and origin are poorly investigated compared to SOC stocks. Here, using soil fractionation and neutral sugars, we find that long-term drainage increased both plant- and microbial-dominated (i.e., light and mineral-associated, respectively) fractions in a fen while microbial residues increased at the expense of plant residues in a drained bog, accompanied by divergent changes of SOC contents. These findings highlight stimulated soil microbial carbon pump in drained wetlands, whose efficiency deserves further investigation related to wetland SOC persistence

Experimental Study of a Novel Non-Packing Closed Evaporative Cooling Tower with Vertical 3D Deformation Tubes

The closed evaporative cooling tower (CECT) is widely used in the field of industrial cooling. At present, most CECTs still mainly adopt the horizontal-tube falling-film cooling method. In this paper, a novel vertical CECT using 3D deformation tubes is developed. To investigate the vertical surface falling-film evaporative cooling effect of this novel cooling equipment, a traditional horizontal CECT was modified to produce a prototype of vertical non-packing CECT. The cooling performance of the novel vertical CECT has been investigated and compared to the previous traditional horizontal CECT by experimental method. The results show that the convective heat transfer coefficient of the water film outside the tube was increased by 5.87~12.95% and the overall cooling performance was increased by 7.31% on average. This indicates that the cooling load can be increased by changing the traditional horizontal-tube falling-film evaporative cooling method to the vertical falling-film evaporative cooling method. Moreover, the heat flux of the novel vertical CECT decreases by about 7% when the wet bulb temperature increases by 1 °C under the test range of wet bulb temperature, which indicates that the ambient wet bulb temperature has an obvious influence on the cooling load. The research results can provide reference for the optimization design of the CECT

Experimental Study of a Novel Non-Packing Closed Evaporative Cooling Tower with Vertical 3D Deformation Tubes

The closed evaporative cooling tower (CECT) is widely used in the field of industrial cooling. At present, most CECTs still mainly adopt the horizontal-tube falling-film cooling method. In this paper, a novel vertical CECT using 3D deformation tubes is developed. To investigate the vertical surface falling-film evaporative cooling effect of this novel cooling equipment, a traditional horizontal CECT was modified to produce a prototype of vertical non-packing CECT. The cooling performance of the novel vertical CECT has been investigated and compared to the previous traditional horizontal CECT by experimental method. The results show that the convective heat transfer coefficient of the water film outside the tube was increased by 5.87~12.95% and the overall cooling performance was increased by 7.31% on average. This indicates that the cooling load can be increased by changing the traditional horizontal-tube falling-film evaporative cooling method to the vertical falling-film evaporative cooling method. Moreover, the heat flux of the novel vertical CECT decreases by about 7% when the wet bulb temperature increases by 1 °C under the test range of wet bulb temperature, which indicates that the ambient wet bulb temperature has an obvious influence on the cooling load. The research results can provide reference for the optimization design of the CECT

Spatial-temporal variations in riverine carbon strongly influenced by local hydrological events in an alpine catchment

Headwater streams drain >70 % of global land areas but are poorly monitored compared with large rivers. The small size and low water buffering capacity of headwater streams may result in a high sensitivity to local hydrological alterations and different carbon transport patterns from large rivers. Furthermore, alpine headwater streams on the Asian water tower, i.e., Qinghai-Tibetan Plateau, are heavily affected by thawing of frozen soils in spring as well as monsoonal precipitation in summer, which may present contrasting spatial-temporal variations in carbon transport compared to tropical and temperate streams and strongly influence the export of carbon locked in seasonally frozen soils. To illustrate the unique hydro-biogeochemistry of riverine carbon in Qinghai-Tibetan headwater streams, here we carry out a benchmark investigation on the riverine carbon transport in the Shaliu River (a small alpine river integrating headwater streams) based on annual flux monitoring, sampling at a high spatial resolution in two different seasons and hydrological event monitoring. We show that riverine carbon fluxes in the Shaliu River were dominated by dissolved inorganic carbon, peaking in the summer due to high discharge brought by the monsoon. Combining seasonal sampling along the river and monitoring of soil-river carbon transfer during spring thaw, we also show that both dissolved and particulate forms of riverine carbon increased downstream in the pre-monsoon season due to increasing contribution of organic matter derived from thawed soils along the river. By comparison, riverine carbon fluctuated in the summer, likely associated with sporadic inputs of organic matter supplied by local precipitation events during the monsoon season. Furthermore, using lignin phenol analysis for both riverine organic matter and soils in the basin, we show that the higher acid-to-aldehyde (Ad/Al) ratios of riverine lignin in the monsoon season reflect a larger contribution of topsoil likely via increased surface runoff compared with the pre-monsoon season when soil leachate lignin Ad/Al ratios were closer to those in the subsoil than topsoil solutions. Overall, these findings highlight the unique patterns and strong links of carbon transport in alpine headwater catchments with local hydrological events. Given the projected climate warming on the Qinghai-Tibetan Plateau, thawing of frozen soils and alterations of precipitation regimes may significantly influence the alpine headwater carbon transport, with critical effects on the biogeochemical cycles of the downstream rivers. The alpine headwater catchments may also be utilized as sentinels for climate-induced changes in the hydrological pathways and/or biogeochemistry of the small basin

The linkage between vegetation and soil nutrients and their variation under different grazing intensities in an alpine meadow on the eastern Qinghai-Tibetan Plateau

The grassland degradation caused by overgrazing is the result of imbalance of energy flow and material cycle under grazing pressure in the ecosystem, and numerous ecological restoration and ecological engineering techniques widely used to restore some ecosystem. However, little research known focused on the relationship (energy flow and material cycle) between plant and soil under different grazing intensity, especially at the rhizosphere which is the most powerful area of plant and soil interaction. In this research we conducted a 5-year grazing experiment including 4 different grazing intensities (no grazing, UG; light grazing, LG; moderate grazing, MG; and heavy grazing, HG) in an alpine meadow on the northeastern margin of Qinghai-Tibetan Plateau (QTP). Plants and soil materials were sampled in July 2015, and to examine the nutrients concentration and ecological stoichiometric of vegetation and soil, the soil microbial biomass and activity, as well as their relationship with vegetation and soil characteristics. We found that grazing increased vegetation and soil nutrient concentrations and increased the ratio of microbial biomass carbon (MBC) to microbial biomass nitrogen (MBC), but the grazing intensity did not significantly influence the nutrients enrichment ratio of rhizosphere soil. There was significant relationship between total carbon (TC) concentration of vegetation and soil, and between vegetation total phosphorus (TP) and soil PO43--P concentration. These results suggested that vegetation and soil nutrient concentrations respond differently to the grazing intensity. Coupling relationship exists in specific nutrients of vegetation and soil and rhizosphere is a powerful tool to understanding the linkage between plant and soil

Leaching of organic carbon from grassland soils under anaerobiosis

The projected increase of extreme precipitation and freeze-thawing events may lead to frequent occurrence of anaerobiosis in upland soils, which has significant impacts on biogeochemical processes affecting soil carbon loss. However, compared to mineralization, the impacts of anaerobiosis (potentially accompanied by fermentation) on soil organic carbon (SOC) leaching is limited. Here we conducted microcosm and intact soil column incubation experiments to examine processes influencing SOC leaching from four typical Chinese grassland soils under simulated anaerobiosis. Compared to aerobiosis, non-fermenting anaerobiosis increased the pH, dissolved phenol concentrations and aromaticity of soil leachates. In contrast, fermenting anaerobiosis induced acetate accumulation, lowered pH, stimulated phenol oxidative activity and generally decreased aromaticity in soil leachates in both microcosm and soil column experiments relative to aerobiosis. Both anaerobiosis potentially induced a strong release of dissolved organic carbon (DOC) accompanied by iron and nitrate reduction, especially with fermentation. However, DOC in soil leachates decreased in alpine subsoils under fermentation relative to aerobiosis. This interesting phenomenon was mainly attributed to (i) minimal iron reduction and dissolution in the alpine subsoils and (ii) enhanced DOC oxidation by elevated phenol oxidative activity in the fermentation relative to aerobiosis treatments. These results collectively indicate that anaerobiosis may increase SOC leaching and its magnitude is dependent on the extent of iron reduction and pH variations. Fermentation-enhanced release of ferrous iron and acetate may have an even stronger influence on the downstream biogeochemistry. Hence, temporary anaerobiosis warrants better recognition and investigation in the Mongolian (relative to Qinghai-Tibetan) grasslands that show high soil iron reduction potentials and are predicted to experience increased extreme precipitation in the future

Inactive and inefficient: Warming and drought effect on microbial carbon processing in alpine grassland at depth

Subsoils contain >50% of soil organic carbon (SOC) globally yet remain under-investigated in terms of their response to climate changes. Recent evidence suggests that warmer, drier conditions in alpine grasslands induce divergent responses in SOC decomposition and carbon accrual in top- versus subsoils. However, longer term effects on microbial activity (i.e., catabolic respiration vs. anabolic growth) and belowground carbon cycling are not well understood. Here we utilized a field manipulation experiment on the Qinghai-Tibetan Plateau and conducted a 110-day soil incubation with and without C-13-labeled grass litter to assess microbes' role as both SOC decomposers and contributors in the top- (0-10 cm) versus subsoils (30-40 cm) after 5 years of warming and drought treatments. Microbial mineralization of both SOC and added litter was examined in tandem with potential extracellular enzyme activities, while microbial biomass synthesis and necromass accumulation were analyzed using phospholipid fatty acids and amino sugars coupled with C-13 analysis, respectively. We found that warming and, to a lesser extent, drought decreased the ratio of inorganic nitrogen (N) to water-extractable organic carbon in the subsoil, intensifying N limitation at depth. Both SOC and litter mineralization were reduced in the subsoil, which may also be related to N limitation, as evidenced by lower hydrolase activity (especially leucine aminopeptidase) and reduced microbial efficiency (lower biomass synthesis and necromass accumulation relative to respiration). However, none of these effects were observed in the topsoil, suggesting that soil microbes became inactive and inefficient in subsoil but not topsoil environments. Given increasing belowground productivity in this alpine grassland under warming, both elevated root deposits and diminished microbial activity may contribute to new carbon accrual in the subsoil. However, the sustainability of plant growth and persistence of subsoil SOC pools deserve further investigation in the long term, given the aggravated N limitation at depth

Compositional Characteristics of Fluvial Particulate Organic Matter Exported From the World's Largest Alpine Wetland

Wetlands are hot spots for particulate organic matter (POM) supply into rivers, which link the land-ocean transfer in the global carbon cycle. However, the source, composition, and seasonal variability of POM carried by wetland-draining rivers are poorly constrained. Here we combine bulk and source-specific biomarker analyses to investigate the fluvial POM biogeochemistry of the Black and White Rivers draining from the Zoige wetland. We find that POM was dominated by terrestrial organic matter including high-molecular-weight (HMW) lipids, branched glycerol dialkyl glycerol tetraethers, and lignin phenols. However, fluvial POM was rich in HMW lipids and poor in lignin phenols compared to the catchment soils, possibly due to hydrodynamic sorting and dissolution processes. While lignin phenol concentrations were higher in the wet season, HMW lipid concentrations were lower. Additionally, lignin phenols increased with total suspended solids, while HMW lipids decrease. These contrasts imply an enhanced input of lignin-rich particles from soil surface layers in the wet season, diluting HMW lipids. Compared with that in other rivers around the world with a higher forest coverage in the catchment, POM in the Black and White Rivers draining grass-dominated wetlands had a much higher ratio of HMW fatty acids to lignin phenols. Our results represent a benchmark study highlighting compositional characteristics of fluvial POM exported from the Zoige wetland and the divergent behavior of molecular components during fluvial transfer. Such information is vital for assessing future changes in the Zoige wetland, given its high vulnerability to climatic and land use changes