[Optical Composition and Potential Driving Factors of Chromophoric Dissolved Organic Matter in Large Lakes and Reservoirs in the Eastern Region of China].

Intensified urbanization has been occurring in the eastern region of China in recent decades, and excessive industrial and household sewage has been discharged into lakes and reservoirs, which has directly lowered water quality and destructed the functions of aquatic ecosystems. Lakes and reservoirs are typically drinking water sources supplying water for metropolitan areas as well as large- and medium-sized cities. Chromophoric dissolved organic matter (CDOM) is the colored fraction of DOM, and its source and optical composition strongly affect water supply safety and the health of surrounding citizens. In April 2021, we collected 68 samples from Reservoir Changtan (n=11), Lake Taihu (n=25), Lake Hongze (n=18), and Lake Gaoyou (n=14), and we further carried out 28 days of laboratory bio-incubation, together with optical measurements and parallel factor analysis (EEMs-PARAFAC) to analyze the bio-degradability ω(BDOC), sources, and optical composition of CDOM in these waters. The results showed that after 28 days of laboratory bio-incubation, the bioavailability of dissolved organic carbon (BDOC) of the four lakes and reservoirs were all higher than 50%. PARAFAC results showed that CDOM collected from the four lakes was composed of four fluorescent components, including a terrestrial humic-like C1, a tryptophan-like C2, and tyrosine-like C3 and C4. Protein-like components (C2, C3, and C4) contributed importantly to the CDOM pool in the four waterbodies, and in Lake Hongze the contribution of C2-C4 was as high as (90.0±2.2)%. In Lake Hongze, total phosphorus (TP) correlated closely with C1-C3, indicating that those components can be used to trace the variability of TP. Among the four waterbodies, a254 was positively correlated with DOC (R2=0.96, P<0.01), indicating that a254 can be used to estimate the dynamics of DOC in these waters. In Lake Taihu, we found a low level of humification index (HIX) and a high level of biological index (BIX), indicating that autochthonous substances contributed importantly to the CDOM pool in this lake. In comparison, allochthonous sources contributed importantly to the CDOM pool in the remaining three waters. The enhanced monitoring of the discharge of industrial and agricultural effluents in the upstream watersheds of the four waters can help to protect the water quality and maintain healthy aquatic ecosystems.

[CH4 Emissions Characteristics and Its Influencing Factors in an Eutrophic Lake].

In order to identify methane (CH4) diffusion emissions characteristics and their impact factors in an eutrophic lake, CH4 flux across the lake-air interface was observed in Meiliang Bay and the central zone of Lake Taihu over one year. The relationships between CH4 flux and environmental factors and water quality indices were analyzed. The results indicated that the annual mean CH4 diffusion flux in the eutrophic zone was significantly higher than that in the central zone, which were 0.140 mmol·(m2·d)-1 and 0.024 mmol·(m2·d)-1, respectively. Additionally, the highest CH4 flux appeared in the eutrophic littoral zone. The CH4 flux varied seasonally, which was consistent with water temperature that peaked in summer. Furthermore, the difference in CH4 flux between seasons was an order of magnitude. The temporal variation in CH4 flux was mostly driven by wind speed and water temperature. The spatial correlation between CH4 flux and dissolved organic carbon concentration was highly significant (R2=0.62, P<0.01). Observing temporal and spatial patterns of CH4 flux was necessary to accurately estimate whole-lake CH4 emissions due to large variability across time and space.

[Greenhouse Gas Fluxes at Water-Air Interface in Small Pond Using Flux-Gradient Method Based on Spectrum Analyzer].

As an important part of inland waters,small pond is a neglected source of greenhouse gas.The main objective of the study was to quantify greenhouse gas fluxes (CO2 and CH4) from small pond in the Yangtze Delta using flux-gradient method.The results showed that:① zero-gradient test indicated that the flux measurement precision for water vapor,CO2,and CH4 was 7.525 W·m-2,0.022 mg·(m2·s)-1,and 0.054 µg·(m2·s)-1,respectively.During the test period,84%,80%,and 94% of half-hourly flux data for H2O,CO2,and CH4 were higher than the zero-gradient measurement precision.② Based on the measurement,the small pond was the source of CO2 and CH4 for the atmosphere in summer,the mean emission flux of CO2 and CH4 was 0.038 mg·(m2·s)-1 and 0.889 µg·(m2·s)-1,respectively.The CH4 emission fluxes from the small pond were more higher than the median value of emission for global lakes.The results indicated that greenhouse gas emission from small pond was an important part for estimating inland water greenhouse gas emissions,especially for CH4 emission.These results can provide scientific reference for making emission inventory of regional greenhouse gas.

[Temporal Dynamics of Stable Isotopic Composition in Lake Taihu and Controlling Factors].

The composition of hydrogen and oxygen stable isotopes in lake water is important to the researches in hydrology, meteorology and paleoclimatology. In this study, long-term and continuous measurement on the compositions of HDO and H218O in lake water (δDL and δ18OL) was conducted over Lake Taihu, the deuterium excess (dL) was calculated, and the temporal variability and controlling factors were analyzed. The results indicated that ① the variation of isotopic enrichment in lake water was significant, ranging from -59.8‰ to -24.2‰ for δDL, from -8.6‰ to -2.6‰ for δ18OL, and from -7.9‰ to 12.9‰ for dL, respectively. In comparison to cold season, δDL and δ18OL were higher and dL was lower during warm season. ② On monthly time-scale, lake evaporation and the ratio of total water inputs lost by evaporation controlled the isotopic enrichment in lake water. When lake evaporation or the ratio increased, δDL and δ18OL increased, but dL decreased. ③ Over Lake Taihu, the isotopic composition in precipitation and water temperature did not control the isotopic enrichment. The results provide scientific reference for isotope hydrology and the researches related to the isotopic enrichment in lake water in meteorology and paleoclimate.

[Impacts of elevated ozone concentration on N2O emission from arid farmland].

To investigate the impact of elevated surface ozone (O3) concentration on nitrous oxide (N2O) emission from arid farmland, field experiments were carried out during winter-wheat and soybean growing seasons under the condition of simulating O3 concentrations, including free air (CK), 100 nL x L(-1) O3 concentration (T1), and 150 nL x L(-1) O3 concentration (T2). N2O emission fluxes were measured by static dark chamber-gas chromatograph method. The results showed that the accumulative amount of N2O (AAN) were decreased by 37.8% (P = 0.000 ) and 8.8% (P = 0.903 ) under T1 and T2 treatments, respectively, in the turning-green stage of winter wheat. In the elongation-booting stage, ANN were decreased by 15.0% (P = 0.217) and 39.1% (P = 0.000) under T1 and T2 treatments, respectively. ANN were decreased by 18.9% (P = 0.138) and 25.6% (P = 0.000) under T1 and T2 treatments, respectively, during the whole winter-wheat growing season. No significant impact of elevated O3 concentration on N2O emission from soil-soybean system was found due to the less rainfall during the soybean growing season, drought had a stronger stress on soybean than O3 concentration. The results of this study suggested that elevated O3 concentration could reduce N2O emission from arid farmland.

[Temporal and Spatial Characteristics of Lake Taihu Surface Albedo and Its Impact Factors].

Lake surface albedo determines energy balance of water-atmospheric interface and water physical environment. Solar elevation angle, cloudiness, wind speed, water quality and other factors can affect lake surface albedo. Using solar radiation, wind speed, and water quality data (turbidity and chlorophyll-a concentration) which were observed in four eddy covariance sites (Meiliangwan, Dapukou, Bifenggang and Xiaoleishan i. e. MLW, DPK, BFG and XLS) in Lake Taihu and clearness index (k(t)), the influence of these factors on Lake Taihu surface albedo and the reasons that led to its spatial difference were investigated. The results showed that solar elevation angle played a leading role in the diurnal and seasonal change of lake surface albedo; lake surface albedo reached two peaks in 0 < k(t) < 0.1 and 0.4 < k(t) < 0.6 respectively, when solar elevation angle was below 35 degrees. The surface albedo increased with the increasing wind speed, turbidity and chlorophyll-a concentration. However, wind could indirectly affect surface albedo through leading to the changes in sediment resuspension and chlorophyll-a distribution. The sequence of albedo in the four sites was XLS > BFG > DPK > MLW. XLS and BFG belonged to the higher albedo group, while DPK and MLW belonged to the lower albedo group. The different biological environments caused by aquatic macrophytes and algae resulting in the spatial variation of Lake Taihu surface albedo. The relationship between albedo and chlorophyll-a concentration was not a very sensitive factor for indicating the outbreak of algae. This study can provide theoretical reference for lake albedo parameterization.

[Effects of conservation tillage on soil CO2 and N2O emission during the following winter-wheat season].

In order to study the effect of conservation tillage on soil CO2 and N2O emissions in the following crop-growing season, field experiments were conducted in the winter wheat-growing season. Four treatments were conventional tillage (T), no-tillage with no straw cover (NT), no-tillage with straw cover (NTS), and conventional tillage with straw incorporation (TS), respectively. The CO2 and N2O fluxes were measured using a static chamber-gas chromatograph technique. The results showed that in the following winter wheat-growing season, conservation tillage did not change the seasonal pattern of CO2 and N2O emission fluxes from soil, and had no significant effect on crop biomass. Conservation tillage significantly reduced the accumulative amount of CO2 and N2O. Compared with the T treatment, the accumulative amount of CO2 under TS, NT, and NTS treatments were reduced by 5.95% (P = 0.132), 12.94% (P = 0.007), and 13.91% (P = 0.004), respectively, and the accumulative amount of N2O were significantly reduced by 31.23% (P = 0.000), 61.29% (P = 0.000), and 33.08% (P = 0.000), respectively. Our findings suggest that conservation tillage significantly reduced CO2 and N2O emission from soil in the following winter wheat-growing season.

[Effects of antiseptic on the analysis of greenhouse gases concentrations in lake water].

To gain insight into antiseptic effects on the concentrations of CO2, CH4, and N2O in lake water, antisepetic (CuSO4 and HgCl2) were added into water sample, and concentrations of greenhouse gases were measured by the gas chromatography based on water equilibrium method. Experiments were conducted as following: the control group without antisepetic (CK), the treatment group with 1 mL CuSO4 solution (T1), the treatment group with 5 mL CuSO4 solution (T2), and the treatment group with 0.5 mL HgCl2 solution (T3). All groups were divided into two batches: immediately analysis (I), and after 2 days analysis (II). Results showed that CuSO4 and HgCl2 significantly increased CO2 concentration, the mean CO2 concentration (Mco2) of CK (I) and CK (II) were (11.5 +/- 1.47) micromol x L(-1) and (14.38 +/- 1.59) micromol x L(-1), respectively; the Mco2 of T1 (I) and T1 (II) were (376 +/- 70) micromol x L(-1) and (448 +/- 246.83) micromol x L(-1), respectively; the Mco2 of T2 (I) and T2 (II) were (885 +/- 51.53) micromol x L(-1) and (988.83 +/- 101.96) micromol x L(-1), respectively; the Mco2 of T3 (I) and T3 (II) were (287.19 +/- 30.01) micromol x L(-1) and (331.33 +/- 22.06) micromol x L(-1), respectively. The results also showed that there was no difference in CH4 and N2O concentrations among treatments. Water samples should be analyzed as soon as possible after pretreatment. Our findings suggest that adding antiseptic may lead an increase in CO2 concentration.

[Effects of diurnal warming on soil N2O emission in soybean field].

To investigate the impact of experimental warming on N2O emission from soil of soybean field, outdoor experiments with simulating diurnal warming were conducted, and static dark chamber-gas chromatograph method was used to measure N2O emission fluxes. Results indicated that: the diurnal warming did not change the seasonal pattern of N2O emissions from soil. In the whole growing season, comparing to the control treatment (CK), the warming treatment (T) significantly enhanced the N2O flux and the cumulative amount of N2O by 17.31% (P = 0.019), and 20.27% (P = 0.005), respectively. The significant correlations were found between soil N2O emission and soil temperature, moisture. The temperature sensitivity values of soil N2O emission under CK and T treatments were 3.75 and 4.10, respectively. In whole growing stage, T treatment significantly increased the crop aboveground and total biomass, the nitrate reductase activity, and total nitrogen in leaves, while significantly decreased NO3(-) -N content in leaves. T treatment significantly increased soil NO3(-) -N content, but had no significant effect on soil organic carbon and total nitrogen contents. The results of this study suggested that diurnal warming enhanced N2O emission from soil in soybean field.