Plankton play a vital role in energy flow and material cycling in marine ecosystems. Based on a survey in the southwestern waters of Fangchenggang in September 2021, 21 environmental factors and plankton community composition were analyzed. A total of 67 phytoplankton species (3 phyla) were identified, dominated by Bacillariophyta, and 62 zooplankton species (17 phyla), dominated by Copepods. Spatially, nearshore waters exhibited high nutrients, heavy metals, and Chl-a, with Bacillariophyta dominance; offshore waters showed increased transparency and lower nutrients, favoring motile Pyrrophyta. Surface-layer heavy metals decreased offshore due to terrestrial dilution. In the bottom layer, Pb, Cd, and As accumulated nearshore via sedimentation of metal-laden particles, whereas Zn increased offshore, likely driven by sediment remobilization or external water mass inputs. Mantel test and RDA revealed that depth, Secchi depth, and mid-layer suspended solids were the key factors shaping plankton community distribution. These findings highlight the contrasting spatial patterns of heavy metals between surface and bottom layers and the nutrient–light driven shift in phytoplankton composition along the nearshore–offshore gradient.

The mangrove forest constitutes an important part of blue carbon sink. A summary of carbon accumulation rates of vegetation and soil in mangrove forests and their influencing factors is lacking at the national scale of China. Based on literature collection and data mining, we analyzed the carbon accumulation rate and influencing factors of mangrove vegetation, soil, and the entire ecosystem in China. The results showed that vegetation carbon accumulation rate changed with the age of mangrove forests, with the pattern differing among different communities. Soil carbon accumulation rate differed little among forest ages. The mangrove community types with carbon accumulation rate being commonly reported were Avicennia marina, Kandelia obovata, Aegiceras corniculatum, Rhizophora stylosa, Bruguiera gymnorrhiza, Sonneratia apetala and Sonneratia caseolaris. Among them, S. apetala community and S. caseolaris community had the highest vegetation carbon accumulation rate, and soil carbon accumulation rate was not significantly different among communities. The mean carbon accumulation rates of vegetation and soil in mangroves of China were 766.9 and 201.1 g·m-2·a-1, respectively, with Guangdong Province having the highest average values. Stand origin (natural or planted) and quantification methods had no significant effect on the carbon accumulation rates of vegetation, soil or entire ecosystem. In general, the vegetation carbon accumulation rate of mangroves is mainly influenced by forest age and community type at the national scale, which can be regulated by tree species selection and forest age management in mangrove restoration. However, the main factors influencing soil carbon accumulation rate need to be investigated by further research. More measurement of carbon accumulation rate of mangroves in Zhejiang and Hainan provinces are needed.

Widespread applications of Internet of Things (IoT) generate massive periodic time series data. The time series forecasting (TSF) enables people to perceive things before hand. Inspired by the classical autoregressive TSF models, we conclude that the future sequences are influenced by the regular periodic variations, the long-term trend changes, the short-term trend changes, and the noises. In this article, a novel lightweight interpretable deep TSF model, 2-D convolution and linear mapping-based model (CLinear), is proposed. CLinear decomposes time series into periods, and models the periodic patterns, long-term trends, short-term trends, and noise in a progressive manner. The model extracts features with 2-D convolution, and then generates periodic patterns and long-term trends with fully connected layers. Combining short-term trends generated from the nearest sequence, preliminary predictions are produced. Finally, the model generates the prediction noise according to the feedback noise from the preliminary predictions. Short-term trend enhances the model's adaptability for nonperiodic datasets. The feedback noise is utilized to fit complex factors that cannot be effectively expressed in complex environments. Experiments on five time series datasets and three raw meteorological datasets demonstrate the superior performance of our method while maintaining a smaller parameter scale. Ablation experiments validate the necessity of each component. Finally, experiments on the meteorological temperature dataset demonstrate the practical significance of each component, and visualize the interpretability. CLinear verifies that classical methods are referential for constructing deep learning models. Code is available at https://github.com/LoneLoser/CLinear.

Biochar as an effective adsorbent can be used for the removal of triclocarban from wastewater. Biochar-derived dissolved organic carbon (BC-DOC) is an important carbonaceous component of biochar, nonetheless, its role in the interaction between biochar and triclocarban remains little known. Hence, in this study, sixteen biochars derived from pine sawdust and corn straw with different physico-chemical properties were produced in nitrogen-flow and air-limited atmospheres at 300–750 °C, and investigated the effect of BC-DOC on the interaction between biochar and triclocarban. Biochar of 600∼750 °C with low polarity, high aromaticity, and high porosity presented an adsorption effect on triclocarban owing to less BC-DOC release as well as the strong π-π, hydrophobic, and pore filling interactions between biochar and triclocarban. In contrast and intriguingly, biochar of 300∼450 °C with low aromaticity and high polarity exhibited a significant solubilization effect rather than adsorption effect on triclocarban in aqueous solution. The maximum solubilization content of triclocarban in biochar-added solution reached approximately 3 times its solubility in biochar-free solution. This is mainly because the solubilization effect of BC-DOC surpassed the adsorption effect of biochar though the BC-DOC only accounted for 0.01–1.5 % of bulk biochar mass. Furthermore, the high solubilization content of triclocarban induced by biochar was dependent on the properties of BC-DOC as well as the increasing BC-DOC content. BC-DOC with higher aromaticity, larger molecular size, higher polarity, and more humic-like matters had a greater promoting effect on the water-solubility of triclocarban. This study highlights that biochar may promote the solubility of some organic pollutants (e.g., triclocarban) in aqueous environment and enhance their potential risk.

Hydroxylamine (NH 2 OH) is an important intermediate of nitrification and can contribute to nitrous oxide (N 2 O) production through abiotic and biotic pathways. NH 2 OH concentrations and associated biogeochemical controls in estuarine water and sediments remain poorly understood. Here, we investigated spatial and seasonal variability of NH 2 OH concentrations in water and sediments and determined the correlation between NH 2 OH and N 2 O in the Min River Estuary off southeast China. NH 2 OH concentrations in water and sediments ranged from 0.22 to 140 nmol N L −1 and from 0.17 to 9.22 μmol N kg −1 , respectively. High differences in NH 2 OH concentrations between sediments and water could drive diffusion of NH 2 OH from sediments to water. NH 2 OH concentrations were significantly higher in warm than in cold seasons, suggesting that NH 2 OH production is highly dependent on temperature. Upper estuary (urban section) showed higher NH 2 OH concentrations in water and sediments, followed by lower and middle estuary. Temperature, dissolved oxygen, and pH were the major factors driving spatial and seasonal variability in NH 2 OH concentrations. Water dissolved N 2 O concentrations and air‐water N 2 O fluxes varied between 1.52 and 55.9 nmol N L −1 and between −57.7 and 78.7 nmol N m −2 h −1 , respectively, with switches from net sinks in spring to net sources of N 2 O emissions in other seasons. Although N 2 O concentrations and fluxes were linearly correlated with water NH 2 OH concentrations, the contribution of NH 2 OH to N 2 O production could not be addressed. Therefore, the dynamics of NH 2 OH concentrations are a useful tool to assess potential production and consumption pathways of N 2 O in estuarine and coastal ecosystems.

Biochar-derived dissolved organic carbon (BDOC), as a highly activated carbonaceous fraction of biochar, significantly affects the environmental effect of biochar. This study systematically investigated the differences in the properties of BDOC produced at 300–750 °C in three atmosphere types (including N2 and CO2 flows and air limitation) as well as their quantitative relationship with biochar properties. The results showed that BDOC in biochar pyrolyzed in air limitation (0.19–2.88 mg/g) was more than that pyrolyzed in N2 (0.06–1.63 mg/g) and CO2 flows (0.07–1.74 mg/g) at 450–750 °C. The aliphaticity, humification, molecular weight, and polarity of BDOC strongly depended on the atmosphere types as well as the pyrolysis temperatures. BDOC produced in air limitation contained more humic-like substances (0.65–0.89) and less fulvic-like substances (0.11–0.35) than that produced in N2 and CO2 flows. The multiple linear regression of the exponential form of biochar properties (H and O contents, H/C and (O+N)/C) could be used to quantitatively predict the bulk content and organic component contents of BDOC. Additionally, self-organizing maps could effectively visualize the categories of fluorescence intensity and components of BDOC from different pyrolysis atmospheres and temperatures. This study highlights that pyrolysis atmosphere types are a crucial factor controlling the BDOC properties, and some characteristics of BDOC can be quantitatively evaluated based on the properties of biochar.

Abiotic nitrogen transformation processes can generate N 2 O in estuarine wetlands. The abiotic processes of hydroxylamine（NH 2 OH） and nitrite（NO 2 - ） can produce N 2 O, but production potential and affecting factors remain unclear. We conducted an experiment with three treatments（without nitrogen addition（CK）, NH 2 OH and NO 2 - additions） to determine N 2 O production rates from abiotic processes in wetland soils of Min River Estuary. The results showed that N 2 O production rates significantly differed among three treatments（P＜0.05）. The potential production rates of N 2 O from abiotic processes under NH 2 OH addition, NO 2 - addition, and without nitrogen treatments ranged from 16.88 to 307.99,-0.50 to 27.51 and-1.20 to 2.97 ng·g -1 ·h -1 , respectively. The contributions of N 2 O production from abiotic processes of NH 2 OH and NO 2 - additions were 20.74%-98.73% and 1.27%-79.26%, respectively. Average N 2 O production rates of abiotic processes for all the five soil types varied significantly among the three treatments（P＜0.05）. The average N 2 O production rates of abiotic process of NO 2 - were in the order of Kandelia candel＞Spartina alterniflora＞Phragmites australis＞Scirpus mariqueter＞Cyperus malaccensis, while the rates of abiotic processes of NH 2 OH were S. mariqueter＞S. alterniflora＞P. australis＞C. malaccensis＞K. candel. These results suggest that abiotic processes of NH 2 OH have higher N 2 O production rates compared to NO 2 - , and the N 2 O production from abiotic processes of soil NH 2 OH and NO 2 - varied largely among different vegetation soils. Soil pH, NO 2 - , NO 3 - , Fe 2+ , Fe 3+ and C/N ratio were crucial factors affecting N 2 O production rates from abiotic processes of NH 2 OH and NO 2 - .

Biochar-derived water-soluble organic carbon (BWSOC) plays important roles in the environmental effect of biochar. The environmental behavior and fate of BWSOC are closely related to its size distribution and chemical components. However, the molecular size-dependent BWSOC components and properties remain little known. To evaluate molecular size-dependent BWSOC characteristics, BWSOC samples were prepared by pyrolyzing biomasses in air-limitation and N2-flow atmospheres at 300-600 °C and fractionated through a series of membranes with different pore sizes including 0.7 μm, 0.45 μm, 100 kDa, 10 kDa, 3 kDa, and 1 kDa. In all BWSOCs, <1 kDa and 0.45-0.7 μm fractions had the maximum abundance (mean: 40.6 %) and the minimum abundance (mean: 4.4 %), respectively. The spectral characteristics of BWSOC including polarity index, spectral slope, and humification index varied significantly with molecular size. The fluorescence excitation-emission matrix parallel factor (EEM-PARAFAC) analysis indicated that BWSOC was mainly composed of three organic components (humic-like, fulvic-like, and aromatic protein/polyphenol-like substances). Humic-like and fulvic-like substances mainly existed in <1 kDa fraction, while aromatic protein/polyphenol-like substances mainly existed in medium-size fractions (3 kDa-0.45 μm). The different locations of <1 kDa, 1 kDa-0.45 μm, and 0.45-0.7 μm fractions in EEM and PARAFAC self-organizing maps indicated self-organizing maps could effectively distinguish 0.45-0.7 μm, 1 kDa-0.45 μm, and < 1 kDa fractions via the variations of fluorescence intensity and organic components. Additionally, the distribution ratio of different molecular size fractions as well as the abundances of organic components in different molecular size fractions were strongly controlled by pyrolysis atmospheres (air-limitation and N2-flow). This study systematically clarified the organic components and properties of different molecular size fractions in BWSOC, and the results are helpful to understand the possible environmental behavior and fate of BWSOC.

Biochar-derived dissolved organic carbon (BDOC) and smoke-derived dissolved organic carbon (SDOC) are two different biomass-pyrogenic DOCs. They inevitably enter soil and water, then potentially pose different impacts on the chemistry of these media. This study systemically investigated the emissions and spectral characteristics of BDOC and SDOC as well as their differences from natural DOC. The results showed that the emission of SDOC was 1-3 orders of magnitude greater than that of BDOC after biomass pyrolysis. UV-vis spectra indicated that BDOC had higher aromaticity and molecular weight as well as lower polarity than SDOC. The two-dimensional correlation infrared spectrum (2D-PCIS) matrix indicated that BDOC contained more chemical groups with stronger temperature-dependence than SDOC. Fluorescence EEM-PARAFAC analysis showed that BDOC was dominated by macromolecular humic-like substances, while SDOC was primarily composed of small molecules of aromatic protein/polyphenols-like compounds. The fluorescence indicators including humification index (HIX) (0.08-0.76) and biological index (BIX) (1.18-1.72) of SDOC were significantly different from those of BDOC (HIX: 1.64-12.68, and BIX: 0.17-1.62). The higher BIX and more small molecules of aromatic protein/polyphenols-like compounds indicated SDOC had potentially higher bioavailability and turnover rate in the environment than BDOC. Furthermore, the UV-vis spectral indicator (S275-295) and fluorescence spectral indicators (HIX, and BIX) of BDOC were equivalent to those of natural DOC, whereas these indicators of SDOC were significantly different from those of natural DOC. This study demonstrated that BDOC and SDOC had significantly different components and properties and they might present different environmental behaviors and effects.

This study systemically investigated the characteristics of biochars derived from thermo-conversion of pine sawdust and wheat straw in air-limitation, CO2, and N-2 atmospheres at the temperatures of 300-750 ?degrees C. Meanwhile, their energy and C stability parameters were also evaluated here. The results showed that biochar produced in air-limitation had less yield, fixed C and bulk C, as well as more volatile matter and inorganic elements than that produced in CO2 and N-2. Biochars derived from thermo-conversion of pine sawdust in CO2 and N-2 at 450-750 ?degrees C had the greatest energy densification ratios (EDR) (range: 1.40-1.61), because pine sawdust contained more lignin than wheat straw, and the thermo-conversion of lignin in N-2 and CO2 at 450-750 ?degrees C benefited for the formation of fixed C. Recalcitrance potential (R-50) results showed that the biochars produced in CO2 and N-2 at 600-750 ?degrees C had the highest carbon stability (R-50: 0.54-0.64) for given biomass, owing to the thermo-conversion of biomass in CO2 and N2 at 600-750 ?degrees C preferring to form the organic C with high aromaticity and low polarity. Nonetheless, thermo-conversion of biomass in CO2 and N-2 at 300 ?degrees C presented the greatest C sequestration potential, owing to high biochar yields under these conditions. Generally, the temperature-variability for the composition, EDR, and C sequestration potential followed the order: air -limitation > CO2 > N-2, whereas carbon stability presented an opposite order. Our results contributed to selecting the appropriate atmosphere to optimize the properties and performances of biochars.

Long‐term patterns of dissolved oxygen (DO) in estuarine and coastal waters remain poorly understood. Here we summarized DO concentrations and analyzed the crucial drivers of hypoxia in northwestern and southern Hong Kong and Mirs Bay over the past three decades. Deoxygenation was weak in the bottom water in northwestern Hong Kong, although DO was consistently undersaturated, whereas the annual minimum DO in the bottom water exhibited a significant decrease in southern Hong Kong (−0.06 ± 0.01 mg L −1 yr −1 ) and Mirs Bay (−0.10 ± 0.02 mg L −1 yr −1 ). Seasonal hypoxia in the bottom water was accompanied by supersaturated DO and high Chl ‐a in surface waters of southern Hong Kong, indicating a crucial role of local extensive productivity in the oxygen depletion of the bottom water. The rapid deoxygenation was also attributed to the water stratification preventing oxygen replenishment in southern Hong Kong and the predeoxygenation of bottom water retained in Mirs Bay. The dissolved inorganic nitrogen concentrations have increased from 1 to 3 mg N L −1 , which increased the primary productivity contributing to the decrease in water DO over the past three decades. Therefore, these results suggest that biological oxygen consumption and seasonal stratification are mainly driving the formation and maintenance of hypoxia in the Pearl River Estuary and adjacent areas.

以闽江口水体为研究对象,研究了闽江河口上段(城市河口段)、河口中段和河口下段(口外海滨段)不同季节水体N2O的溶存浓度、水-气界面通量及其环境影响因子.结果表明,闽江口水体N2O溶存浓度为0.99～55.92 nmol·L-1,N2O饱和度为8.0％～396％,水-气界面N2O释放通量为-5.21～7.91 μg·m-2·h-1.从季节差异看,7月(夏季)、9月(秋季)和12月(冬季)水体中N2O过饱和,表现为N2O的排放"源";4月(春季)水体中N2O不饱和,表现为N2O"汇".水-气界面N2O释放通量呈夏、秋和冬季高,春季低的季节变化规律.在空间变化上,水-气界面N2O释放通量从河口上段到下段降低,与氮含量变化趋势一致.N2O间接性释放因子为0.004％～0.128％,低于IPCC推荐值(0.25％).温度、盐度、pH、NO3-、DO、悬浮颗粒物浓度是影响N2O溶存浓度与释放通量的主要影响因素.因此,季节性变化和人类活动导致河口N2O释放存在较大的时空差异性,从而增加了河口 N2O排放评估的不确定性.

Dissolved organic carbon derived from biomass-pyrogenic smoke (SDOC) can be transported and deposited with atmospheric aerosols, enter aqueous environments, and possibly alter aqueous chemistry and quality. However, the characteristics of SDOC in aqueous environments and their effects on the fate of hydrophobic organic pollutants are poorly understood. In this study, we found that the emitted SDOC is 7.2 similar to 19.6 wt.% of biochar retained in situ after biomass pyrolysis, and the emitted SDOC is approximately 1-3 orders of magnitude greater than dissolved organic carbon (DOC) released from biochar in a short term, which indicates that SDOC is a more important source of DOC in aqueous environments relative to biochar-released DOC after a biomass burning/pyrolysis event. The characteristics of SDOC in aqueous environments are dominated by the <1000 Da fraction, which accounts for >96 wt.% of bulk SDOC. In comparison with DOC in biochar, natural water, and soil, the S275-295 value of SDOC (0.037-0.053) is significantly greater, further indicating that SDOC has a smaller molecular size. Moreover, fluorescence EEM suggests that a fluorescence component located at the Ex/Em of 205/310 nm and the combinational ranges of fluorescence index (1.28-2.28), humification index (0.07-0.80), and biological index (1.16-1.72) can be used to identify SDOC from DOC in other media. Solubilization experiments indicated that SDOC (20 mg/L) improved the solubility of hydrophobic pollutants (pyrene and triclocarban) by 2-6 folds in aqueous environments, which potentially enhances the mobility of pollutants and enlarges the potential risk region. This study indicates that SDOC may cause a severe harm to aqueous environments in addition to the atmosphere. The results have profound implications for comprehensive assessments of the environmental effects of SDOC while promoting its identification and elucidating its behavior in aqueous environments.

How the partitioning between dissimilatory nitrate reduction to ammonium (DNRA) and N 2 production responds to hypoxia in seasonally hypoxic estuaries has not been well understood. In this study, 15 N tracer incubations were used to investigate benthic denitrification, anammox and DNRA rates for three years (2016–2018) across seasonally hypoxic areas off the Changjiang Estuary. Dissolved oxygen contents in bottom water were significantly lower in July than in February/March. Concentrations of NH 4 + and NO x – in bottom water throughout the sampling sites did not vary between seasons. Rates of benthic denitrification (0.45–6.55 nmol N g −1 h −1 ), anammox (0.02–0.30 nmol N g −1 h −1 ) and DNRA (0.06–0.59 nmol N g −1 h −1 ) varied largely along the estuary. Denitrification, anammox and DNRA rates did not vary significantly between inshore and offshore sites. However, denitrification rates were significantly lower and DNRA rates were significantly higher in consistently than transiently hypoxic sites. Anammox rates were slightly lower in consistently than transiently hypoxic sites. Ratios of DNRA to N 2 production were in a range of 0.015–0.53, and were significantly higher in consistently than transiently hypoxic sites, but did not vary significantly between inshore and offshore sites. The lower temperature sensitivity of denitrification and anammox compared to their Q 10 values further indicated that hypoxia may inhibit N 2 production. These results suggest that high incidence hypoxia can decrease denitrification and anammox rates but increase DNRA rates with important implications on N retention versus removal, thereby exacerbating eutrophication and hypoxia in estuarine and coastal environments.

Spatial and temporal variations in soil denitrification and anaerobic ammonium oxidation (anammox) across the freshwater-oligohaline wetlands in subtropical estuary have not been well understood. In this study, continuous-flow soil core incubation combined with nitrogen isotope tracer was used to determine denitrification and anammox rates across freshwater-oligohaline tidal wetlands in Min River Estuary, Southeast China. Areal rates of denitrification and anammox varied from 3.89 to 19.0 μmol m−2 h−1 and from 0.15 to 1.11 μmol m−2 h−1, respectively, across these wetlands and throughout sampling months. Denitrification rates were higher in warm months (July, September) than in cool months (November, January), whereas anammox did not vary significantly across the sampling months. Average denitrification rates throughout the sampling months were higher in freshwater than in oligohaline wetlands, while anammox rates did not vary among the wetlands. Relative contribution of anammox (Ra) to N2 production (including denitrification and anammox) varied from 1.03 to 18.3% across the sampling months and wetlands. Denitrification rates differed significantly across the wetlands and sampling months. Anammox rates and Ra did not vary significantly among the sampling months. Denitrification rates were positively correlated with water content, total organic carbon (TOC), total nitrogen, dissolved organic nitrogen, NH4+, NOx–, Fe2+, and Fe2+/Fe3+, but negatively related to pH. Anammox rates showed negative relationships with water content and TOC. Water content, temperature, and pH were crucial for organic carbon and Fe2+ availability with important implications on denitrification and anammox. Therefore, denitrification rates vary significantly, whereas anammox rates do not vary significantly across freshwater-oligohaline wetlands in the Min River Estuary.

Estuarine sediment denitrification and anammox in response to increased nitrogen (N) loads remain poorly understood. In this study, we used N isotope tracer approach to investigate the spatial distribution of denitrification and anammox and identified the crucial controls on the partitioning of dinitrogen gas (N-2) production along the Min River Estuary (MRE), a highly impacted estuary in southeast China. The results indicated that denitrification and anammox rates ranged from 10.5 to 70.7 nmol g(-1) h(-1) and from 0.44 to 4.31 nmol g(-1) h(-1), respectively. Relative contribution of anammox to N-2 production (R-a) was in a range of 1.04-15.1%, tending to increase toward estuary mouth. Denitrification rates were significantly higher in upper (high N loads) than in lower estuary (low N loads), while anammox rates and R-a showed inverse distributions along the MRE. Wastewater discharge caused the N point pollution triggering denitrification but inhibiting anammox. The best predictor of the variations in denitrification rates was total nitrogen, whereas pH and NH4+ could explained the variations in anammox rates across the estuary. The crucial predictors for the partitioning of N-2 production between denitrification and anammox were NH4+ and NOx. These results suggest that the increase in human activities intensity can alter the partitioning of N-2 production between denitrification and anammox, and the magnitude of this switch can be predicted by N loads in MRE and other highly impacted estuaries. (c) 2021 Elsevier Ltd. All rights reserved.

Human activities are dramatically increasing estuarine nitrogen (N) loads, further altering N processes. However, relative importance of denitrification and anaerobic ammonium oxidation (anammox) in response to human activities intensity gradient remains poorly understood for estuaries. In this study, we used a N-15 isotope tracer approach to characterize the variations in sediment denitrification and anammox rates and determined the crucial factors controlling the partitioning of N-2 production and regulating N2O production across five subtropical estuaries in southeast China. Denitrification rates increased significantly from 8.82 +/- 3.89 nmol N g(-1) h(-1) (low human activity intensity) to 41.2 +/- 11.5 nmol N g(-1) h(-1) (high human activity intensity) across the studied estuaries. Anammox rates not varied significantly between low (2.37 +/- 0.66 nmol N g(-1) h(-1)), moderate (3.96 +/- 0.91 nmol N g(-1) h(-1)), and high (4.04 +/- 1.09 nmol N g(-1) h(-1)) human activity intensity estuaries. Relative contribution of anammox to total N-2 production (R-a) decreased toward estuary mouth within each estuary. The Ra was also significantly lower in the estuaries characterized by high N loads and low DO. N2O production rates were in a range of 0.192-1.92 nmol N2O g(-1) h(-1) across the estuaries and controlled by water NO2-, salinity and TOC. DO, NH4+, and NO3- were the best predictors of the partitioning of N-2 production between denitrification and anammox. The high human activities intensity increased NH4+ and NO3- loads and further enhanced denitrification, leading to the decrease in Ra and increase in N2O production. These findings suggest that low DO and high N loads estuaries can increase denitrification and N2O emissions, whereas not affect anammox substantially under the higher intensity of human activities.

Quality and source of soil organic carbon controlling DNRA in estuarine wetlands have not been well understood. We used a 15N isotopic tracer approach to investigate DNRA rates and determined the effects of organic carbon quality and source on magnitudes and distribution of DNRA along a freshwater-oligohaline gradient of Min River estuary, Southeast China. DNRA rates ranged from 0.45 to 2.92 nmol g-1 h-1 and were significantly higher in summer than in winter. DNRA was well predicted by dissolved organic carbon (DOC), organic carbon isotope (δ13Corg), total OC as the main predictor variables, which explained 63%, 6.0% and 7.0% of DNRA variances, respectively. Water content and temperature were crucial for DOC availability with important implications on DNRA. Soil δ13Corg increased from freshwater to oligohaline wetlands and showed a positive correlation with DNRA. We therefore conclude that high DOC availability enhances DNRA and can best predict spatial distribution in subtropical estuary.

The alteration of rainfall pattern under the background of global climate change may affect the quantity and quality of soil dissolved organic matter (DOM). To better understand the responses of soil DOM to rainfall reduction in subtropical forests, we conducted a 6-year rainfall reduction experiment. There were three treatments: control (CK), 30% rainfall reduction (-30%), and 60% rainfall reduction (-60%). With ultraviolet-visible, infrared and three-dimensional fluorescence spectroscopy, we investigated the effects of rainfall reduction on the quantity and structure of DOM from different soil layers of a natural Castanopsis carlesii forest in subtropical China. Rainfall significantly reduced the content of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in the 0-10 cm soil layer. Specifically, less decreases of DOC than DON were found under the -30% treatment, while more reduction of DOC than DON for the -60% treatment. Under each treatment, the contents of DOC and DON were significantly higher in the 0-10 cm soil layer than that in the 10-20 cm soil layer. Relatively more microbial metabolite with complex structures, such as aromatic humus and alkanes, contributed to soil DOM from the -30% treatment than that from the -60% treatment. Beyond water availability, microbial activity was a key factor regulating the quantity and structure of soil DOM in response to rainfall reduction.

Soil erosion and runoff generation are a major concern in forest and agricultural ecosystem management. However, the tangled coeffects of rainfall and vegetation on runoff and erosion generation are regionally dependent and remain unclear in regions with high erosivity, such as Southeast China where the large-scale afforestation is being implemented. Here, we present the 4-year observations of runoff and erosion responses to various storms following three afforestation (assisted natural regeneration [ANR; windrowing harvest residues on surface]; Chinese fir cultivation [YCF]; and Castanopsis carlesii cultivation [YCP] after slash-and-burn) since 2012. Stepwise power-law regressions were used to identify the key variables and formulate the responses. Results show that the YCF and YCP following slash and burn have extremely high erosion rates, over 20 t ha(-1) yr(-1) (twofold higher than the global tolerable threshold), during the first year. In contrast, the erosion rate is only approximately 1.3 t ha(-1) yr(-1) in the ANR. The high erosion rates reveal the environmental vulnerability and the necessity of management in this region. Stepwise regressions suggest total rainfall rather than rainfall intensity as key variable probably because the forest canopy substantially offsets raindrop kinetics. The nonlinear runoff and erosion responses indicate that the risk of soil erosion is extremely high when vegetation cover is less than 40% and rainfall is greater than 80 mm. This study, therefore, suggests that windrowing harvest residues into strips or maintaining a vegetation cover greater than 40% is a sound management practice for mitigating land degradation in this region.