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Identifying and eliminating pollutant sources of water bodies is critical for drinking water safety. In this research, river water, reservoir water and groundwater samples ( n  = 259) were collected from November 2015 to January 2017. Spatial Analysis was made of the isotopic compositions of potential nitrate sources ( i.e. , manure, sewage, chemical nitrogen fertilizer, soil organic nitrogen and rainfall) so as to obtain the site source isotopic signatures. Different sources pools and fractionation factors were loaded to a Bayesian isotope mixing model to ensure posterior estimates with less uncertainty. Results showed that the total nitrogen (TN) concentrations in Hexi Reservoir watershed were higher than the Environmental Quality Standards for Surface Water of China (GB 3838-2002), and NO 3 − -N was the dominant form of TN (accounting for 68.63% on average). There are significant spatio-temporal variations in the isotope data (δ 15 N-NO 3 − and δ 18 O-NO 3 − ) and the dominant nitrate sources, which were related to the land use types. Loading the site source isotopic signatures to the Bayesian isotope mixing model effectively improved the accuracy and precision of nitrate source apportionment. Chemical nitrogen fertilizer (NF) was the foremost largest contributor of NO 3 − -N (38.82%), especially for Hexi North Stream (34.19%) and Yangmei Stream (44.39%), while atmospheric deposition (AD) contributed the least to NO 3 − -N (0.47%) of river water in the watershed; soil organic nitrogen (NS) contributed more to NO 3 − -N in the dry season than in the wet season; and manure and sewage (M&S) contributed approximately 30.22% in the whole study period, 53.60% in September 2016 and 41.33% in Hexi South Stream. This research suggests that combination of Spatial Analysis and the Bayesian isotope mixing model with the measured isotopic signatures of potential nitrate sources accurately apportion the nitrate source contributions. [ABSTRACT FROM AUTHOR]

Abstract Microorganisms play crucial roles in the nitrogen removal processes of wetlands. However, the key functional genes and microbes related to the nitrogen removal remain largely unknown in the free water surface constructed wetland (FWS CW). Here we studied the abundances of denitrifiers by targeting the key functional genes (nirS , nirK and nosZ) and investigated the community compositions of denitrifiers and their correlations with the abiotic variables in a FWS CW. The increase of nos Z /(nir S + nir K) and nirS / nirK ratios in the outlet indicated a shift of denitrifiers' communities which tended to release less nitrous oxide at the genetic potential level. The denitrifiers dominated the bacterial community which also remarkably changed from the inlet to the outlet. PICRUSt analysis revealed that the denitrifiers contributed to 39.1% of the nitrogen metabolism, 38.9% of the amino acid metabolism and 25.6% of the amino acid related enzymes. Four bacterial genera including Hydrogenophaga , Hylemonella , Aquabacterium and Cellvibrio were detected as the putative keystone denitrifiers. The abundance (nirS , nirK and nosZ) and the relative abundance of putative keystone denitrifiers were significantly correlated with total organic carbon, oxidation-reduction potential and C/N ratio, which could be regarded as the determinants for the denitrification process in the free water. Graphical abstract Unlabelled Image Highlights • Denitrifiers were investigated in a multi-stage free water surface constructed wetland. • The abundance of nos Z -type denitrifiers increased in the outlet. • The denitrifiers' community composition remarkably changed from the inlet to outlet. • Four putative keystone denitrifers were predictors of nitrogen related functions. [ABSTRACT FROM AUTHOR]

• Cr(VI) was adsorbed and photo-reduced into Cr(III) by metal-free N-doped carbon. • sp2 -hybridized structures and graphic-N were responsible for photo-reduction effect. • The prepared N-doped carbon was derived from the abundant biomass cellulose. The efficient removal of toxic hexavalent chromium (Cr(VI)) is crucial for waste water treatment. Herein, we report a new strategy to couple adsorption and in situ photo-reduction of Cr(VI) to Cr (III) using metal-free, N-doped carbon facilely derived from naturally abundant biomass cellulose. Experimental results exhibited the removal rate of Cr(VI) can be significantly enhanced from 43.25 mg/g to 98.25 mg/g after visible light irradiation under acidic conditions. We demonstrated that toxic Cr(VI) ions were firstly adsorbed on N-doped carbon via electrostatic attraction, and then photo-reduced into Cr(III), followed by re-adsorption through chemical complexation. The carbon sp2 -hybridized structures and electro-attracting graphic-N groups (N-(C) 3) are proposed to be responsible for this photo-reduction effect. This work reveals the efficient removal of heavy metals through the cooperative adsorption and photo-reduction using the materials synthesized from biomass waste. [ABSTRACT FROM AUTHOR]