Collateral implications of carbon and metal pollution on carbon dioxide emission at land-water interface of the Ganga River

Abstract

\n Atmospheric CO2- source- and- sink is among the most debated issues that have puzzled climate change geochemist for decades. Here, we tested whether heavy metal pollutants in river sediments favor preservation of organic matter through shielding microbial degradation. We measured CO2 emission and extracellular enzyme activities at land-water interface (LWI) of 7 sites along a 285 km main stem of the Ganga River and 60 locations up- and downstream of two contrasting point sources discharging urban (Assi drain; Asdr) and industrial (Ramnagar drain; Rmdr) wastewaters to the river. We found the lowest CO2 flux at Rmdr mouth characterized by the highest concentrations of Cu, Cr, Zn, Pb, Ni, and Cd. The fluxes were relatively higher at locations up-and down-stream Rmdr. Substrate induced respiration (SIR), protease, FDAase and β-D-glucosidase all showed a similar trend but phenol oxidase and alkaline phosphatase showed opposite trend at the main river stem and Asdr. Sites rich in terrestrially derived organic matter have high phenol oxidase activity with low CO2 emission. The CO2 emission in the main river stem showed curvilinear relationships with total heavy metals (∑THM; R2\u2009=\u20090.68; P\u2009<\u20090.001) and TOC (R2\u2009=\u20090.65; p\u2009<\u20090.001). The dynamic fit model of main stem data showed that the ∑THM above 337.4 µg g−\u20091 were able to significantly decrease the activities of protease, FDAase and β-D-glucosidase. The study has implications for understanding C-cycling in human-impacted river sediments where metal pollution shields microbial degradation consequently carbon and nutrient release; and merits attention towards river management decisions.