Abstract
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 downstream 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 = 0.68; p < 0.001) and TOC (R2 = 0.65; p < 0.001). The dynamic fit model of main stem data showed that the ∑THM above 337.4 µg g-1 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.
Highlights
The loading of terrestrially-derived organic carbon (OC) influences ecosystem functioning in rivers and streams along hydrologic continuum, through the processes controlled by microorganisms (Tezuka 1990; Taylor and Townsend 2010, Berggren and Giorgio 2015)
Many lines of evidence presented here indicate that, CO2 emission in human-impacted large rivers is controlled by multiplicity of factors including chemical recalcitrance, high concentration of heavy metals suppresses the microbial-extracellular enzyme activities responsible for organic matter decomposition and the magnitude of CO2 emission
We still have very less knowledge of how and to what magnitude metal enrichment will effect organic matter decomposition and CO2 release, our study clearly indicates that at source oriented locations, total metal concentration was sufficient to suppress microbial/enzyme activities vis-à-vis CO2 release
Summary
The loading of terrestrially-derived organic carbon (OC) influences ecosystem functioning in rivers and streams along hydrologic continuum, through the processes controlled by microorganisms (Tezuka 1990; Taylor and Townsend 2010, Berggren and Giorgio 2015). Point sources releasing sewage and industrial effluents and non-point sources such as surface runoff are continuing to add massive amount of carbon and nutrients to streams and rivers (Bernot et al 2010; Yadav and Pandey 2017). The presence of both the C-sources (allochthonous and autochthonous) allows uncoupling of primary- and secondary Cproduction and has implications for food web dynamics, CO2 emission and riverine transport (Van den Meersche et al 2009; Lau et al 2009)
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