Integrating atmospheric deposition-driven nutrients (N and P), microbial and biogeochemical processes in the watershed with carbon and nutrient export to the Ganga River

Abstract

There has been considerable debate on how atmospheric deposition (AD) in the watershed alters the supply of dissolved organic carbon (DOC) to river and coastal ecosystems. Here we show how AD-N and –P in the Ganga River Basin (GRB) drives the delivery of DOC. We conducted three sets of studies, a 3 year (2016–2018) in situ trial, a 5 year (2014–2018) soil-spray experiment and a decadal-scale (2009–2018) in situ coupled pot-culture study to show how AD-N and –P stimulates soil microbial activity and consequently enhances the DOC export to the Ganga River. The AD-input increased overtime, and on an average, the basin receives ~ 1.55 Tg Nr and ~ 0.088 Tg P annually. Our results show that AD–N and –P increases soil microbial biomass (Cmic), substrate-induced respiration (SIR), FDAase and β-D-glucosidase activity to enhance water-soluble organic carbon (WSOC) in the watershed. We found significant correlations (r = 0.83–0.93; p < 0.001) of Cmic, SIR, enzyme activities, WSOC and DOC with AD–N and –P in the watershed. Although the land use was found to be the dominant contributor of variations, the AD-input did appear an important driver enhancing the delivery of DOC. The runoff flux of DOC was found to the highest for built-up land (that include urban wash) followed by wasteland, grassland, agricultural land, scrubland, mixed forest and deciduous forest. The study has relevance understanding the mechanism of AD-driven DOC delivery and designing an action plan for integrated river basin management (IRBM).