Export of Dissolved Organic Matter, Nutrients and Carbon from Himalayan River System in Central Nepal

Abstract:

Chemical weathering is a vital ecosystem process and plays a central role in regulation of global carbon cycles. Weathering from Himalayan landscape supply high amount of major ions, nutrients and suspended sediments to the oceans. Surface water samples were collected from sixteen stations at different altitude along the Langtnag-Narayani Himalayan river system in central Nepal on a monthly basis for one year. This study aims to investigate spatiotemporal variations of dissolved organic matter, nutrients and carbonic species and to evaluate their controlling factors within the basin. The fluxes of these species appeared several fold higher at low elevation than at mid mountains and high elevation Himalaya sites. Seasonality appeared to exert major control on concentrations and fluxes of major solutes along the drainage network. The highest export rate of chemical species corresponded to the monsoon season, followed by the ones corresponding to post-monsoon and pre-monsoon seasons. Carbonate has major control on the flux of major solutes within the basin. The export rate of dissolved organic carbon and total dissolved nitrogen were about three and seventeen times higher respectively at the Narayani basin than its headwater at Langtang basin within the high Himalaya. Nitrate and phosphate export rates in the Narayani basin were 5.07 and 0.34 tons km^-2 yr^-1 respectively which is several fold higher than the rates in the high Himalaya probably due to input from agricultural activities. The export of dissolved inorganic carbon from the Narayani basin was 101.87 tons km^-2 yr^-1 of which bicarbonate appeared to be the dominant fraction (94.9%) followed by carbonic acid (4.7%) and carbonate (0.4%). Partial pressure of carbon dioxide (pCO₂) resulted under-saturated in the high elevation Himalayan basin and supersaturated at the low elevation Narayani basin. The concentration of pCO₂ is considered to be an important factor for regulating weathering rates of any landscape.