Sources and transport of carbon and nitrogen in the River Sava watershed, a major tributary of the River Danube

Carbon and nitrogen dynamics were examined throughout the River Sava watershed, a major tributary of the River Danube, in 2005 and 2006. The River Sava exported 2.1 x 10(11) mol C/yr as dissolved inorganic carbon (DIC), and emitted 2.5 x 10(10) mol C/yr as CO2 to the atmosphere. Stable carbon isotope ratios indicate that up to 42% of DIC originated from carbonate weathering and similar to 23% from degradation of organic matter. Loads of dissolved and particulate organic carbon increased with discharge and export rates were calculated to be 2.1 x 10(10) mol C/yr and up to 4.1 x 10(9) mol C/yr, respectively. Isotopic compositions (delta C-13 and delta N-15) and C/N ratios indicated that soil organic matter was the dominant source of particulate organic matter for 59% of the samples. Eighteen percent of the samples were dominated by plankton, 12% by periodic inputs of fresh terrestrial plant detritus with C/N >15, and about 11% of the samples were dominated by the contribution of aquatic vascular plants. Nitrate inputs were controlled by land use in the River Sava watershed. delta N-15(NO3) values <6 parts per thousand were found in predominantly forested watersheds, while values >6 parts per thousand typically represented watersheds with a higher percentage of agricultural and/or urban land use. Elevated delta N-15(NO3) values (up to +25.5 parts per thousand) at some sites were probably due to the combined effects of low-flow and inputs from sewage and/or animal waste. (c) 2008 Elsevier Ltd. All rights reserved.