Agricultural catchments are potentially important but often neglected
sources of dissolved organic matter (DOM), of which a large part is
dissolved organic carbon (DOC) and nitrogen (DON). DOC is an important
source of aquatic microbial respiration and DON may be an important
source of nitrogen to aquatic ecosystems. However, there is still a lack
of comprehensive studies on the amount, composition and seasonality of
DOM export from agricultural catchments in different climates. The aim
of our study was to assess the amount, composition and seasonality of
DOM in a total of four streams in the wet-temperate and subtropical
climate of Denmark and Uruguay, respectively. In each climate, we
investigated one stream with extensive agriculture (mostly pasture) and
one stream with intensive agriculture (mostly intensively used arable
land) in the catchment. We sampled each stream taking grab samples
fortnightly for two years and measured DOC and DON concentration, as
well as molecular composition by size-exclusion chromatography. We used
absorbance, fluorescence and parallel factor analysis to gather
additional information on the sources and composition of the DOM. The
results were coupled to measurements of precipitation, water
temperature, discharge, water residence time and physicochemical data
measured at each study site to investigate the effects these
environmental variables have on the amount and composition of DOM in the
streams. Average annual DOM concentration and seasonality were highest
in the stream with intensive agriculture in Uruguay and lowest in the
stream with extensive agriculture in Denmark. In all streams, the
molecular-size composition of DOC and DON were similar and most DOC and
DON were exported as humic substances with low C:N ratio, which
indicates high bioavailability. Moreover, DON was of higher relative
importance in the Uruguayan streams than in the Danish streams, as can
be seen from the lower dissolved inorganic to total dissolved nitrogen
ratios. The high seasonality of DOM, especially in the Uruguayan streams
implies high influence of variation of the environmental conditions,
both temperature and precipitation. Our study shows the importance of
agriculture as DOM source in both climate zones, with a high seasonality
particularly in Uruguayan streams. Future investigations and models of
DOC and DON export in agricultural catchments need to include this
seasonality to give correct information on the catchment export.
Moreover, our results clearly show that DON is increasingly important
for the nitrogen export from agricultural catchments when moving from a
temperate to a warmer climate.