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Understanding the relationship between land use and the dynamics of nitrate
(NO3-) is the key to constrain sources of NO3-
export in order to aid effective management of waterways. In this study,
isotopic compositions of NO3-
(δ15N–NO3- and
δ18O–NO3-) were used to elucidate the effects of
land use (agriculture in particular) and rainfall on the major sources and
sinks of NO3- within the Western Port catchment, Victoria,
Australia. This study is one of the very few studies carried out in temperate
regions with highly stochastic rainfall patterns, enabling a more
comprehensive understanding of the applications of NO3- isotopes
in catchment ecosystems with different climatic conditions. Longitudinal
samples were collected from five streams with different agriculture land use
intensities on five occasions – three during dry periods and two during wet
periods. At the catchment scale, we observed significant positive
relationships between NO3- concentrations
(p < 0.05), δ15N–NO3-
(p < 0.01) and percentage agriculture (particularly during the
wet period), reflecting the dominance of anthropogenic nitrogen inputs within
the catchment. Different rainfall conditions appeared to be major controls on
the predominance of the sources and transformation processes of
NO3- in our study sites. Artificial fertiliser was the dominant
source of NO3- during the wet periods. In addition to artificial
fertiliser, nitrified organic matter in sediment was also an apparent source
of NO3- to the surface water during the dry periods.
Denitrification was prevalent during the wet periods, while uptake of
NO3- by plants or algae was only observed during the dry periods
in two streams. The outcome of this study suggests that effective reduction
of NO3- load to the streams can only be achieved by prioritising
management strategies based on different rainfall conditions.