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Understanding the relationship between land use and the dynamics of nitrate (NO-3) is the key to constrain sources of NO-3 export in order to aid effective management of waterways. In this study, isotopic compositions of NO-3 (δ15N-NO-3 and δ18N-NO-3) were used to elucidate the effects of land use (agriculture in particular) and rainfall on the major sources and sinks of NO-3 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 NO-3 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 NO-3 concentrations (p < 0.05), δ15N-NO-3 (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 NO-3 in our study sites. Artificial fertiliser was the dominant source of NO-3 during the wet periods. In addition to artificial fertiliser, nitrified organic matter in sediment was also an apparent source of NO-3 to the surface water during the dry periods. Denitrification was prevalent during the wet periods, while uptake of NO-3 by plants or algae was only observed during the dry periods in two streams. The outcome of this study suggests that effective reduction of NO-3 load to the streams can only be achieved by prioritising management strategies based on different rainfall conditions. [ABSTRACT FROM AUTHOR]

Abstract: Wetlands are often biogeochemical hotspots, and they can remove excess N via denitrification and assimilatory uptake. Wetlands are also susceptible to plant invasions, but the effect of invasive plants on denitrification in freshwater wetland sediments is not well‐studied. Two distinct mechanisms suggest the potential for invasive plants to alter denitrification. First, invasive plants often produce more biomass than non‐invasive species, thus potentially providing additional carbon (C) for denitrifiers. Second, some invasive wetland plants funnel more oxygen into the root zone than non‐invasive plants, potentially stimulating coupled nitrification–denitrification. Using the push–pull isotope pairing technique, we measured denitrification and coupled nitrification–denitrification in the sediments beneath monoculture plots of Phragmites australis and Typha domingensis, and beneath unvegetated sediments, in an urban wetland in Melbourne, Australia. We also measured pore water nutrient concentrations and calculated the diffusive flux of nutrients from the sediments into the overlying water column. We hypothesised that plots containing P. australis would have the highest denitrification and coupled nitrification–denitrification rates, followed by plots containing T. domingensis, with the lowest rates in the unvegetated plots, as a result of higher C and oxygen availability. Instead, we found that denitrification and coupled nitrification–denitrification rates were highly variable, with no difference among plot type. However, we did find that diffusive flux of ammonium from the sediments into the water column was lower in the vegetated plots than in the unvegetated plots, suggesting that vegetation enhances wetland N retention via plant assimilatory uptake. [ABSTRACT FROM AUTHOR]

We used δ15N-nitrate () and δ18O-to unravel the provenance and fate of in a groundwater-fed estuary. A total of 13 monthly and two time series surveys were undertaken in the Werribee River estuary near Melbourne, Australia. The different survey timescales provided a comprehensive evaluation of the hydrological effects on the biogeochemistry of, which accounted for tidal variability and episodic runoff events. The distribution of δ15N and δ18O of the estuarine along a mixing line between shallow and deep groundwater provided strong evidence for the predominance of groundwater-derived in the estuary. During dry periods when water residence time in the estuary was extended, shallow groundwater contributed 60% to 76% of the (calculated from δ15N- and δ18O-) to the estuary, and assimilation removed ~70% of this groundwater-derived . During wet periods, deep groundwater provided more (62%) than shallow groundwater, and there was no indication of consumption. Occasional sources of, which were also reflected by their δ15N- and δ18O- values, included from nitrification and sewage-derived particularly at the bay entrance to the estuary. Greater emphasis should be placed upon the role of groundwater as a substantial end member when assessing biogeochemistry using methods relying on the dual isotopic composition of. [ABSTRACT FROM AUTHOR]

The long-term efficacy of stormwater treatment systems requires continuous pollutant removal without substantial re-release. Hence, the division of incoming pollutants between temporary and permanent removal pathways is fundamental. This is pertinent to nitrogen, a critical water body pollutant, which on a broad level may be assimilated by plants or microbes and temporarily stored, or transformed by bacteria to gaseous forms and permanently lost via denitrification. Biofiltration systems have demonstrated effective removal of nitrogen from urban stormwater runoff, but to date studies have been limited to a ‘black-box’ approach. The lack of understanding on internal nitrogen processes constrains future design and threatens the reliability of long-term system performance. While nitrogen processes have been thoroughly studied in other environments, including wastewater treatment wetlands, biofiltration systems differ fundamentally in design and the composition and hydrology of stormwater inflows, with intermittent inundation and prolonged dry periods. Two mesocosm experiments were conducted to investigate biofilter nitrogen processes using the stable isotope tracer 15NO3− (nitrate) over the course of one inflow event. The immediate partitioning of 15NO3− between biotic assimilation and denitrification were investigated for a range of different inflow concentrations and plant species. Assimilation was the primary fate for NO3− under typical stormwater concentrations (∼1–2 mg N/L), contributing an average 89–99% of 15NO3− processing in biofilter columns containing the most effective plant species, while only 0–3% was denitrified and 0–8% remained in the pore water. Denitrification played a greater role for columns containing less effective species, processing up to 8% of 15NO3−, and increased further with nitrate loading. This study uniquely applied isotope tracing to biofiltration systems and revealed the dominance of assimilation in stormwater biofilters. The findings raise important questions about nitrogen release upon plant senescence, seasonally and in the long term, which have implications on the management and design of biofiltration systems. [ABSTRACT FROM AUTHOR]

Organic matter provides energy and nutrients to aquatic systems. Alterations to its sources and processing have repercussions for water quality and food-web stability and structure. Despite worldwide recognition of the impacts of urbanisation, there is limited understanding of the relative importance of catchment-scale urban stormwater drainage connection and reach-scale riparian vegetation on organic matter sources., We investigated the effects of catchment-scale urban stormwater drainage connection and reach-scale riparian vegetation cover on organic matter sources in small streams. Using stable isotopes and elemental ratios (i.e. δ13C, δ15N and C : N), we traced the origin of microbially respired carbon and standing stocks of dissolved organic carbon ( DOC), suspended particulate organic matter ( POM) and benthic coarse particulate organic matter ( CPOM)., Catchment-scale urban stormwater drainage connection significantly increased the contribution of labile organic matter to POM and DOC standing stocks. Greater POM lability was a product of increased inputs of autochthonous organic matter in more heavily urbanised streams, although the origin of labile DOC was less clear., While reach-scale riparian vegetation was the most likely source of the terrestrially dominated CPOM observed across most sites, increasing cover had no significant effect on the origin of POM or DOC standing stocks. We conclude that catchment-scale stormwater drainage impacts overwhelm the effects of reach-scale riparian vegetation on the sources and lability of POM and DOC in small streams., Our results suggest that the protection or restoration of riparian vegetation, in the absence of modifications to catchment-scale stormwater drainage connection, is insufficient to mitigate the effects of urbanisation on organic matter sources, lability and processing in these small streams. [ABSTRACT FROM AUTHOR]