Native vegetation of the Canterbury Plains of South Island in New Zealand has been heavily modified by agriculture and now occupies less than 0.5% of the total land area. With recent large-scale conversion to intensive dairy farming, restoration of native plants and biodiversity into a modern agricultural matrix creates a significant challenge. Native species are adapted to low nitrogen (N) environments but fertilizers and effluents have substantially raised soil N loadings. We investigated the interactions of selected native species to elevated soil N, using field studies and glasshouse-based nutrient trials. Growth and uptake of N by perennial ryegrass provided a reference. At restoration sites, several native species had similar foliar N concentrations to ryegrass. Deciduous and N-fixing species of tree had highest concentrations. There was significant inter-species variation in soil mineral N concentrations in native plant rhizospheres, differing substantially to the root zone of ryegrass. Pot trials revealed that native species tolerate high N-loadings (up to 1600 kg ha-1), although there was a negligible or no significant growth response. Among the native plants, monocots (tussock grass, sedge and NZ flax) assimilated most N, although total N assimilation by ryegrass would exceed that of native species at field productivity rates. Nevertheless, the deeper rhizospheres of native species may reduce nitrate leaching when planted on the margins of agricultural land or for effluent disposal. Selected native plant species could contribute to the sustainable management of N in intensive agricultural landscapes. Highlights:  Native species are tolerant to elevated soil N, with negligible growth response  Low-N adapted plants show species-specific traits that include luxury N uptake  Differences rhizosphere N pools exist between native species and with ryegrass  Planting natives may help to provide sustainable agricultural management of N

Excess nitrogen (N) leading to the eutrophication of water and impacts on ecosystems is a serious environmental challenge. Wetlands can remove significant amounts of N from the water, primarily through the process of denitrification. Most of our knowledge on wetland denitrification is from temperate climates; studies in natural tropical wetlands are very scarce. We measured denitrification rates during a dry and a wet season in five floodplain forests dominated by Melaleuca spp., a coastal freshwater wetland of tropical Australia. We hypothesised that the denitrification potential of these wetlands would be high throughout the year and would be limited by N and carbon (C) availability. Mean potential denitrification rates (Dt) were 5.0±1.7mgm2h–1, and were within the reported ranges for other tropical and temperate wetlands. The rates of Dt were similar between the dry and the wet seasons. From the total unamended denitrification rates (Dw, 3.1±1.7mgm2h–1), 64% was derived from NO3– of the water column and the rest from coupled nitrification–denitrification. The factor most closely associated with denitrification was background water NO3–-N concentrations. Improved management and protection of wetlands could play an important role in improving water quality in tropical catchments.

. 2017. Leaf litter additions enhance stream metabolism, denitrification, and restoration prospects for agricultural catchments. Ecosphere 8(11):e02018. 10. 1002/ecs2.2018 Abstract. Globally intensive agriculture has both increased nitrogen pollution in adjacent waterways and decreased availability of terrestrially derived carbon frequently used by stream heterotrophs in nitrogen cycling. We tested the potential for carbon additions via leaf litter from riparian restoration plantings to act as a tool for enhancing denitrification in agricultural streams with relatively high concentrations of nitrate (1.3-8.1 mg/L) in Canterbury, New Zealand. Experimental additions of leaf packs (N = 200, mass = 350 g each) were carried out in 200-m reaches of three randomly selected treatment streams and compared to three control streams receiving no additional leaf carbon. Litter additions increased ecosystem respiration in treatment streams compared to control streams but did not affect gross primary production, indicating the carbon addition boosted heterotrophic activity, a useful gauge of the activities of microbes involved in denitrification. Bench-top assays with denitrifying enzymes using acetylene inhibition techniques also suggested that the coarse particulate organic matter added from leaf packs would have provided substrates suitable for high rates of denitrification. Quantifying denitrification directly in experimental reaches by open-channel methods based on membrane inlet mass spectrophotometry indicated that denitrification was around three times higher in treatment streams where litter was added compared to control streams. We further assessed the potential for riparian plantings to reduce large-scale downstream nitrogen losses through increasing in-stream denitrification by modeling the effects of increasing riparian vegetation cover on nitrogen fluxes. Here, we combined estimates of in-stream ecosystem processes derived from our experiment with a network model of catchment-scale nitrogen retention and removal based on empirical measurements of nitrogen flux in this typical agricultural catchment. Our model indicated leaf inputs associated with increased riparian cover had the potential to double the catchment level rate of denitrification, offering a promising way to mitigate nitrate pollution in agricultural streams. Altogether, our study indicates that overcoming carbon limitation and boosting heterotrophic processes will be important for reducing nitrogen pollution in agricultural streams and that combining empirical approaches for predictions suggests there are large potential benefits from riparian re-vegetation efforts at catchment scales.

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Terrestrial particulate nutrients transported during flood events are known to indirectly fuel phytoplankton blooms in rivers, lakes and coastal waters, although the mechanisms are poorly understood. Quantifying the response of phytoplankton to nutrients in sediments eroded from catchments is fundamental to prioritizing areas for erosion control. This study developed a novel bioassay technique for rapidly assessing the effects of nutrients released from suspended sediments on the growth of marine and freshwater phytoplankton communities. A range of sediment slurries were placed in bioassay bottles within dialysis tubing in the presence of phytoplankton and their photosynthetic efficiency (F/F) was measured over 72 h. This allowed an assessment of the effects of dissolved nutrients released from sediments without the confounding effects of suspended sediments. Chlorophyll a concentrations were also measured for comparison with F/F. Our study showed F/F was an effective method for measuring phytoplankton responses to sediment slurries. Photosynthetic efficiency was a more sensitive response metric than chlorophyll a. Applying the method to a range of suspended sediments from two tropical catchments in Australia that drain into Great Barrier Reef coastal waters, we identified a subset of sediment types (~40%) that increased F/F under the bioassay conditions. These sediments have the potential to stimulate marine and freshwater phytoplankton growth under the loads simulated in this study. The bioassay has the advantage of being a rapid and relatively simple method where a large number of sediments can be simultaneously tested for a phytoplankton response. To our knowledge this is the first time F/F has been used to assess phytoplankton responses to sediments in a bioassay. This approach advances the use of F/F as a sensitive indicator of phytoplankton responses to nutrients and could be used to develop indices of the relative risk various sediments pose, hence support decision making for erosion control measures.