A conceptual model of nitrogen dynamics for the Great Barrier Reef catchments

Nitrogen (N) from anthropogenic sources has been identified as a major pollutant of the Great Barrier Reef (GBR), Australia. We developed a conceptual framework to synthesise and visualise the fate and transport of N from the catchments to the sea from a literature review. The framework was created to fit managers and policymakers' requirements to reduce N in the GBR catchments. We used this framework to determine the N stocks and transformations (input, sources, and outputs) for ecosystems commonly found in the GBR: rainforests, palustrine wetlands, lakes, rivers (in-stream), mangroves and seagrasses. We included transformations of N such as nitrogen fixation, nitrification, denitrification, mineralisation, anammox, sedimentation, plant uptake, and food web transfers. This model can be applied to other ecosystems to understand the transport and fate of N within and between catchments. Importantly, this approach can guide management actions that attenuate N at different scales and locations within the GBR ecosystems. Finally, when combined with local hydrological modelling, this framework can be used to predict outcomes of management activities.

Nitrogen processing by treatment wetlands in a tropical catchment dominated by agricultural landuse

Agriculture is a major contributor to marine nitrogen pollution, and treatment wetlands can be a strategy to reduce it. However, few studies have assessed the potential of treatment wetlands to mitigate nitrogen pollution in tropical regions. We quantify the nitrogen removal rates of four recently constructed treatment wetlands in tropical Australia. We measured denitrification potential (D_t), the inflow-outflow of nutrients, and tested whether the environment in these tropical catchments is favourable for nitrogen removal. D_t was detected in three of the four systems with rates between 2.0 and 12.0 mg m^-2 h^-1; the highest rates were measured in anoxic soils (ORP -100 to 300 mV) that were rich in carbon and nitrogen (>2% and >0.2%, respectively). The highest nitrogen removal rates were measured when NO₃^--N concentrations were >0.4 mg L^-1 and when water flows were slow. Treatment wetlands in tropical regions can deliver high removal rates of nitrogen and other pollutants when adequately managed. This strategy can reduce nutrient loads and their impacts on sensitive coastal zones such as the Great Barrier Reef.

Mangrove sinkholes (cenotes) of the Yucatan Peninsula, a global hotspot of carbon sequestration

Mangroves are among the most carbon-dense ecosystems on the planet. The capacity of mangroves to store and accumulate carbon has been assessed and reported at regional, national and global scales. However, small-scale sampling is still revealing 'hot spots' of carbon accumulation. This study reports one of these hotspots, with one of the largest-recorded carbon stocks in mangroves associated with sinkholes (cenotes) in the Yucatan Peninsula, Mexico. We assessed soil organic carbon (SOC) stocks, sequestration rates and carbon origin of deep peat soils (1 to 6 m). We found massive amounts of SOC up to 2792 Mg C ha-1, the highest value reported in the literature so far. This SOC is primarily derived from highly preserved mangrove roots and has changed little since its deposition, which started over 3220 years ago (±30 BP). Most cenotes are owned by Mayan communities and are threatened by increased tourism and the resulting extraction and pollution of groundwater. These hot spots of carbon sequestration, albeit small in area, require adequate protection and could provide valuable financial opportunities through carbon-offsetting mechanisms and other payments for ecosystem services.

Soil nitrogen accumulation, denitrification potential, and carbon source tracing in bioretention basins

Bioretention basins are one of the most commonly used green stormwater features, with the potential to accumulate significant levels of nitrogen (N) in their soil and to permanently remove it through denitrification. Many studies have investigated the N removal potential of bioretention basins through the assessment of inflow and outflow concentrations. However, their long-term N removal through soil accumulation and denitrification potential is less known. This study investigated the temporal variation of total N (TN) accumulation and denitrification potential in soils of 25 bioretention basins within a 13-year soil chronosequence, in a subtropical climate in Australia. The denitrification potential of a subset of seven bioretention basins was investigated in accompaniment with nutrient and soil characteristics. Additionally, stable isotopes (δ¹³C and δ¹⁵N) were used to assess temporal changes in the soil composition as well as to identify the sources of carbon (C) into these basins. Over 13 years of operation, TN accumulated faster in the top 5 cm of soil than deeper soils. Soil TN density was highest in the top 5 cm with an average of 1.4 kg N m^-3, which was about two times higher than deeper soils. Site age and soil texture were the best predictors of soil TN density and denitrification (1 to 9.7 mg N m^-2 h^-1). The isotope values were variable among basins. Low δ¹⁵N values in young basins (-1.02‰) suggested fixation as the main source of N, while older basins had higher δ¹⁵N, indicating higher denitrification. Bioretention plants were the primary source of soil C; although the occurrence of soil amendment also contributed to the C pool. To improve the performance of these bioretention basins, we recommend increasing vegetation at initial years after construction, and enhancing more frequent anaerobic conditions in the high soil profile. These two conditions can improve denitrification potential, and thus the performance of these basins for improving water quality.

Avoided emissions and conservation of scrub mangroves: potential for a Blue Carbon project in the Gulf of California, Mexico

Mangroves are considered ideal ecosystems for Blue Carbon projects. However, because of their short stature, some mangroves ('scrub' mangroves, less than 2 m) do not fulfil the current definition of 'forests', which makes them ineligible for emission reduction programmes such as REDD+. Short stature mangroves can be the dominant form of mangroves in arid and nutrient-poor landscapes, and emissions from their deforestation and degradation could be substantial. Here, we describe a Blue Carbon project in the Gulf of California, Mexico, to illustrate that projects that avoid emissions from deforestation and degradation could provide financial resources to protect mangroves that cannot be included in other emission reduction programmes. The goal of the project is to protect 16 058 ha of mangroves through conservation concessions from the Mexican Federal Government. The cumulative avoided emissions of the project are 2.84 million Mg CO₂ over 100 years, valued at $US 426 000 per year (US$15 per Mg CO₂ in the California market). The funds could be used for community-based projects that will improve mangrove management, such as surveillance, eradication of invasive species, rehabilitation after tropical storms and environmental education. The strong institutional support, secure financial status, community engagement and clear project boundaries provide favourable conditions to implement this Blue Carbon project. Financial resources from Blue Carbon projects, even in mangroves of short stature, can provide substantial resources to enhance community resilience and mangrove protection.

Selecting cost-effective areas for restoration of ecosystem services

Selection of areas for restoration should be based on cost-effectiveness analysis to attain the maximum benefit with a limited budget and overcome the traditional ad hoc allocation of funds for restoration projects. Restoration projects need to be planned on the basis of ecological knowledge and economic and social constraints. We devised a novel approach for selecting cost-effective areas for restoration on the basis of biodiversity and potential provision of 3 ecosystem services: carbon storage, water depuration, and coastal protection. We used Marxan, a spatial prioritization tool, to balance the provision of ecosystem services against the cost of restoration. We tested this approach in a mangrove ecosystem in the Caribbean. Our approach efficiently selected restoration areas that at low cost were compatible with biodiversity targets and that maximized the provision of one or more ecosystem services. Choosing areas for restoration of mangroves on the basis carbon storage potential, largely guaranteed the restoration of biodiversity and other ecosystem services.

Direct estimate of 1:1 stoichiometry of K(+)-Cl(-) cotransport in rabbit erythrocytes

This work was undertaken to obtain a direct measure of the stoichiometry of Na(+)-independent K(+)-Cl(-) cotransport (KCC), with rabbit red blood cells as a model system. To determine whether (86)Rb(+) can be used quantitatively as a tracer for KCC, (86)Rb(+) and K(+) effluxes were measured in parallel after activation of KCC with N-ethylmaleimide (NEM). The rate constant for NEM-stimulated K(+) efflux into isosmotic NaCl was smaller than that for (86)Rb(+) by a factor of 0.68 +/- 0.11 (SD, n = 5). This correction factor was used in all other experiments to calculate the K(+) efflux from the measured (86)Rb(+) efflux. To minimize interference from the anion exchanger, extracellular Cl(-) was replaced with SO, and 4,4'-diisothiocyanothiocyanatodihydrostilbene-2,2'-disulfonic acid was present in the flux media. The membrane potential was clamped near 0 mV with the protonophore 2,4-dinitrophenol. The Cl(-) efflux at 25 degrees C under these conditions is approximately 100,000-fold smaller than the uninhibited Cl(-)/Cl(-) exchange flux and is stimulated approximately 2-fold by NEM. The NEM-stimulated (36)Cl(-) flux is inhibited by okadaic acid and calyculin A, as expected for KCC. The ratio of the NEM-stimulated K(+) to Cl(-) efflux is 1.12 +/- 0.26 (SD, n = 5). We conclude that K(+)-Cl(-) cotransport in rabbit red blood cells has a stoichiometry of 1:1.

Characterization of oxalate transport by the human erythrocyte band 3 protein

This paper describes characteristics of the transport of oxalate across the human erythrocyte membrane. Treatment of cells with low concentrations of H2DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonate) inhibits Cl(-)-Cl- and oxalate-oxalate exchange to the same extent, suggesting that band 3 is the major transport pathway for oxalate. The kinetics of oxalate and Cl- self-exchange fluxes indicate that the two ions compete for a common transport site; the apparent Cl- affinity is two to three times higher than that of oxalate. The net exchange of oxalate for Cl-, in either direction, is accompanied by a flux of H+ with oxalate, as is also true of net Cl(-)-SO4(2-) exchange. The transport of oxalate, however, is much faster than that of SO4(2-) or other divalent anions. Oxalate influx into Cl(-)-containing cells has an extracellular pH optimum of approximately 5.5 at 0 degrees C. At extracellular pH below 5.5 (neutral intracellular pH), net Cl(-)-oxalate exchange is nearly as fast as Cl(-)-Cl- exchange. The rapid Cl(-)-oxalate exchange at acid extracellular pH is not likely to be a consequence of Cl- exchange for monovalent oxalate (HOOC-COO-; pKa = 4.2) because monocarboxylates of similar structure exchange for Cl- much more slowly than does oxalate. The activation energy of Cl(-)-oxalate exchange is about 35 kCal/mol at temperatures between 0 and 15 degrees C; the rapid oxalate influx is therefore not a consequence of a low activation energy. The protein phosphatase inhibitor okadaic acid has no detectable effect on oxalate self-exchange, in contrast to a recent finding in another laboratory (Baggio, B., L. Bordin, G. Clari, G. Gambaro, and V. Moret. 1993. Biochim. Biophys. Acta. 1148:157-160.); our data provide no evidence for physiological regulation of anion exchange in red cells.