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Denitrifying woodchip bioreactors (DBRs) are an established nitrate mitigation technology, but uncertainty remains on their viability for phosphorus (P) removal due to inconsistent source-sink behaviour in field trials. We investigated whether iron (Fe) redox cycling could be the missing link needed to explain P dynamics in these systems. A pilot-scale DBR (Aotearoa New Zealand) was monitored for the first two drainage seasons (2017-2018), with supplemental in-field measurements of reduced solutes (Fe 2+ , HS - /H 2 S) and their conjugate oxidised species (Fe 3+ /SO 4 2- ) made in 2021 to constrain within-reactor redox gradients. Consistent with thermodynamics, the dissolution of Fe 3+ (s) to Fe 2+ ( aq ) within the DBR sequentially followed O 2 , NO 3 , but at ⁓50 % lower P concentrations relative to inlet Fe:P ratios. In season 2 the reactor became a net P sink, coinciding with declining outlet Fe - and MnO 2 (s) reduction, but occurred before SO 4 2- reduction. Monitoring of inlet and outlet chemistry revealed tight coupling between Fe and P (inlet R 2 0.94, outlet R 2 0.85), but distinct dynamics between drainage seasons. In season one, outlet P exceeded inlet P (net P source), and coincided with elevated outlet Fe 2+ , but at ⁓50 % lower P concentrations relative to inlet Fe:P ratios. In season 2 the reactor became a net P sink, coinciding with declining outlet Fe 2+ concentrations (indicating exhaustion of Fe 3+ (s) hydroxides and associated P). In order to characterize P removal under varying source dynamics (i.e. inflows vs in-situ P releases), we used the inlet Fe vs P relationship to estimate P binding to colloidal Fe (hydr)oxide surfaces under oxic conditions, and the outlet Fe 2+ concentration to estimate in-situ P releases associated with Fe (hydr)oxide reduction. Inferred P-removal rates were highest early in season 1 (k = 0.60 g P m 3 d -1 ; 75-100 % removal), declining significantly thereafter (k = 0.01 ± 0.02 g P m 3 d -1 ; ca. 3-67 % removal). These calculations suggest that microbiological P removal in DBRs can occur at comparable magnitudes to nitrate removal by denitrification, depending mainly on P availability and hydraulic retention efficiency., Competing Interests: Declaration of competing interest The authors declare no competing interests., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)

Diffusive Gradients in Thin-Films (DGT) have traditionally been used to measure time-weighted average concentration in water. We tested whether Br - -DGT in combination with the trace-dilution flow rate method, could be used as a new approach for measuring water flow rate. A novel bromide selective DGT based on the Purolite Bromide Plus anion exchange resin (Br - -DGT) was developed, which provided environmental bromide concentrations comparable to grab samples. The Br - -DGT provided quantitative bromide concentrations at a range of pH, competing ion concentrations, and in synthetic natural solution. The uptake efficiency was 95.7 ± 3.4%, and the elution efficiency was 95.5 ± 4.7%. The absorption maximum/saturation point of each binding disk was 0.684 ± 0.001 mg. Bromide adsorption to the binding layer was linear to 44.1% of the total binding capacity, 0.302 mg. The determined diffusion coefficient through the agarose cross-linked polyacrylamide (APA) hydrogels was 1.05 × 10 -5 cm 2 s -1 at 17.9 °C, temperature corrected to 25 °C was 1.29 × 10 -5 cm 2 s -1 . DGT flow rates were between -14.7 and 6.50% of the flow independently monitored flow rate (weir). In comparison, grab sample flow rates diverged by 5.52 to 58.9% from the weir flow rate., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Drained peatlands are major sources of CO 2 to the atmosphere, yet the effects of land management and hydrological extremes have been little-studied at spatial scales relevant to agricultural enterprises. We measured fluxes of CO 2 using the eddy covariance (EC) technique at two adjacent dairy farms on a drained peatland in Aotearoa New Zealand with remaining peat depths 5.5-8 m. One site (SD) had shallow surface drains and mean water table depth (WTD) -657 mm, while the other site (BD) had deep field border drains and mean WTD -838 mm. Net ecosystem CO 2 production (NEP) was similar at the two sites when the soils were moist but diverged during late-summer drying, with site BD having 4.56 t C ha -1 greater CO 2 emission than site SD over the four-month dry period. Soil drying reduced gross primary production (GPP) at both sites, while ecosystem respiration (ER) was reduced at site SD but not at site BD. The low dry season respiration rates at site SD contributed to near-zero annual NEP, while higher respiration rates at site BD led to annual CO 2 loss of -4.95 ± 0.59 t C ha -1  yr -1 . Accounting for other imports and exports of carbon, annual net ecosystem carbon balances were -2.23 and -8.47 t C ha -1  yr -1 at sites SD and BD, respectively. It is likely that the contrasting dry season respiration rates resulted from differences in soil physical properties affecting soil moisture vertical redistribution and availability to plants and microbes rather than from the relatively small differences in WTD. These differences could be caused by soil physical disturbances during pasture renewal or paddock recontouring, or time since initial drainage. Therefore, improved soil management might provide practical mitigation against excessive CO 2 emissions during dry conditions, including droughts., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

The increase in environmental nutrient availability as a result of human activities has necessitated the development of mitigation strategies for nutrient removal, such as nitrate. Current methods for determining the efficiency of different mitigation strategies required measurement of changes in nitrate concentrations, however, these methods can be expensive or do not account fully for the temporal variability of nitrate concentration. This study evaluated the utility of Diffusive Gradients in Thins-Films (DGT) for determining nitrate removal in two denitrifying bioreactors, and compared DGT performance to traditional approaches for determining performance, including high and low frequency water grab sampling. The binding layer was produced using the Purolite® A520E anion exchange resin. The uptake and elution efficiencies were 98.8% and 93.4% respectively. DGTs of three material diffusion layer thicknesses were placed in piezometers along longitudinal transects, to enable calculation of the diffusive boundary layer and provide replicates. These were removed after 16, 24 and 36 h, and the accumulated nitrate masses were extracted and quantified to calculate nitrate concentration. Concentrations were subsequently utilised to calculate nitrate removal rates in both bioreactors. Grab samples were taken at 30 and 60 min intervals over those periods, nitrate concentrations were also measured to determine nitrate removal. DGTs provided nitrate removal rates at bioreactor site one (controlled flow, wastewater treatment) of 14.83-30.75 g N m -3  d -1 , and 1.22-3.63 g N m -3  d -1 at site two (variable flow, agricultural run-off). DGT determined nitrate concentrations and removal rates were in strong accordance with high frequency grab sampling, but data collection via DGTs was considerably easier. Utilising DGTs for the measurement of bioreactor performance overcame many of the challenges associated with high frequency grab sampling, and other methods, such as accounting for temporal variation in nitrate concentration and reduced analytical requirements., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Peatland biological, physical and chemical properties change over time in response to alterations in long-term water table position. Such changes complicate our ability to predict the response of peatland carbon stocks to sustained drying. In order to better understand the effect of sustained lowering of the water table on peatland carbon dynamics, we re-visited a drainage-affected bog, repeating eddy covariance measurements of CO 2 flux after a 16-year interval. We found the ecosystem CO 2 sink to have strengthened across the intervening period, despite a deep and fluctuating water table. This was mostly due to an increase in CO 2 uptake through photosynthesis associated with increased shrub growth. We also observed a decline in CO 2 loss through ecosystem respiration. These changes could not be attributed to environmental conditions. Air temperature was the only significant contemporaneous driver of monthly anomalies in CO 2 fluxes, with higher temperatures decreasing the net CO 2 sink via increased ecosystem respiration. However, the effect of air temperature was weak in comparison to the underlying differences between time periods. Therefore, we demonstrate that for drying peatlands, long-term changes within the ecosystem can be of primary importance as drivers of CO 2 exchange. In this peatland, the ecosystem carbon sink has shown resilience to water table drawdown, with internal feedbacks leading to a recovery of the CO 2 sink after a 16-year interval., Competing Interests: Declaration of Competing Interest The authors declare that there is no conflict of interest regarding the publication of this article., (Copyright © 2019 Elsevier B.V. All rights reserved.)

High water tables (WT) stabilise peatland carbon (C) through regulation of biogeochemical processes. The impact of peatland WT on ecosystem function, including C exchange, alters over time, and the factors that cause some peatlands to display resilience and others to undergo degradation are poorly understood. Here we use CO 2 flux measurements, measured by eddy covariance, to compare ecosystem function between two raised bogs; one drainage-affected, with a deep and fluctuating water table and the other near-natural, with a shallow and stable water table. The drainage-affected bog was found to be a moderate sink for CO 2 (69 g C m -2  yr -1 ), which was 134 g C m -2  yr -1 less than the near-natural bog (203 g C m -2  yr -1 ). Greater ecosystem productivity has allowed the drainage-impacted bog to act as a CO 2 sink despite higher ecosystem respiration; most likely due to an increase in photosynthetic capacity caused by expansion of ericaceous shrub cover. The tolerance of the vegetation community, particularly the main peat former Empodisma robustum (Restionaceae), to low and fluctuating WT appears to have been key in allowing the site to remain a sink. Despite the current resilience of the ecosystem CO 2 sink, we found gross primary production to be limited under both high and low water tables, even in a year with typical rainfall. This is best explained by the limited physiological ability of ericaceous shrubs to tolerate a fluctuating WT. As such we hypothesise that if the WT continues to drop and become even more unstable, then without further vegetation change, a reduction in gross primary production is likely which may in turn cause the site to become a source for CO 2 ., (Copyright © 2018 Elsevier B.V. All rights reserved.)

A possible agricultural climate change mitigation option is to increase the amount of soil organic carbon (SOC). Conversely, some factors might lead to inadvertent losses of SOC. Here, we explore the effect of various management options and environmental changes on SOC storage and milk production of dairy pastures in New Zealand. We used CenW 4.1, a process-based ecophysiological model, to run a range of scenarios to assess the effects of changes in management options, plant properties and environmental factors on SOC and milk production. We tested the model by using 2years of observations of the exchanges of water and CO 2 measured with an eddy covariance system on a dairy farm in New Zealand's Waikato region. We obtained excellent agreement between the model and observations, especially for evapotranspiration and net photosynthesis. For the scenario analysis, we found that SOC could be increased through supplying supplemental feed, increasing fertiliser application, or increasing water availability through irrigation on very dry sites, but SOC decreased again for larger increases in water availability. Soil warming strongly reduced SOC. For other changes in key properties, such as changes in soil water-holding capacity and plant root:shoot ratios, SOC changes were often negatively correlated with changes in milk production. The work showed that changes in SOC were determined by the complex interplay between (1) changes in net primary production; (2) the carbon fraction taken off-site through grazing; (3) carbon allocation within the system between labile and stabilised SOC; and (4) changes in SOC decomposition rates. There is a particularly important trade-off between carbon either being removed by grazing or remaining on site and available for SOC formation. Changes in SOC cannot be fully understood unless all four factors are considered together in an overall assessment., (Copyright © 2016 The Authors. Published by Elsevier B.V. All rights reserved.)

We used two years of eddy covariance (EC) measurements collected over an intensively grazed dairy pasture to better understand the key drivers of changes in soil organic carbon stocks. Analysing grazing systems with EC measurements poses significant challenges as the respiration from grazing animals can result in large short-term CO2 fluxes. As paddocks are grazed only periodically, EC observations derive from a mosaic of paddocks with very different exchange rates. This violates the assumptions implicit in the use of EC methodology. To test whether these challenges could be overcome, and to develop a tool for wider scenario testing, we compared EC measurements with simulation runs with the detailed ecosystem model CenW 4.1. Simulations were run separately for 26 paddocks around the EC tower and coupled to a footprint analysis to estimate net fluxes at the EC tower. Overall, we obtained good agreement between modelled and measured fluxes, especially for the comparison of evapotranspiration rates, with model efficiency of 0.96 for weekly averaged values of the validation data. For net ecosystem productivity (NEP) comparisons, observations were omitted when cattle grazed the paddocks immediately around the tower. With those points omitted, model efficiencies for weekly averaged values of the validation data were 0.78, 0.67 and 0.54 for daytime, night-time and 24-hour NEP, respectively. While not included for model parameterisation, simulated gross primary production also agreed closely with values inferred from eddy covariance measurements (model efficiency of 0.84 for weekly averages). The study confirmed that CenW simulations could adequately model carbon and water exchange in grazed pastures. It highlighted the critical role of animal respiration for net CO2 fluxes, and showed that EC studies of grazed pastures need to consider the best approach of accounting for this important flux to avoid unbalanced accounting., (Copyright © 2015. Published by Elsevier B.V.)

Denitrification walls are a practical approach for decreasing non-point source pollution of surface waters. They are constructed by digging a trench perpendicular to groundwater flow and mixing the aquifer material with organic matter, such as sawdust, which acts as a carbon source to stimulate denitrification. For efficient functioning, walls need to be permeable to groundwater flow. We examined the functioning of a denitrification wall constructed in an aquifer consisting of coarse sands. Wells were monitored for changes in nitrate concentration as groundwater passed through the wall and soil samples were taken to measure microbial parameters inside the wall. Nitrate concentrations upstream of the wall ranged from 21 to 39 g N m(-3), in the wall from 0 to 2 g N m(-3) and downstream from 19 to 44 g N m(-3). An initial groundwater flow investigation using a salt tracer dilution technique showed that the flow through the wall was less than 4% of the flow occurring in the aquifer. Natural gradient tracer tests using bromide and Rhodamine-WT confirmed groundwater bypass under the wall. Hydraulic conductivity of 0.48 m day(-1) was measured inside the wall, whereas the surrounding aquifer had a hydraulic conductivity of 65.4 m day(-1). This indicated that during construction of the wall, hydraulic conductivity of the aquifer had been greatly reduced, so that most of the groundwater flowed under rather than through the wall. Denitrification rates measured in the center of the wall ranged from 0.020 to 0.13 g N m(-3) day(-1), which did not account for the rates of nitrate removal (0.16-0.29 g N m(-3) day(-1)) calculated from monitoring of groundwater nitrate concentrations. This suggested that the rate of denitrification was greater at the upstream face of the wall than in its center where it was limited by low nitrate concentrations. While denitrification walls can be an inexpensive tool for removing nitrate from groundwater, they may not be suitable in aquifers with coarse textured subsoils where simple inexpensive construction techniques result in major decreases in hydraulic conductivity.