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Even small increases in the large pool of soil organic carbon could result in large reductions in atmospheric CO 2 concentrations sufficient to limit global warming below the threshold of 2 °C required for climate stability. Globally, grasslands occupy 70% of the world’s agricultural area, so interventions to farm management practices to reduce losses or increase soil carbon stocks in grassland are highly relevant. Here, we review the literature with particular emphasis on New Zealand and report on the effects of management practices on changes in soil carbon stocks for temperate grazed grasslands. We include findings from models that explore the trade-offs between multiple desirable outcomes, such as increasing soil carbon stocks and milk production. Farm management practices can affect soil carbon stocks through changes in net primary production, the proportions of biomass removed, the degree of stabilisation of carbon in the soil and changes to the rate of soil carbon decomposition. The carbon saturation deficit defines the potential for a soil to stabilise additional carbon. Earlier reviews have concluded that, while labile carbon is the dominant substrate for soil carbon decomposition, a fraction of soil carbon stocks is stabilised and protected from decomposition by the formation of organo-mineral complexes. Recent evidence shows that the rate of organic carbon decomposition is determined primarily by the extent of soil organic carbon protection and, therefore, the availability of substrates to microbial activity. New Zealand grassland systems have moderate to high soil carbon stocks in the surface layers (i.e., upper 0.15 m) where most roots are located, so the carbon saturation deficit is relatively low and the scope to increase soil carbon stocks by carbon inputs from primary production may be limited. International studies have shown that the addition of fertilisers, feed imports, and applications of manure and effluent can increase soil carbon stocks, especially for degraded soils, but the responses in New Zealand soils are uncertain because of the limited number of studies. However, recent evidence shows that irrigation can reduce soil carbon stocks in New Zealand, but neither the processes nor the long-term trends are known. Studies of sward renewal have shown that short-term losses of carbon losses resulting from the disturbance can be mitigated using rapid replacement of the new sward, minimum tillage and avoidance of times when the soil water content is high. Swards comprising multiple species have also shown that soil carbon stocks may be increased after periods of several years. Model simulations have shown that the goal of increasing both soil carbon and milk production could be achieved best by increasing carbon inputs from supplementary animal feed. However, losses of carbon at feed export sites need to be minimised to achieve overall net gains in soil carbon. Grazing intensity can have a big influence on soil carbon stocks but the magnitude and direction of the effects are not consistent between studies. Biochar addition could possibly increase soil carbon stocks but it is not yet an economical option for large-scale application in New Zealand. There is some evidence that the introduction of earthworms and dung beetles could potentially increase soil carbon stabilisation, but the greenhouse gas benefits are confounded by possible increases in nitrous oxide emissions. The new practice of full inversion tillage during grassland renewal has the potential to increase soil carbon stocks under suitable conditions but full life-cycle analysis including the effects of the disruptive operations has yet to be completed. We conclude with a list of criteria that determine the success and suitability of management options to increase soil carbon stocks and identify priority research questions that need to be addressed using experimental and modelling approaches to optimise management options to increase soil carbon stocks. [ABSTRACT FROM AUTHOR]

Cadmium (Cd) is a toxic metal which has accumulated in New Zealand agricultural soils due to phosphate fertilizer application. Understanding the contribution of plant uptake or leaching of Cd to observed Cd losses from soil is important. The concentration and distribution of Cd in irrigated and unirrigated soils with the same phosphate fertilizer history were investigated. Twenty-two pairs of soil samples from four depths (0–0.1, 0.1–0.2, 0.2–0.3 and 0.3–0.4 m) were taken from irrigated and unirrigated areas in the same field on dairy farms in three regions of New Zealand. The mean concentration of Cd at depths of 0–0.1 m and 0.1–0.2 m, as well as the cumulative masses of Cd (0–0.2, 0–0.3 and 0–0.4 m) in unirrigated soils were significantly higher ( P < 0.05) than in irrigated soils. The concentration of phosphorus (P) at all depths (except for 0.2–0.3 m), as well as the cumulative mass of P in all depths of unirrigated soils, was also significantly higher ( P < 0.05) than irrigated soils. However, no significant difference was detected in the concentrations of uranium (U) between irrigated and unirrigated soils. Irrigation induced a ∼7% Cd loss from topsoil (0–0.1 m), with the average rate of Cd loss from the top 0.1 m (due to irrigation) being 2.3 g ha −1 yr −1 . This study therefore confirms that irrigation can enhance Cd mobilization, however Cd is mainly adsorbed to the surface soil. [ABSTRACT FROM AUTHOR]

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 2 years 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. [ABSTRACT FROM AUTHOR]

Abstract: Denitrification beds, utilising fragmented wood particles as the carbon source, have been successfully used to remove nitrate from point source discharge. Other more labile carbonaceous solids have provided higher short-term nitrate removal rates than wood in laboratory scale trials, but the longevity of these media is unproven. In addition, the nitrate removal rate of a bed is indicated to be temperature dependent. Improving the hydraulic efficiency of a denitrification bed, by reducing short-circuit flow, may also provide for increased long-term nitrate removal rate and reduced bed size and lower installation cost. In this study, we compared the hydraulic properties and hydraulic efficiency of nine carbon media, including five grain sizes of wood particles, in 0.2m3 barrels, at two temperatures (14°C and 23.5°C). The relationship between hydraulic efficiency and nitrate removal of the different media was also investigated. We found that carbon substrate and temperature were more influential on nitrate removal rate than hydraulic efficiency of the media. While larger grain-sizes of wood media were less hydraulically efficient than smaller grain-sizes, the difference in hydraulic efficiency was small. We also found that primary porosity of the wood media increased with temperature, which may have been caused by contraction of the wood particles with increasing temperature due to loss of water from the cellulose to the liquid phase. While hydraulic properties and hydraulic efficiencies varied between carbon media, the variation did not cause significant difference in nitrate removal rate. The results indicate that future work on improving nitrate removal performance of denitrification beds should focus on carbon availability of the substrate and increasing bed temperature, rather than on identifying more hydraulically efficient media. [Copyright &y& Elsevier]

Abstract: Denitrification beds are containers filled with wood by-products that serve as a carbon and energy source to denitrifiers, which reduce nitrate (NO3 −) from point source discharges into non-reactive dinitrogen (N2) gas. This study investigates a range of alternative carbon sources and determines rates, mechanisms and factors controlling NO3 − removal, denitrifying bacterial community, and the adverse effects of these substrates. Experimental barrels (0.2 m3) filled with either maize cobs, wheat straw, green waste, sawdust, pine woodchips or eucalyptus woodchips were incubated at 16.8 °C or 27.1 °C (outlet temperature), and received NO3 − enriched water (14.38 mg N L−1 and 17.15 mg N L−1). After 2.5 years of incubation measurements were made of NO3 −–N removal rates, in vitro denitrification rates (DR), factors limiting denitrification (carbon and nitrate availability, dissolved oxygen, temperature, pH, and concentrations of NO3 −, nitrite and ammonia), copy number of nitrite reductase (nirS and nirK) and nitrous oxide reductase (nosZ) genes, and greenhouse gas production (dissolved nitrous oxide (N2O) and methane), and carbon (TOC) loss. Microbial denitrification was the main mechanism for NO3 −–N removal. Nitrate–N removal rates ranged from 1.3 (pine woodchips) to 6.2 g N m−3 d−1 (maize cobs), and were predominantly limited by C availability and temperature (Q 10 = 1.2) when NO3 −–N outlet concentrations remained above 1 mg L−1. The NO3 −–N removal rate did not depend directly on substrate type, but on the quantity of microbially available carbon, which differed between carbon sources. The abundance of denitrifying genes (nirS, nirK and nosZ) was similar in replicate barrels under cold incubation, but varied substantially under warm incubation, and between substrates. Warm incubation enhanced growth of nirS containing bacteria and bacteria that lacked the nosZ gene, potentially explaining the greater N2O emission in warmer environments. Maize cob substrate had the highest NO3 −–N removal rate, but adverse effects include TOC release, dissolved N2O release and substantial carbon consumption by non-denitrifiers. Woodchips removed less than half of NO3 − removed by maize cobs, but provided ideal conditions for denitrifying bacteria, and adverse effects were not observed. Therefore we recommend the combination of maize cobs and woodchips to enhance NO3 − removal while minimizing adverse effects in denitrification beds. [Copyright &y& Elsevier]

Abstract: Denitrification beds are a simple and relatively inexpensive technology for removing nitrate from point source discharges. To date, operational beds have used wood media as the carbon source, as it provides a sustained nitrate removal rate (2–10gNm−3 ofmediad−1) while maintaining permeability. In pilot-scale (2.9m−3) denitrification beds receiving municipal wastewater effluent dosed with KNO3, we looked at improving nitrate removal by using alternative carbon media (maize cobs) and increasing bed temperature through passive solar heating. The influence of flow regime (horizontal-point, horizontal-diffuse, downflow and upflow) on short-circuit flow was also investigated. The long-term nitrate removal rate (21.8gNm−3 d−1) of the maize cob beds over the 15-month period of the trial was 2–11-fold higher than sustained removal rates reported by other researchers for wood-based beds. While passive solar heating raised the mean bed temperature by 3.4°C, it did not cause a measurable increase in the nitrate removal rate due to the variability in the removal rate exceeding the expected increase due to temperature. Horizontal flow had more short-circuiting than vertical flow. Short-circuiting in the horizontal flow was attributed to flow being concentrated near the top surface due to the buoyancy effect of warmer water. Greater short-circuiting in the solar heated horizontal and upflow beds than in the corresponding unheated beds was attributed to the buoyancy effect being more pronounced in the solar heated beds. Overall, downflow was deemed the most effective of the four tested flow regimes. It provided the highest increase in bed temperature due to solar heating, had the highest nitrate removal rate in the latter part of the trial and had more plug-flow characteristics. While passive solar heating raised bed temperature, we were unable to demonstrate a significant difference (at 95% CL) in nitrate removal rate between the unheated and solar heated beds because of the high variability in nitrate removal rate and the increase in short-circuiting in the solar heated horizontal and upflow beds. [Copyright &y& Elsevier]

Abstract: Denitrifying woodchip bioreactors (denitrification beds) are increasingly used to remove excess nitrate () from point-sources such as wastewater effluent or subsurface drains from agricultural fields. removal in these beds is assumed to be due to microbial denitrification but direct measurements of denitrification are lacking. Our objective was to test four different approaches for measuring denitrification rates in a denitrification bed that treated effluent discharged from a glasshouse. We compared these denitrification rates with the rate of removal along the length of the bed. The removal rate was 8.73 ± 1.45 g m−3 d−1. In vitro acetylene inhibition assays resulted in highly variable denitrification rates (DRAI) along the length of the bed and generally 5 times greater than the measured ( removal rate. An in situ push–pull test, where enriched was injected into 2 locations along the bed, resulted in rates of 23.2 ± 1.43 g N m−3 d−1 and 8.06 ± 1.64 g N m−3 d−1. The denitrification rate calculated from the increase in dissolved N2 and N2O concentrations (DRN2) along the length of the denitrification bed was 6.7 ± 1.61 g N m−3 d−1. Lastly, denitrification rates calculated from changes in natural abundance measurements of δ15N–N2 and δ15 along the length of the bed yielded a denitrification rate (DRNA) of 6.39 ± 2.07 g m−3 d−1. Based on our experience, measurements were the easiest and most efficient approach for determining the denitrification rate and N2O production of a denitrification bed. However, the other approaches were useful for testing other hypotheses such as factors limiting denitrification or may be applied to determine denitrification rates in environmental systems different to our study site. does require very careful sampling to avoid atmospheric N2 contamination but could be used to rapidly determine denitrification rates in a variety of aquatic systems with high N2 production and even water flows. These measurements demonstrated that the majority of removal was due to heterotrophic denitrification. [Copyright &y& Elsevier]

Abstract: Denitrification beds are a simple approach for removing nitrate (NO3 −) from a range of point sources prior to discharge into receiving waters. These beds are large containers filled with woodchips that act as an energy source for microorganisms to convert NO3 − to nitrogen (N) gases (N2O, N2) through denitrification. This study investigated the biological mechanism of NO3 − removal, its controlling factors and its adverse effects in a large denitrification bed (176m×5m×1.5m) receiving effluent with a high NO3 − concentration (>100gNm−3) from a hydroponic glasshouse (Karaka, Auckland, New Zealand). Samples of woodchips and water were collected from 12 sites along the bed every two months for one year, along with measurements of gas fluxes from the bed surface. Denitrifying enzyme activity (DEA), factors limiting denitrification (availability of carbon, dissolved organic carbon (DOC), dissolved oxygen (DO), temperature, pH, and concentrations of NO3 −, nitrite (NO2 −) and sulfide (S2−)), greenhouse gas (GHG) production – as nitrous oxide (N2O), methane (CH4), carbon dioxide (CO2) – and carbon (C) loss were determined. NO3 −-N concentration declined along the bed with total NO3 −-N removal rates of 10.1kgNd−1 for the whole bed or 7.6gNm−3 d−1. NO3 −-N removal rates increased with temperature (Q 10 =2.0). In laboratory incubations, denitrification was always limited by C availability rather than by NO3 −. DO levels were above 0.5mgL−1 at the inlet but did not limit NO3 −-N removal. pH increased steadily from about 6 to 7 along the length of the bed. Dissolved inorganic carbon (C-CO2) increased in average about 27.8mgL−1, whereas DOC decreased slightly by about 0.2mgL−1 along the length of the bed. The bed surface emitted on average 78.58μgm−2 min−1 N2O-N (reflecting 1% of the removed NO3 −-N), 0.238μgm−2 min−1 CH4 and 12.6mgm−2 min−1 CO2. Dissolved N2O-N increased along the length of the bed and the bed released on average 362g dissolved N2O-N per day coupled with N2O emission at the surface about 4.3% of the removed NO3 −-N as N2O. Mechanisms to reduce the production of this GHG need to be investigated if denitrification beds are commonly used. Dissolved CH4 concentrations showed no trends along the length of the bed, ranging from 5.28μgL−1 to 34.24μgL−1. Sulfate (SO4 2−) concentrations declined along the length of the bed on three of six samplings; however, declines in SO4 2− did not appear to be due to SO4 2− reduction because S2− concentrations were generally undetectable. Ammonium (NH4 +) (range: <0.0007mgL−1 to 2.12mgL−1) and NO2 − concentrations (range: 0.0018mgL−1 to 0.95mgL−1) were always very low suggesting that anammox was an unlikely mechanism for NO3 − removal in the bed. C longevity was calculated from surface emission rates of CO2 and release of dissolved carbon (DC) and suggested that there would be ample C available to support denitrification for up to 39 years. This study showed that denitrification beds can be an efficient tool for reducing high NO3 − concentrations in effluents but did produce some GHGs. Over the course of a year NO3 − removal rates were always limited by C and temperature and not by NO3 − or DO concentration. [Copyright &y& Elsevier]

Abstract: Denitrification beds are a cost-effective technology for removing nitrate from point source discharge. To date, field trials and operational beds have primarily used wood media as the carbon source; however, the use of alternative more labile carbon media could provide for increased removal rate, lower installation costs and reduced bed size. While previous laboratory experiments have investigated the potential of alternative carbon sources, these studies were typically of short duration and small scale and did not necessarily provide reliable information for denitrification bed design purposes. To address this issue, we compared nitrate removal, hydraulic and nutrient leaching characteristics of nine different carbon substrates in 0.2m3 barrels, at 14 and 23.5°C over a 23-month period. Mean nitrate removal rates for the period 10–23 months were 19.8 and 15gNm−3 d−1 (maize cobs), 7.8 and 10.5gNm−3 d−1 (green waste), 5.8 and 7.8gNm−3 d−1 (wheat straw), 3.0 and 4.9gNm−3 d−1 (softwood), and 3.3 and 4.4gNm−3 d−1 (hardwood) for the 14 and 23.5°C treatments, respectively. Maize cobs provided a 3–6.5-fold increase in nitrate removal over wood media, without prohibitive decrease in hydraulic conductivity, but had higher rates of nutrient leaching at start-up. Significant difference in removal rate occurred between the 14 and 23.5°C treatments, with the mean Q 10 temperature coefficient=1.6 for all media types in the period 10–23 months. [Copyright &y& Elsevier]

Abstract: Low-cost and simple technologies are needed to reduce watershed export of excess nitrogen to sensitive aquatic ecosystems. Denitrifying bioreactors are an approach where solid carbon substrates are added into the flow path of contaminated water. These carbon (C) substrates (often fragmented wood-products) act as a C and energy source to support denitrification; the conversion of nitrate (NO3 −) to nitrogen gases. Here, we summarize the different designs of denitrifying bioreactors that use a solid C substrate, their hydrological connections, effectiveness, and factors that limit their performance. The main denitrifying bioreactors are: denitrification walls (intercepting shallow groundwater), denitrifying beds (intercepting concentrated discharges) and denitrifying layers (intercepting soil leachate). Both denitrifcation walls and beds have proven successful in appropriate field settings with NO3 − removal rates generally ranging from 0.01 to 3.6gNm−3 day−1 for walls and 2–22gNm−3 day−1 for beds, with the lower rates often associated with nitrate-limitations. Nitrate removal is also limited by the rate of C supply from degrading substrate and removal is operationally zero-order with respect to NO3 − concentration primarily because the inputs of NO3 − into studied bioreactors have been generally high. In bioreactors where NO3 − is not fully depleted, removal rates generally increase with increasing temperature. Nitrate removal has been supported for up to 15 years without further maintenance or C supplementation because wood chips degrade sufficiently slowly under anoxic conditions. There have been few field-based comparisons of alternative C substrates to increase NO3 − removal rates but laboratory trials suggest that some alternatives could support greater rates of NO3 − removal (e.g., corn cobs and wheat straw). Denitrifying bioreactors may have a number of adverse effects, such as production of nitrous oxide and leaching of dissolved organic matter (usually only for the first few months after construction and start-up). The relatively small amount of field data suggests that these problems can be adequately managed or minimized. An initial cost/benefit analysis demonstrates that denitrifying bioreactors are cost effective and complementary to other agricultural management practices aimed at decreasing nitrogen loads to surface waters. We conclude with recommendations for further research to enhance performance of denitrifying bioreactors. [Copyright &y& Elsevier]

Gradients in stressed areas potentially provide a powerful tool to interpret relations between soil biodiversity and site quality. We measured soil chemistry, soil microbiology and nematodes along three transects representing a fertility gradient and at a disturbed site near a road in a Dracophyllum subulatum-dominated shrubland in which frosts are a major factor in preventing succession to forest; we used D. subulatum size as a site-quality index. Significant correlations between both shrub height and shrub growth rate and volumetric measures of total soil phosphorus and anaerobically mineralisable nitrogen indicate that nitrogen and phosphorus regulate plant growth. Microbial biomass and total nematode abundance significantly increased with greater plant growth, presumably in response to greater litter input. Conversely, neither heterotrophic microbial diversity nor nematode diversity was correlated with shrub performance along the transects. Litter was from a single species and thus likely similar in quality so changes in microbial or nematode diversity might not be expected. In this oligotrophic environment, nutrient levels were not only the important regulators of plant growth but also appeared to have an indirect influence on the size of the microbial and nematode populations. [Copyright &y& Elsevier]

• Irrigated pastures had lower soil C and respiration than non-irrigated pasture. • Soil respiration at the same temperature was lower and Q 10 higher in irrigated soils. • The change in temperature response was attributed lower labile C under irrigation. • Soil management can alter the temperature dependence of respiration. Land management practices that increase food production are needed to match demand from a growing global population. Adoption of these practices needs to be balanced by potential adverse consequences such as nutrient losses and production of greenhouse gases. We previously demonstrated that pasture soils irrigated during summer-dry conditions had significantly less soil carbon than adjacent unirrigated pastures, despite increased plant production. Precise reasons for lower carbon under irrigation were not clear but both inputs (photosynthesis) and losses (respiration) of carbon are regulated by soil moisture and temperature. Our objective was to determine whether the temperature and moisture response of soil respiration differed between 13 adjacent irrigated and unirrigated soils (0-0.1 m). Soil respiration rates were measured in the laboratory using a temperature block where rates of respiration were measured within 5 h at ˜3 °C increments between 6 and 60 °C (20 temperatures) and at 5 different moisture contents. Temperature response, sensitivity and key temperature parameters (temperature optimum (T opt) and inflection point temperature (T inf)) were calculated using macromolecular rate theory (MMRT). Respiration rates increased with increasing moisture content similarly for both irrigated and unirrigated soils. However, soil respiration at the same temperature was significantly (P < 0.05) lower and Q 10 higher in irrigated soils. T inf and T opt were greater in irrigated soils. We attribute the lower respiration in irrigated soils to a disproportionate loss of available carbon, total soil carbon loss was ˜14% while the differences in respiration were between 58% at 10 °C and 41% at 20 °C. The lower carbon availability in irrigated soils was likely responsible for the increased Q 10 , T inf and T opt as substrate decomposability and availability became more limiting so that ongoing decomposition became increasingly dependent on solubilisation and diffusion of remaining carbon substrates to micro-organisms. We postulate that as respiration becomes increasingly limited by substrate supply through physical chemistry processes (diffusion, and sorption/desorption) rather than substrate biodegradability, the temperature response curve will shift from a MMRT dominated response (with a temperature optimum) to an Arrhenius dominated response (exponential). Our data suggest that commencement of irrigation removed moisture limitation during normally dry summers and led to a loss of soil carbon due to an initially increased microbial activity that has now decreased as carbon availability declined. [ABSTRACT FROM AUTHOR]

• Denitrification alone does not govern N removal in woodchip and coconut husk bioreactors. • Substantial N removal can be achieved via the anammox pathway. • Relative contributions of denitrification and anammox vary between media types used. • Anammox becomes more dominant when denitrification is carbon limited. Denitrification has been considered the main pathway converting nitrate (NO 3 –) to dinitrogen gas (N 2) in denitrifying bioreactors. Here, the importance of an alternative nitrogen (N) removal process, namely anaerobic ammonium oxidation (anammox), was assessed by monitoring the removal of N species from partially nitrified municipal wastewater passing through fifteen mesocosm scale (∼700 L) bioreactors containing woodchip, coconut husk or gravel during their initial and eighth year of operation. Additionally, lab experiments using a 15N isotope-pairing technique were performed to partition production of N 2 to these different microbial processes. The effective removal of both NO 3 – and ammonium (NH 4 +) and the formation of hybrid N 2 (i.e. 29N 2) observed in this study demonstrated that, along with denitrification, anammox was an effective pathway for N removal when both NO 3 – and NH 4 + were present. Anammox removal rates ranged from 0.6 to 3.8 g N per m3 per day, while denitrification rates ranged from 0.7 to 2.6 g N per m3 per day. The contributions of anammox to N removal was dependent on media, with anammox becoming more dominant in bioreactors where denitrification was carbon limited. Designing denitrifying bioreactors to support both denitrification and anammox expands the utility of these passive approaches for improving treatment of wastewater. [ABSTRACT FROM AUTHOR]

The nitrate removal efficiency of denitrifying bioreactors decreases when the carbon supply from woodchips is insufficient, particularly during large nitrate pulses. This study aimed to assess the effects of methanol dosing, as an external carbon source, on nitrate removal rates in mesocosm-scale bioreactors while monitoring the secondary effects of dosing that may occur. A secondary goal was to quantify sulfate reduction rates and methanol consumption in the presence and absence of nitrate to aid in determining the dosing load to minimize undesirable effects such as methanol entering receiving waters and minimizing sulfate reduction. We continuously dosed the bioreactors with nitrate (∼20 mg NO 3 −-N L−1), methanol (∼35 mg CH 3 OH-C L−1), and sulfate (∼9 mg SO 4 2−-S L−1), which was already present in the tap water. In a long-term controlled mesocosm experiment, we established three bioreactor treatments to investigate nitrate, methanol, and sulfate removal rates with and without the presence of nitrate and methanol. Compared to the woodchip control treatment, methanol dosing resulted in an approximately fourfold increase in nitrate removal rates from 7 to 27 g N m−3 day−1. Methanol dosing increased sulfate removal rates, from average sulfate removal rates of 1.5 g SO 4 2−-S m−3 day−1 under nitrate-prevailing conditions to 5.5 g SO 4 2−-S m−3 day−1 removal rates under nitrate-limiting conditions compared to woodchip control removal of 0.3 g SO 4 2−-S m−3 day−1. Mean methanol removal rates were 23 g CH 3 OH-C m−3 day−1 under nitrate-prevailing conditions compared to 18 g CH 3 OH-C m−3 day−1 in the woodchip control experiment. Improved nitrate removal rates, as well as methanol consumption and sulfate removal rates, might be leveraged to develop innovative low-footprint bioreactors. [Display omitted] • Methanol dosage increased nitrate removal fourfold in denitrifying bioreactors. • Sulfate removal increased in bioreactors with methanol dosing. • Methanol was consumed in bioreactors with and without nitrate. • Methanol dosing offers a low-cost approach to improving bioreactor performance. [ABSTRACT FROM AUTHOR]

Contemporary measurements of carbon (C) accumulation rates in peatlands around the world often show the C sink to be stronger on average than at times in the past. Alteration of global nutrient cycles could be contributing to elevated carbon accumulation in the present day. Here we examine the effect of volcanic inputs of nutrients on peatland C accumulation in Moanatuatua Bog, New Zealand, by examining a high-resolution Late Holocene C accumulation record during which powerful volcanic eruptions occurred, depositing two visible rhyolitic tephra layers (Taupo, 232 ± 10 CE; Kaharoa, 1314 ± 12 CE). Carbon accumulation rates since c. 50 CE, well before any human presence, increased from a background rate of 23 g C m−2 yr−1 up to 110 g C m−2 yr−1 following the deposition of the Taupo Tephra, and 84 g C m−2 yr−1 following the deposition of the Kaharoa Tephra. Smaller but nevertheless marked increases in C accumulation additionally occurred in association with the deposition of three andesitic-dacitic cryptotephras (each ≤ ∼1 mm thick) of the Tufa Trig Formation between the Taupo and Kaharoa events. These five periods of elevated C uptake, especially those associated with the relatively thick Taupo and Kaharoa tephras, were accompanied by shifts in nutrient stoichiometry, indicating that there was greater availability of phosphorus (P) relative to nitrogen (N) and C during the period of high C uptake. Such P was almost certainly derived from volcanic sources, with P being present in the volcanic glass at Moanatuatua, and many of the eruptions described being associated with the local deposition of the P rich mineral apatite. We found peatland C accumulation to be tightly coupled to N and P accumulation, suggesting nutrient inputs exert a strong control on rates of peat accumulation. Nutrient stoichiometry indicated a strong ability to recover P within the ecosystem, with C:P ratios being higher than most other peatlands in the literature. We conclude that nutrient inputs, deriving from volcanic eruptions, have been very important for C accumulation rates in the past. Therefore, the elevated nutrient inputs occurring in the present day could offer a more plausible explanation, as opposed to a climatic component, for observed high contemporary C accumulation in New Zealand peatlands. Image 1 • Peatland carbon accumulation rates (CAR) greatly increased during periods of tephra input. • Nutrient stoichiometry suggests greater phosphorus abundance during periods of high CAR associated with tephra deposition. • P potentially derives from aerosols as well as from dissolution of freshly-deposited glass and apatite. • Nutrient stoichiometry suggests C accumulation is tightly coupled to P inputs. • P inputs appear to exert a strong control on C dynamics in the peatland studied as well others in the literature. [ABSTRACT FROM AUTHOR]

• CO 2 sink underwent recovery after 16 year interval between measurements. • Primary productivity increased in all seasons other than winter. • Ecosystem respiration declined. • Changes could not be attributed to climatic variability. 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. [ABSTRACT FROM AUTHOR]

Denitrifying bioreactors are simple, low-cost ecotechnologies designed to reduce nitrate (NO 3 −) present in septic tank effluent and drainage water. Recent studies indicate that, in addition to significant reduction in NO 3 − loads, these systems are also able to remove microbial contaminants from municipal wastewater. However, the removal of microbial contaminants in denitrifying bioreactors remains poorly characterised and factors that control removal in denitrifying bioreactors remain unexplored. In this study, the removal efficiency of faecal indicator bacteria Escherichia coli (E. coli) and total coliforms (TC) as a model for bacterial pathogens and F-specific RNA bacteriophage (FRNA bacteriophage) as a model for viruses was assessed for mesocosm-scale (∼700 L) bioreactors receiving municipal wastewater. Systems were filled with two different slow-release carbon sources: woodchip and coconut husk. The effect of media age on attenuation of microbial contaminants was assessed by comparing the performance of 8-year old systems with equivalent newly constructed woodchip and coconut husk bioreactors. Additionally, removal performance of these carbon substrates was compared to that of gravel, a non-carbon substrate commonly used in subsurface flow (SSF) constructed wetlands. Substantial reduction of E. coli , TC and FRNA bacteriophage from primary treated municipal wastewater was achieved in all bioreactors. Mean annual log 10 removal efficiencies were similar between microbial indicators ranging from 1.4 to 1.9 for TC, 1.3 to 1.8 for E. coli and 1.3 to 2.0 for FRNA bacteriophage. All denitrifying bioreactors showed consistent year-round performance and long-term performance which did not markedly change in the ninth year of operation. The woodchip or coconut husk bioreactors achieved microbial effluent quality within the same range of log 10 removal rates achieved in gravel-based systems. This suggests that denitrifying bioreactors, as well as reducing N loads, can effectively reduce microbial contaminants in wastewater, providing a complimentary disinfection role. Further research is needed to increase understanding of factors affecting removal of microbial contaminants in denitrifying bioreactors to support design of these systems for microbial contaminant removal. [ABSTRACT FROM AUTHOR]

Abstract 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. Graphical abstract Unlabelled Image Highlights • Weaker CO 2 sink in the dry bog • Greater ecosystem productivity and respiration in the dry bog • CO 2 flux in wet and dry bogs responds differently to changes in water table. • Plant functional traits explain the diverse response of CO 2 exchange to water table. • Tolerance of a fluctuating WT is important to maintain plant productivity in dry bogs. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Continuous N 2 O flux measurements at ecosystem scales are currently limited. • We observed N 2 O fluxes from an intensively grazed pasture using eddy covariance. • Diurnal and seasonal patterns of N 2 O fluxes were identified at ecosystem scales. • We found strong combined effects of soil moisture and temperature on N 2 O fluxes. • N 2 O fluxes varied daily and seasonally thus continuous observations are needed. Abstract Nitrous oxide (N 2 O) is a prominent greenhouse gas. Our understanding of environmental controls on N 2 O fluxes has mainly come from small-scale experiments, for example, static chamber measurements on plots or lab incubations. However, studies of the environmental controls for N 2 O fluxes at ecosystem scales have been limited. Using eddy covariance (EC) measurements, this study evaluated the environmental drivers of N 2 O fluxes for a one-year period at a farm grazed year-round by dairy cows in the Waikato region, New Zealand. We identified an optimum soil moisture/temperature zone that favours maximal N 2 O emissions, demonstrating maximum N 2 O fluxes at ∼70% water-filled pore space (WFPS) and moderate soil temperatures. Our measurements consistently identified significant N 2 O flux pulses associated with rainfall following grazing events in warm-dry months. In contrast, during cold-wet months when WFPS was consistently high, pulses after rainfall did not occur. A clear positive temperature response for N 2 O fluxes was observed above 70% WFPS while a negative relationship was detected when WFPS was less than 70%. Distinctive diurnal flux patterns emerged in both pulses and background fluxes, implying that soil temperature regulates N 2 O fluxes at sub-daily timescales. Over the annual period, N 2 O emissions were 6.5 kg N 2 O-N ha−1. We found the highest cumulative rates (maximum 35.7 g N 2 O-N ha−1 day−1) in autumn but the rates were low during both summer and winter. Our results highlighted the combined effects of environmental factors on N 2 O fluxes, and quantified N 2 O flux variations at seasonal and daily scales, suggesting that continuous measurement techniques, such as EC, could serve as an alternative in national N 2 O inventories. [ABSTRACT FROM AUTHOR]

The temperature response of soil microbial respiration ( R h ) is of significance, with the optimum temperature of R h being the key parameter for accurately modeling how it responds to temperature change under climate warming scenarios. However, knowledge about T opt in natural ecosystems remains limited, especially at large scales, which increases the uncertainty of climate projections. Here, we collected 25 soils from tropical to cold-temperate forests in the northern hemisphere to quantify regional variation in T opt and the controls underlying this variation. R h was measured at high frequency using a novel system under the mode, with temperature gradually increasing from 5 to 50 °C. The results showed that T opt ranged from 38.5 to 46.0 °C (mean: 42.4 °C). Of note, this study is the first to demonstrate that T opt is far higher than the assumed value used in models (35 °C), varying greatly across different climatic zones and increasing with latitude from tropical to cold-temperate forest soils. To some extent, our results supported the substrate supply hypothesis, and contrast with the climate adaption hypothesis. In addition, climate, nutrient, and soil microorganisms jointly regulate regional variation in T opt , together explaining 53% of variation in T opt . The higher T opt in northern regions indicated that these regions have a greater potential to release more CO 2 from soil, which might lead to a positive feedback to global warming. In conclusion, process-based models should incorporate the high variability of T opt across regions to improve predictions of the carbon dynamics of terrestrial ecosystems under climate warming scenarios. [ABSTRACT FROM AUTHOR]

Understanding the temperature response of soil microbial respiration is essential for predicting carbon (C) losses as the planet warms. As fresh, labile C inputs can further accelerate soil C loss (priming effect), determining if priming is temperature sensitive has important implications for global C cycling and remains relatively unexplored. We conducted a series of 5-h incubations for five different soil orders at 40 discrete temperatures with added 13C-labelled glucose and measured soil microbial respiration. We then estimated the temperature response of microbial respiration attributable to (1) the added glucose, (2) the soil organic matter (SOM), and (3) soil priming. The relative proportion of the priming response varied with temperature and the magnitude of these changes differed by soil type. We found that the temperature response of microbial respiration attributable to priming and to the added glucose were unimodal and could be modelled using Macromolecular Rate Theory (MMRT). This suggests that biological mechanisms play a strong role in shaping the temperature response of priming. In contrast, respiration derived from SOM typically increased continuously with increasing temperature. Using MMRT we estimated a temperature optimum (T o p t ) and inflection point (T inf) from each of the temperature response curves for microbial respiration derived from the added glucose and from soil priming. The temperature response of respiration from soil priming (T o p t = 30.6 °C and T inf = 12.8 °C) was significantly lower than from the added glucose (T o p t = 42.4 °C and T inf = 14.5 °C), which indicates that priming is more temperature sensitive. This study demonstrates that soil priming itself is temperature sensitive and responds differently to warming than the bulk soil, which may alter soil C stocks in ways not previously predicted. Further exploration of the temperature sensitivity of priming therefore warrants inclusion in future discussions of soil microbial responses to climate change. [Display omitted] • Temperature response of soil priming was measured for five soil orders. • The temperature optima of respiration attributed to soil priming was distinct. • Temperature response of soil priming can be modelled by Macromolecular Rate Theory. • The relative proportion of priming varied with temperature and soil type. [ABSTRACT FROM AUTHOR]

This study provides fundamental knowledge about the interaction of allophane, deoxyribonucleic acid (DNA), and organic matter in soils, and how allophane sequesters DNA. The adsorption capacities of salmon-sperm DNA on pure synthetic allophane (characterised morphologically and chemically) and on humic-acid-rich synthetic allophane were determined, and the resultant DNA–allophane complexes were characterised using synchrotron-radiation-derived P X-ray absorption near-edge fine structure (XANES) spectroscopy and infrared (IR) spectroscopy. The synthetic allophane adsorbed up to 34 μg mg − 1 of salmon-sperm DNA. However, the presence of humic acid significantly lowered the DNA uptake on the synthetic allophane to 3.5 μg mg − 1 by occupying the active sites on allophane so that DNA was repulsed. Both allophane and humic acid adsorbed DNA chemically through its phosphate groups. IR spectra for the allophane–DNA complex showed a chemical change of the Si–O–Al stretching of allophane after DNA adsorption, possibly because of the alteration of the steric distance of the allophane outer wall, or because of the precipitation of aluminium phosphate on allophane after DNA adsorption on it, or both. The aluminol groups of synthetic allophane almost completely reacted with additions of small amounts of DNA (~ 2–6 μg mg − 1 ), but the chemical adsorption of DNA on allophane simultaneously led to the formation of very porous allophane aggregates up to ~ 500 μm in diameter. The formation of the allophane nano- and microaggregates enabled up to 28 μg mg − 1 of DNA to be adsorbed (~ 80% of total) within spaces (pores) between allophane spherules and allophane nanoaggregates (as “physical adsorption”), giving a total of 34 μg mg − 1 of DNA adsorbed by the allophane. The stability of the allophane–DNA nano- and microaggregates likely prevents encapsulated DNA from exposure to oxidants, and DNA within small pores between allophane spherules and nanoaggregates may not be accessible to enzymes or microbes, hence enabling DNA protection and preservation in such materials. By implication, substantial organic carbon is therefore likely to be sequestered and protected in allophanic soils (Andisols) in the same way as demonstrated here for DNA, that is, predominantly by encapsulation within a tortuous network of nanopores and submicropores amidst stable nanoaggregates and microaggregates, rather than by chemisorption alone. [ABSTRACT FROM AUTHOR]

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 CO 2 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 CO 2 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. [ABSTRACT FROM AUTHOR]

It is commonly assumed that agricultural peatlands are net sources of CO 2 to the atmosphere because of lowered water tables and intensive land management altering the balance of plant productivity and respiration. Yet actual farm-scale fluxes of CO 2 have been infrequently quantified. We measured net ecosystem exchange of CO 2 (NEE) using a permanent and a mobile eddy covariance tower installed over dairy farms with year-round rotational grazing on deep peats in New Zealand. The permanent tower was in place for one year and the mobile tower was deployed for periods of 3–4 weeks at three other farms on peat between spring and autumn. At all sites, grazing cycles caused large variations in pasture biomass and consequent daytime NEE and we accounted for these variations using an index of photosynthesising biomass (phytomass index, Lohila et al., 2004 ) automatically derived from daily CO 2 flux measurements. We estimated annual CO 2 loss of 190 gC m −2 yr −1 for the permanent site, which is in broad agreement with other agricultural peatland studies. Including other farm-scale exports of C, overall net ecosystem carbon loss estimated for the permanent site was 294 gC m −2 yr −1 . Accounting for changes in phytomass index, daytime NEE was similar for permanent-mobile site farm pairings, except when there were very large differences in water table depths between farms in autumn. In contrast, night-time respiration losses were almost identical between farms even when water tables were markedly different, suggesting that spatial differences in NEE in these agricultural peatlands are caused by reduced photosynthesis in dry periods, due to plant water stress, rather than increased respiration. Comparisons between permanent and mobile towers appeared a useful approach for determining spatial variability of CO 2 fluxes from peat soils. Taken together, our results suggested that the CO 2 losses measured at the permanent site were representative of CO 2 losses for farmed peats in the Waikato region when the water table was within ∼0.5 m of the surface. Where water tables were deeper net CO 2 losses would be expected to be greater due to reduced pasture photosynthesis and production. Maintaining higher water tables might achieve dual benefits of increasing pasture productivity and reducing CO 2 losses. [ABSTRACT FROM AUTHOR]

Highlights: [•] We measured net CO2 exchange (NEE) in a remnant raised peat bog for two years. [•] Mean annual NEE was −234gCm−2 yr−1 indicating substantial CO2 sink strength. [•] Year-round growing conditions accounted for large component CO2 fluxes and NEE. [•] Summertime light response of NEE was similar to northern hemisphere peatlands. [•] Small differences in cool season NEE accounted for differences in annual NEE. [Copyright &y& Elsevier]

The rate of development of topsoil is an important characteristic for soil resilience and sustainable use. We located a chronosequence (1–59 yr) of recovering landslip scars in erodible siltstone hill country under permanent pasture for sheep farming in New Zealand. We measured the rates of recovery in microbial C, respiration, catabolic diversity, phosphatase, sulphatase and invertase activities, pH, total C, total N, C/N ratio, potentially mineralizable N, total P, Olsen P, cation-exchange capacity, bulk density, particle density, porosity, available water and aggregate stability (0–10 cm depth). A subset of the same sites was sampled again after a 14-yr interval, enabling us to test whether rates of change estimated by resampling the same sites were the same as those estimated from a single time sample from the chronosequence (the space-for-time hypothesis).Most topsoil characteristics had recovered to 71–85% of those in the non-slipped sites after 59 yr. Exceptions were soil respiration, invertase and sulphatase activities, and bulk density, which recovered to 94–110% of the values of the non-slipped sites. There was little change in soil pH, total P, Olsen P, exchangeable cations and water storage along the chronosequence. An asymptote model fitted the patterns of recovery in biochemical characteristics, organic matter, bulk density and particle density. Recovery (to 90% of the asymptote value) was most rapid for the C/N ratio (5 yr) and longest for particle density (79 yr); most other characteristics fell in an 18–50 yr range. Overall, a single sampling of a chronosequence of matched landslip scars was as reliable to estimate rates of recovery as was resampling individual sites through time. Total C and N were as effective as more complicated biochemical measures to monitor the recovery of topsoil. [Copyright &y& Elsevier]

Woodchip bioreactors have the capability to promote the reduction of reactive nitrogen in the nitrate (NO 3 −) form to dinitrogen (N 2), a harmless gas in the atmosphere. Nevertheless, during the reaction the potent greenhouse gas nitrous oxide (N 2 O) is produced and can be released if denitrification is not complete. The aim of this experiment was to quantify the effect of a soil cap, the concentration of NO 3 − inflow and drying-rewetting cycles (DRW) on N 2 O emissions from bench top bioreactors (BTBs, 36.2 × 24.2 × 16.8 cm). The soil cap effect was quantified by comparing the performance of two treatments (n = 3): soil cap (CAP) and soil cap free (UNCAP). The NO 3 − inflow was simulated by feeding the BTBs with two NO 3 − concentrations (10 and 5 mg N L−1), and DRW were simulated by saturating and draining the BTBs. Nitrous oxide was quantified in the water samples as well as measured from the surface of the BTBs. The soil cap proved effective at decreasing surface N 2 O emissions with a reduction of total N 2 O emissions (calculated as the sum of dissolved N 2 O and surface N 2 O emissions) ranging from 30.4 to 42.9%. The NO 3 − inflow affected dissolved N 2 O and surface N 2 O emissions with higher values (average of 3.41 and 0.36 mg m−2 d−1, respectively for CAP, and average of 2.92 and 2.52 mg m−2 d−1, respectively for UNCAP) measured at high NO 3 − inflow. Drying-rewetting cycles influenced dissolved N 2 O and surface N 2 O emissions, with values following rewetting that accounted for more than 56% of the total N 2 O emissions for both treatments. This study confirmed that soil caps are effective at mitigating N 2 O emissions and contributed to a better understanding of N 2 O dynamics induced by two different NO 3 − inflow concentrations and DRW. [ABSTRACT FROM AUTHOR]

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. Unlabelled Image • Contrasting annual CO 2 and carbon losses at two dairy farms on drained peat • Contrasting respired CO 2 fluxes during dry season explain annual differences • Water table depth alone could not explain differences in ecosystem respiration. • Soil water dynamics implicated in respired CO 2 differences [ABSTRACT FROM AUTHOR]

Denitrification walls are a low-cost technology with the capability to reduce nitrogen (N) loading in shallow groundwater beneath agricultural systems. The aims of this study were to quantify the effect of different carbon (C) substrates on nitrate removal rate (NRR) and greenhouse gas (GHG) production in two soil-capped denitrification walls (volume ≈ 27 m3) under subtropical climate conditions. The relative performance of softwood and hardwood woodchips to promote denitrification was tested over a 2-year program of weekly monitoring, during which water samples were collected for nitrate (NO 3 −) and dissolved GHG analysis. Both the softwood and the hardwood wall had similar average NRR (2.0 and 1.6 g N m−3 d−1, respectively) but were NO 3 − limited, and acted as a sink for nitrous oxide (N 2 O) produced in the walls and dissolved in the aquifer. Both walls produced carbon dioxide (CO 2) and methane (CH 4), with the hardwood producing respectively 3-fold and 2.5-fold higher fluxes compared to the softwood. Calculation of the Global Warming Potential (GWP) permitted a comparison of the GHG emissions within the walls in terms of CO 2 equivalents (CO 2 -eq). Both the walls emitted CO 2 -eq lower than natural environments, with the softwood producing null emissions and the hardwood emitting 65-fold higher than softwood. The results of the present study suggest that woodchip bioreactors may be used to reduce nutrient loading from agricultural areas into surrounding aquatic environments as well as to decrease GHG emissions under subtropical climates, with softwood being a preferable substrate. Unlabelled Image [ABSTRACT FROM AUTHOR]

• Diffusive Gradients in Thin-Films (DGT) measured NO 3 − along two bioreactors. • DGT NO 3 − concentration data were comparable to high frequency grab sampling. • DGTs more easily accounted for temporal variation of NO 3 − than grab sampling. • DGTs enabled the calculation of bioreactor performance. • NO 3 − removal rates were 1.2–30.8 g N m−3 d−1 for two bioreactors. 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. [ABSTRACT FROM AUTHOR]

• Emission factor and eddy covariance derived annual N 2 O budgets were comparable. • Seasonal variabilities were not adequately expressed by emission factors. • Nitrogen excretion from supplementary feed contributed to annual N 2 O emissions. • Low magnitude fluxes made up 76% of total N 2 O measured by eddy covariance. • Eddy covariance measurements resulted in a N 2 O emission factor of 1.57%. Estimates of regional and national nitrous oxide (N 2 O) emissions rely on emission factors (EFs) commonly derived from measurements using static chambers. These measurements can include high uncertainties and might obscure the quantification of N 2 O fluxes. Advances in micrometeorological eddy covariance technique (EC) now allow direct measurements of N 2 O fluxes at the field scale. Here, we compared N 2 O emissions calculated from site-specific EFs with N 2 O flux data derived from year-round EC measurements on an intensively grazed dairy pasture in the Waikato region, NZ. Annual N 2 O emissions of 7.30 kg N 2 O-N ha−1 yr−1 determined using gap-filled EC flux data were greater than N 2 O estimates of 3.82 kg N 2 O-N ha−1 yr−1 based on site-specific EFs for cattle urine (1.53%), cattle dung (0.24%) and urea fertiliser (0.16%). Likely reasons for this difference were that the EF approach did not take into account the seasonal variability of EFs, the effect of supplementary feed on cattle nitrogen (N) excretion and background N 2 O emissions (BNE). Including calculated emissions from supplementary feed N (0.92 kg N 2 O-N ha−1 yr−1) and BNE (1.09 kg N 2 O-N ha−1 yr−1) increased annual EF-based emissions to 5.83 kg N 2 O-N ha−1 yr−1. The site-specific EFs were established in spring 2017 and may not have adequately represented summer, winter and particularly autumn N 2 O emissions. The EF approach, therefore, did not fully account for the seasonal variability of N 2 O fluxes as measured by EC but, if quantified, could have led to further agreement between measurements. Using EC measurements to complement static chambers and EF approaches altered annual N 2 O emissions estimates from intensively grazed pastoral land. Hence, we conclude that N 2 O budgets derived from EFs need to better capture the effect of seasonal variability, supplementary feed and BNE. [ABSTRACT FROM AUTHOR]