Nitrate attenuation in a restored river floodplain system: River Cole (Oxfordshire - UK)

PhDRestoring river-floodplain connectivity has been proposed as an alternative management measure for natural flood defence through the temporary storage of floodwaters and the attenuation of flood peaks downstream. Whilst several studies have documented the associated ecological and landscape amenity values of such hydrological measures, the water quality benefits to the adjacent water bodies have been inadequately studied. To date, the focus of scientific research and natural resource management has been on the role of riparian buffer zones for the alleviation of agricultural diffuse nitrate pollution. This research investigated the potential for nitrate attenuation in a restored riverfloodplain system, the River Cole (Coleshill, England), with the aim of informing future restoration schemes of the best management practices for enhanced nitrate removal. Following restoration, the increased river-floodplain connectivity has encouraged overbank flooding of the different land use zones throughout the year. The flood pulse supplies the floodplain soil with river water nitrate and creates the necessary anaerobic conditions for the effective removal of nitrate via heterotrophic denitrification, while organic carbon is supplied mainly through the traditional land use management practices of grazing and mowing. The conservation of nitrogen via DNRA is of minimal importance in this lowland agricultural catchment setting, mainly due to the nonlimiting nitrate supply from the surrounding agricultural land but also the intermittent saturation regime that restricts the low redox conditions to the low elevation riparian areas. This presents the added benefit of restricting methane emission to the more frequently waterlogged riparian soils, while denitrification is effective across the whole floodplain area. Additionally, more than 90% of nitrate removal occurs in the top 30 cm of the soil during the flood, while the role of subsurface denitrification is restricted by the limited availability of organic carbon and nitrate. Based on these findings, this study demonstrates that, for similar catchments, the nitrate removal capacity of a floodplain can be assessed by the denitrification capacity of the surface soil. The assessment of the denitrification capacity can be undertaken inexpensively using a simple empirical model that requires a single microbial denitrification potential measurement, and a seasonal or monthly record of soil nitrate content, soil moisture, and temperature. Assessments can be undertaken as part of the design process to optimise nitrate removal or post restoration to appraise the functioning of the scheme

Stimulation of soil gross nitrogen transformations and nitrous oxide emission under Free air CO₂ enrichment in a mature temperate oak forest at BIFoR-FACE

Forest ecosystems are considered globally important sinks for offsetting increasing anthropogenic atmospheric carbon dioxide (CO2), however, this may be limited by the soil nutrient supply, predominantly nitrogen and phosphorus. Uncertainty remains regarding how soil N cycling in mature forests may respond to changes in carbon availability, arising from enhanced photosynthesis under elevated CO2 (eCO2) due to lack of experimental data. Further, potential positive feedbacks of nitrous oxide emissions may offset benefits of additional carbon sequestration under eCO2. The Birmingham Institute of Forest Research Free Air Carbon Enrichment experiment (BIFoR-FACE) started fumigating a mature temperate deciduous forest in 2017 at +150 ppm CO2 above ambient. Soil N cycling responses to eCO2 were investigated using the 15N pool dilution approaches to assess gross N mineralisation, immobilisation and nitrification rates, in combination with the 15N-gas flux method to quantify and source partition N2O production from 2018 to 2020 (2nd to 4th year of fumigation). Soil gross N mineralisation increased by 20% under eCO2 (6.6 μg N g−1 d−1) compared to the control treatment (5.3 μg N g−1 d−1) and despite the trends being consistent over the three years (2018–2020), the high variability between arrays reduced statistical significance except in 2019. Ammonium immobilisation by microbes increased by 20% under eCO2 (3.5 μg N g−1 d−1) as well. Overall, gross mineralisation was 4 times higher than nitrification, indicating a much higher ammonium turnover rate compared to nitrate (1.5 vs. 12 days mean residence time). N2O emission from denitrification (0.18 ng N g−1 h−1) was significantly higher under eCO2. After four years of CO2 fumigation, there are modest indications of enhanced soil N transformation rates and N availability to support the observed enhanced canopy CO2 uptake. Increased N2O fluxes under eCO2 indicated the potential for positive feedbacks on C sequestration under rising atmospheric CO2. The overall implications for C sequestration will depend on how long upregulation of soil N transformations and N bioavailability will last to meet plant demands before manifestation of N limitation, if any

Warming drove the expansion of marine anoxia in the equatorial Atlantic during the Cenomanian leading up to Oceanic Anoxic Event 2

Oceanic Anoxic Event 2 (OAE 2) (∼ 93.5 Ma) is characterized by widespread marine anoxia and elevated burial rates of organic matter. However, the factors that led to this widespread marine deoxygenation and the possible link with climatic change remain debated. Here, we report long-term biomarker records of water-column anoxia, water column and photic zone euxinia (PZE), and sea surface temperature (SST) from Demerara Rise in the equatorial Atlantic that span 3.8 Myr of the late Cenomanian to Turonian, including OAE 2. We find that total organic carbon (TOC) content is high but variable (0.41 wt %–17 wt %) across the Cenomanian and increases with time. This long-term TOC increase coincides with a TEX86-derived SST increase from ∼ 35 to 40 ◦C as well as the episodic occurrence of 28,30- dinorhopane (DNH) and lycopane, indicating warming and expansion of the oxygen minimum zone (OMZ) predating OAE 2. Water-column euxinia persisted through much of the late Cenomanian, as indicated by the presence of C35 hopanoid thiophene but only reached the photic zone during OAE 2, as indicated by the presence of isorenieratane. Using these biomarker records, we suggest that water-column anoxia and euxinia in the equatorial Atlantic preceded OAE 2 and this deoxygenation was driven by global warming

Elevated temperature and nutrients lead to increased N2O emissions from salt marsh soils from cold and warm climates

Salt marshes can attenuate nutrient pollution and store large amounts of ‘blue carbon’ in their soils, however, the value of sequestered carbon may be partially offset by nitrous oxide (N2O) emissions. Global climate and land use changes result in higher temperatures and inputs of reactive nitrogen (Nr) into coastal zones. Here, we investigated the combined effects of elevated temperature (ambient + 5℃) and Nr (double ambient concentrations) on nitrogen processing in marsh soils from two climatic regions (Quebec, Canada and Louisiana, U.S.) with two vegetation types, Sporobolus alterniflorus (= Spartina alterniflora) and Sporobolus pumilus (= Spartina patens), using 24-h laboratory incubation experiments. Potential N2O fluxes increased from minor sinks to major sources following elevated treatments across all four marsh sites. One day of potential N2O emissions under elevated treatments (representing either long-term sea surface warming or short-term ocean heatwaves effects on coastal marsh soil temperatures alongside pulses of N loading) offset 15–60% of the potential annual ambient N2O sink, depending on marsh site and vegetation type. Rates of potential denitrification were generally higher in high latitude than in low latitude marsh soils under ambient treatments, with low ratios of N2O:N2 indicating complete denitrification in high latitude marsh soils. Under elevated temperature and Nr treatments, potential denitrification was lower in high latitude soil but higher in low latitude soil as compared to ambient conditions, with incomplete denitrification observed except in Louisiana S. pumilus. Overall, our findings suggest that a combined increase in temperature and Nr has the potential to reduce salt marsh greenhouse gas (GHG) sinks under future global change scenarios

Chronic atmospheric reactive N deposition has breached the N sink capacity of a northern ombrotrophic peatbog increasing the gaseous and fluvial N losses

Peatlands play an important role in modulating the climate, mainly through sequestration of carbon dioxide into peat carbon, which depends on the availability of reactive nitrogen (Nr) to mosses. Atmospheric Nr deposition in the UK has been above the critical load for functional and structural changes to peatland mosses, thus threatening to accelerate their succession by vascular plants and increasing the possibility of Nr export to downstream ecosystems. The N balance of peatlands has received comparatively little attention, mainly due to the difficulty in measuring gaseous N losses as well as the Nr inputs due to biological nitrogen fixation (BNF). In this study we have estimated the mean annual N balance of an ombrotrophic bog (Migneint, North Wales) by measuring in situ N2 + N2O gaseous fluxes and also BNF in peat and mosses. Fluvial N export was monitored through a continuous record of DON flux, while atmospheric N deposition was modelled on a 5 × 5 km grid. The mean annual N mass balance was slightly positive (0.7 ± 4.1 kg N ha−1 y−1) and varied interannually indicating the fragile status of this bog ecosystem that has reached N saturation and is prone to becoming a net N source. Gaseous N losses were a major N output term accounting for 70% of the N inputs, mainly in the form of the inert N2 gas, thus providing partial mitigation to the adverse effects of chronic Nr enrichment. BNF was suppressed by 69%, compared to rates in pristine bogs, but was still active, contributing ~2% of the N inputs. The long-term peat N storage rate (8.4 ± 0.8 kg N ha−1 y−1) cannot be met by the measured N mass balance, showing that the bog catchment is losing more N than it can store due its saturated status