Circulating PACAP levels are associated with altered imaging measures of entorhinal cortex neurite density in posttraumatic stress disorder

Introduction: Pituitary adenylate cyclase-activating polypeptide (PACAP) regulates plasticity in brain systems underlying arousal and memory and is associated with posttraumatic stress disorder (PTSD). Research in animal models suggests that PACAP modulates entorhinal cortex (EC) input to the hippocampus, contributing to impaired contextual fear conditioning. In PTSD, PACAP is associated with higher activity of the amygdala to threat stimuli and lower functional connectivity of the amygdala and hippocampus. However, PACAP-affiliated structural alterations of these regions have not been investigated in PTSD. Here, we examined whether peripheral PACAP levels were associated with neuronal morphology of the amygdala and hippocampus (primary analyses), and EC (secondary) using Neurite Orientation Dispersion and Density Imaging.Methods: Sixty-four (44 female) adults (19 to 54 years old) with DSM-5 Criterion A trauma exposure completed the Clinician-Administered PTSD Scale (CAPS-5), a blood draw, and magnetic resonance imaging. PACAP38 radioimmunoassay was performed and T1-weighted and multi-shell diffusion-weighted images were acquired. Neurite Density Index (NDI) and Orientation Dispersion Index (ODI) were quantified in the amygdala, hippocampus, and EC. CAPS-5 total score and anxious arousal score were used to test for clinical associations with brain structure.Results: Higher PACAP levels were associated with greater EC NDI (β = 0.0099, q = 0.032) and lower EC ODI (β = -0.0073, q = 0.047), and not hippocampal or amygdala measures. Neither EC NDI nor ODI was associated with clinical measures.Conclusions: Circulating PACAP levels were associated with altered neuronal density of the EC but not the hippocampus or amygdala. These findings strengthen evidence that PACAP may impact arousal-associated memory circuits in PTSD.

Comparison of sediment biomarker signatures generated using time-integrated and discrete suspended sediment samples

Sediment source fingerprinting using biomarker properties has led to new insights in our understanding of land use contributions to time-integrated suspended sediment samples at catchment scale. A time-integrated mass-flux sampler (TIMS; also known as the 'Phillips' sampler), a cost-effective approach for suspended sediment collection in situ. Such samplers are widely being used to collect sediment samples for source fingerprinting purposes, including studies using biomarkers as opposed to more conventional tracer properties. Here, we assessed the performance of TIMS for collecting representative sediment samples for biomarkers during high discharge events in a small lowland grassland-dominated catchment. Concentrations of long odd-chain n-alkanes (> C23) and both saturated free and bound fatty acids (C14-C32), as well as compound-specific 13C were compared between sediment collected by both TIMS and autosamplers (ISCO). The results showed that concentrations of alkanes, free fatty acids, and bound fatty acids are consistently comparable between TIMS and ISCO suspended sediment samples. Similarly, compound-specific 13C signals were not found to be significantly different in the suspended sediment samples collected using the different samplers. However, different magnitudes of resemblance in biomarker concentrations and compositions between the samples collected using the two sediment collection methods were confirmed by overlapping index and symmetric coordinates-based correlation analysis. Here, the difference is attributed to the contrasting temporal basis of TIMS (time-integrated) vs. ISCO (discrete) samples, as well as potential differences in the particle sizes collected by these different sediment sampling methods. Nevertheless, our findings suggest that TIMS can be used to generate representative biomarker data for suspended sediment samples collected during high discharge events.

Anhedonia and Delay Discounting: Differing Patterns of Brain-Behavior Relationships in Healthy Control Participants Versus Individuals With Posttraumatic Stress Disorder

BACKGROUND

Anhedonia may contribute to individual differences in delay discounting (DD). In prior work, we found that higher anhedonia was associated with shallower DD in healthy control (HC) participants but steeper DD in individuals with posttraumatic stress disorder (PTSD). In this study, we aimed to directly compare the relationship between anhedonia and DD across groups and to identify functional brain correlates of this interaction.

METHODS

Participants (HC group: n = 23, DSM-5 PTSD group: n = 23) completed a questionnaire assessing anhedonia (Snaith-Hamilton Pleasure Scale [SHAPS]), task-based functional magnetic resonance imaging of decision making including DD, and resting-state functional magnetic resonance imaging. Task-based activity and resting-state functional connectivity were evaluated in reward-related regions that have also been implicated in PTSD (nucleus accumbens [NAcc], right anterior insula).

RESULTS

Higher SHAPS scores were associated with steeper DD in PTSD, but there was no relationship between DD and SHAPS in the HC group. There was a significant group-by-SHAPS interaction for NAcc activity, t31 = 2.92, p = .007: Greater NAcc activity when immediate rewards were chosen was associated with higher SHAPS in the PTSD group but lower SHAPS in the HC group. In resting-state functional connectivity, there was a group-by-SHAPS interaction between the NAcc seed and right parietal and frontal pole clusters.

CONCLUSIONS

These results extend prior findings that anhedonia is associated with steeper DD in PTSD and demonstrate that this behavioral finding occurs in the context of NAcc hyperactivity to immediate rewards and hyperconnectivity in anhedonic individuals with PTSD.

Circulating PACAP levels are associated with altered imaging measures of entorhinal cortex neurite density in posttraumatic stress disorder

Background

Pituitary adenylate cyclase-activating polypeptide (PACAP) regulates plasticity in brain systems underlying arousal and memory and is associated with posttraumatic stress disorder (PTSD). Research in animal models suggests that PACAP modulates entorhinal cortex (EC) input to the hippocampus, contributing to impaired contextual fear conditioning. In PTSD, PACAP is associated with higher activity of the amygdala to threat stimuli and lower functional connectivity of the amygdala and hippocampus. However, PACAP-affiliated structural alterations of these regions have not been reported. Here, we examined whether peripheral PACAP levels were associated with neuronal morphology of the amygdala and hippocampus (primary analysis), and EC (secondary analysis) using Neurite Orientation Dispersion and Density Imaging.

Methods

Sixty-four (44 female) adults (19 to 54 years old) with DSM-5 Criterion A trauma exposure completed the Clinician-Administered PTSD Scale (CAPS-5), a blood draw, and magnetic resonance imaging. PACAP38 radioimmunoassay was performed and T1-weighted and multi-shell diffusion- weighted images were acquired. Neurite Density Index (NDI) and Orientation Dispersion Index (ODI) were quantified in the amygdala, hippocampus, and EC. CAPS-5 total score and anxious arousal score were used to test for clinical associations with brain structure.

Results

Higher PACAP levels in blood were associated with greater EC NDI (β=0.31, q=0.034) and lower EC ODI (β=-0.30, q=0.042) and not hippocampal or amygdala measures. Neither EC NDI nor ODI was associated with clinical measures.

Conclusions

Circulating PACAP levels were associated with altered neuronal density of the EC but not hippocampus or amygdala. These findings strengthen evidence that PACAP may impact arousal- associated memory circuits.

Hydro-chemical responses at different scales in a rural catchment, UK, and implications for managing the unintended consequences of agriculture

Diffuse pollutant transfers from agricultural land often constitute the bulk of annual loads in catchments and storm events dominate these fluxes. There remains a lack of understanding of how pollutants move through catchments at different scales. This is critical if the mismatch between the scales used to implement on-farm management strategies, compared to those used for assessment of environmental quality, is to be addressed. The aim of this study was to understand how the mechanisms of pollutant export may change when assessed at different scales and the corresponding implications for on-farm management strategies. A study was conducted within a 41 km2 catchment containing 3 nested sub-catchments, instrumented to monitor discharge and various water quality parameters. Storm data over a 24-month period were analysed and hysteresis (HI) and flushing (FI) indices calculated for two water quality variables that are typically of environmental significance; NO3-N and suspended sediment (SSC). For SSC, increasing spatial scale had little effect on the mechanistic interpretation of mobilisation and the associated on-farm management strategies. At the three smallest scales NO3-N was chemodynamic with the interpretation of dominant mechanisms changing seasonally. At these scales, the same on-farm management strategies would be recommended. However, at the largest scale, NO3-N appeared unaffected by season and chemostatic. This would lead to a potentially very different interpretation and subsequent on-farm measures. The results presented here underscore the benefits of nested monitoring for extracting mechanistic understanding of agricultural impacts on water quality. The application of HI and FI indicates that monitoring at smaller scales is crucial. At large scales, the complexity of the catchment hydrochemical response means that mechanisms become obscured. Smaller catchments more likely represent critical areas within larger catchments where mechanistic understanding can be extracted from water quality monitoring and used to underpin the selection of on-farm mitigation measures.

Reduced structural connectivity of the medial temporal lobe including the perforant path is associated with aging and verbal memory impairment

The perforant path, the white matter bundle connecting the entorhinal cortex (ERC) with the hippocampal formation deteriorates with age-related cognitive decline. Previous investigations using diffusion-weighted MRI to quantify perforant path integrity in-vivo have been limited due to image resolution or have quantified the perforant path using methods susceptible to partial volume effects such as the tensor model and without consideration of its 3-dimensional morphology. In this investigation, we use quantitative-anisotropy informed tractography derived from ultra-high resolution diffusion imaging (ZOOMit) to investigate structural connectivity of the perforant path and other medial temporal lobe (MTL) pathways in older adults (63 to 98 years old, n = 51). We show that graph density within the MTL declines with age and is associated with lower delayed recall performance. We also show that older age and poorer delayed recall are associated with reduced streamlines connecting the ERC and dentate gyrus of the hippocampus (the putative perforant path). This work suggest that intra-MTL connectivity may new candidate biomarkers for age-related cognitive decline.

Posterior white matter hyperintensities are associated with reduced medial temporal lobe subregional integrity and long-term memory in older adults

White matter hyperintensities are a marker of small vessel cerebrovascular disease that are strongly related to cognition in older adults. Similarly, medial temporal lobe atrophy is well-documented in aging and Alzheimer's disease and is associated with memory decline. Here, we assessed the relationship between lobar white matter hyperintensities, medial temporal lobe subregional volumes, and hippocampal memory in older adults. We collected MRI scans in a sample of 139 older adults without dementia (88 females, mean age (SD) = 76.95 (10.61)). Participants were administered the Rey Auditory Verbal Learning Test (RAVLT). Regression analyses tested for associations among medial temporal lobe subregional volumes, regional white matter hyperintensities and memory, while adjusting for age, sex, and education and correcting for multiple comparisons. Increased occipital white matter hyperintensities were related to worse RAVLT delayed recall performance, and to reduced CA1, dentate gyrus, perirhinal cortex (Brodmann area 36), and parahippocampal cortex volumes. These medial temporal lobe subregional volumes were related to delayed recall performance. The association of occipital white matter hyperintensities with delayed recall performance was fully mediated statistically only by perirhinal cortex volume. These results suggest that white matter hyperintensities may be associated with memory decline through their impact on medial temporal lobe atrophy. These findings provide new insights into the role of vascular pathologies in memory loss in older adults and suggest that future studies should further examine the neural mechanisms of these relationships in longitudinal samples.

Hippocampal dentate gyrus integrity revealed with ultrahigh resolution diffusion imaging predicts memory performance in older adults

Medial temporal lobe (MTL) atrophy is a core feature of age-related cognitive decline and Alzheimer's disease (AD). While regional volumes and thickness are often used as a proxy for neurodegeneration, they lack the sensitivity to serve as an accurate diagnostic test and indicate advanced neurodegeneration. Here, we used a submillimeter resolution diffusion weighted MRI sequence (ZOOMit) to quantify microstructural properties of hippocampal subfields in older adults (63-98 years old) using tensor derived measures: fractional anisotropy (FA) and mean diffusivity (MD). We demonstrate that the high-resolution sequence, and not a standard resolution sequence, identifies dissociable profiles for CA1, dentate gyrus (DG), and the collateral sulcus. Using ZOOMit, we show that advanced age is associated with increased MD of the CA1 and DG as well as decreased FA of the DG. Increased MD of the DG, reflecting decreased cellular density, mediated the relationship between age and word list recall. Further, increased MD in the DG, but not DG volume, was linked to worse spatial pattern separation. Our results demonstrate that ultrahigh-resolution diffusion imaging enables the detection of microstructural differences in hippocampal subfield integrity and will lead to novel insights into the mechanisms of age-related memory loss.

Prolonged heavy rainfall and land use drive catchment sediment source dynamics: Appraisal using multiple biotracers

Excessive sediment loss degrades freshwater quality and is prone to further elevation and variable source contributions due to the combined effect of extreme rainfall and differing land uses. To quantify erosion and sediment source responses across scales, this study integrated work at both field and catchment scale for two hydrologically contrasting winters (2018-19 and 2019-20). Sediment load was estimated at the field scale (grassland-arable conversion system). Sediment source apportionment work was undertaken at the catchment scale (4.5 km2) and used alkanes, and both free and bound fatty acid carbon isotope signatures as diagnostic fingerprints to distinguish sediment sources: arable, pasture, woodland and stream banks. Sediment source apportionment based on bound fatty acids revealed a substantial shift in contributions, from stream banks dominating (70 ± 5%) in winter 2018-19, to arable land dominating (52 ± 7%) in the extreme wet winter 2019-20. Increases in sediment contributions from arable (∼3.9 times) and pasture (∼2.4 times) land at the catchment outlet during the winter 2019-20 were consistent with elevated sediment losses monitored at the field scale which indicated that low-magnitude high frequency rainfall alone increased sediment loss even from pasture by 350%. In contrast, carbon isotope signatures of alkanes and free fatty acids consistently estimated stream banks as a dominant source (i.e., ∼36% and ∼70% respectively) for both winters regardless of prolonged rainfall in winter 2019-20. Beyond quantifying the shifts in field scale sediment load and catchment scale sediment sources due to the changes in rainfall patterns, our results demonstrate valuable insight into how the fate of biotracers in soil and sediment manifests in the δ13C values of homologues and, in turn, their role in information gain for estimating sediment source contributions. Discrepancies in the estimated sediment source contributions using different biotracers indicate that without a careful appreciation of their biogeochemical limitations, erroneous interpretation of sediment source contributions can undermine management strategies for delivering more sustainable and resilient agriculture.

Latent anxiety in clinical depression is associated with worse recognition of emotional stimuli

BACKGROUND

Major Depressive Disorder, characterized by cognitive affective biases, is a considerable public health challenge. Past work has shown that higher depressive symptoms are associated with augmented memory of negative stimuli. In contrast, anxiety symptoms have been associated with overgeneralization of emotional memories. Given the high comorbidity of depression and anxiety, it is critical to understand how cognitive affective biases are differentially associated with clinical symptoms.

METHOD

We used continuous measures of depression (Beck Depression Inventory [BDI-II]) and anxiety (Beck Anxiety Inventory [BAI]) to evaluate an adult sample (N = 79; 18-41 years old, 58 female). Emotional memory discrimination and recognition memory were tested using an emotional discrimination task. We applied exploratory factor analysis to questions from the BAI and BDI-II to uncover latent constructs consisting of negative affect, anhedonia, somatic anxiety, and cognitive anxiety.

RESULTS

We report evidence that anxious symptoms were associated with impaired recognition of negative items after accounting for age and sex. Our exploratory factor analysis revealed that impaired negative item recognition is largely associated with somatic and cognitive anxiety factors.

LIMITATIONS

Interpretations in a mixed pathology sample, especially given collinearity among factors, may be difficult.

CONCLUSIONS

We provide evidence that somatic and cognitive anxiety are related to impaired recognition memory for negative stimuli. Future clinical investigations should uncover the neurobiological basis supporting the link between recognition of negative stimuli and somatic/cognitive symptoms of anxiety.

Why do mothers never stop grieving for their deceased children? Enduring alterations of brain connectivity and function

A child's death is a profound loss for mothers and affects hundreds of thousands of women. Mothers report inconsolable and progressive grief that is distinct from depression and impacts daily emotions and functions. The brain mechanisms responsible for this relatively common and profound mental health problem are unclear, hampering its clinical recognition and care. In an initial exploration of this condition, we used resting state functional MRI (fMRI) scans to examine functional connectivity in key circuits, and task-based fMRI to examine brain network activity in grieving mothers in response to pictures of their deceased child and as well as recognizable deceased celebrities and unfamiliar individuals. We compared nine mothers who had lost an adult child and aged-matched control mothers with a living child of a similar age. Additionally, we collected diffusion imaging scans to probe structural connectivity and complemented the imaging studies with neuropsychological assessments. Increased functional activation in Ventral Attention/Salience Networks accompanied by a reduced activation in the medial prefrontal cortex in response to the deceased child's picture robustly distinguished the grieving mothers from controls. Heightened resting-state functional connectivity between the paraventricular thalamic nucleus (PVT) and the amygdala distinguished the grieving mothers from the controls and correlated with subjective grief severity. Structurally, maternal grief and its severity were associated with alterations in corticolimbic white matter tracts. Finally, grieving mothers performed worse than controls on neuropsychological tests of learning, memory, and executive function, linked with grief severity. Reduced activation in cortical regions inhibiting emotions and changes in the PVT circuitry-a region involved in long-term emotional memories and decision making under conflict-distinguish grieving mothers from controls. Notably, the magnitude of neurobiological changes correlates with the subjective severity of grief. Together, these new discoveries delineate a prevalent and under-recognized mental health syndrome and chart a path for its appreciation and care.

Dynamics of fluvial hydro-sedimentological, nutrient, particulate organic matter and effective particle size responses during the U.K. extreme wet winter of 2019–2020

The floc size distribution of suspended sediment is a critical driver for in-channel sedimentation and sediment-associated contaminant and nutrient transfer and fate in river catchments. Real-time, in situ, floc size characterisation is possible using available technology but, to date, limited high resolution floc data have been published for fluvial systems draining upland extensive grassland catchments. To that end, suspended sediment floc size distribution and turbidity were characterised at 15-minute intervals using Laser In-Situ Scattering and Transmissometry (LISST) diffraction and a YSI turbidity sonde for six storm events in the upper River Taw (15 km²) catchment in SW England. Maximum event discharges (Q) ranged between 4.3 and 20.0 m³ s⁻¹, with clockwise hysteretic responses (HI = 0.18–0.48) of total suspended solid concentrations (TSS) and Q. The sediment flushing index was highest in the early autumn (0.93) and storm event TSS fluxes varied from 0.04 to 2.9 t km⁻². This suggests a change in sources or composition of sediment during higher Q and highly variable patterns of sediment flux from event-to-event. The proportion of particulate organic matter (POM) to TSS was highly variable (5–89%) and did not increase with Q, indicating POM source limitation. The fine-grained tail (D₁₀ and D₁₆) of the floc size distributions decreased during hydrograph rising limbs, with the finest floc sizes associated with the highest TN and TP concentrations at peak Q. The results suggest that dynamic interactions between wet ground and extreme rainfall events can flush significant amounts of sediment from the relatively undisturbed extensive grassland upland catchment. We strongly encourage a sensors-based approach to reveal the spatio-temporal complexity of floc size and associated pollutant export during high Q generated by extreme rainfall since this can help to elucidate processes and mechanisms and generate high-resolution data for water quality modelling without significant user intervention.

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In situ flocs size and turbidity were studied in sequential high discharge events.

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Hydrographs characterised by clockwise hysteresis with a secondary sediment peak.

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Proportion of POM to TSS in the physical samples was highly variable (5–89%).

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Fine (D₁₀ and D₁₆) floc size diameter was lowest during peak discharges.

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Floc size exerted less influence on TP and TN concentrations.

Integrity of the uncinate fasciculus is associated with emotional pattern separation-related fMRI signals in the hippocampal dentate and CA3

Alterations in white matter integrity have been demonstrated in a number of psychiatric disorders involving emotional disruptions. One such pathway - the uncinate fasciculus - connects the orbitofrontal cortex (OFC) to the medial temporal lobes (MTL) and has been associated with early life adversity, maltreatment, anxiety, and depression. While it is purported to play a role in episodic memory and discrimination, its exact function remains poorly understood. We have previously described the role of the amygdala and dentate (DG)/CA3 fields of the hippocampus in the mnemonic discrimination of emotional experiences (i.e. emotional pattern separation). However, how this computation may be modulated by connectivity with the orbitofrontal cortex remains unknown. Here we asked if the uncinate fasciculus plays a role in influencing MTL subregional activity during emotional pattern separation. By combining diffusion imaging with high-resolution fMRI, we found that reduced integrity of the UF is related to elevated BOLD fMRI activation of the DG/CA3 subregions of the hippocampus during emotional lure discrimination. We additionally report that higher levels of DG/CA3 activity are associated with poorer memory performance, suggesting that greater activation in this network (possibly driven by CA3 recurrent collaterals) is associated with memory errors. Based on this work we suggest that the UF is one pathway that may allow the OFC to exert control on this network and improve discrimination of emotional experiences, although further work is necessary to fully evaluate this possibility. This work provides novel insight into the role of prefrontal interactions with the MTL, particularly in the context of emotional memory.

A rapid ammonium fluoride method to determine the oxygen isotope ratio of available phosphorus in tropical soils

RATIONALE

The isotopic composition of oxygen bound to phosphorus (δ¹⁸ O_P value) offers an opportunity to gain insight into P cycling mechanisms. However, there is little information for tropical forest soils, which presents a challenge for δ¹⁸ O_P measurements due to low available P concentrations. Here we report the use of a rapid ammonium fluoride extraction method (Bray-1) as an alternative to the widely used anion-exchange membrane (AEM) method for quantification of δ¹⁸ O_P values of available P in tropical forest soils.

METHODS

We compared P concentrations and δ¹⁸ O_P values of available and microbial P determined by AEM and Bray-1 extraction for a series of tropical forest soils from Panama spanning a steep P gradient. This involved an assessment of the influence of extraction conditions, including temperature, extraction time, fumigation time and solution-to-soil ratio, on P concentrations and isotope ratios.

RESULTS

Depending on the extraction conditions, Bray-1 P concentrations ranged from 0.2 to 66.3 mg P kg^-1 across the soils. Extraction time and temperature had only minor effects on Bray-1 P, but concentrations increased markedly as the solution-to-soil ratio increased. In contrast, extraction conditions did not affect Bray-1 δ¹⁸ O_P values, indicating that Bray-1 provides a robust measure of the isotopic composition of available soil P. For a relatively high P soil, available and fumigation-released (microbial) δ¹⁸ O_P values determined by Bray-1 extraction (20‰ and 16‰, respectively) were higher than those determined by the AEM method (18‰ and 12‰, respectively), which we attribute to slightly different P pools extracted by the two methods and/or differences resulting from the longer extraction time needed for the AEM method.

CONCLUSIONS

The short extraction time, insensitivity to extraction conditions and smaller mass of soil required to extract sufficient P for isotopic analysis make Bray-1extraction a suitable alternative to the AEM method for the determination of δ¹⁸ O_P values of available P in tropical soils.

Changes of oxygen isotope values of soil P pools associated with changes in soil pH

Field data about the effect of soil pH on phosphorus (P) cycling is limited. A promising tool to study P cycling under field conditions is the ¹⁸O:¹⁶O ratio of phosphate (δ¹⁸O_P). In this study we investigate whether the δ¹⁸O_P can be used to elucidate the effect of soil pH on P cycling in grasslands. Soils and plants were sampled from different fertilisation and lime treatments of the Park Grass long term experiment at Rothamsted Research, UK. The soils were sequentially extracted to isolate different soil P pools, including available P and corresponding δ¹⁸O_P values were determined. We did not observe changes in plant δ¹⁸O_P value, but soil P δ¹⁸O_P values changed, and lower δ¹⁸O_P values were associated with higher soil pH values. At sites where P was not limiting, available P δ¹⁸O_P increased by up to 3‰ when lime was applied. We show that the δ¹⁸O_P method is a useful tool to investigate the effect of pH on soil P cycling under field conditions as it highlights that different soil processes must govern P availability as pH shifts. The next challenge is now to identify these underlying processes, enabling better management of soil P at different pH.

The stable oxygen isotope ratio of resin extractable phosphate derived from fresh cattle faeces

RATIONALE

Phosphorus losses from agriculture pose an environmental threat to watercourses. A new approach using the stable oxygen isotope ratio of oxygen in phosphate (δ¹⁸ O_PO4 value) may help elucidate some phosphorus sources and cycling. Accurately determined and isotopically distinct source values are essential for this process. The δ¹⁸ O_PO4 values of animal wastes have, up to now, received little attention.

METHODS

Phosphate (PO₄ ) was extracted from cattle faeces using anion resins and the contribution of microbial PO₄ was assessed. The δ¹⁸ O_PO4 value of the extracted PO₄ was measured by precipitating silver phosphate and subsequent analysis on a thermal conversion elemental analyser at 1400°C, with the resultant carbon monoxide being mixed with a helium carrier gas passed through a gas chromatography (GC) column into a mass spectrometer. Faecal water oxygen isotope ratios (δ¹⁸ O_H2O values) were determined on a dual-inlet mass spectrometer through a process of headspace carbon dioxide equilibration with water samples.

RESULTS

Microbiological results indicated that much of the extracted PO₄ was not derived directly from the gut fauna lysed during the extraction of PO₄ from the faeces. Assuming that the faecal δ¹⁸ O_H2O values represented cattle body water, the predicted pyrophosphatase equilibrium δ¹⁸ O_PO4 (Eδ¹⁸ O_PO4 ) values ranged between +17.9 and +19.9‰, while using groundwater δ¹⁸ O_H2O values gave a range of +13.1 to +14.0‰. The faecal δ¹⁸ O_PO4 values ranged between +13.2 and +15.3‰.

CONCLUSIONS

The fresh faecal δ¹⁸ O_PO4 values were equivalent to those reported elsewhere for agricultural animal slurry. However, they were different from the Eδ¹⁸ O_PO4 value calculated from the faecal δ¹⁸ O_H2O value. Our results indicate that slurry PO₄ is, in the main, derived from animal faeces although an explanation for the observed value range could not be determined.

Phosphate stable oxygen isotope variability within a temperate agricultural soil

In this study, we conduct a spatial analysis of soil total phosphorus (TP), acid extractable phosphate (PO₄) and the stable oxygen (O) isotope ratio within the PO₄ molecule (δ¹⁸O_PO4 ) from an intensively managed agricultural grassland site. Total P in the soil was found to range from 736 to 1952 mg P kg^- 1, of which between 12 and 48% was extractable using a 1 M HCl (HCl_PO4 ) solution with the two variables exhibiting a strong positive correlation. The δ¹⁸O_PO4 of the extracted PO₄ ranged from 17.0 to 21.6‰ with a mean of 18.8‰ (± 0.8). While the spatial variability of Total P has been researched at various scales, this is the first study to assess the variability of soil δ¹⁸O_PO4 at a field-scale resolution. We investigate whether or not δ¹⁸O_PO4 variability has any significant relationship with: (i) itself with respect to spatial autocorrelation effects; and (ii) HCl_PO4 , elevation and slope - both globally and locally. Results indicate that δ¹⁸O_PO4 was not spatially autocorrelated; and that δ¹⁸O_PO4 was only weakly related to HCl_PO4 , elevation and slope, when considering the study field as a whole. Interestingly, the latter relationships appear to vary in strength locally. In particular, the δ¹⁸O_PO4 to HCl_PO4 relationship may depend on the underlying soil class and/or on different field managements that had operated across an historical north-south field division of the study field, a division that had been removed four years prior to this study.

The oxygen isotopic composition of phosphate in river water and its potential sources in the Upper River Taw catchment, UK

The need to reduce both point and diffuse phosphorus pollution to aquatic ecosystems is widely recognised and in order to achieve this, identification of the different pollutant sources is essential. Recently, a stable isotope approach using oxygen isotopes within phosphate (δ¹⁸O_PO4) has been used in phosphorus source tracing studies. This approach was applied in a one-off survey in September 2013 to the River Taw catchment in south-west England where elevated levels of phosphate have been reported. River water δ¹⁸O_PO4 along the main channel varied little, ranging from +17.1 to +18.8‰. This was no >0.3‰ different to that of the isotopic equilibrium with water (Eδ¹⁸O_PO4). The δ¹⁸O_PO4 in the tributaries was more variable (+17.1 to +18.8‰), but only deviated from Eδ¹⁸O_PO4 by between 0.4 and 0.9‰. Several potential phosphate sources within the catchment were sampled and most had a narrow range of δ¹⁸O_PO4 values similar to that of river Eδ¹⁸O_PO4. Discharge from two waste water treatment plants had different and distinct δ¹⁸O_PO4 from one another ranging between +16.4 and +19.6‰ and similar values to that of a dairy factory final effluent (+16.5 to +17.8‰), mains tap water (+17.8 to +18.4‰), and that of the phosphate extracted from river channel bed sediment (+16.7 to +17.6‰). Inorganic fertilizers had a wide range of values (+13.3 to +25.9‰) while stored animal wastes were consistently lower (+12.0 to +15.0‰) than most other sources and Eδ¹⁸O_PO4. The distinct signals from the waste water treatment plants were lost within the river over a short distance suggesting that rapid microbial cycling of phosphate was occurring, because microbial cycling shifts the isotopic signal towards Eδ¹⁸O_PO4. This study has added to the global inventory of phosphate source δ¹⁸O_PO4 values, but also demonstrated the limitations of this approach to identifying phosphate sources, especially at times when microbial cycling is high.

A novel application of natural fluorescence to understand the sources and transport pathways of pollutants from livestock farming in small headwater catchments

This paper demonstrates the application of a low-cost and rapid natural fluorescence technique for tracing and quantifying the transport of pollutants from livestock farming through a small headwater catchment. Fluorescence intensities of Dissolved Organic Matter (DOM) present in different pollutant sources and drainage waters in the Den Brook catchment (Devon, UK) were monitored through storm events occurring between January 2007 and June 2008. Contrasting fluorescence signals from different sources confirmed the technique's usefulness as a tracer of pollutants from livestock farming. Changes in fluorescence intensities of drainage waters throughout storm events were used to assess the dynamics of key pollutant sources. The farmyard area of the catchment studied was shown to contribute polluted runoff at the onset of storm events in response to only small amounts of rain, when flows in the Den Brook first-order channel were low. The application of slurry to a field within the catchment did not elevate the fluorescence of drainage waters during storm events suggesting that when slurry is applied to undrained fields the fluorescent DOM may become quickly adsorbed onto soil particles and/or immobilised through bacterial breakdown. Fluorescence intensities of drainage waters were successfully combined with discharge data in a two component mixing model to estimate pollutant fluxes from key sources during the January 2007 storm event. The farmyard was shown to be the dominant source of tryptophan-like material, contributing 61-81% of the total event flux at the catchment outlet. High spatial and temporal resolution measurements of fluorescence, possibly using novel in-situ fluorimeters, may thus have great potential in quickly identifying and quantifying the presence, dynamics and sources of pollutants from livestock farming in catchments.

The hydrological response of heavy clay grassland soils to rainfall in south-west England using delta2H

Stable isotopes of water have been previously used in catchment studies to separate rain-event water from pre-event groundwater. However, there are a lack of studies at the smaller scale looking at the separation of event water from pre-event water. This is particularly relevant for heavy clay soil systems through which the movement of water is uncertain but is thought to be rainwater-dominated. The data presented here were collected at a rural site in the south-west of England. The historic rainfall at the site was isotopically varied but similar to the global meteoric water line, with annual weighted means of -37 per thousand for delta(2)H and -5.7 per thousand for delta(18)O and with no seasonal variation. Drainage was sampled from the inter-flow (surface runoff + lateral through-flow) and drain-flow (55 cm deep mole drains) pathways of two 1 ha lysimeters during two rainfall events, which had delta(2)H values of -68 per thousand and -92 per thousand, respectively. The delta(2)H values of the lysimeter drainage water suggest that there was no contribution of event water during the first, small discharge (Q) event; however, the second larger event did show isotopic variation in delta(2)H values negatively related to Q indicating that rainwater was contributing to Q. A hydrograph separation indicated that only 49-58% of the inter-flow and 18-25% of the drain-flow consisted of event water. This was surprising given that these soil types are considered retentive of soil water. More work is needed on heavy clay soils to understand better the nature of water movement from these systems.

Applications of stable isotope ratio mass spectrometry in cattle dung carbon cycling studies

Understanding the fate of dung carbon (C) in soils is challenging due to the ubiquitous presence of the plant-derived organic matter (OM), the source material from which both dung-derived OM and soil organic matter (SOM) predominantly originate. A better understanding of the fate of specific components of this substantial source of OM, and thereby its contribution to C cycling in terrestrial ecosystems, can only be achieved through the use of labelled dung treatments. In this short review, we consider analytical approaches using bulk and compound-specific stable carbon isotope analysis that have been utilised to explore the fate of dung-derived C in soils. Bulk stable carbon isotope analyses are now used routinely to explore OM matter cycling in soils, and have shown that up to 20% of applied dung C may be incorporated into the surface soil horizons several weeks after application, with up to 8% remaining in the soil profile after one year. However, whole soil delta(13)C values represent the average of a wide range of organic components with varying delta(13)C values and mean residence times in soils. Several stable (13)C isotope ratio mass spectrometric methods have been developed to qualify and quantify different fractions of OM in soils and other complex matrices. In particular, thermogravimetry-differential scanning calorimetry-isotope ratio mass spectrometry (TG-DSC-IRMS) and gas chromatography-combustion-IRMS (GC-C-IRMS) analyses have been applied to determine the incorporation and turnover of polymeric plant cell wall materials from C(4) dung into C(3) grassland soils using natural abundance (13)C isotope labelling. Both approaches showed that fluxes of C derived from polysaccharides, i.e. as cellulose or monosaccharide components, were more similar to the behaviour of bulk dung C in soil than lignin. However, lignin and its 4-hydroxypropanoid monomers were unexpectedly dynamic in soil. These findings provide further evidence for emerging themes in biogeochemical investigations of soil OM dynamics that challenge perceived concepts of recalcitrance of C pools in soils, which may have profound implications for the assessment of the potential of agricultural soils to influence terrestrial C sinks.

Using delta15N and delta18O to evaluate the sources and pathways of NO3- in rainfall event discharge from drained agricultural grassland lysimeters at high temporal resolutions

The origin of NO(3) (-) yielded in drainage from agricultural grasslands is of environmental significance and has three potential sources; (i) soil organic mater (SOM), (ii) recent agricultural amendments, and (iii) atmospheric inputs. The variation in delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) was measured from the 'inter-flow' and 'drain-flow' of two 1 ha drained lysimeter plots, one of which had received an application of 21 m(3) of NH(4) (+)-N-rich agricultural slurry, during two rainfall events. Drainage started to occur 1 month after the application of slurry. The concentrations of NO(3) (-)-N from the two lysimeters were comparable; an initial flush of NO(3) (-)-N occurred at the onset of drainage from both lysimeters before levels quickly dropped to <1 mg NO(3) (-)-N L(-1). The isotopic signature of the delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) during the first two rainfall events showed a great deal of variation over short time-periods from both lysimeters. Isotopic variation of delta(15)N-NO(3) (-) during rainfall events ranged between -1.6 to +5.2 per thousand and +0.4 to +11.1 per thousand from the inter-flow and drain-flow, respectively. Variation in the delta(18)O-NO(3) (-) ranged from +2.0 to +7.8 per thousand and from +3.3 to +8.4 per thousand. No significant relationships between the delta(15)N-NO(3) (-) or delta(18)O-NO(3) (-) and flow rate were observed in most cases although delta(18)O-NO(3) (-) values indicated a positive relationship and delta(15)N-NO(3) (-) values a negative relationship with flow during event 2. Data from a bulked rainfall sample when compared with the theoretical delta(18)O-NO(3) (-) for soil microbial NO(3) (-) indicated that the contribution of rainfall NO(3) (-) accounted for 8% of the NO(3) (-) in the lysimeter drainage at most. The calculated contribution of rainfall NO(3) (-) was not enough to account for the depletion in delta(15)N-NO(3) (-) values observed during the duration of the rainfall event 2. The relationship between delta(15)N-NO(3) (-) and delta(18)O-NO(3) (-) from the drain-flow indicated that denitrification was causing enrichment in the isotopes from this pathway. The presence of slurry seemed to cause a relative depletion in delta(18)O-NO(3) (-) in the inter-flow and delta(15)N-NO(3) (-) in the drain-flow compared with the zero-slurry lysimeter. This may have been caused by increased microbial nitrification stimulated by the presence of increased NH(4) (+)-N.