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 (δ18OPO4) 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 δ18OPO4 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 (Eδ18OPO4). The δ18OPO4 in the tributaries was more variable (+17.1 to +18.8‰), but only deviated from Eδ18OPO4 by between 0.4 and 0.9‰. Several potential phosphate sources within the catchment were sampled and most had a narrow range of δ18OPO4 values similar to that of river Eδ18OPO4. Discharge from two waste water treatment plants had different and distinct δ18OPO4 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 Eδ18OPO4. 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 Eδ18OPO4. This study has added to the global inventory of phosphate source δ18OPO4 values, but also demonstrated the limitations of this approach to identifying phosphate sources, especially at times when microbial cycling is high.

A multi-stable isotope framework to understand eutrophication in aquatic ecosystems.

Eutrophication is a globally significant challenge facing aquatic ecosystems, associated with human induced enrichment of these ecosystems with nitrogen (N) and phosphorus (P). However, the limited availability of inherent labels for P and N has constrained understanding of the triggers for eutrophication in natural ecosystems and appropriate targeting of management responses. This paper proposes and evaluates a new multi-stable isotope framework that offers inherent labels to track biogeochemical reactions governing both P and N in natural ecosystems. The framework couples highly novel analysis of the oxygen isotope composition of phosphate (δ(18)OPO4) with dual isotope analysis of oxygen and N within nitrate (δ(15)NNO3, δ(18)ONO3) and with stable N isotope analysis in ammonium (δ(15)NNH4). The River Beult in England is used as an exemplar system for initial evaluation of this framework. Our data demonstrate the potential to use stable isotope labels to track the input and downstream fate of nutrients from point sources, on the basis of isotopic differentiation for both P and N between river water and waste water treatment work effluent (mean difference = +1.7‰ for δ(18)OPO4; +15.5‰ for δ(15)NNH4 (under high flow); +7.3‰ for δ(18)ONO3 and +4.4‰ for δ(15)NNO3). Stable isotope data reveal nutrient inputs to the river upstream of the waste water treatment works that are consistent with partially denitrified sewage or livestock sources of nitrate (δ(15)NNO3 range = +11.5 to +13.1‰) and with agricultural sources of phosphate (δ(18)OPO4 range = +16.6 to +19.0‰). The importance of abiotic and metabolic processes for the in-river fate of N and P are also explored through the stable isotope framework. Microbial uptake of ammonium to meet metabolic demand for N is suggested by substantial enrichment of δ(15)NNH4 (by 10.2‰ over a 100 m reach) under summer low flow conditions. Whilst the concentration of both nitrate and phosphate decreased substantially along the same reach, the stable isotope composition of these ions did not vary significantly, indicating that concentration changes are likely driven by abiotic processes of dilution or sorption. The in-river stable isotope composition and the concentration of P and N were also largely constant downstream of the waste water treatment works, indicating that effluent-derived nutrients were not strongly coupled to metabolism along this in-river transect. Combined with in-situ and laboratory hydrochemical data, we believe that a multi-stable isotope framework represents a powerful approach for understanding and managing eutrophication in natural aquatic ecosystems.

Stable Carbon Isotope Evidence for Neolithic and Bronze Age Crop Water Management in the Eastern Mediterranean and Southwest Asia.

In a large study on early crop water management, stable carbon isotope discrimination was determined for 275 charred grain samples from nine archaeological sites, dating primarily to the Neolithic and Bronze Age, from the Eastern Mediterranean and Western Asia. This has revealed that wheat (Triticum spp.) was regularly grown in wetter conditions than barley (Hordeum sp.), indicating systematic preferential treatment of wheat that may reflect a cultural preference for wheat over barley. Isotopic analysis of pulse crops (Lens culinaris, Pisum sativum and Vicia ervilia) indicates cultivation in highly varied water conditions at some sites, possibly as a result of opportunistic watering practices. The results have also provided evidence for local land-use and changing agricultural practices.

Crop manuring and intensive land management by Europe's first farmers.

The spread of farming from western Asia to Europe had profound long-term social and ecological impacts, but identification of the specific nature of Neolithic land management practices and the dietary contribution of early crops has been problematic. Here, we present previously undescribed stable isotope determinations of charred cereals and pulses from 13 Neolithic sites across Europe (dating ca. 5900-2400 cal B.C.), which show that early farmers used livestock manure and water management to enhance crop yields. Intensive manuring inextricably linked plant cultivation and animal herding and contributed to the remarkable resilience of these combined practices across diverse climatic zones. Critically, our findings suggest that commonly applied paleodietary interpretations of human and herbivore δ(15)N values have systematically underestimated the contribution of crop-derived protein to early farmer diets.

Evidence for bias in measured δ15N values of terrestrial and aquatic organic materials due to pre-analysis acid treatment methods.

We investigate the effect of acid treatment methods on δ(15)N values from a range of environmental organic materials in the context of the increased application of 'dual-mode' isotope analysis (the simultaneous measurement of δ(13)C and δ(15)N from the same acid-treated sample). Three common methods are compared; (i) untreated samples; (ii) acidification followed by sequential water rinse (rinse method); and (iii) acidification in silver capsules (capsule method). The influence of capsule type (silver and tin) on δ(15)N is also independently assessed (as the capsule and rinse methods combust samples in different capsules; silver and tin, respectively). We find significant differences in δ(15)N values between methods and the precision of any one method varies significantly between sample materials and above the instrument precision (>0.3‰). The δ(15)N values of untreated samples did not produce the most consistent data on all sample materials. In addition, the capsule type appears to influence the measured δ(15)N value of some materials, particularly those combusted only in silver capsules. We also compare the new δ(15)N data with previously published δ(13)C data on the same materials. The response of δ(13)C and δ(15)N within and between methods and sample materials to acidification appears to be relatively disproportionate, which can influence the environmental interpretation of the measured data. In addition, statistical methods used to estimate inorganic nitrogen are shown to be seriously flawed.

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.

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.

Multiple stable isotope (18O, 13C, 15N and 34S) analysis of human hair to identify the recent migrants in a rural community in SW England.

Relationships between recent migration and hair delta(18)O values were examined for 40 people living in a rural community in SW England. The isotopic contents of 35 'local' hair samples were compared with those of 5 recently arrived individuals (from Australia, Canada, Chile, Germany and the USA). The hair delta(18)O values of these 'visitors' were +7.9 (Omaha, USA), +11.2 (Jena, Germany), +12.1 (Osorno, Chile), +12.6 (Montreal, Canada) and +14.3 per thousand (Adelaide, Australia). The hair value for the USA visitor (+7.9 per thousand) fell outside the range for the 33 local adult residents, +10.5 to +14.3 per thousand (+12.7 +/- 0.8 per thousand). Hair delta(18)O values did not identify the individuals from Adelaide, Montreal and Osorno as 'visitors', but hair delta(13)C or delta(34)S data did. Combining the hair delta(18)O, delta(13)C and delta(34)S values using principal components analysis (two components explained 89% of the overall variation among the 40 subjects) helped to more clearly distinguish European from non-European individuals, indicating the existence of global overall isotope (geo-origin) relationships.

Carbon isotope variations in a plantation of Sitka spruce, and the effect of acid mist.

Intra- and inter-tree variations in 13C/12C ratios were studied within a single clone plantation of 20-year-old Sitka spruce, some of which were treated with mist simulating acidic cloud water. For groups of trees of similar height and the same treatment, sampled at the same whorl height, δ13C values for current year needles showed variations (1 SD) of between 0.2 and 0.7‰. The variations reflect the seasonally averaged influences, on intercellular CO2 concentrations, of slight variations in the microhabitat within a group. For a typical intra-group variation of 0.4‰ one may be able to distinguish between groups whose mean intercellular CO2 concentrations differ by only 8 ppm. Acid misting resulted in a lowering of δ13C values by c. 0.7‰ (significant at the P≤0.05 level). This reflects higher intercellular CO2 concentrations for acid misted trees, which can be interpreted in terms of their having assimilation rates c. 10% lower than those of control trees, and might explain the observed reduction in stem growth for acid-misted trees. Without careful attention to sampling strategy, however, these small inter-tree δ13C variations can be easily masked by the much larger intra-tree variations with height. Large gradients of increasing needle δ13C with height, of c. 0.5‰ m-1, were observed in two untreated trees of different total height. The gradient was similar for both trees so, though δ13C values of both trees were identical close to their leaders (-27‰), the taller tree displayed much lower values close to the ground (-31‰). The gradients are believed to reflect lower light levels close to the ground, rather than the accumulation of respired CO2 in the atmosphere. The different height response of stems versus needles, reflected by an increase in δ13Cstems-δ13Cneedles with height (for cellulose), is discussed in terms of stem photosynthetic recapture of internally respired CO2.