Biogeochemical impacts of sewage effluents in predominantly rural river catchments: Are point source inputs distinct to background diffuse pollution?

Discharge of treated sewage effluent to rivers can degrade aquatic ecosystem quality, interacting with multiple stressors in the wider catchment. In predominantly rural catchments, the river reach influence of point source effluents is unknown relative to complex background pressures. We examined water column, sediment and biofilm biogeochemical water quality parameters along river transects (200 m upstream to 1 km downstream) during summer at five wastewater treatment works (WWTW) in Scotland. Treated sewage effluent (subset, n = 3) pollutant concentrations varied between sites. Downstream concentration profiles of water and sediment biogeochemical parameters showed complex spatial changes. A hypothesised point source signature of elevated concentrations of pollution immediately downstream of WWTW then a decaying pollution 'plume' did not commonly occur. Instead, elevated soluble reactive phosphorus (SRP), ammonium and coliforms (maximum 0.23 mgP/l, 0.33 mgN/l and >2 × 106 MPN/100 ml) occurred immediately downstream of two WWTW, whereas some downstream pollutant concentrations decreased. Microbial substrate respiration responses only differed 1 km downstream. Significantly greater concentrations of sediment metal occurred >500 m downstream, likely due to the redeposition of historic contaminated sediments. Significantly lowered chlorophyll-a downstream of one WWTW coincided with elevated metals, despite water SRP and sediment P increases. Overall, stress caused to microbes and algae by effluent contaminants outweighed the subsidy effect of WWTW nutrients. We observed variable effluent flows to the rivers limited localised pollution downstream of WWTW and overall influence of arable land cover on river water quality. Together, this challenges views of consistently discharging point sources impacting low dilution sensitive rivers in summer contrasting with 'diffuse' sources. Thus, river water column and benthic compartments are altered at varying scales by point source effluents in combination with rural catchment pollution sources, both discrete (e.g. farmyards and septic tanks) and diffuse.

Spatial representation of in-stream sediment phosphorus release combining channel network approaches and in-situ experiments.

Impairment of rivers by elevated phosphorus (P) concentration is an issue often studied at outlets of mesoscale catchments. Our objective was to evaluate within-catchment spatio-temporal processes along connected reaches to understand processes of internal P loading associated with sediment input, accumulations in channels and sediment-water column P exchange. Our overall hypothesis was that heterogeneous sediment residence within the channel of a 52 km2 mixed land cover catchment resulted in key zones for sediment-water P exchange. We evaluated the channel network through ground-survey, spatial data methods establishing connectivity and energy gradients. This gave a background to understand sampling of sediments and P release/uptake to the water column using 90 s in-situ resuspension isolating a portion of streambed over five sets of three-location transects in May (spring storms, recent active erosion) and September (summer low flow, longer sediment residence). Simple transect position models (top, mid, bottom) predicted increased sediment resuspension yields and P contents in lower settings. Sediment P release following resuspension were mean (and range) 0.5 (-0.8 to 1.8) and 0.5 (-2.5 to 3.6) mg soluble reactive P/m2 bed in May and September, respectively, strengthening generally down the transects but inconsistently. Relationships (log form) showed a steepening rise in fine sediments, P content, background and disturbance-released dissolved P, with specific stream power < 40 W/m2. In-situ methods showed sediments dominantly (12 cases May, 13 cases Sep) as P sources capable of influencing dissolved P concentrations and with potential explanation that heterogeneous locations of internal P loading influence the systems longer-term observed P trends. Combining channel network, stream power assessment and in-situ sorption studies improved the understanding of influential zones of sediment-water P exchange within this mesoscale catchment. Such methods have potential to inform P model development and management.

The interactions of site-specific factors on riparian buffer effectiveness across multiple pollutants: A review.

Following decades of riparian buffer zone (RBZ) studies there remains a need to look across individual site data for collective evidence on the site-specific pollution mitigation and river water quality. We explored primary study evidence on runoff, sediment, P, N, coliforms and pesticides using complimentary styles of metadata interpretation. A quantitative assessment of pollution retention (75 studies, 474 data rows) derived relationships for retention versus width, including significant covariates of clay particle size and buffer slope for sediment, total and dissolved P. Total N and coliforms related to texture and slope but were independent of width. Other factors across pollutants were inconsistently reported. With limitations on quantitative studies a second approach examined factor significance (formal testing versus inferred; 93 studies) across source pressure, transport/physical, vegetation and soil biogeochemical factors on pollution effectiveness. The RBZ evidence showed considerable disagreement and bias in shorter term study implications on longer-term processes. Screening for stronger evidence by study number and agreement left fifteen factors informing on at least one pollutant, whereas only rainfall intensity, preferential deposition, tree planting, soil infiltration remained addressing three or more pollutants. Key messages were that: data complexities, from short-term trapping in upper buffer edges and so-called 'negative effectiveness' associated with internal recycling and/or errors in constraining mass inputs for dissolved pollutant and subsurface transport require careful interpretation; RBZ intervention and study durations were limited compared to effect times (particularly vegetation management and changing soil conditions); factors affect pollutants with particulate and dissolved phases differently and must be understood to limit RBZ pollution swapping. Buffer functioning is highly site-specific. To understand this better attention should be given to revisiting studies of vegetation management to extend timeframes, wider study of belowground (soil biogeochemical and transport) processes and studies should document site contexts across source pressures, riparian hydrological, soil and vegetation factors.

The utility of spatial data to delineate river riparian functions and management zones: A review.

Riparian zones of rivers are transitional environments between land and water ecosystems with distinct hydrological gradients, soils and habitats strongly related to their functioning. When these functions are intact, they integrate multi-directional processes across the land-river channel (e.g. canopy shade effects on the stream, flood inundation effects on the land) with mutual beneficial effects. In many managed landscapes these functions have been degraded. To restore them, considerable efforts have been directed over the last 20 years to understand and place effective riparian 'buffer' zones, particularly to enhance water quality and biodiversity. Since water quality targets are not easily met by current practices in many managed landscapes (as additive pressures increase), catchment managers will have to increasingly restore riparian functions to enhance aquatic ecosystem resilience to land and climate change. Targeting effective restoration within site-specific contexts requires availability of spatial data, in combinations that inform on individual and multiple functions. There are accelerating developments with spatial data, arising from increased spatial resolution of key underlying datasets, availability of soil and landcover data and increasing secondary derived attributes. Hence, a review is timely into the best practices in the use of these data for delineating riparian functions and management zones for rivers. Our review evaluates the application of spatial data and is structured around three conceptual methods of riparian delineation; fixed width, variable width by river corridor features and variable width by context of local pressures or required outcomes. We explore process representation and incorporation into management across main riparian functions (hydrological connectivity, water quality, shading, resource transfers and habitat provision). Translating spatial data into functions informs the ability to go beyond contemporary, generally fixed width approaches using basic structural components towards planning to better target functional attributes to optimise ecosystem protection.

Phosphorus leaching from riparian soils with differing management histories under three grass species.

Plants release carbon-based exudates from their roots into the rhizosphere to increase phosphorus (P) supply to the soil solution. However, if more P than required is brought into solution, additional P could be available for leaching from riparian soils. To investigate this further, soil columns containing a riparian arable and buffer strip soil, which differed in organic matter contents, were sown with three common agricultural and riparian grass species. The P loads in leachate were measured and compared with those from unplanted columns, which were 0.17 ± 0.01 and 0.89 ± 0.04 mg kg-1 for the arable and buffer strip soil, respectively. A mixture of ryegrass and red fescue significantly (p ≤ .05) increased dissolved inorganic P loads in leachate from the arable (0.23 ± 0.01 mg kg-1 ) and buffer strip soil (1.06 ± 0.05 mg kg-1 ), whereas barley significantly reduced P leaching from the buffer strip soil (0.53 ± 0.08 mg kg-1 ). This was dependent on the dissolved organic C released under different plant species and on interactions with soil management history and biogeochemical conditions, rather than on plant uptake of P and accumulation into biomass. This suggested that the amount and forms of P present in the soil and the ability of the plants to mobilize them could be key factors in determining how plants affect leaching of soil P. Selecting grass species for different stages of buffer strip development, basing species selection on root physiological traits, and correcting soil nutrient stoichiometry in riparian soils through vegetative mining could help to lower this contribution.

Catchment effects of a future Nordic bioeconomy: From land use to water resources.

In the future, the world is expected to rely increasingly on renewable biomass resources for food, fodder, fibre and fuel. The sustainability of this transition to bioeconomy for our water systems depends to a large extent on how we manage our land resources. Changes in land use together with climate change will affect water quantity and quality, which again will have implications for the ecosystem services provided by water resources. These are the main topics of this Ambio special issue on "Environmental effects of a green bio-economy". This paper offers a summary of the eleven papers included in this issue and, at the same time, outlines an approach to quantify and mitigate the impacts of bioeconomy on water resources and their ecosystem services, with indications of useful tools and knowledge needs.

Nordic Bioeconomy Pathways: Future narratives for assessment of water-related ecosystem services in agricultural and forest management.

Further development of the bioeconomy, the substitution of bioresources for fossil resources, will lead to an increased pressure on land and water resources in both agriculture and forestry. It is important to study whether resultant changes in land management may in turn lead to impairment of water services. This paper describes the Nordic Bioeconomy Pathways (NBPs), a set of regional sectoral storylines nested within the global Shared Socioeconomic Pathways (SSP) framework developed to provide the BIOWATER research program with land management scenarios for projecting future developments to explore possible conflicts between land management changes and the Water Framework Directive (WFD). The NBPs are a set of narrative storylines capturing a range of plausible future trajectories for the Nordic bioeconomy until 2050 and that are fit for use within hydrological catchment modelling, ecosystem service studies and stakeholder dialogue about possible changes in agricultural and forestry management practices.

Coupled macronutrient cycling in stream biofilms: Effects of stoichiometry, light and temperature.

Stream biofilms have the capacity to modify the passage of macronutrients through catchments as they respond to nutrient compositions and ratios from different sources. Knowledge of coupled cycling of N, P and organic C in flowing freshwaters is essential to understanding and predicting aquatic ecosystems responses to environmental change comprising multiple chemical and physical stressors. Colonisation on nutrient diffusing substrates (glucose-C, inorganic NP, combined CNP and control applied in-situ in an oligotrophic, upland stream) led to biofilms differing in community and element compositions. The 72 biofilms were transferred to replicated recirculating water chambers (1 L volume) for 4-days where additional effects of light and temperature treatments were investigated on nutrient exchange with the water column. Chemical (nutrient analyses, 13C, 15N tracing, stoichiometry) and biological (chlorophyll, TRFLP) analyses were performed to understand the biofilm composition changes and interaction with the water column. Biofilms combining C with NP incorporated more N and P relative to controls than did those with NP alone. During the chamber phase C-treated biofilms resulted in lower water column N, P concentrations with CNP relative to NP treatments. The effects of the light and temperature were manifested mainly in impaired net nutrient uptake at temperature deviating from ambient stream temperatures. The effects of organic C on N, P cycling (and vice-versa) in mixed biofilms and their interaction with waters is a developing field. Combining in-stream and chamber tests has shown potential for studying in controlled and replicated systems such complex interactions.

Modeling the Ecological Impact of Phosphorus in Catchments with Multiple Environmental Stressors.

The broken phosphorus (P) cycle has led to widespread eutrophication of freshwaters. Despite reductions in anthropogenic nutrient inputs that have led to improvement in the chemical status of running waters, corresponding improvements in their ecological status are often not observed. We tested a novel combination of complementary statistical modeling approaches, including random-effect regression trees and compositional and ordinary linear mixed models, to examine the potential reasons for this disparity, using low-frequency regulatory data available to catchment managers. A benthic Trophic Diatom Index (TDI) was linked to potential stressors, including nutrient concentrations, soluble reactive P (SRP) loads from different sources, land cover, and catchment hydrological characteristics. Modeling suggested that SRP, traditionally considered the bioavailable component, may not be the best indicator of ecological impacts of P, as shown by a stronger and spatially more variable negative relationship between total P (TP) concentrations and TDI. Nitrate-N ( < 0.001) and TP ( = 0.002) also showed negative relationship with TDI in models where land cover was not included. Land cover had the strongest influence on the ecological response. The positive effect of seminatural land cover ( < 0.001) and negative effect of urban land cover ( = 0.030) may be related to differentiated bioavailability of P fractions in catchments with different characteristics (e.g., P loads from point vs. diffuse sources) as well as resilience factors such as hydro-morphology and habitat condition, supporting the need for further research into factors affecting this stressor-response relationship in different catchment types. Advanced statistical modeling indicated that to achieve desired ecological status, future catchment-specific mitigation should target P impacts alongside multiple stressors.

Assessing Risk of E. coli Resuspension from Intertidal Estuarine Sediments: Implications for Water Quality.

Estuarine sediments are a reservoir for faecal bacteria, such as E. coli, where they reside at greater concentrations and for longer periods than in the overlying water. Faecal bacteria in sediments do not usually pose significant risk to human health until resuspended into the water column, where transmission routes to humans are facilitated. The erosion resistance and corresponding E. coli loading of intertidal estuarine sediments was monitored in two Scottish estuaries to identify sediments that posed a risk of resuspending large amounts of E. coli. In addition, models were constructed in an attempt to identify sediment characteristics leading to higher erosion resistance. Sediments that exhibited low erosion resistance and a high E. coli loading occurred in the upper- and mid-reaches of the estuaries where sediments had higher organic content and smaller particle sizes, and arose predominantly during winter and autumn, with some incidences during summer. Models using sediment characteristics explained 57.2% and 35.7% of sediment shear strength and surface stability variance respectively, with organic matter content and season being important factors for both. However large proportions of the variance remained unexplained. Sediments that posed a risk of resuspending high amounts of faecal bacteria could be characterised by season and sediment type, and this should be considered in the future modelling of bathing water quality.

Simultaneous Quantification of Soil Phosphorus Labile Pool and Desorption Kinetics Using DGTs and 3D-DIFS.

The buffering of phosphorus concentrations in soil solution by the soil-solid phase is an important process for providing plant root access to nutrients. Accordingly, the size of labile solid phase-bound phosphorus pool and the rate at which it can resupply phosphorous into the dissolved phase can be important variables in determining when the plant availability of the nutrient may be limited. The phosphorus labile pool (Plabile) and its desorption kinetics were simultaneously evaluated in 10 agricultural UK soils using the diffusive gradients in thin-films (DGT) technique. The DGT-induced fluxes in the soil and sediments model (DIFS) was fitted to the time series of DGT deployments (1-240 h), which allowed the estimation of Plabile, and the system response time ( Tc). The Plabile concentration was then compared to that obtained by several soil P extracts including Olsen P, FeO-P, and water extractable P, in order to assess if the data from these analytical procedures can be used to represent the labile P across different soils. The Olsen P concentration, commonly used as a representation of the soil labile P pool, overestimated the desorbable P concentration by 6-fold. The use of this approach for the quantification of soil P desorption kinetic parameters found a wide range of equally valid solutions for Tc. Additionally, the performance of different DIFS model versions working in different dimensions (1D, 2D, and 3D) was compared. Although all models could provide a good fit to the experimental DGT time series data, the fitted parameters showed a poor agreement between different model versions. The limitations of the DIFS model family are associated with the assumptions taken in the modeling approach and the three-dimensional (3D) version is here considered to be the most precise among them.

Management Options to Reduce Phosphorus Leaching from Vegetated Buffer Strips.

Vegetated buffer strips (VBS) between agricultural areas and surface waters are important retention areas for eroded particulate P through which they may obtain critically high degrees of P saturation imposing high risk of soluble P leaching. We tested topsoil removal and three harvesting frequencies (once, twice, or four times per year) of natural buffer vegetation to reduce P leaching with the aim to offset erosional P accumulation and high degrees of P saturation. We used a simple numerical time-step model to estimate changes in VBS soil P levels with and without harvest. Harvesting offset erosional deposition as it resulted in an annual ammonium oxalate-extractable P reduction of 0.3 to 2.8% (25-cm topsoil content) in soils of the VBS and thus, with time, reduced potential P leaching below a baseline of 50 µg L. Topsoil removal only marginally reduced potential leaching at two sites and not anywhere near this baseline. The harvest frequency only marginally affected the annual P removal, making single annual harvests the most economical. We estimate 50 to 300 yr to reach the P leaching baseline, due to substantial amounts of P accumulated in the soils. Even in high-erosion-risk situations in our study, harvesting reduced soil P content and the P leaching risk. We suggest harvesting as a practical and efficient management to combat P leaching from agricultural VBS, not just for short-term reductions of dissolved P, but also for reductions of the total soil P pool and for possible multiple benefits for VBS.

Current Insights into the Effectiveness of Riparian Management, Attainment of Multiple Benefits, and Potential Technical Enhancements.

Buffer strips between land and waters are widely applied measures in diffuse pollution management, with desired outcomes across other factors. There remains a need for evidence of pollution mitigation and wider habitat and societal benefits across scales. This paper synthesizes a collection of 16 new primary studies and review papers to provide the latest insights into riparian management. We focus on the following areas: (i) diffuse pollution removal efficiency of conventional and saturated buffer strips, (ii) enhancing biodiversity of buffers, (iii) edge-of-field technologies for improving nutrient retention, and (iv) potential reuse of nutrients and biomass from buffers. Although some topics represent emerging areas, for other well-studied topics (e.g., diffuse pollution), it remains that effectiveness of conventional vegetated buffer strips for water quality improvement varies. The collective findings highlight the merits of targeted, designed buffers that support multiple benefits, more efficiently interrupting surface and subsurface contaminant flows while enhancing diversity in surface topography, soil moisture and C, vegetation, and habitat. This synthesis also highlights that despite the significant number of studies on the functioning of riparian buffers, research gaps remain, particularly in relation to (i) the capture and retention of soluble P and N in subsurface flows through buffers, (ii) the utilization of captured nutrients, (iii) the impact of buffer design and management on terrestrial and aquatic habitats and species, and (iv) the effect of buffers (saturated) on greenhouse gas emissions and the potential for pollution swapping.

Is Green Manure from Riparian Buffer Strip Species an Effective Nutrient Source for Crops?

Agriculture needs to reduce inputs of inorganic fertilizers and close the loop on nutrients that can otherwise become environmental pollutants. This can be achieved by promoting recycling of nutrients within the agricultural landscape. We investigated the extent to which plants found in riparian buffer zones have the potential to provide nutrients to crops as a green manure, through plant growth and decomposition studies. Under controlled conditions, species typical of Scottish riparian buffer strips were tested for their ability to accumulate biomass and nutrients in tissue under N- and P-replete conditions and whether this ability enhanced the utility of the resulting green manure in promoting crop growth. In this proof-of-concept study, we found that green manure derived from riparian buffer strips did not effectively replace inorganic fertilizer and only had a significant positive effect on growth, yield, and nutrient accumulation in barley ( L.) when it was integrated with the addition of inorganic fertilizers. The individual species tested varied in the amount of P they accumulated in their tissue (1.38-52.73 mg P plant), but individual species did not differ in their ability to promote yield when used as a green manure. Our results indicate that selecting certain species in the buffer strip on the basis of their nutrient accumulating abilities is not an effective way to increase the utility of buffer strip green manure as a nutrient source for crops.

An Assessment of the Multifunctionality of Integrated Buffer Zones in Northwestern Europe.

Integrated buffer zones (IBZs) have recently been introduced in the Northwestern Europe temperate zone to improve delivery of ecosystem services compared with the services associated with long-established vegetated buffer zones. A common feature of all the studied IBZ sites is that tile drainage, which previously discharged directly into the streams, is now intercepted within the IBZ. Specifically, the design of IBZs combines a pond, where soil particles present in drain water or surface runoff can be deposited, and a planted subsurface flow infiltration zone. Together, these two components should provide an optimum environment for microbial processes and plant uptake of nutrients. Nutrient reduction capacities, biodiversity enhancement, and biomass production functions were assessed with different emphasis across 11 IBZ sites located in Denmark, Great Britain, and Sweden. Despite the small size of the buffer zones (250-800 m) and thus the small proportion of the drained catchment (mostly <1%), these studies cumulatively suggest that IBZs are effective enhancements to traditional buffer zones, as they (i) reduce total N and P loads to small streams and rivers, (ii) act as valuable improved habitats for aquatic and amphibian species, and (iii) offer economic benefits by producing fast-growing wetland plant biomass. Based on our assessment of the pilot sites, guidance is provided on the implementation and management of IBZs within agricultural landscapes.

Factors affecting the spatial and temporal distribution of E. coli in intertidal estuarine sediments.

Microbiological water quality monitoring of bathing waters does not account for faecal indicator organisms in sediments. Intertidal deposits are a significant reservoir of FIOs and this indicates there is a substantial risk to bathers through direct contact with the sediment, or through the resuspension of bacteria to the water column. Recent modelling efforts include sediment as a secondary source of contamination, however, little is known about the driving factors behind spatial and temporal variation in FIO abundance. E. coli abundance, in conjunction with a wide range of measured variables, was used to construct models to explain E. coli abundance in intertidal sediments in two Scottish estuaries. E. coli concentrations up to 6 log10 CFU 100 g dry wt-1 were observed, with optimal models accounting for E. coli variation up to an adjusted R2 of 0.66. Introducing more complex models resulted in overfitting of models, detrimentally affected the transferability of models between datasets. Salinity was the most important single variable, with season, pH, colloidal carbohydrates, organic content, bulk density and maximum air temperature also featuring in optimal models. Transfer of models, using only lower cost variables, between systems explained an average deviance of 42%. This study demonstrates the potential for cost-effective sediment characteristic monitoring to contribute to FIO fate and transport modelling and consequently the risk assessment of bathing water safety.

The potential use of natural vs commercial biosorbent material to remediate stream waters by removing heavy metal contaminants.

The presence of high level of heavy metals in aquatic environment is a cause of ecological and environmental concern and thus their removal from water courses is environmentally essential. Four natural inexpensive biosorbents: macro algae (Fucus vesiculosus), crab shells (Cancer pagurus), wood chippings and iron-rich soil were tested for copper (Cu2+) and zinc (Zn2+) removal from aqueous solutions. Batch equilibrations were performed at 1:100 w/v with different initial metal concentrations. Three macro algae pre-treatments (unmodified (UM algae), chemically treated (Ca-T algae) and thermally treated (T-T algae)) were additionally investigated for performance. The sorption capacities were compared with the commercial material biochar and activated carbon. The maximum level of the sorbents for Cu2+ uptake at 15.7 mM/l was attained by the natural material of UM algae (72.37 ±â€¯0.37 mg/g) > Ca-T algae (66.77 ±â€¯0.19 mg/g) > T-T algae (63.06 ±â€¯0.82 mg/g), followed by the commercial material activated carbon (36.71 ±â€¯2.20 mg/g). The maximum level of the sorbents for Zn2+ uptake at 15.3 mM/l was also achieved by the natural material of UM algae (52.40 ±â€¯0.80 mg/g) > Ca-T algae (48.83 ±â€¯2.01 mg/g) > T-T algae (39.57 ±â€¯0.80 mg/g) followed by the commercial material activated carbon (20.78 ±â€¯1.63 mg/g) and biochar (18.07 ±â€¯1.09 mg/g). The results demonstrated that Cu2+ and Zn2+ were effectively removed by these biosorbents at all concentrations. However, at high metals concentrations, the natural material macro algae had greater Cu2+ and Zn2+ sorption capacity than the conventional sorbent activated carbon, and the affinity of these natural biosorbents were greater for Cu2+ than Zn2+. Hence, inexpensive natural and readily available materials showed potential as biosorbents to remediate polluted stream water of toxic metal contaminants.

The role of zeta potential in the adhesion of E. coli to suspended intertidal sediments.

The extent of pathogen transport to and within aquatic systems depends heavily on whether the bacterial cells are freely suspended or in association with suspended particles. The surface charge of both bacterial cells and suspended particles affects cell-particle adhesion and subsequent transport and exposure pathways through settling and resuspension cycles. This study investigated the adhesion of Faecal Indicator Organisms (FIOs) to natural suspended intertidal sediments over the salinity gradient encountered at the transition zone from freshwater to marine environments. Phenotypic characteristics of three E. coli strains, and the zeta potential (surface charge) of the E. coli strains and 3 physically different types of intertidal sediments was measured over a salinity gradient from 0 to 5 Practical Salinity Units (PSU). A batch adhesion microcosm experiment was constructed with each combination of E. coli strain, intertidal sediment and 0, 2, 3.5 and 5 PSU. The zeta potential profile of one E. coli strain had a low negative charge and did not change in response to an increase in salinity, and the remaining E. coli strains and the sediments exhibited a more negative charge that decreased with an increase in salinity. Strain type was the most important factor in explaining cell-particle adhesion, however adhesion was also dependant on sediment type and salinity (2, 3.5 PSU > 0, 5 PSU). Contrary to traditional colloidal (Derjaguin, Landau, Vervey, and Overbeek (DLVO)) theory, zeta potential of strain or sediment did not correlate with cell-particle adhesion. E. coli strain characteristics were the defining factor in cell-particle adhesion, implying that diverse strain-specific transport and exposure pathways may exist. Further research applying these findings on a catchment scale is necessary to elucidate these pathways in order to improve accuracy of FIO fate and transport models.

Phosphorus acquisition by citrate- and phytase-exuding Nicotiana tabacum plant mixtures depends on soil phosphorus availability and root intermingling.

Citrate and phytase root exudates contribute to improved phosphorus (P) acquisition efficiency in Nicotiana tabacum (tobacco) when both exudates are produced in a P deficient soil. To test the importance of root intermingling in the interaction of citrate and phytase exudates, Nicotiana tabacum plant-lines with constitutive expression of heterologous citrate (Cit) or fungal phytase (Phy) exudation traits were grown under two root treatments (roots separated or intermingled) and in two soils with contrasting soil P availability. Complementarity of plant mixtures varying in citrate efflux rate and mobility of the expressed phytase in soil was determined based on plant biomass and P accumulation. Soil P composition was evaluated using solution 31 P NMR spectroscopy. In the soil with limited available P, positive complementarity occurred in Cit+Phy mixtures with roots intermingled. Root separation eliminated positive interactions in mixtures expressing the less mobile phytase (Aspergillus niger PhyA) whereas positive complementarity persisted in mixtures that expressed the more mobile phytase (Peniophora lycii PhyA). Soils from Cit+Phy mixtures contained less inorganic P and more organic P compared to monocultures. Exudate-specific strategies for the acquisition of soil P were most effective in P-limited soil and depended on citrate efflux rate and the relative mobility of the expressed phytase in soil. Plant growth and soil P utilization in plant systems with complementary exudation strategies are expected to be greatest where exudates persist in soil and are expressed synchronously in space and time.