Karl G. Richards

Long-Term Persistence and Leaching of Escherichia coli in Temperate Maritime Soils

ABSTRACT Enteropathogen contamination of groundwater, including potable water sources, is a global concern. The spreading on land of animal slurries and manures, which can contain a broad range of pathogenic microorganisms, is considered a major contributor to this contamination. Some of the pathogenic microorganisms applied to soil have been observed to leach through the soil into groundwater, which poses a risk to public health. There is a critical need, therefore, for characterization of pathogen movement through the vadose zone for assessment of the risk to groundwater quality due to agricultural activities. A lysimeter experiment was performed to investigate the effect of soil type and condition on the fate and transport of potential bacterial pathogens, using Escherichia coli as a marker, in four Irish soils (n = 9). Cattle slurry (34 tonnes per ha) was spread on intact soil monoliths (depth, 1 m; diameter, 0.6 m) in the spring and summer. No effect of treatment or the initial soil moisture on the E. coli that leached from the soil was observed. Leaching of E. coli was observed predominantly from one soil type (average, 1.11 ± 0.77 CFU ml−1), a poorly drained Luvic Stagnosol, under natural rainfall conditions, and preferential flow was an important transport mechanism. E. coli was found to have persisted in control soils for more than 9 years, indicating that autochthonous E. coli populations are capable of becoming naturalized in the low-temperature environments of temperate maritime soils and that they can move through soil. This may compromise the use of E. coli as an indicator of fecal pollution of waters in these regions.

Evaluation of cover crop and reduced cultivation for reducing nitrate leaching in Ireland.

Nitrate (NO(3)) loss from arable systems to surface and groundwater has attracted considerable attention in recent years in Ireland. Little information exists under Irish conditions, which are wet and temperate, on the effects of winter cover crops and different tillage techniques on NO(3) leaching. This study investigated the efficacy of such practices in reducing NO(3) leaching from a spring barley (Hordeum vulgare L.) system in the Barrow River valley, southeast Ireland. The study compared the effect of two tillage systems (plow-based tillage and noninversion tillage) and two over-winter alternatives (no vegetative cover and a mustard cover crop) on soil solution NO(3) concentrations at 90 cm depth over two winter drainage seasons (2003/04 and 2004/05). Soil samples were taken and analyzed for inorganic N. During both years of the study, the use of a mustard cover crop significantly reduced NO(3) losses for the plowed and reduced cultivation treatments. Mean soil solution NO(3) concentrations were between 38 and 70% lower when a cover crop was used, and total N load lost over the winter was between 18 and 83% lower. Results from this study highlight the importance of drainage volume and winter temperatures on NO(3) concentrations in soil solution and overall N load lost. It is suggested that cover crops will be of particular value in reducing NO(3) loss in temperate regions with mild winters, where winter N mineralization is important and high winter temperatures favor a long growing season.

Factors affecting nitrate distribution in shallow groundwater under a beef farm in South Eastern Ireland

Groundwater contamination was characterised using a methodology which combines shallow groundwater geochemistry data from 17 piezometers over a 2 yr period in a statistical framework and hydrogeological techniques. Nitrate-N (NO3-N) contaminant mass flux was calculated across three control planes (rows of piezometers) in six isolated plots. Results showed natural attenuation occurs on site although the method does not directly differentiate between dilution and denitrification. It was further investigated whether NO3-N concentration in shallow groundwater (<5 m below ground level) generated from an agricultural point source on a 4.2 ha site on a beef farm in SE Ireland could be predicted from saturated hydraulic conductivity (Ksat) measurements, ground elevation (m Above Ordnance Datum), elevation of groundwater sampling (screen opening interval) (m AOD) and distance from a dirty water point pollution source. Tobit regression, using a background concentration threshold of 2.6 mg NO3-N L(-1) showed, when assessed individually in a step wise procedure, Ksat was significantly related to groundwater NO3-N concentration. Distance of the point dirty water pollution source becomes significant when included with Ksat in the model. The model relationships show areas with higher Ksat values have less time for denitrification to occur, whereas lower Ksat values allow denitrification to occur. Areas with higher permeability transport greater NO3-N fluxes to ground and surface waters. When the distribution of Cl- was examined by the model, Ksat and ground elevation had the most explanatory power but Ksat was not significant pointing to dilution having an effect. Areas with low NO3 concentration and unaffected Cl- concentration points to denitrification, low NO3 concentration and low Cl- chloride concentration points to dilution and combining these findings allows areas of denitrification and dilution to be inferred. The effect of denitrification is further supported as mean groundwater NO3-N was significantly (P<0.05) related to groundwater N2/Ar ratio, redox potential (Eh), dissolved O2 and N2 and was close to being significant with N2O (P=0.08). Calculating contaminant mass flux across more than one control plane is a useful tool to monitor natural attenuation. This tool allows the identification of hot spot areas where intervention other than natural attenuation may be needed to protect receptors.

Agriculture, meteorology and water quality in Ireland: a regional evaluation of pressures and pathways of nutrient loss to water

The main environmental impact of Irish agriculture on surface and ground water quality is the potential transfer of nutrients to water. Soil water dynamics mediate the transport of nutrients to water, and these dynamics in turn depend on agro-meteorological conditions, which show large variations between regions, seasons and years. In this paper we quantify and map the spatio-temporal variability of agro-meteorological factors that control nutrient pressures and pathways of nutrient loss. Subsequently, we evaluate their impact on the water quality of Irish rivers. For nitrogen, pressure and pathways factors coincide in eastern and southern areas, which is reflected in higher nitrate levels of the rivers in these regions. For phosphorus, pathway factors are most pronounced in north-western parts of the country. In south-eastern parts, high pressure factors result in reduced biological water quality. These regional differences require that farm practices be customised to reflect the local risk of nutrient loss to water. Where pathways for phosphorus loss are present almost year-round * as is the case in most of the north-western part of the country * build-up of pressures should be prevented, or ameliorated where already high. In south-eastern areas, spatio-temporal coincidence of nutrient pressures and pathways should be prevented, which poses challenges to grassland management.

Characterization of environmentally persistent Escherichia coli isolates leached from an Irish soil.

ABSTRACT Soils are typically considered to be suboptimal environments for enteric organisms, but there is increasing evidence that Escherichia coli populations can become resident in soil under favorable conditions. Previous work reported the growth of autochthonous E. coli in a maritime temperate Luvic Stagnosol soil, and this study aimed to characterize, by molecular and physiological means, the genetic diversity and physiology of environmentally persistent E. coli isolates leached from the soil. Molecular analysis (16S rRNA sequencing, enterobacterial repetitive intergenic consensus PCR, pulsed-field gel electrophoresis, and a multiplex PCR method) established the genetic diversity of the isolates (n = 7), while physiological methods determined the metabolic capability and environmental fitness of the isolates, relative to those of laboratory strains, under the conditions tested. Genotypic analysis indicated that the leached isolates do not form a single genetic grouping but that multiple genotypic groups are capable of surviving and proliferating in this environment. In physiological studies, environmental isolates grew well across a broad range of temperatures and media, in comparison with the growth of laboratory strains. These findings suggest that certain E. coli strains may have the ability to colonize and adapt to soil conditions. The resulting lack of fecal specificity has implications for the use of E. coli as an indicator of fecal pollution in the environment.

Denitrification and indirect N2O emissions in groundwater: Hydrologic and biogeochemical influences

Identification of specific landscape areas with high and low groundwater denitrification potential is critical for improved management of agricultural nitrogen (N) export to ground and surface waters and indirect nitrous oxide (N₂O) emissions. Denitrification products together with concurrent hydrogeochemical properties were analysed over two years at three depths at two low (L) and two high (H) permeability agricultural sites in Ireland. Mean N₂O-N at H sites were significantly higher than L sites, and decreased with depth. Conversely, excess N₂-N were significantly higher at L sites than H sites and did not vary with depth. Denitrification was a significant pathway of nitrate (NO₃⁻-N) reduction at L sites but not at H sites, reducing 46-77% and 4-8% of delivered N with resulting mean NO₃⁻-N concentrations of 1-4 and 12-15 mg N L⁻¹ at L and H sites, respectively. Mean N₂O-N emission factors (EF₅g) were higher than the most recent Intergovernmental Panel on Climate Change (IPCC, 2006) default value and more similar to the older IPCC (1997) values. Recharge during winter increased N₂O but decreased excess dinitrogen (excess N₂-N) at both sites, probably due to increased dissolved oxygen (DO) coupled with low groundwater temperatures. Denitrifier functional genes were similar at all sites and depths. Data showed that highly favourable conditions prevailed for denitrification to occur--multiple electron donors, low redox potential (Eh<100 mV), low DO (<2 mg L⁻¹), low permeability (k(s)<0.005 m·d⁻¹) and a shallow unsaturated zone (<2 m). Quantification of excess N₂-N in groundwater helps to close N balances at the local, regional and global scales.

Confirmation of co-denitrification in grazed grassland

Pasture-based livestock systems are often associated with losses of reactive forms of nitrogen (N) to the environment. Research has focused on losses to air and water due to the health, economic and environmental impacts of reactive N. Di-nitrogen (N2) emissions are still poorly characterized, both in terms of the processes involved and their magnitude, due to financial and methodological constraints. Relatively few studies have focused on quantifying N2 losses in vivo and fewer still have examined the relative contribution of the different N2 emission processes, particularly in grazed pastures. We used a combination of a high 15N isotopic enrichment of applied N with a high precision of determination of 15N isotopic enrichment by isotope-ratio mass spectrometry to measure N2 emissions in the field. We report that 55.8 g N m−2 (95%, CI 38 to 77 g m−2) was emitted as N2 by the process of co-denitrification in pastoral soils over 123 days following urine deposition (100 g N m−2), compared to only 1.1 g N m−2 (0.4 to 2.8 g m−2) from denitrification. This study provides strong evidence for co-denitrification as a major N2 production pathway, which has significant implications for understanding the N budgets of pastoral ecosystems.

Slurry 15NH4-N recovery in herbage and soil: effects of application method and timing.

The effects of slurry application method and weather conditions after application on ammonia volatilisation are well documented, however, the effect on slurry N recovery in herbage is less evident due to large variability of results. The objective of this field experiment was to determine the recovery of cattle slurry NH4-N in herbage and soil in the year of application as affected by application method (trailing shoe versus broadcast) and season of application (spring versus summer), using 15N as a tracer. In 2007 and 2008, 15N enriched slurry was applied on grassland plots. N recovery in herbage and soil during the year of application was determined. Both spring and trailing shoe application resulted in significantly higher herbage DM yields, N uptake and an increased recovery of 15NH4-N in herbage. Additionally, the recovery of slurry 15NH4-N in the soil at the end of the growing season was increased. Spring and trailing shoe application reduced the losses of slurry 15NH4-N by on average 14 and 18 percentage points, respectively, which corresponded closely to ammonia volatilisation as predicted by the ALFAM model. It was concluded that slurry N recovery in temperate pasture systems can be increased by adjusting the slurry application method or timing.

A review of nitrous oxide mitigation by farm nitrogen management in temperate grassland-based agriculture

Nitrous oxide (N2O) emission from grassland-based agriculture is an important source of atmospheric N2O. It is hence crucial to explore various solutions including farm nitrogen (N) management to mitigate N2O emissions without sacrificing farm profitability and food supply. This paper reviews major N management practices to lower N2O emission from grassland-based agriculture. Restricted grazing by reducing grazing time is an effective way to decrease N2O emissions from excreta patches. Balancing the protein-to-energy ratios in the diets of ruminants can also decrease N2O emissions from excreta patches. Among the managements of synthetic fertilizer N application, only adjusting fertilizer N rate and slow-released fertilizers are proven to be effective in lowering N2O emissions. Use of bedding materials may increase N2O emissions from animal houses. Manure storage as slurry, manipulating slurry pH to values lower than 6 and storage as solid manure under anaerobic conditions help to reduce N2O emissions during manure storage stage. For manure land application, N2O emissions can be mitigated by reducing manure N inputs to levels that satisfy grass needs. Use of nitrification inhibitors can substantially lower N2O emissions associated with applications of fertilizers and manures and from urine patches. N2O emissions from legume based grasslands are generally lower than fertilizer-based systems. In conclusion, effective measures should be taken at each step during N flow or combined options should be used in order to mitigate N2O emission at the farm level.

Evaluating the utility of 15N and 18O isotope abundance analyses to identify nitrate sources: A soil zone study

(15)N and (18)O isotope abundance analyses in nitrate (NO(3)(-)) (expressed as δ(15)N-NO(3)(-) and δ(18)O-NO(3)(-) values respectively) have often been used in research to help identify NO(3)(-) sources in rural groundwater. However, questions have been raised over the limitations as overlaps in δ values may occur between N source types early in the leaching process. The aim of this study was to evaluate the utility of using stable isotopes for nitrate source tracking through the determination of δ(15)N-NO(3)(-) and δ(18)O-NO(3)(-) in the unsaturated zone from varying N source types (artificial fertiliser, dairy wastewater and cow slurry) and rates with contrasting isotopic compositions. Despite NO(3)(-) concentrations being often elevated, soil-water nitrate poorly mirrored the (15)N content of applied N and therefore, δ(15)N-NO(3)(-) values were of limited assistance in clearly associating nitrate leaching with N inputs. Results suggest that the mineralisation and the nitrification of soil organic N, stimulated by previous and current intensive management, masked the cause of leaching from the isotopic prospective. δ(18)O-NO(3)(-) was of little use, as most values were close to or within the range expected for nitrification regardless of the treatment, which was attributed to the remineralisation of nitrate assimilated by bacteria (mineralisation-immobilisation turnover or MIT) or plants. Only in limited circumstances (low fertiliser application rate in tillage) could direct leaching of synthetic nitrate fertiliser be identified (δ(15)N-NO(3)(-)<0‰ and δ(18)O-NO(3)(-)>15‰). Nevertheless, some useful differences emerged between treatments. δ(15)N-NO(3)(-) values were lower where artificial fertiliser was applied compared with the unfertilised controls and organic waste treatments. Importantly, δ(15)N-NO(3)(-) and δ(18)O-NO(3)(-) variables were negatively correlated in the artificial fertiliser treatment (0.001≤p≤0.05, attributed to the varying proportion of fertiliser-derived and synthetic nitrate being leached) while positively correlated in the dairy wastewater plots (p≤0.01, attributed to limited denitrification). These results suggest that it may be possible to distinguish some nitrate sources if analysing correlations between δ variables from the unsaturated zone. In grassland, the above correlations were related to N input rates, which partly controlled nitrate concentrations in the artificial fertiliser plots (high inputs translated into higher NO(3)(-) concentrations with an increasing proportion of fertiliser-derived and synthetic nitrate) and denitrification in the dairy wastewater plots (high inputs corresponded to more denitrification). As a consequence, nitrate source identification in grassland was more efficient at higher input rates due to differences in δ values widening between treatments.