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1. Background and Objectives The flux of nutrients to water bodies is increasing world wide as a function of climatic warming, the intensification of agriculture, car use and fossil fuel combustion in both developing and developed countries. Such nutrient enrichment (particularly carbon and nitrogen) is leading to substantial environmental degradation including: changes in species composition within freshwater and marine plant and animal communities, excessive growth of filamentous/ blue-green alga and the formation of carcinogenic N-nitrosodimethylamine and trihalomethane as disinfection byproducts during drinking water treatment. Current research and policy development surrounding catchment scale macronutrient balances has focussed mainly on inorganic nutrient concentration data. Much of our current understanding of biogeochemical cycling has been gained through research conducted within developed countries whose river systems have suffered high anthropogenic alteration such as land use change and river channel engineering (Caraco and Cole, 1999; Vega et al., 1998). As a result it has long since been assumed that organic nutrient fractions are relatively recalcitrant when compared to a dominance of labile inorganic fractions.
2. Materials and Methods Two sub catchments within the Hampshire Avon basin (southern England) were selected for study due to their wide variety of both diffuse and point source nutrient inputs (River Wylye and Millersford Brook). Daily samples were collected from six stations located across both study catchments and were analysed for: Inorganic and N and P fraction determination using segmented flow colourimetry along with total, particulate and dissolved fraction determination using persulphate oxidation and non purgeable organic carbon concentration.
Weekly samples were also collected from these sites plus 12 spatially important locations to allow analysis of samples for determinands which are unstable over periods of more than 24 hours from collection. This included the determination of soluble reactive phosphorus Analysis was also conducted using novel optical characterisation techniques such as excitation emission fluorescence spectroscopy, which allow identification of the dominant groups which constitute the dissolved organic component, and distinction between the more labile low molecular weight material (amino acids, proteins) which are directly available for plant and algal uptake, and the higher molecular weight components (for example, human and fulvic acids) which are less labile, though an important substrate for microbial decomposition, and lend much to the generation of colour in natural and potable waters.
Nitrogen Workshop 2012
3. Results and discussion Current results demonstrate further the relative importance of organic nutrient fractions finding that in areas not under anthropogenic influence organic nitrogen dominates the nutrient pool. In anthropogenically altered systems nitrate was found to be the dominant form of nitrogen present (Figure 1). Organic macronutrient fractions were found to be elevated with concentrations varying both over time and as a function of their source areas. Results demonstrate source to be a key factor in determining organic matter composition and potential reactivity with features such as; sewage treatment works, poor agricultural practises and an abundance of septic tanks found to be locally important macronutrient source areas.
Within the chalk dominated Wylye catchment organic nitrogen concentrations were found to be elevated in relation to non purgeable organic carbon concentrations, generating a very low C:N ratio, suggesting compounds such as Urea to be important in optically clear low organic carbon systems dominated by anthropogenic inputs. This contrasts markedly with the findings for the Millersford Brook, which drains a peatland catchment in the New Forest, where higher molecular weight compounds dominated the dissolved organic fraction, contributing to high water colour in the stream, and the carbon to nitrogen ratio approximates to rates reported for other peatland systems in the UK and internationally.
Figure 1. Nitrogen species concentration in the River Wylye
4. Conclusions The dissolved organic fraction of macronutrient flux varies widely in character, depending on the distribution of contributing source areas in the catchment. In low intensity agricultural catchments dominated by peaty soils, high C:N ratios and a high proportion of high molecular weight components dominate the DOM signal, and DON and DOP are the dominant fractions of the TN and TP load, respectively. In more intensive agricultural systems, with septic tanks serving the rural population, and with predominantly mineral soils in the catchment, DOM contributes a smaller proportion of the TN and TP load, and is dominated by labile, low molecular weight compounds which contribute little to water colour, and generate a relatively low C:N ratio. These findings have implications, both for the ecosystem functional role of DOM in contrasting system types, and for the potential for carcinogenic carbonaceous and nitrogenous disinfection byproduct formation in potable waters abstracted from different water supply intake waters.
References Caraco, N.F. and Cole, J.J. 1999. Human impact on nitrate export: An analysis using major world rivers. Ambio 28, 167-170.
Durand, P., Breur, L, Johnes, P.J. et al.2011. Nitrogen Cycling in Aquatic Ecosystems. Ch. 7, European N Assessment, Cambridge University Press.
Perakis, S.S. and Hedin, L.O. 2002. Nitrogen loss from unpolluted South American forests mainly via dissolved organic compounds (vol 415, pg 416, 2002). Nature 418, 665-665.
Vega, M., Pardo, R., Barrado, E. and Deban, L. 1998. Assessment of seasonal and polluting effects on the quality of river water by exploratory data analysis. Water Research 32, 3581-3592.
Nitrogen Workshop 2012
Investigating the efficacy of soil nitrogen tests to predict soil nitrogen supply across a range of Irish soil types under controlled environmental conditions.
McDonald, N.T.a,b, Watson, C.J.cb, Laughlin, R.J.c, Lalor, S.T.J.a, Hoekstra, N.Ja, Wall, D.P.a a Teagasc, Crops Environment and Land Use Programme, Johnstown Castle,Wexford, Rep. of Ireland. b School of Biological Sciences, Queen’s University Belfast, Belfast, N. Ireland c Agri-Food and Biosciences Institute, Newforge Lane, Belfast, N. Ireland
1. Background & Objectives Nitrogen (N) fertiliser usage on Irish farms is constrained under the European Union (EU) Nitrates Directive (S.I.610, 2010) which is part of the larger EU Water Framework Directive aimed at improving water quality. These constraints and increasing fertiliser prices at farm level coupled with concerns over food security and climate change at international level have placed N use efficiency high up the agri-environmental agenda. In order to maximise the recovery and yield potential derived from N fertiliser, these inputs must be balanced with mineralised N (No) from soil reserves. The potential No may vary considerably between different soil types, and in Ireland No recovery over a range of grassland soils was shown to range from 74 to 212 kg N ha-1 yr-1 (Humphreys, 2007). However, the variability in soil N supply between soils is not reflected in current N recommendations in Ireland and in many fields N fertilisers are either under- or oversupplied compared to requirements for crop growth. The objective of this study was to evaluate soil N tests for predicting soil N supply, grass DM yield and grass N uptake for a range of Irish soil types. This research aims to develop a soil N testing system for Ireland, as a basis for new soil specific N fertiliser advice to help farmers achieve grass production targets while conserving N fertiliser resources, and minimising N losses to the environment.
2. Materials & Methods A soil microcosm experiment was established to compare grass growth across 28 different soil types. Soils were collected throughout Ireland to a depth of 10cm, potted in 11.3 L pots, optimised with key macronutrients (i.e. P, K & S) and seeded with ryegrass (Lolium perenne L.). No N fertiliser was added to these soils over the duration of the experiment. Four replications of each soil type were placed into a controlled environment facility in randomised blocks (where shelf position was the blocking factor). The temperature was fixed at 15oC, relative humidity at 80%, soil water maintained at 65% field capacity, and day-length at 16 hours light per day. Four grass harvests from each pot were taken at five week growth intervals and the grass DM yield, N content and N uptake were determined. The soils were also sampled to a depth of 10cm at each harvest time and analysed within 24 hours for mineral N (Total Oxidized N (TON= NO3-N + NO2-N) & NH4+-N) using 2M KCl extraction. The remainder of the soil (approx 40g) was dried at 40oC and sieved to 2mm. The No potential was analysed using a standard seven-day anaerobic incubation method (AI-7) (Waring and Bremner, 1964) and the Illinois soil N test (ISNT) (Khan et al., 2001). Soils were analysed for a range of physical, chemical and biological properties, e.g. Total C and N, texture, pH and soil organic matter levels. Regression and stepwise regression analysis was performed on these data in SAS. JMP. version 9, to model the relationships between AI-7 and ISNT, and between mineral N, ISNT and grass DM yield and grass N uptake across the 28 soil types.
3. Results & Discussion There was a large range in No potential (73 to 396 mg NH4+-N kg-1) over the 28 different soil types in this study as measured by AI-7. This shows that some soil types have the capacity to supply more of the grass N requirement than others. Therefore less N fertiliser may be required to reach
Nitrogen Workshop 2012
optimum grass growth on these soils. Illinois soil N test (ISNT) values were correlated with AI-7 values across all soils (r2 = 0.68). This result supports previous laboratory studies where six rapid chemical N tests were compared to the time consuming AI-7 method across 35 similar soil types.
Here the ISNT was found to be the best rapid soil test for predicting of No (McDonald et al., 2011).
However, ISNT does not measure TON and by itself was a poor predictor of grass yield across all 28 soil types (Figure 1). Thirteen out of 28 soils had high residual TON levels (6 mg kg-1) after harvest (Figure 2). These soils produced higher grass DM yields and grass N uptake compared to the soils with low residual N levels. Total Oxidised N was a poor predictor of grass yield for sites with low residual N reserves, as these sites relied on the supply of N through No. However, when both ISNT and TON were combined to predict grass DM yield and grass N uptake across these soil types, the r2 for these prediction models were 0.78 and 0.87 respectively.
Figure 1. Illinois soil N test (ISNT) versus grass DM yield Figure 2.
Residual Total Oxidised N (TON) versus grass for 28 soil types. DM yield for 28 soil types.
4. Conclusion Irish soil types have the capacity to supply high levels of N through No. This shows that there is scope to reduce N fertiliser application rates on some soils without compromising grass DM yields.
The ISNT was correlated with AI-7 and is suitable for routine soil analysis to predict No. Where high levels of residual TON was present in some soils, this contributed to higher grass DM yield and grass N uptake above what would have been achieved from No alone. When combined in a model, ISNT and TON were able to predict both grass DM yield and grass N uptake with a high degree of accuracy. This work shows the potential to better manage N fertiliser inputs based on soil N supply potential in order to increase N use efficiency.
References Humphreys, J. 2007. Cutting back on fertilizer in 2007. In: The Fertilizer Association of Ireland 2007-2008, pp. 2-35. Khan, S.A., Mulvaney, R.L. and Hoeft, R.G. 2001. A simple soil test for detecting sites that are nonresponsive to nitrogen fertilization. Soil Science Society of America Journal 65, 1751-1760.
McDonald, N.T., Watson, C.J., Laughlin, R.J. Lalor, S.T.J., Hoekstra, N.J. and Wall, D.P. 2011. Accounting for the potential supply of nitrogen from Irish grassland soils. The British Grassland Society 10th Research Conference, Hillsborough, Belfast.
Waring, S.A. and Bremner, J.M. 1964. Ammonium production in soil under waterlogged conditions as an index of nitrogen availability. Nature (London) 201, 951.
A new approach for measuring ammonia volatilization in the field: First results of the French research project “VOLAT’NH3” Cohan J.P.a, Charpiot A.b, Morvan T.c, Eveillard P.d, Trochard R.a, Champolivier L.e, De Chezelles E.f, Espagnol S.g, Génermont S. h, Loubet B.h a ARVALIS-Institut du végétal, Station expérimentale de La Jaillière, 44370 La Chapelle St Sauveur, France b Institut de l’élevage, Monvoisin- BP 85225, 35652 Le Rheu Cedex, France c INRA UMR SAS, 4 rue de Stang Vihan, 29 000 Quimper, France d UNIFA, Le diamant A, 92909Paris La Défense, France e CETIOM, BP 52627, 31326 Castanet Tolosan Cedex, France f ACTA, 149 rue de Bercy, 75595 PARIS Cedex 12, France g IFIP, La Motte au Vicomte, BP 35104, 35651 Le Rheu Cedex, France h AgroParisTech, UMR 1091 EGC, F-78850 Thiverval-Grignon, France