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THE POTENTIAL FOR SOIL PHOSPHORUS TESTS TO PREDICT PHOSPHORUS LOSSES IN OVERLAND
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FLOW
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4
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John N Quinton1*, Peter Strauss2, Nicola Miller3, Erol Azazoglu4,
Markku Yli-Halla5, Risto Uusitalo5
1Department
of Environmental Science, Institute of Environmental
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and Natural Sciences, Lancaster University, Lancaster,LA1 4YQ , United
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Kingdom (J.Quinton@Lancaster.ac.uk).
9
2
Institut für Kulturtechnik und Bodenwasserhaushalt, Bundesamt
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für Wasserwirtschaft, Pollnbergstraße 1, A-3252 Petzenkirchen, Austria
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(peter.strauss@baw.at).
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13
14
15
16
17
3
National Soil Resources Institute, Cranfield University, Silsoe,
Bedford MK45 4DT, United Kingdom
(n.miller.s00@cranfield.ac.uk).
Institute for Soil Science, University of Agricultural Sciences,
A-1180 Vienna, Austria (erol.azazoglu@baw.at).
5
MTT
Agrifood
Research
Finland,
31600
Jokioinen,
Finland
(markku.yli-halla@mtt.fi).
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Key words: Phosphorus, Risk assessment, Diffuse pollution, Soil P
tests, Overland flow, Sediment.
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
Paper given at the COST 832 Workshop on ‘ the development of a
risk assessment methodology for predicting phosphorus losses at the
field scale.’ 16 –19 October 2002, Zurich, Switzerland.
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ABSTRACT
Soil phosphorus tests offer a potentially powerful tool for land
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managers trying to predict the areas which will contribute diffuse
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losses of phosphorus (P) to surface water bodies through the overland
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flow vector – but do they work? We address this question at a range of
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scales, from patch (<1 m2), through plot (several m2) to small
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watershed (several hectares). Our hypothesis is that as we increase
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the scale, and therefore the complexity of the system, soil P tests
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will predict P concentrations and losses associated with overland flow
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less well, and that this is partly due to a shift from dissolved P
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losses to P losses associated with eroded soil material.
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At the patch scale soil P tests were used to predict the P
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concentration and load from 24 European soils exposed to simulated
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rainfall under controlled conditions in the laboratory. Results showed
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that soil P tests were generally good predictors of reactive P <0.45
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m, but did less well at predicting total P >0.45 m. By combining the
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soil P test with measured sediment concentrations predictions of total
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P concentrations improved. Outdoor rainfall simulation experiments on
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bare soil plots (10 m2) revealed the overwhelming influence of particle
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bound P losses compared with P losses in the water phase. Soil P
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tests, which relate primarily to the dissolved P fractions in soil,
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were not able to predict total P losses, but were related to reactive
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P < 0.45 µm losses. At the watershed scale soil P tests were able to
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predict reactive P <0.45 µm losses, but
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uncertainty.
with considerable
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We conclude that soil P tests, in combination with sediment
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concentration provide a useful means of assessing the mobilisation of
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P in overland flow, but should not be expected to provide watershed
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scale predictions of the movement of P into overland flow.
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INTRODUCTION
As the problem of phosphorus pollution of surface waters is
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increasingly recognised, research efforts are focussing on
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understanding the mechanisms of P transfer and trying to predict it.
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Phosphorus can move to surface waters by surface and subsurface
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pathways and in a variety of forms. In this paper we concentrate on
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the surface pathway.
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Phosphorus transfer from soil surfaces to watercourses relies on
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the connection of a source of phosphorus with a surface water body.
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Phosphorus can move from a source either in solution or attached to
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suspended sediment particles. The phosphorus may be of inorganic or
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organic nature. The dissolved fraction is commonly determined as P
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<0.45m (Haygarth and Sharpley 2000) and described as molybdate or
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non-molybdate reactive dissolved P. Particulate P is operationally
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defined as P >0.45m and is again split into reactive and non-reactive
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forms. As dissolved and particulate forms of P can cause
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eutrophication of freshwater ecosystems we need to be able to predict
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the transport of both. However, when soils have been tilled, by far
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the greater mass of P tends to be exported as particulate P (Quinton
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et al., 2001).
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We hypothesise that soil P tests are useful predictors of the
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sources of P: areas with high soil P test values representing
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potential problem areas. Indeed several authors have found good
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correlations between soil P test results and dissolved losses of P,
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see for example Sharpley (1995), Pote et al (1996) and Sims et al.,
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(2000). However, predicting particulate P transfer in overland flow
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requires us to also predict how much particulate associated P will be
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detached and transported. This requires us to know something of where
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and how much overland flow is generated in a landscape, and how much
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and which fraction of the soil it will detach and transport. The
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generation of overland flow is dependent on soil, landscape and
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vegetation properties; soil detachment is related to the energy of the
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rainfall or flow, and the dispersability of the soil. For the P to be
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transferred to surface water bodies a connection must be made between
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the overland flow and the surface water. This connectivity will depend
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upon a range of soil, topographic and rainfall factors. None of these
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factors controlling overland flow, soil detachment or connectivity is
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directly related to soil phosphorus and we would therefore not expect
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soil P tests to predict particulate P transfer with any confidence.
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However, as demonstrated by a number of authors (Hooda et al, 2000;
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Pote et al 1996; Sharpley 1995) soil P tests are related to the
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solubilization of P. We would therefore expect that there would be
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some relationship between the soil P tests and dissolved phosphorus
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moving in overland flow. We would expect this relationship to be
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demonstrated most clearly at the point of detachment, before the
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complexity of the water running over different sources and receiving
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inputs from different parts of the catchment is added, and less well
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at the mouth of the watershed, where spatially dispersed sources have
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been integrated and the signal becomes blurred. This paper uses data
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from a range of scales, from patch (<1m2), through plot (several m2) to
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small watershed (several hectares) to test these ideas. Our hypothesis
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is that as we increase the scale, and therefore the complexity of the
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system, soil P tests will predict P concentrations and losses
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associated with overland flow less well, and that this is partly due
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to a shift from dissolved P losses to P losses associated with eroded
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soil material.
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METHODOLOGY
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Site and soil characteristics
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Soils for the patch scale experiments were taken from 24
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agricultural soils from six EU countries. Some analytical information
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is provided in Table 1. The soils had Olsen-P values from 8 to 96 mg
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kg-1 and water soluble-P of 0.26 to 65 mg kg-1. To limit other sources
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of variability the same pre-treatment was used for all soil samples.
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Soil samples from all sites were taken from the surface 50 mm of the
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soil, air-dried and sieved to
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metal trays measuring 250 x 500 mm with a depth of 90 mm. Soil was
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packed to a density of 1.3 g cm-3.
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capillary rise and drained for a further 24 hours before rainfall was
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applied.
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<5 mm. The sieved soil was packed into
Soil was pre-wetted for 24 hours by
The plot scale study was conducted at selected sites also used for
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the patch scale experiment, namely at Nagyhorvati, Somogybabod, Tetto
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Frati and Riva. At each site a seedbed was prepared about 5 days
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before the rainfall simulation experiments started.
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The watershed scale work was carried out in the catchments of 10
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lakes in an area covering about 10,000 km2 in the Kokemäenjoki river
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basin in southern Finland. Agriculture in this area consists of cereal
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production and dairy farming. Soil texture, organic matter, pH, and
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nutrient status vary widely. The soils are formed on glacial tills or
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clayey sediments of the ancient Baltic Sea.
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sediment samples were taken on the assumptions that they represented
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sediment from: forested areas; fields used for cereal production; cow-
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sheds, stables, or piggeries - denoted as animal farm ditches;
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pastures, which may receive wastewaters from milking centres; and
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wastewater-affected ditches, clearly affected by septic tanks, and
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milking centre wastewater.
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Soil P tests
One hundred and two ditch
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For the patch and plot scale experiments the amount of P which is
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deemed potentially plant available was determined using three
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different extraction methods which are commonly used in different
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European countries. Olsen P was determined after 30 min of extraction
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with sodium bicarbonate and filtering through a paper filter (Olsen et
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al., 1954). Water soluble P (WSP) was determined
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soil with 60 ml of water for one hour in an orbital shaker and
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allowing the to stand for 23 hours. The suspension was then reshaken
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for another ten minutes and filtered through a 0.02 mm filter before
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being analysed for P colormetrically. Total phosphorus (TP) was
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determined by sodium hydroxide fusion (Smith and Bain, 1982). Soil and
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sodium hydroxide pellets are placed into a nickel crucible and heated
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to 350ºC for one hour.
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water and covered for two hours.
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the TP concentration of the filtrate is determined
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spectrophotometrically using the molybdate ascorbic method.
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by shaking 1 g of
Once cool the crucible is placed in deionised
The suspension is then filtered and
In the Finnish experiments, P was extracted from 20 mineral soil
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samples with an acetate solution and deionised water (Pw, soil-to-
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water 1:100, 18 h). The acid acetate extractant (AAAc; 0.5 M ammonium
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acetate, 0.5 M acetic acid, pH 4.65; Vuorinen and Mäkitie, 1955) is
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used to extract P and macronutrient cations in Finland. The extraction
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time was one hour and the volumetric soil-to-solution ratio wass 1:10.
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This extractant
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solution 1:60) from the predominantly acidic Finnish soils (Yli-Halla,
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1990).
dissolves similar amounts of P to water (soil-to-
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Table 1. Analysis of soils used in the patch and plot scale
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studies. WSP – Water soluble phosphorus; NaOH-P – sodium hydroxide
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extractable P; Aqua Regia -
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– Olsen P.
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Total P after aqua regia digestion; Olsen
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Rainfall simulation
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At the patch scale rainfall was simulated using a nine-metre
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indoor gravity fed rainfall simulator.
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deionised water being fed through a tray of hypodermic needles at 9
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metres above the soil surface. A splash screen at 6 m decreases the
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uniformity of the drops producing a variety of drop sizes which better
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imitates natural rainfall.
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1
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chosen, after trial runs, as an intensity that would generate runoff
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from all soil samples. The soil tray was placed beneath the rainfall
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on a 8.7 % slope and subjected to 30 minutes of rainfall. Overland
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flow was collected separately at 0-10, 10-20 and 20-30 minutes.
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the simulation was complete the soil tray was then left to stand for
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five days and the simulation was repeated. Experiments were replicated
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three times.
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The rainfall is formed by
Rainfall was applied at a rate of 60 mm hr-
with a mean drop diameter of 1.0 mm for 30 minutes. 60 mm hr-1 was
After
Plot scale experiments were carried out on plots measuring 2 m x 5
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m using a rainfall simulator, described in Strauss et al. (2000), with
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a median volumetric drop size of 2.0 mm and a Christiansen uniformity
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coefficient of about 90%. Slopes at the different sites were 7.5 % at
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Nagyhorvati, 13 % at Somogybabod and 0.5 % at Tetto Frati and Riva
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respectively. Prior to each rainfall simulation 30 mm of rain was
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applied to the plots, which had been covered with a permeable mesh to
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prevent destruction of the soil surface, to obtain equal initial water
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contents for the plots. Consecutive rainfall simulations were carried
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out at day 1 (rain 1), day 5 (rain 2), and day 10 (rain 3). Reported
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results are mean values of four independent replicates and the three
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different rains applied. To aid comparison to the patch scale
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experiments the same rainfall intensity of 60 mm h-1 was used.
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Experiments continued until constant runoff rates were achieved.
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Sediment analysis
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For the patch and plot scale experiments volume and sediment
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concentration of overland flow were determined before analysis for P
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content (House et al., pers. com.). Reactive P (<0.45) was determined
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on undigested samples and phosphorus concentration in all were
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measured colourimetrically using the molybdenum blue method (Murphy
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and Riley 1962) on a UV/VIS Spectrometer. Total P and TP (<0.45) were
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determined for the patch samples by sulphuric acid-persulphate digest
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adapted from (Eiseneich et al, 1975) and for the plot scale
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experiments using aqua regia digests (12N HCl and 15N HNO3 mixed 3:1).
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Analysis was carried out within 24 hours of sampling. Ditch sediment
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samples, from the watershed study, were analysed for AAAc-P using the
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method of Jansson et al. (2000).
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RESULTS
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Patch scale
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The total overland flow produced from each of the 24 soils ranged
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from a low of 940 ml to a high of 3290 ml, with a mean of 2481 ml.
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This variability was related to differences in infiltration capacity.
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For all soils overland flow volume increased during the rainfall
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event, with some soils reaching a steady state of runoff.
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The general trend was for TP concentration to decrease through
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time, probably due to the dilution effect of the increasing volume of
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flow. However, TP load did not vary significantly through time due to
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the greater carrying capacity of the increased flow.
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of soils TP (<0.45) and RP (<0.45) concentration did not vary
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significantly through time.
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For the majority
The mean concentration of TP for the entire event for each soil
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ranged from a low of 0.41 mg l-1 to a high of 5.46 mg l-1, with a mean
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of 2.04 mg l-1.
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high of 0.65 mg l-1 to a low of 0.01 mg l-1, with a mean of 0.14 mg l-1
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RP (<0.45) contributed between 44-100% of TP (<0.45). Total P (>0.45)
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contributed by far the greatest proportion of P to overland flow with
The mean concentration of RP (<0.45) ranged from a
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between 65-98% of the total.
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particulate bound P transport by overland flow and highlights the need
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to predict the potential for particulate P transport by this pathway.
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This indicates the significance of
Table 2 indicates that the usefulness of soil P tests as
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predictors of P in overland flow depends on the extraction method and
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on the form of P being predicted.
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correlated with the <0.45 m fractions. The WSP gives significant
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(P<0.05) correlations with both the loads and concentrations of all
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fractions, while Olsen-P does not correlate with the total P load
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results. Correlations are highest in all cases for the WSP soil test
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and lowest in all cases for the TP extracted by sodium hydroxide
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fusion.
All soil P tests are better
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Table 2. Correlation coefficients (r ) for soil P tests correlated
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to TP, TP (<0.45) and RP (<0.45) load and concentration in overland
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flow from the patch scale experiments. * indicates significant at
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P<0.05.
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Plot scale
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Mean runoff rates at constant flow conditions varied between 0.4 l
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m-2 min-1 (Nagyhorvati) and 0.8 l m-2 min-1 (Tetto Frati). Erosion rates
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at constant runoff varied between 5.2 g m-2 min-1 (Riva and Tetto Frati)
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and 70.3 g m-2 min-1 (Somogybabod). No total values are given because
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the experiments were carried out until steady state conditions had
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been achieved, and therefore durations and consequently total runoff
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and erosion were different at each site. Table 3 summarises mean
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concentrations and loads for TP (<0.45 µm) and TP under steady state
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conditions. For all sites concentrations and loads of RP<0.45µm were
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negligible compared to TP. This emphasises the importance of soil
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erosion for P transport at this scale. The amount of eroded soil was
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significantly (P<0.01 for Nagyhorvati, Riva and Tetto Frati, and
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P<0.05 for Somogybabod) correlated to the amount of surface runoff at
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each site.
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Table 3: Mean total P (TP) and total P < 0.45µm (RP<0.45)
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concentrations and loads for three consecutive rains and different
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sites.
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Watershed
scale
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Uusitalo and Jansson (2002) investigated the relationship between
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AAAc-extractable soil P and MRP in runoff at the field scale, in the
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3.3 km2 catchment of Lake Rehtijärvi in Jokioinen, Southern Finland.
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They found that the AAAc test results were related to MRP
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concentrations in runoff (Figure 1). High MRP in runoff was always
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from fields with a high value of soil test P. However, soils with a
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high soil test P periodically produced runoff low in MRP. The soil P
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test did not perform well in predicting P in turbid samples, since the
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concentration of particulate P was highly dependent on the
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concentration of suspended solids (Uusitalo et al., 2000).
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Figure 1. Soil P extracted by ammonium acetate buffer (AAAc-P)
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versus concentration of dissolved molybdate-reactive P (MRP) in
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overland flow (n = 18) in Lake Rehtijärvi catchment, southern Finland.
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Flow-weighted MRP concentrations are averaged over the three study
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years, and the error bars represent SE of the mean. From Uusitalo and
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Jansson (2002), with the kind permission of Agriculture and Food
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Science in Finland.
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We have less evidence to relate soil P test results to the
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concentration of P in the ditch sediments. The average P
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concentrations of the four source area classes clearly differed from
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each other (Table 4), and were typically about 4 mg l-1 in the ditches
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carrying sediment and water from forested areas, and about 8 mg l-1 in
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the ditches connected to fields. According to Jansson et al. (2000),
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ditch sediment AAAc-P concentrations of less than 4 mg l-1 typically
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correspond to MRP concentration of less than 40 g l-1. The AAAc-P
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values greater than 12.6 mg l-1, in our survey found were all in
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ditches connected to animal farm and waste water sources (Table 4),
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which
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concentrations greater than 80 g l-1. Anecdotal evidence from the
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watershed suggests that fields with higher soil P test results
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contribute to higher sediment P concentrations. However, in some
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catchments a considerable part of P load originates from point
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sources, like confined animal operations and paddocks. In these cases,
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soil test P of the fields are likely to underestimate the P load to
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the watercourse.
according to Jansson et al., (2000) correspond to MRP
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Table 4. Ditch sediment P (mg P l-1 sediment) extractable by
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ammonium acetate buffer; the material is classified according to
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source areas.
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DISCUSSION
Our results demonstrate the problems of using soil-P tests to
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predict P transport in overland flow. At the patch scale significant
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correlations were found between WSP and total P (both less than and
315
greater than 0.45m)
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found with the less than 0.45 m fractions. Since RP makes up the
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greatest proportion of the <0.45m fraction (a mean of 88% for the
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samples used) the WSP P test appears to be a useful tool for assessing
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the potential for mobilisation of dissolved P.
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well at predicting total P concentrations. This is not surprising
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since it would be expected that total P concentrations will be the
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product of
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sediment in the runoff. In fact if the residuals of a linear
and RP. However, the best relationships are
The test does less
P concentrations in the sediment and the concentration of
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regression of total P against WSP are plotted against sediment
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concentration (Figure 3) a significant (r2 = 0.85; p<0.05) agreement is
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found. Using a linear model that includes sediment concentration and
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WSP 81% of the variance in total P concentrations can be explained
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(p<0.05).
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Figure 2. Sediment concentration potted against the residuals of a
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best fit regression line of WSP against total P concentration in the
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overland flow of all samples. The regression line illustrated (y=
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0.888 + 0.262x) explains 85 % of the variance and is significant at
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the 0.05% level.
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If we relate concentrations of RP<0.45 µm to results of soil P
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tests (Olsen P, water soluble P) at the plot scale, we recognise a
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general trend of increasing concentrations with increasing soil P test
339
values (Figure 3) indicating the potential of soil P tests in
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predicting soluble P fractions in surface runoff. At sites with a low
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risk of erosion, such as grassland dominated areas, dissolved P
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fractions may constitute a major part of P losses in surface runoff
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(Schønning et al., 1995; Ulen, 1997) and
344
valuable tool for predicting P mobilisation (Table 2). Soil P tests do
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less well at predicting total losses or total loads when particle
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bound P transport is the dominant process, such as in the plot scale
347
study (see Table 3).
soil P tests may be a
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349
Figure 3. Relationship between soil P test results (represented by
350
Olsen P and water soluble P) and RP<0.45µm concentrations in surface
351
runoff for the studied sites
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353
At the watershed scale the variability present in the predictions
354
of MRP from the AAAc-P soil test results (Figure 2) could be the
355
result of several phenomena. Depths of water on the soil surface will
356
be variable, making the ratios of water to the soil variable.
357
Watersheds will also encompass areas with differing amounts of soil
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phosphorus and areas contributing different volumes of water. The
359
mixing of sediment and water from different sources makes it difficult
360
to use a soil P test to make predictions at this scale with any degree
361
of precision. However, Figure 2 does suggest that soil P tests can be
362
a useful risk assessment tool for identifying the potential release of
363
dissolved phosphorus to overland flow of different soils within a
364
catchment.
365
366
CONCLUSION
Overall our results show that soil P tests are capable of
367
predicting RP losses at a range of scales, albeit with increasing
368
uncertainty,
369
patch scale. This should not surprise us and conforms with our stated
370
hypothesis. As we increase scale, the system we are dealing with
371
becomes more complex and the processes which are controlling the
372
amount of P in surface waters change and shift in their relative
373
dominance. Soil P status is one key component required to predict the
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phosphorus content of overland flow, but without knowledge of the
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factors governing, overland flow generation, sediment production and
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connectivity to surface waters it remains one piece of a complex
377
jigsaw.
and fail to predict total P concetrations at all but the
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ACKNOWLEDGEMENTS
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The authors would like to acknowledge the assistance of Elissabeta
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Barbaris, Istvan Sisak and Helina Hartikainen
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the DESPRAL project (EVK1-CT-1999-00007) for the patch and plot cale
382
studies.
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44, 165-177.
432
Uusitalo, R. and H. Jansson. (2002): Phosphorus concentration in
433
runoff assessed by soil extraction using an acetate buffer.
434
Agricultural and Food Science in Finland 11, 343-353.
435
Uusitalo, R.,Yli-Halla, M.& Turtola, E. (2000): Suspended soil as
436
a source of potentially bioavailable phosphorus in surface runoff
437
waters from claysoils. Water Res. 34, 2477-2482.
438
439
440
Vuorinen, J. and O. Mäkitie. (1955): The method of soil testing in
use in Finland. Agrogeological Publications 63, 1-44.
Yli-Halla, M. (1990): Comparison of a bioassay and three chemical
441
methods for determination of plant-available P in cultivated soils of
442
Finland. Journal of Agricultural Science in Finland 62, 213-219.
443
444
445
TABLES
Table 1. Analysis of soils used in the patch and plot scale
446
studies. WSP – Water soluble phosphorus; NaOH-P – sodium hydroxide
447
extractable P; Aqua Regia -
448
– Olsen P.
Country
Site
United
Kingdom
United
Kingdom
United
Kingdom
United
Kingdom
United
Kingdom
United
Kingdom
Italy
Italy
Italy
Italy
Italy
Italy
Finland
Finland
Austria
Austria
Austria
Austria
Austria
Austria
Hungary
Hungary
Hungary
Hungary
449
450
Total P after aqua regia digestion; Olsen
%
%
WSP
sand silt
17
58
3.79
NaOH-P Aqua
Regia
487
800
OLSEN
Bridgets
%
clay
25
Boxworth
39
27
34
2.75
585
524
6.27
Rosemaund
25
9
66
12.76 652
435
12.67
Pwllpeiran
25
35
40
2.46
708
6.60
Gleadthorpe
10
65
25
21.42 830
645
26.60
Olcenengo
18
27
55
0.26
595
601
20.00
Villafranca
Piemonte
Mantova
Bonavia
Tetto Frati
Sale
Riva
Jokioinen
Turka
Gross-ens Min
Gross-ens Org
Rit-4
Rit-5
Rottenhaus 5
Rottenhaus 6
Keszthely
Nagyhorvati
Somogybabod
Szentgyorgyvolgy
13
41
46
44.7
1288
1122
69.13
33
24
12
37
20
61
48
20
22
23
23
39
33
22
20
17
21
14
24
27
10
11
20
11
36
30
17
16
4
7
38
30
27
8
53
52
61
53
69
19
41
44
48
60
61
57
60
40
50
56
71
20.46
10.3
20.14
38.41
31.92
13.09
28.59
13.44
19.32
30.71
64.74
6.45
2.95
1.08
5.64
0.99
2.73
998
503
940
703
882
1343
1903
640
603
953
1425
1033
917
367
525
775
712
1315
749
897
617
684
1284
1581
732
736
1062
1249
823
876
363
500
537
669
48.13
23.93
31.20
44.60
40.93
41.00
62.47
22.27
24.07
57.13
95.47
13.27
40.73
8.00
14.20
0.47
6.67
1365
9.73
451
Table 2. Correlation coefficients (r ) for soil P tests correlated
452
to TP, TP (<0.45) and RP (<0.45) load and concentration in overland
453
flow from the patch scale experiments. * indicates significant at
454
P<0.05.
Load (mg)
TP
TP (<0.45)
RP (<0.45)
Concentration (mg l-1)
TP
TP (<0.45)
RP (<0.45)
455
456
WSP
Olsen-P
mg l-1
mg l-1
TP
(NaoH)
mg l-1
0.36*
0.87*
0.90*
0.25
0.76*
0.80*
0.01
0.30*
0.37*
0.53*
0.89*
0.91*
0.45*
0.83*
0.85*
0.17
0.47*
0.49*
457
458
Table 3: Mean total P (TP) and total P < 0.45µm (RP<0.45)
concentrations and loads for 3 consecutive rains and different sites.
Site
Riva
Tetto Frati
Somogybabod
Nagyhorvati
459
460
Concentrations
TP
RP<0.45
mg l-1
mg l-1
8.8
0.23
7.6
0.12
79.0
0.01
10.8
0.14
Loads
TP
mg min-1 m-2
7.0
5.9
52.2
4.3
RP<0.45
mg min-1 m-2
0.18
0.09
0.01
0.06
461
Table 4. Ditch sediment P (mg P l-1 sediment) extractable by
462
ammonium acetate buffer; the material is classified according to
463
source areas.
n
Mean
Min
Max
464
465
466
Forested
Field
22
3.9
1.7
5.8
51
7.2
2.1
12.6
Animal
farm
14
15.7
10.9
24.1
Wastewater
15
33.9
10.0
107
467
FIGURES
468
MRP, mg l
-1
1
0.1
0.01
1
10
100
AAAc-P, mg l -1 soil
469
470
Figure 1. Soil P extracted by ammonium acetate buffer (AAAc-P)
471
versus concentration of dissolved molybdate-reactive P (MRP) in
472
overland flow (n = 18) in Lake Rehtijärvi catchment, southern Finland.
473
Flow-weighted MRP concentrations are averaged over the three study
474
years, and the error bars represent SE of the mean. From Uusitalo and
475
Jansson (2002), with the kind permission of Agriculture and Food
476
Science in Finland.
Residual of total P predicited by WSP
4.5
3.5
2.5
1.5
0.5
-0.5
-1.5
-2.5
0
4
8
12
16
20
Sediment concentration
477
Figure 2. Sediment concentration potted against the residuals of a
478
best fit regression line of WSP against total P concentration in the
479
overland flow of all samples. The regression line illustrated ( y=
480
0.888 + 0.262x)
481
the 0.05% level.
482
explains 85 % of the variance and is significant at
483
35
50
45
Olsen P
WSP
30
-1
Olsen P (mg kg )
30
20
25
15
20
15
10
-1
25
35
Water soluble P (mg kg )
40
10
5
5
0
0
0.05
0.15
0.2
0
0.25
RP<0.45µm (mg l-1)
484
485
0.1
Figure 3. Relationship between soil P test results (represented by
486
Olsen P and water soluble P) and RP<0.45µm concentrations in surface
487
runoff for the studied sites.