View of Release of soil phosphorus during runoff as affected by ionic strength and temperature

Release of soil phosphorus during runoff as affected by ^ionic strength ^and temperature

Markku Yli-Halla1 and Helinä Hartikainen

DepartmentofApplied ChemistryandMicrobiology, P.O. Box27,FIN-00014 Universityof^{Helsinki,Finland}

Dissolved reactivephosphorus (DRP) from two cultivatedclaysoil samples (Vertic Cambisols)was extracted under conditions simulatingthe variationintheproperties of surfacerunoff^waterinthe

field.^DRPwasextracted^at three temperatures (5, 15and25°C), and at different ionic strengths by usingdeionized water and CaCl₂ solutions (0.00005-0.005 M) asextractants.The solution-to-soil

ratiovaried^from50to2000 1 kg

1

^{.Sorptiontoanddesorption}from the soils werestudied at different temperaturesand ionicstrengths by determining quantity-intensity (Q/I) plotsatthe solution-to-soil ratio of50 1kg 1^,and the resultswerefitted^toamodifiedLangmuir equation:

Q⁼ Q_maxl/⁽1/K⁺I)-Qo

whereQisPsorbedor desorbed,Q_max⁼maximumPsorption, I= Pconcentrationintheequilibrium solution, K⁼sorption/desorption equilibriumconstant, andQ 0^{=instantlylabileP.Thedesorptionof}

DRPwasdepressed byincreasesinthe CaCl₂concentration of the extractant andpromoted bywiden- ingof the solution-to-soil ratio. At thesolution-to-soil ratio of50 1 kg ^',the increase inthe tempera- turefrom5to25°C raisedtheDRPreleaseto water from12.6to20.7mgkg*

1

^{intheAurajoki soil and}

from 1.8to3.4mgkg^' inthe Jokioinen soil. In^theAurajoki soil,the constant

Q 0 of

^{the Langmuir}

equation responded to thechanges of ionic strength and temperature in thesame way as did DRP extracted at wide solution-to-soil ratios. However,thePreleasecapacityof both soils^wasunderesti- matedby the constantQO.

Key words: waterextraction,CaCl

2extraction, solution-to-soilratio,modified Langmuir equation, phosphorus loading of surface waters

1^{Current address:} Agricultural Research Centre of^{Finland, Institute} of^{Soils and} Environment, FIN-31600 Jokioinen,Finland

Introduction

and in the particulate material. In areas of fro- zen soil and snow coverage in ^winter, the vol- umeof surface runoff^waterpeaks in spring dur- ing thethaw, another peak often occurring in

autumn.Erosion and removal of dissolved Pare Surface runoffwatertransports phosphorus(P)

from soil to watercourses in the dissolved form

©Agricultural and Food ScienceinFinland Manuscriptreceived May 1996

alsomostintensive during these periods ^(Turto- la and Jaakkola 1995).The equilibrium ^P con- centration (EPC) in the solution where the par- ticulate material collected from river waternei- ther released_oradsorbed Pwasshowntobe low- erthan 0.010 mg P f which is decisively below the EPC obtained for cultivated soils (e.g. Har- tikainen 1982^b, Yli-Halla 1991, Yli-Hallaetal.

1995). Pietiläinen and Ekholm (1992) showed that90% of particulate material in asmall agri- culturally loaded river in southern Finlandwas recently eroded from the surface soil of the fields of the drainage basin. Comparison of the results above thus reveals that P must be effectively desorbed during the erosion process.

Dissolved P in the runoff water originates partly from the bulk of the soil remaining in the field and partly from suspended particles. Soil P status, tosome extent,explains the average dis- solved reactive P(DRP)concentration in runoff waterbutnotthe temporal variation of DRP (Yli- Hallaetal. 1995).The temperature during the runoffpeaks inautumnand particularly in spring is much lower(between0 and5°C)ascompared

totemperatures prevailing during the occasion- al summerrainstorms (commonly around ^15°C) and toroom temperatures atwhich laboratory experiments are usually done. There is also a marked seasonal variation in other externalcon- ditions (ionic strength and volume of runoff water)prevailing during runoffevents.

In this study, a set of desorption tests was carried out to quantify the impact of environ- mental factors _on the P loading risk due to the surface runoff from cultivated soils. The tem- perature as well asionic strength and solution- to-soil ratiowerevariedtosimulate their changes during the runoff and erosion process and mater- ial_transportinwatercourses.The impact oftem- perature and external ionic strengthon the dy- namic equilibrium between solution and solid materialwasinvestigated bymeansof the quan- tity-intensity (Q/I) plots. The instantly labile P derived from these graphswas usedas oneesti- mate for the P loading risk duetorunoff_water.

The suitability of the Q/I plots topredict P re- lease from soil was also evaluated.

Table 1.^Propertiesof the experimental soils.

Characteristic Aurajoki soil Jokioinen soil

Clay,^% 60.1 58.5

OrganicC,^% 2.8 1.4

SoilpH(CaCl₂⁾ 5.72 5.14

Al_oi,gkg" 1.05 2.22

Fe_ox^,gkg" 7.23 7.03

Pw,mgkg

1

^{" 27.5 3.6}

27.1 5.1

NH₄CI-P, mgkg

1

^{"" 5.2 0.2}

NH₄F-P,mgkg

1

^{"" 132 80}

NaOH-P, mgkgI**"1^{**" 527 423}

H,S0₄-P,mgkg'"" 475 172

Sumof fractions, mgkg 1139 675

* Extracted with0.05 Mammoniumoxalate,pH3.3(Nis- kanen 1989)

** Pextracted with water, solution-to-soil ratio 50 1 kg 1

*** Pextracted withanammonium acetatesolution,pH 4.65 (Vuorinen and Mäkitie 1955)

Changand Jackson fractions of inorganicP(Hartikai- nen ¹⁹⁷⁹⁾

Material and methods

Soil samples

The soil samples ofAurajoki and Jokioinen^(Ta- ble 1)were taken from experimental fields set

up for studiesonsurface runoff. Theyrepresent fields with high and low level of P concentra- tion in runoff, respectively (Yli-Halla ^etal.

1995).Both soils, located in southwestern Fin- land, are classified as Vertic Cambisols (FAO 1988). The soil samples were taken from the 0-10 cm layer. After air-drying and homogeni- zation, the sampleswere rewetted toa moisture content of20% and stored at 5°C for several weeks before the analyses. Air-dry sampleswere analyzed for pH in ^a 0.01 M CaCl2 suspension and for water-extractable P(lg ofsoil, 50 ml of deionizedwater, 17 hours of equilibration; Har-

likainen 1982^a)

Desorption tests

To study the effect of ionic strength on P de- sorption fromsoil,deionizedwater,0.0005 M and 0. M CaCl₂wereusedasextracting solutions

at room temperature (25°C). The selection of CaCl₂is based on the dominance of Ca among the exchangeable cations of the experimental soils(results notpresented). For theextractions, moist soil samples (four replicates) were weighed togive dry soil concentrations of 0.5, 1,2,5 and 20 g I'¹of the_extractant(solution-to- soil ratio 2000-50 1 kg ^').The soil suspensions were shaken for 17 h in an orbital shaker_{at a} speed of 250 rotations min According _topre- liminary experiments, this reaction timewaswell sufficient^toreach a semi equilibrium. The su- pernatant solutions_werefiltered througha mem- brane filter(0.2pm, Nuclepore polycarbonate) and analyzed for dissolved reactive P ^(DRP)by amolybdenum blue method using ascorbic acid asthe reducing agent.The effect oftemperature on P desorption was studied by extracting soil with deionizedwater at5, 15 and 25°C. At each temperature, the extractionswere carriedout at solution-to-soilratios from50to2000 I kg

1

^.The

soil samples and the solutionstobe added_were adaptedtothe respective temperaturesbefore the extraction. Phosphorus wasdeterminedas men- tioned above.

and0.00005 M CaCl₂as the supporting electro- lytes. The ionic strengths of soil extracts were estimated from the electrical conductivity ac- cordingtoGriffin and Jurinak (1973).

Sorptiontoordesorption from soil(Y) in mg P kg’

1

^wascalculated from the changes in Pcon- centration of the contacting solution and fitted

toa modified Langmuir equation(Hartikainen and Simojoki 1994):

Q

⁼

Q

ma,I/⁽1/K⁺

D-Qo

where Qis P desorbed or sorbed, Q_'-max⁼ maxi- mumP sorption, I⁼P concentration in the final equilibrium solution,K ⁼ sorption/desorption equilibriumconstant, and

Q 0

⁼ instantly labile P. Mathematically, the isotherm will intersect the y-axis when 1 ⁼0. According toBeckett and White(1964),the intercept^(term

Q 0 in

^{the equa-}

tion)_represents what_was termed instantly la- bile^P that would haveto be removed from the soil toreduce I to zero ata given solution-to- soil ratio. The intersecting point of the graphon the x-axis (Y ^{= 0),}the equilibrium phosphate concentration(EPC), represents the zero point of P exchange atwhich nonetdesorption from orsorptiontosoil occurs.The slope of the sorp- tion-desorption curve atthe EPC wasreferred toby Holford and Mattingly (1976)_asthe equi- librium buffer capacity (EBC).

Q/l plots

The Q/I plotswere applied toexpress the sorp- tionordesorptionas afunction of thePconcen- tration in the equilibrium solution. They were determined in three replicates by adding 50 ml ofKH,P0₄solution (0-4 mg PT

1

for Aurajoki samples, 0-5 mgP I’

1

for Jokioinen samples) to moist samples corresponding to 1 g of dry soil (solution-to-soilratio50 1 kg’^1,orconcentration of suspended solids 20 g I’

1

^{). Theextracts were}

obtainedas described above and analyzed for DRP. The

Q/I

^{plotsweredeterminedatthreetem-}

peratures: 5, 15 and 25°C. At 25°C, the plots werealso determined using 0.005 M, 0.0005 M

Results

Desorption tests

At every solution-to-soil ratio, the two CaCl₂ solutions extracted much less P than didwater (Fig. 1).At the solution-to-soil ratio of50 1 kg

1

the P concentration in the extract was lowered from0.42to0.14 mg 1

1

in the Aurajoki soil and in the Jokioinen soil from 0.068 ^to0.015 mg 1 ¹ when the ionic strength of the soilextract, shown in Table 2, increased from 0.3-0.5 mmol I’

1

^(wa-

ter extract) ^to 16 mmol I’

1

^{(0.005 M CaCI, ex-}

tract).Relatively, the decrease in P desorption

wasequal in both soils. If desorptionto wateris denoted as 100, the relative desorptionto0.0005 M and0.005 ^MCaCl, was66 and 33 in the^Au-

rajokisoil, and 68 and 30 in the Jokioinensoil, respectively.

Fig. 1.^Phosphorusextracted from theAurajokiand Jokioi- nensoil with deionized water,0.0005 Mand0.005 MCaCl₂ solutions at different solution-to-soil ratios (Ex).Inthe cal- culation of theequations, the ionic strengths(S,mmolI ¹⁾ estimated from the measured electrical conductivities of the soil extracts^wereused.

Fig. 2. Phosphorusextracted from theAurajokiand Jokioi- nensoil at three temperatures (t) with deionized water at different solution-to-soil ratios (Ex).

Table2. lonic strengths of water extracts and the CaCl₂ extracts atthesolution-to-soil^ratio 501 kg 1^.

Extractant lonicstrength,mmol1'

Aurajoki Jokioinen

Water 0.51 0.29

0.0005 M CaCl

2 2.13 1.94

O.OOSMCaCl₂ 15.92 15.70

At the solution-to-soil ratio of50 1 kg

1

^{, ele-}

vation oftemperaturefrom 5to25°C enhanced P desorption markedly. The DRP concentration in thewater extracts increased in the Aurajoki soil from0.25_to 0.42 mg f (by ^68%)and in the Jokioinen soil from 0.037 _to 0.068 mg 1

1

^(by

84%).The quantities ofP desorbed (mg kg'¹⁾ with_{water at}the threetemperaturesand the hon- estsignificant differences _atP=0.05 (HSD

00J)

were:

Aurajoki ^Jokioinen

5°C 12.6 1.8

16.6 2.2

20.7 3.4

3.5 0.3

15°C 25°C

HSD,_0.05

Increasing the volume of water around the soil particles lowered the ionic strength and the DRP concentration in theextract(Table 3). Ow- ingtothestrongP buffer power of the soil,how- ever, the decrease of DRP concentration was even less linear than that of the ionic strength.

Consequently,^the desorption^ofP, expressed as mg kg^-1, was strongly promoted (Fig. 2). De- sorption increased from 20.7 _to 119 mg kg'¹in the Aurajoki soil and from3.4to45.0 mg kg'¹in the Jokioinen soilat25°C when the solution-to- soil ratio increased from50to2000 1 kg

1

^.

Q/l plots

The Q/I plots crossed fromnetdesorptionto net sorption, and the results conformed accurately to the modified Langmuir equation ^(r2>0.99) (Fig. 3 and Fig. 4). However, for Jokioinensoil, the graphs intersected the x-axis closetothe ori- gin and the desorption remained very smallat the solution-to-soil ratio of 50 1 kg'¹ at which the Q/I plotsweredetermined. In both^soils,sorp- tion increased and desorption decreased when CaCl2solutions_wereused_asextractants(Fig. 3).

TheÉPCvalues obtained in 0.005 M CaCl₂were less than one fifth of that measured with out asupporting electrolyte (i.e., in water),and the EBC increased substantially upon increase of the ionic strength(Table 4). Despite the sim- ilar level of ionic strengths(0.3 mmol1¹ inwa- ter extracts, 0.5 mmol I'

1

in 0.00005 M CaCl₂ extracts) in the Jokioinensoil, the

Q/I

^{plot in}

the CaCl₂solutionwas markedly steeper.

The Q/I plots determinedat5, 15 and 25°C (Fig. 4) showed that both desorption and sorp- tion ofP werepromoted by gradual elevation of

temperature. Both EBC and EPC increasedupon

Table3. DRPconcentration and ionicstrengthof the water extracts obtained at different solution-to-soil ratios at 25°C.'

Solution-to-soil DRP,mg 1' ^{lonicstrength,mmoll"'}

° Aurajoki Jokioinen Aurajoki Jokioinen

50 0.415' 0.068" 0.51' 0.29^c

200 0.243" 0.065" 0.23" 0.19"

500 0.149' 0.052' 0.1 5^ah 0.12»

1000 0.094^b 0.036" 0.09»^b 0.10^s

2000 0.060^a 0.023" 0.09» 0.09"

HSD00S 0.030 0.011 0.12 0.05

Each columnwastestedseparately.Means marked with thesame superscriptdo not differ at P⁼0.05.

increase ⁱⁿtemperature^{(Table 4).}The plots in- tersected at 1.2 and 0.2 mg^PI^' in the Aurajoki and Jokioinen soil,respectively, i.e. clearly

above the respective EPC values^(see Table 4).

In Jokioinen^soil,dominated by amarked sorp- tion tendency, the effect oftemperature onthe P exchangewas small^atthe low P concentrations in the equilibrium solution. Therefore the graphs

Fig.3.Q/I^plotsof the Aurajoki and Jokioinen soil deter- minedindeionized water and inCaCl₂solutions at the^so- lution-to-soil ratio of50 I kg^'.The molarities intheleg- end refer to the concentration of CaCl₂ inthe extractant.

Thecurves displayed ^arecalculatedusing the modified Langmuir equation.The constants of theequations of the curves arepresented inTable4.

Fig.4,Q/I^plotsof the Aurajoki and Jokioinen soil deter- minedindeionized water at three temperatures at theso- lution-to-soil ratio of50 1 kg^'.The curvesdisplayed are calculatedusingthe modifiedLangmuir equation.Thecon- stantsof theequationsof thecurves arepresented inTa- ble4.

Table4.Constantsof theQ/lplotscalculated fromamodifiedLangmuir equation.

EPC EBC O, Q K

mg^["' 1 kg"' mgkg^' mgkg^' 1mg'

Equilibrationwithoutasupporting electrolyte at5. 15and 25°C:

Aurajoki

5°C 0.793 19.6 21.3 79 0.47

15°C 0.845 24.4 30.9 92 0.60

25°C 0.945 31.1 55.3 118 0.93

Jokioinen

5°C 0.024 186 4.6 140 1.42

15°C 0.025 168 4.4 248 0.71

25°C 0.050 212 11.0 272 1.18

EquilibrationinwaterorCaCl. at 25°C:

Aurajoki

H₂O 0.945 31.1 55.3 118 0.93

0.Ö0005M1 ^0.790 41.1 59.5 131 1.06

0.0005 M 1 0.460 66.0 38.5 183 0.58

0.005 M' 0.174 150.3 31.7 179 1.24

Jokioinen

H₂O 0.050 212 11.0 272 1.18

0.00005 M 1 0.051 309 16.6 324 1.06

0.0005 M^' 0.028 567 16.9 322 1.96

0.005 M 1 0.010 888 9.4 430 2.16

1

^CaCl2concentration of the extractant.

are very close to each other and thecross-over points cannot be clearly seen in the scale used in Fig. 4.

In the Aurajoki^soil,the^constant

Q 0 increased

upon elevations oftemperature and in general decreased upon increasing ionic strength^(Table 4). The changes of

Q 0 were

thus in accordance with the influence of temperature and ionic strengthon desorption obtained in the desorp- tion^tests.In the Jokioinensoil, onthecontrary, the response of

Q 0 to

the changes oftempera- tureorionic strengthwasless consistent. In both

soils,the values of

Q 0 of

^the

Q/I

plots determined without a supporting electrolyte were less than half of the observed desorption to water at the widest solution-to-soil ratio. The values of

Q 0

determined in CaCl₂ were 56-94% of the meas- ured maximum desorption to the respective CaCl₂ solution(0.005 M or0.0005 ^M;0.00005 Mnot used in the desorption^test).

Discussion

The quantity of labile adsorbed P onsoil parti- cles is the ultimate reserve of P whichcanbe desorbed, but the DRP concentration in runoff water is also controlled substantially by ionic strength andtemperature,andtosomeextentby the solution-to-soil ratio. The P buffer power of soil tends to maintain a constantDRP concen- tration in^water, and thereforemore voluminous runoff markedly increases the total quantity of

DRP removed from the field.

The ionic strength of the^waterextracts atthe solution-to-soil ratio 50 1 kg'¹ corresponded to that of rain and snowmeltwater (0.3 mmol I 1) while those of theextractsobtainedatwider so- lution-to-soil ratios were even lower in salts.

As for ionic strength, the soilextractsobtained with0.0005MCaCl_2weresimilartothe surface

runoff waters of the Jokioinen and Aurajoki fields (Yli-Halla ^etal. 1995),and those obtained with0.005 M CaCl2corresponded tosoil solu- tion (Wiklander and Andersson ^1974).As for the solution-to-soilratio, itwasobserved in an ear- lier study that the average DRP concentration in the surface runoff in these particular fieldswas the same as that of soil extractsobtainedat the solution-to-soil ratio between 250 and 500 1 kg

1

(Yli-Halla etal. ¹⁹⁹⁵⁾but accordingtoEkholm (1994) the concentration of suspended solids in coastal riverwatersof Finland is lower thanwas applied in the present study. The equations in Fig. 1 describing the dependence of P desorp- tion _onionic strength and solution-to-soil ratio

thuscover the range of these factors occurring in the hydrologic environment in the field.

On the ionic strength scale^used,the decrease in the salt concentration proved ^to effectively promote the P release from soil.^In the ^water extracts,all dissolved salts originated from the soil sample, resulting in the decrease of the ion- ic strength of the extracts upon widening the solution-to-soil ratio. Ataconstant solution-to- soilratio, decreasing ionic strengthpromotes P desorption (Hartikainen and Yli-Halla 1982).

Therefore,itcanbe concluded that upon widen- ing the solution-to-soil ratio the decrease of DRP concentration (mg I 1) may have been^more sub- stantial and the increase of P desorption (mg kg^') less marked if the ionic strength had been kept constant. The present results thus give the ^net effect oftwo factors promoting P desorption;

widening solution-to-soil ratio and decreasing ionic strength.

Besides DRP released in the field, surface runoff^watertransports^Pwhich is adsorbedonto the suspended soil material and which canbe releasedasDRP in the recipientwatercourse.As for the total DRP loading from the eroded^mate- rial,thetemperatureduring the runoffeventmay be unimportant because in the ^water body, the eroded material is subject also^tohighertempera- tures during the summer months. Therefore, results obtainedatalowtemperature areneeded toassess the DRP release in the field during the cool and wet season, while those obtained ^at

higher temperatures are applicable toDRP re- lease by thesummerrains and^todesorption tak- ing place in a watercourse during the warmer period of the year.

Increased speed of diffusionatelevatedtem- peraturesexplains thecross-overof the

Q/I

^plots.

Below theEPC, net diffusionoccurs from soil to solution, resulting in _a higher P concentra- tion in the extract when thetemperature is ele- vated. Above theEPC,netdiffusion is fromso- lutionto soil,andanelevatedtemperatureleads toa higher sorption. If diffusionwerethe only factor affected by the temperature, the curves shouldcrossatthe EPC.However, in both soils, elevation of thetemperatureseemed_toshift the EPC toa higher concentration. At higher ^tem- peratures, ahigher P concentrationwasrequired for sorption to start, or, vice versa, desorption continuedtoahigher P concentration. Asa con- sequence, the crossing of the^curves occurredat a P concentration above the EPC. At the EPC

the net diffusion is zero. Based on the shift of theEPC, conclusions _can be madeonthe tem- perature-dependency of thePexchange equilib- rium. The shift of EPCtoahigher concentration indicates thatahightemperaturefavors desorp- tion. Thissuggestssorption ^tobe anexothermic reaction, aspresented by Barrow (1979), and desorption ^to be an endothermic one. The pa- rameter Q stands for P sorption sites availa- ble. Its increaseas aresponse^tothe elevatedtem- perature canbe takento indicate that thesatura- tion of the sorption sites is kinetically control- led.

The Q/I plots^can in principle be utilizedto quantify the instantly labile P of the soil (Pionke and Kunishi 1992).The physical relevance of the constant

Q

^(| as a measure for instantly labile P can be assessed by comparing it with the ob-

served desorption^atwide solution-to-soilratios, e.g. at 2000 1 kg’

1

and with other estimates of^P release from soil. Phosphorus boundto hydrous oxides of Al and Fe are the majorreserves of bioavailable P in thewatercourse(Dorich etal.

1985), and they control the level ofwater-ex- tractable^P in soil (Hartikainen

1982

^{a). Maxi-}

mum desorption in this study (solution-to-soil

ratio 2000 1 kg ^', 25°C) corresponded ^to9 and 18% of the secondary P fractions ^(NH₄CI-P ⁺ NH₄F-P ⁺ NaOH-P, see Table 1) in the Jokioi- nen and Aurajoki soils, respectively, while the constant

Q 0 of

the respective

Q/I

^{plots amount-}

edtoonly 3 and 8% of the secondary Preserves.

Inanexhaustivepotexperiment with_{one soil,} Yli-Halla and Renlund(1990) measureda30%

decrease in these P fractions. If this decrease is taken torepresent a measure of the maximum bioavaliability of soil P reserves,the

Q 0 values

markedly underestimate the potential P loading.

Even the desorption measuredatthe widestso- lution-to-soil ratio (2000 1 kg

1

^{) at}25°C may be smaller than the P amount thatcanbe released from eroded soil in a watercourse. However, it should be mentionedthat, particularly in the Aurajoki soil,

Q 0 and

the P release in the de- sorption testsresponded similarlytochanges of ionic strength andtemperature. This shows that

the Q/I plots qualitatively reflect the dynamic P exchangeeventhough quantitative interpretation of the

Q 0 values

^{may be}questionable.

The presentresults show that dependingon prevailing experimental conditions, awidevar- iation of P desorption results can be measured in _agivensoil,leading todifferent estimates for P loading. In ^most studies _onsoil samples, the Q/I plots have been determined usinga0.01 M supporting electrolyte (e.g. Barrow 1979). The information obtained from those

Q/I

plots is ap- plicable toP fertilization and the nutrition of plants. Phosphorus release to surface runoff_or waterbodies needstobe assessed by experiments performed at a low ionic strength and a wide (above 200 1 kg ^‘)solution-to-soil ratio.

Acknowledgements. This studywasfinancially supported bytheAcademyof Finland.

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SELOSTUS

Ympäristöolosuhteiden vaikutus ^maan fosforin liukenemiseen pintavalunnan aikana

Markku Yli-Halla ja Helinä Hartikainen Helsingin yliopisto

Pellolla kulkevaan pintavaluntaveteen liukenee fos-

foria(P) sekäpaikalleen jäävästämaastaettä veden mukana kulkevasta eroosioaineksesta.Aurajoen ja Jokioisten huuhtoutumiskentiltäotettujen savimaa- näytteiden P-luovutuskykyä tutkittiin laboratorioko- kein. Kokeissa pyrittiinsimuloimaan pintavalunnas- sa vallitsevia olosuhteita uuttamalla maasta P:a eri lämpötiloissa^(5, 15 ja 25 °C),eri suolakonsentraa- tioissa (deionisoitu vesi tai0,00005-0,005 M^CaCI₂⁾ ja käyttäen erilaisia vesi-maasuhteita (50^- 2000 Ikg').

Maa-aineksen kykyä sitoa ja luovuttaa P:a tutkit-

tiinmyös sorptio-desorptioisotermien avulla. Isoter- mit määritettiin ravistelemalla ^maataerivahvuisissa P-liuoksissaeri lämpötiloissa jasuolakonsentraatiois- sa. ^Kun uuttolämpötila nousi 5 °C;sta 25°C:een, vesi-maasuhteella 50 1 kg

1

^Aurajoen maasta veteen uuttuneenP:n määräkasvoi arvosta 12,6mgkg'1^ar-

voon 20,7 mgkg 'ja Jokioisten^{maassa arvosta 1,8}

mgkg

1

^{arvoon3,4mgkg}

1

^. Kun maata uutettiindeio- nisoidunveden asemesta maaveden suolapitoisuutta jäljittelevällä 0,005 MCaCI₂:lla,uuttuneet P-määrät Aurajoen maastapienenivät 6,8 mgkg haanja Jo- kioisten maasta0,7 mgkghaan.Vesi-maasuhteen väl- jentäminen 2000 1 kghaan puolestaan lisäsiP:n ^uut- tumista veteenAurajoen maassa kuusinkertaiseksi (119 mgkg haan) jaJokioisten maassa 14-kertaisek- si (46 mgkg haan). Lämpötilan ja suolapitoisuuden kohotessapidättyi maahan lisätystäP:stayhäsuurem- pi osuus. ^{Saatujen tulosten}perusteella voidaanpää- tellä, että pellolta^tulevan pintavalunnan liukoisen^P:n pitoisuuteen javesistökuormituksen^suuruuteenvai- kuttavat oleellisesti maanhelppoliukoisen P:n pitoi- suuden ohellamyösvalumaveden määrä, sen ^lämpö- tilaja suolapitoisuus.