Vol. 5(1996): 193-202. Release of soil phosphorus during runoff as affected by ionic strength and temperature Markku Yli-Halla 1 and Helinä Hartikainen Department ofApplied Chemistry and Microbiology, P.O. Box 27, FIN-00014 University ofHelsinki, Finland Dissolved reactive phosphorus (DRP) from two cultivated clay soil samples (Vertic Cambisols) was extracted under conditions simulating the variation in the properties of surface runoff water in the field. DRP was extracted at three temperatures (5, 15 and 25°C), and at different ionic strengths by using deionized water and CaCl2 solutions (0.00005-0.005 M) as extractants. The solution-to-soil ratio varied from 50 to 2000 1 kg 1 . Sorption to and desorption from the soils were studied at different temperatures and ionic strengths by determining quantity-intensity (Q/I) plots at the solution-to-soil ratio of 50 1kg 1 , and the results were fitted to a modified Langmuir equation: Q = Q maxl/( 1/K + I)-Qo where Qis P sorbed or desorbed, Qmax =maximum P sorption, I= P concentration in the equilibrium solution, K = sorption/desorption equilibrium constant, and Q 0 = instantly labile P. The desorption of DRP was depressed by increases in the CaCl2 concentration of the extractant and promoted by widen- ing of the solution-to-soil ratio. At the solution-to-soil ratio of 50 1 kg ', the increase in the tempera- ture from 5 to 25°C raised the DRP release to water from 12.6 to 20.7 mg kg* 1 in the Aurajoki soil and from 1.8 to 3.4 mg kg ' in the Jokioinen soil. In the Aurajoki soil, the constant Q 0 of the Langmuir equation responded to the changes of ionic strength and temperature in the same way as did DRP extracted at wide solution-to-soil ratios. However, the Prelease capacity of both soils was underesti- mated by the constant Q O. Key words: water extraction, CaCl 2 extraction, solution-to-soil ratio, 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- ume of surface runoff water peaks in spring dur- ing the thaw, another peak often occurring in autumn. Erosion and removal of dissolved P are Surface runoff water transports phosphorus (P) from soil to watercourses in the dissolved form © Agricultural and Food Science in Finland Manuscript received May 1996 193 AGRICULTURAL AND FOOD SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=gjfXurlk_ZIHxp53.4WQfqW5LK3Fl_mG9sN4r9A.nBSxSzqCzENkoPcgEfMi59C_AOUa7PmMle1M-ulIAGfSyW4RxofRhoakauu1Z6Dc-CLkE5652sh43YFuG3VTFnKJvRe9Zo7VP3-cI8d5SMWJMzoDQi3N5AhORvVedeIRkR_p-VD05lAJhTLT-jVqGeD-gMo8EHtEoTqG9pJUBBV5Whp0JKhQVcXPibqqeAzz7L3aFBlubUOJ1Ks4Ghmi8b0fnpelCDnpe4Ui7shZZC1BJkWTVSWtzhnc5tAY93TPim6XeRO3eqUBmLt4jJIDcdYqtVTuyReFKbtWHt-bwz36kOA Yli-Halla, M. & Hartikainen, H.: Phosphorus desorption to runoff water also most intensive 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 water nei- ther released or adsorbed P was shown to be low- er than 0.010 mg P f which is decisively below the EPC obtained for cultivated soils (e.g. Har- tikainen 1982b, Yli-Halla 1991, Yli-Halla et al. 1995). Pietiläinen and Ekholm (1992) showed that 90% of particulate material in a small agri- culturally loaded river in southern Finland was 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, to some extent, explains the average dis- solved reactive P (DRP) concentration in runoff water but not the temporal variation of DRP (Yli- Halla et al. 1995). The temperature during the runoffpeaks in autumn and particularly in spring is much lower (between 0 and 5°C) as compared to temperatures prevailing during the occasion- al summer rainstorms (commonly around 15°C) and to room temperatures at which laboratory experiments are usually done. There is also a marked seasonal variation in other external con- ditions (ionic strength and volume of runoff water) prevailing during runoff events. 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 as ionic strength and solution- to-soil ratio were varied to simulate their changes during therunoff and erosion process and mater- ial transport in watercourses. The impact oftem- perature and external ionic strength on the dy- namic equilibrium between solution and solid material was investigated by means of the quan- tity-intensity (Q/I) plots. The instantly labile P derived from these graphs was used as one esti- mate for the P loading risk due to runoff water. The suitability of the Q/I plots to predict P re- lease from soil was also evaluated. Table 1.Properties of the experimental soils. Characteristic Aurajoki soil Jokioinen soil Clay, % 60.1 58.5 Organic C, % 2.8 1.4 Soil pH(CaCl2 ) 5.72 5.14 Aloi,gkg" 1.05 2.22 Feox, g kg" 7.23 7.03 P w, mg kg 1" 27.5 3.6 27.1 5.1 NH 4CI-P, mg kg 1"" 5.2 0.2 NH4F-P, mg kg 1"" 132 80 NaOH-P, mg kgI**" 1 **" 527 423 H,S04 -P, mg kg'"" 475 172 Sum of fractions, mg kg 1139 675 * Extracted with 0.05 M ammonium oxalate, pH 3.3 (Nis- kanen 1989) ** P extracted with water, solution-to-soil ratio 50 1 kg 1 *** P extracted with an ammonium acetate solution, pH 4.65 (Vuorinen and Mäkitie 1955) Chang and Jackson fractions of inorganic P (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 studies on surface runoff. They represent fields with high and low level of P concentra- tion in runoff, respectively (Yli-Halla et al. 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 samples were rewetted to a moisture content of 20% and stored at 5°C for several weeks before the analyses. Air-dry samples were analyzed for pH in a 0.01 M CaCl2 suspension and for water-extractable P(lg of soil, 50 ml of deionized water, 17 hours of equilibration; Har- likainen 1982a) 194 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 193-202. Desorption tests To study the effect of ionic strength on P de- sorption from soil, deionizedwater, 0.0005 M and 0. M CaCl2 were used as extracting solutions at room temperature (25°C). The selection of CaCl2 is based on the dominance of Ca among the exchangeable cations of the experimental soils (results not presented). For the extractions, moist soil samples (four replicates) were weighed to give dry soil concentrations of 0.5, 1,2, 5 and 20 g I' 1 of 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 to pre- liminary experiments, this reaction time was well sufficient to reach a semi equilibrium. The su- pernatant solutions were filtered through a mem- brane filter (0.2 pm, Nuclepore polycarbonate) and analyzed for dissolved reactive P (DRP) by a molybdenum blue method using ascorbic acid as the reducing agent. The effect of temperature on P desorption was studied by extracting soil with deionized water at 5, 15 and 25°C. At each temperature, the extractions were carried out at solution-to-soilratios from 50 to 2000 I kg 1 . The soil samples and the solutions to be added were adapted to the respective temperatures before the extraction. Phosphorus was determined as men- tioned above. and 0.00005 M CaCl2 as the supporting electro- lytes. The ionic strengths of soil extracts were estimated from the electrical conductivity ac- cording to Griffin and Jurinak (1973). Sorption to or desorption from soil (Y) in mg P kg’ 1 was calculated from the changes in P con- centration of the contacting solution and fitted to a modified Langmuir equation (Hartikainen and Simojoki 1994): Q = Q ma, I/ ( 1/K + D-Qo where Qis P desorbed or sorbed, Q = maxi-'-max mum P sorption, I = P concentration in the final equilibrium solution, K = sorption/desorption equilibrium constant, and Q 0 = instantly labile P. Mathematically, the isotherm will intersect the y-axis when 1 = 0. According to Beckett and White (1964), the intercept (term Q 0 in the equa- tion) represents what was termed instantly la- bile P that would have to be removed from the soil to reduce I to zero at a given solution-to- soil ratio. The intersecting point of the graph on the x-axis (Y = 0), the equilibrium phosphate concentration (EPC), represents the zero point of P exchange at which no net desorption from or sorption to soil occurs. The slope of the sorp- tion-desorption curve at the EPC was referred to by Holford and Mattingly (1976) as the equi- librium buffer capacity (EBC). Q/l plots The Q/I plots were applied to express the sorp- tion or desorption as a function of the P concen- tration in the equilibrium solution. They were determined in three replicates by adding 50 ml of KH,P0 4 solution (0-4 mg PT 1 for Aurajoki samples, 0-5 mg P I’ 1 for Jokioinen samples) to moist samples corresponding to 1 g of dry soil (solution-to-soil ratio 50 1 kg’ 1 , or concentration of suspended solids 20 g I’ 1). The extracts were obtained as described above and analyzed for DRP. The Q/I plots were determinedat three tem- peratures: 5, 15 and 25°C. At 25°C, the plots were also 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 did water (Fig. 1). At the solution-to-soil ratio of 50 1 kg 1 the P concentration in the extract was lowered from 0.42 to 0.14 mg 1 1 in the Aurajoki soil and in the Jokioinen soil from 0.068 to 0.015 mg 1 1 when the ionic strength of the soil extract, 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 195 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Yli-Halla, M. & Hartikainen, H.: Phosphorus desorption to runoff water was equal in both soils. If desorption to water is denoted as 100, the relative desorption to 0.0005 M and 0.005 M CaCl, was 66 and 33 in the Au- rajoki soil, and 68 and 30 in the Jokioinen soil, respectively. Fig. 1. Phosphorus extracted from the Aurajoki and Jokioi- nen soil with deionized water,0.0005 M and 0.005 M CaCl2 solutions at different solution-to-soil ratios (Ex). In the cal- culation of the equations, the ionic strengths (S, mmol I 1) estimated from the measured electrical conductivities of the soil extracts were used. Fig. 2. Phosphorus extracted from the Aurajoki and Jokioi- nen soil at three temperatures (t) with deionized water at different solution-to-soil ratios (Ex). 196 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 193-202. Table 2. lonic strengths of water extracts and the CaCl2 extracts at the solution-to-soilratio 501 kg 1. Extractant lonic strength, mmol 1' Aurajoki Jokioinen Water 0.51 0.29 0.0005 M CaCl2 2.13 1.94 O.OOSMCaCl 2 15.92 15.70 At the solution-to-soil ratio of 50 1 kg 1, ele- vation of temperature from 5 to 25°C enhanced P desorption markedly. The DRP concentration in the water extracts increased in the Aurajoki soil from 0.25 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 of P desorbed (mg kg' 1 ) with water at the three temperatures and the hon- est significant differences at P=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 the extract (Table 3). Ow- ing to the strong P 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 of P, expressed as mg kg -1 , was strongly promoted (Fig. 2). De- sorption increased from 20.7 to 119 mg kg' 1 in the Aurajoki soil and from 3.4 to 45.0 mg kg' 1 in the Jokioinen soil at 25°C when the solution-to- soil ratio increased from 50 to 2000 1 kg 1. Q/l plots The Q/I plots crossed from net desorption to net sorption, and the results conformed accurately to the modified Langmuir equation (r2 >0.99) (Fig. 3 and Fig. 4). However, for Jokioinen soil, the graphs intersected the x-axis close to the ori- gin and the desorption remained very small at the solution-to-soil ratio of 50 1 kg' 1 at which the Q/I plots were determined. In both soils, sorp- tion increased and desorption decreased when CaCl 2 solutions were used as extractants (Fig. 3). The ÉPC values obtained in 0.005 M CaCl2 were less than one fifth of that measured with out a supporting 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 mmol 1 1 in wa- ter extracts, 0.5 mmol I' 1 in 0.00005 M CaCl 2 extracts) in the Jokioinen soil, the Q/I plot in the CaCl2 solution was markedly steeper. The Q/I plots determined at 5, 15 and 25°C (Fig. 4) showed that both desorption and sorp- tion ofP were promoted by gradual elevationof temperature. Both EBC and EPC increased upon Table 3. DRP concentration and ionic strength of the water extracts obtained at different solution-to-soil ratios at 25°C.' Solution-to-soil DRP, mg 1' lonic strength, mmol l"' ° 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 5ah 0.12» 1000 0.094b 0.036" 0.09»b 0.10s 2000 0.060a 0.023" 0.09» 0.09" HSD00S 0.030 0.011 0.12 0.05 Each column was tested separately. Means marked with the same superscript do not differ at P = 0.05. 197 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Yli-Halla, M. & Hartikainen, H.: Phosphorus desorption to runoff water increase in temperature (Table 4). The plots in- tersected at 1.2 and 0.2 mg P I ' in the Aurajoki and Jokioinen soil, respectively, i.e. clearly above the respective EPC values (see Table 4). In Jokioinen soil, dominated by a marked sorp- tion tendency, the effect of temperature on the P exchange was small at the low P concentrations in the equilibrium solution.Therefore the graphs Fig. 3. Q/I plots of the Aurajoki and Jokioinen soil deter- mined in deionized water and in CaCl2 solutions at the so- lution-to-soil ratio of 50 I kg '. The molarities in the leg- end refer to the concentration of CaCl2 in the extractant. The curves displayed are calculated using the modified Langmuir equation. The constants of the equations of the curves are presented in Table 4. Fig. 4, Q/I plots of the Aurajoki and Jokioinen soil deter- mined in deionized water at three temperatures at the so- lution-to-soil ratio of 50 1 kg '. The curves displayed are calculated using the modified Langmuir equation. The con- stants of the equations of the curves are presented in Ta- ble 4. 198 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 193-202. Table 4. Constants of the Q/l plots calculated from a modified Langmuir equation. EPC EBC O, Q K mg ["' 1 kg"' mg kg ' mg kg ' 1 mg ' Equilibration without a supporting electrolyte at 5. 15 and 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 Equilibration in water or CaCl. at 25°C: Aurajoki H 2O 0.945 31.1 55.3 118 0.93 0.Ö0005M 1 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 H 2O 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 CaCl2 concentration of the extractant. are very close to each other and the cross-over points cannot be clearly seen in the scale used in Fig. 4. In the Aurajoki soil, the constant Q 0 increased upon elevations of temperature 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 strength on desorption obtained in the desorp- tion tests. In the Jokioinen soil, on the contrary, the response of Q 0 to the changes of tempera- ture or ionic strength was less 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 or 0.0005 M; 0.00005 M not used in the desorption test). Discussion The quantity of labile adsorbed P on soil parti- cles is the ultimate reserve of P which can be desorbed, but the DRP concentration in runoff water is also controlled substantially by ionic strength and temperature, and to some extent by the solution-to-soil ratio. The P buffer power of soil tends to maintain a constant DRP concen- tration in water, and therefore more voluminous runoff markedly increases the total quantity of DRP removed from the field. The ionic strength of the water extracts at the solution-to-soil ratio 50 1 kg' 1 corresponded to that of rain and snowmelt water (0.3 mmol I 1) while those of the extracts obtained at wider so- lution-to-soil ratios were even lower in salts. As for ionic strength, the soil extracts obtained with 0.0005 M CaCl 2 were similar to the surface 199 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Yli-Halla, M. & Hartikainen, H.: Phosphorus desorption to runoff water runoff waters of the Jokioinen and Aurajoki fields (Yli-Halla et al. 1995), and those obtained with 0.005 M CaCl2 corresponded to soil solu- tion (Wiklander and Andersson 1974).As for the solution-to-soil ratio, it was observed in an ear- lier study that the average DRP concentration in the surface runoff in these particular fields was the same as that of soil extracts obtained at the solution-to-soil ratio between 250 and 500 1 kg 1 (Yli-Halla et al. 1995) but according to Ekholm (1994) the concentration of suspended solids in coastal river waters ofFinland is lower than was applied in the present study. The equations in Fig. 1 describing the dependence of P desorp- tion on ionic strength and solution-to-soil ratio thus cover 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. At a constant solution-to- soil ratio, decreasing ionic strength promotes P desorption (Hartikainen and Yli-Halla 1982). Therefore, it can be 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 of two factors promoting P desorption; widening solution-to-soil ratio and decreasing ionic strength. Besides DRP released in the field, surface runoff water transports P which is adsorbed onto the suspended soil material and which can be released as DRP in the recipient watercourse. As for the total DRP loading from the eroded mate- rial, the temperature during therunoff event may be unimportant because in the water body, the eroded material is subject also to higher tempera- tures during the summer months. Therefore, results obtained at a low temperature are needed to assess the DRP release in the field during the cool and wet season, while those obtained at higher temperatures are applicable to DRP re- lease by the summer rains and to desorption tak- ing place in a watercourse during the warmer period of the year. Increased speed ofdiffusion at elevated tem- peratures explains the cross-over of the Q/I plots. Below the EPC, net diffusion occurs from soil to solution, resulting in a higher P concentra- tion in the extract when the temperature is ele- vated. Above the EPC, net diffusion is from so- lution to soil, and an elevated temperature leads to a higher sorption. If diffusion were the only factor affected by the temperature, the curves should cross at the EPC. However, in both soils, elevation of the temperature seemed to shift the EPC to a higher concentration. At higher tem- peratures, a higher P concentration was required for sorption to start, or, vice versa, desorption continued to a higher P concentration. As a con- sequence, the crossing of the curves occurred at a P concentration above the EPC. At the EPC the net diffusion is zero. Based on the shift of the EPC, conclusions can be made on the tem- perature-dependency of the P exchange equilib- rium. The shift of EPC to a higher concentration indicates that a high temperature favors desorp- tion. This suggests sorption to be an exothermic reaction, as presented by Barrow (1979), and desorption to be an endothermic one. The pa- rameter Q stands for P sorption sites availa- ble. Its increase as a response to the elevated tem- perature can be taken to indicate that the satura- tion of the sorption sites is kinetically control- led. The Q/I plots can in principle be utilized to 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 at wide solution-to-soil ratios, e.g. at 2000 1 kg’ 1 and with other estimates ofP release from soil. Phosphorus bound to hydrous oxides of Al and Fe are the major reserves of bioavailable P in the watercourse (Dorich et al. 1985), and they control the level of water-ex- tractable P in soil (Hartikainen 1982a). Maxi- mum desorption in this study (solution-to-soil 200 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Vol. 5 (1996): 193-202. ratio 2000 1 kg ', 25°C) corresponded to 9 and 18% of the secondary P fractions (NH 4CI-P + NH 4F-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- ed to only 3 and 8% ofthe secondary P reserves. In an exhaustive pot experiment with one soil, Yli-Halla and Renlund (1990) measured a 30% decrease in these P fractions. If this decrease is taken to represent a measure of the maximum bioavaliability of soil P reserves, the Q 0 values markedly underestimate the potential P loading. Even the desorption measured at the widest so- lution-to-soil ratio (2000 1 kg 1 ) at 25°C may be smaller than the P amount that can be released from eroded soil in a watercourse. However, it should be mentioned that, particularly in the Aurajoki soil, Q 0 and the P release in the de- sorption tests responded similarly to changes of ionic strength and temperature. This shows that the Q/I plots qualitatively reflect the dynamic P exchange even though quantitative interpretation of the Q 0 values may be questionable. The present results show that depending on prevailing experimental conditions, a wide var- iation of P desorption results can be measured in a given soil, leading to different estimates for P loading. In most studies on soil samples, the Q/I plots have been determined using a 0.01 M supporting electrolyte (e.g. Barrow 1979). The information obtained from those Q/I plots is ap- plicable to P fertilization and the nutrition of plants. Phosphorus release to surface runoff or water bodies needs to be assessed by experiments performed at a low ionic strength and a wide (above 200 1 kg ‘) solution-to-soil ratio. Acknowledgements. This study was financially supported by the Academy of Finland. References Barrow, N.J. 1979. Three effects of temperature on the reactions between inorganic phosphate and soil. Jour- nal of Soil Science 30: 271-279. Beckett, P.H.T. & White, R.E. 1964. Studies on the phos- phate potentials of soils. 111.The pool of labile inorganic phosphate. Plant and Soil 21: 253-282. Dorich, R.A., Nelson, D.W. & Sommers, L.E. 1985. Estimating algal available phosphorus in suspended sedi- ments by chemical extraction. Journal of the Environ- mental Quality 14: 400-405. Ekholm, P. 1994. Bioavailability of phosphorus in agri- culturally loaded rivers in southern Finland. Hydrobiologia 287: 179-194. FAO 1988. FAO/UNESCO Soil Map of the World. Re- vised legend with corrections. World Resources Report 60. FAQ, Rome. Reprinted as Technical paper 20, ISRIC, Wageningen, The Netherlands. 140 p. Griffin, R.A. & Jurinak, J.J. 1 973. Estimation of activity coefficients from the electrical conductivity of natural aquatic systems and soil extracts. Soil Science 116: 26- 30. Hartikainen, H. 1979. Phosphorus and its reactions in terrestrial soils and lake sediments. Journal of the Sci- entific Agricultural Society of Finland 51: 537-624, -1982a. Water soluble phosphorus in Finnish mineral soils and its dependence on soil properties. Journal of the Scientific Agricultural Society of Finland 54: 89-98. -1982b. Relationship between phosphorus intensity and capacity parameters in Finnish mineral soils. 11. Sorption- desorption isotherms and their relation to soil character- istics, Journal of the Scientific Agricultural Society of Fin- land 54: 245-250, - & Simojoki, A. 1994. Response of soil phosphorus to acid loading. Transactions of the 15th World Congress of Soil Science 3b: 109-110. - & Yli-Halla, M. 1982. Chloride and sulphate solutions as extractants for soil P. I. Effect of ionic species and ionic strength on P desorption. Journal of the Scientific Agricultural Society of Finland 54: 287-296. Holford, I.C.R. & Mattingly, G.E.G. 1976. A model for the behavior of labile phosphate in soil. Plant and Soil 44: 219-229. Niskanen, R. 1989. Extractable aluminium, iron and manganese in mineral soils. 11. Extractability by oxalate and pyrophosphate. Journal of Agricultural Science in Finland 61; 79-87. Pietiläinen, 0.-P. & Ekholm, P. 1992. Origin of eroded material in a small agricultural drainage basin in south- western Finland. Aqua Fennica 22: 105-110. Pionke, H.B. & Kunishi, H.M. 1992, Phosphorus status and sontent of suspended sediment in a Pennsylvania watershed. Soil Science 153: 452-462. Turtola, E. & Jaakkola, A. 1995. Loss of phosphorus by surface runoff and leaching from a heavy clay soil under barley and grass ley in Finland. Acta Agriculturae Scan- dinavia Section B, Soil and Plant Sciences 45:159-165. 201 AGRICULTURAL AND FOOD SCIENCE IN FINLAND Yli-Halla, M. & Hartikainen, H.: Phosphorus desorption to runoff water Vuorinen, J. & Mäkitie, 0. 1955.The method of soil test- ing in use in Finland. Agrogeological Publications 63:1- 44. Wiklander, L. & Andersson, A. 1974. The composition of the soil solution as influenced by fertilization and nu- trient uptake. Geoderma 11: 157-166. Yli-Halla, M. 1991. Phosphate adsorption characteris- tics of two soils responding differently to P fertilization. Journal of Agricultural Science in Finland 63: 363-369. -, Hartikainen, H., Ekholm, P., Turtola, E., Puustinen, M. & Kallio, K. 1995. Assessment of soluble phospho- rus load in surface runoff by soil analyses. Agriculture, Ecosystems & Environment 56: 53-62. - & Renlund, S. 1990. Solubility of residual P. Transac- tions of the 14th International Congress of Soil Science II: 335-336. SELOSTUS Ympäristöolosuhteiden vaikutus maan fosforin liukenemiseen pintavalunnan aikana Markku Yli-Halla ja Helinä Hartikainen Helsingin yliopisto Pellolla kulkevaan pintavaluntaveteen liukenee fos- foria (P) sekä paikalleen jäävästä maasta että veden mukana kulkevasta eroosioaineksesta. Aurajoen ja Jokioisten huuhtoutumiskentiltä otettujen savimaa- näytteiden P-luovutuskykyä tutkittiin laboratorioko- kein. Kokeissa pyrittiin simuloimaan pintavalunnas- sa vallitsevia olosuhteita uuttamalla maasta P:a eri lämpötiloissa (5, 15 ja 25 °C), eri suolakonsentraa- tioissa (deionisoitu vesi tai 0,00005-0,005 M CaCI2 ) ja käyttäen erilaisia vesi-maasuhteita (50 - 2000 I kg'). Maa-aineksen kykyä sitoa ja luovuttaa P:a tutkit- tiin myös sorptio-desorptioisotermien avulla. Isoter- mit määritettiin ravistelemalla maata erivahvuisissa P-liuoksissa eri lämpötiloissa ja suolakonsentraatiois- sa. Kun uuttolämpötila nousi 5 °C;sta 25 °C:een, vesi-maasuhteella 50 1 kg 1 Aurajoen maasta veteen uuttuneen P:n määrä kasvoi arvosta 12,6 mg kg' 1 ar- voon 20,7 mg kg 'ja Jokioisten maassa arvosta 1,8 mg kg 1 arvoon 3,4 mg kg 1. Kun maata uutettiin deio- nisoidun veden asemesta maaveden suolapitoisuutta jäljittelevällä 0,005 M CaCI 2:lla, uuttuneet P-määrät Aurajoen maasta pienenivät 6,8 mg kg haan ja Jo- kioisten maasta 0,7 mg kg haan. Vesi-maasuhteen väl- jentäminen 2000 1 kghaan puolestaan lisäsi P:n uut- tumista veteen Aurajoen maassa kuusinkertaiseksi (119 mg kg haan) ja Jokioisten maassa 14-kertaisek- si (46 mg kg haan). Lämpötilan ja suolapitoisuuden kohotessa pidättyi maahan lisätystä P:sta yhä suurem- pi osuus. Saatujen tulosten perusteella voidaan pää- tellä, että pellolta tulevan pintavalunnan liukoisen P:n pitoisuuteen ja vesistökuormituksen suuruuteen vai- kuttavat oleellisesti maan helppoliukoisen P:n pitoi- suuden ohella myös valumaveden määrä, sen lämpö- tila ja suolapitoisuus. 202 AGRICULTURAL AND FOOD SCIENCE IN FINLAND