Agricultural phosphorus and water quality; sources, transport and management Andrew Sharpley, William Gburek USDA-ARS, Pasture Systems and Watershed Management Research Laboratory, Curtin Road, University Park, Pennsylvania 16802-3702, USA, e-mail: ans3@psu.edu Louise HealhwaiLe Department ofGeography, University ofSheffield, Winter Street, Sheffield, SlO 2TN, UnitedKingdom Freshwater eutrophication is usually controlled by inputs ofphosphorus (P). To identify critical sources of P export from agricultural catchments we investigated hydrological and chemical factors control- ling P export from a mixed land use (30% wooded, 50% cultivated, 20% pasture) 39.5-ha catchment in east-central Pennsylvania, USA. Mehlich-3 extractable soil P, determined on a 30-m grid over the catchment, ranged from 7to 788 mg kg' 1 . Generally, soils in wooded areas had low Mehlich-3P (<3O mg kg' 1), grazed pasture had Mehlich-3 P values between 100 and 200 mg kg' 1 , and cropped fields receiving manure and fertiliser applications were in most cases above 200 mg kg 1 . Average P con- centrations for ten storms during 1996 decreased 50% downstream from segment 4 to segment 1 (catchment outlet).Flow-weighted streamflow P concentrations were more closely related to the near- stream (within 60 m) than whole catchment distribution of high-P soils. This suggests that near- stream surface runoff and soil P are controlling P export from the catchment. Remedial measures should be targeted to these critical P source areas in a catchment. Measures include source (fertiliser and manure application) and transport management (reduce surface runoff and erosion). Key words: animal manure, catchments, critical source areas, drainflow, erosion, fertiliser, leaching, macropores, nonpoint source pollution, remediation, surface runoff, subsurface flow ntroduction Agricultural management and catchment char- acteristics in the Chesapeake Bay Basin (on the northeastern USA coast) are similar to those in Finland and justify their joint assessment in this paper. Compared with other enclosed seas, the Chesapeake Bay has an appreciably largercatch- ment area relative to water stored in the Bay (2410 km 2 km 3; Fig. 1). The GulfofFinland and Bothnian Bay, which both receive drainage from Finland, have the next highest ratios (380 and 180 km 2 km' 3 , respectively). Fertiliser use in both the Chesapeake Bay Basin and Finland has declined in the last 5 years, © Agricultural and Food Science in Finland Manuscript received February 1998 297 Vol. 7 (1998): 297- AGRICULTURAL AND FOOD SCIENCE IN FINLAND 5 but the growth and concentration of livestock operations has the potential to produce large amounts of manure and excreted nutrients in lo- calised areas. In many areas, there is an insuffi- cient land base available for efficient utilisation of nutrients in manure, resulting in large local- ised nutrient surpluses. In both areas, the more intensive agriculture tends to be located near main tributaries and water bodies, with forests generally occupying the outer perimeters of the catchments. Thus, agricultural management has the potential to have a large impact on the qual- ity of waters associated with the Chesapeake Bay Basin and Finland. Eutrophication has been identified as the main problem in surface waters having impaired water quality in Finland, UK and USA (HEL- COM 1993, Ministry of Agriculture, Fisheries and Food 1991, USEPA 1996). Eutrophication restricts water use for fisheries, recreation, in- dustry and drinking due to the increased growth of undesirable algae and aquatic weeds and ox- ygen shortages caused by their senescence and decomposition. Associated periodic surface blooms of cyanobacteria occur in drinking wa- ter supplies throughout the world and may pose a serious health hazard to livestock and humans (Kotak et al. 1993, Lawton and Codd 1991). Eutrophication also causes the loss of crucial habitats including aquatic plant beds in fresh and marine waters and coral reefs of tropical coasts. Recent outbreaks of the dinoflagellate Pfieste- ria piscicida in the eastern USA, in Chesapeake Bay tributaries in particular, have been linked to excess nutrients in affected waters. Neurologi- cal damage in people exposed to the highly tox- ic volatile chemical produced by this dinoflag- ellate has dramatically increased public aware- ness ofeutrophication and the need for solutions. This need is even greater when one realises that by the time these impacts are manifest, remedi- al strategies are often difficult and expensive to implement, they cross political and regional boundaries, and it can be several years before an improvement in water quality occurs. Eutrophication ofmost freshwater around the world is accelerated by P inputs (Kauppi et al. 1993, Sharpley et al. 1994, Schindler 1977). Al- though nitrogen (N) and carbon (C) are essen- tial to the growth of aquatic biota, most atten- tion has focused on P inputs because of the dif- ficulty in controlling the exchange of N and C between the atmosphere and water, and fixation of atmospheric N by some blue-green algae. Thus, P is often the limiting element and its con- trol is of prime importance in reducing the ac- celerated eutrophication of fresh waters. As sa- linity increases, N generally becomes the ele- Fig. 1. Ratio of catchment area to volume of water in several major lakes, bays and seas of the world. 298 Seminar in honour of the 100th anniversary ofMTT AGRICULTURAL AND FOOD SCIENCE IN FINLAND ment controlling aquatic productivity. This is true for both the Chesapeake Bay and Finnish coast- al waters, where P tends to be the limiting nutri- ent in the upper fresh and brackish water reach- es, while N is limiting in tidal saline waters (Re- kolainen 1993,Thomann and Mueller 1987). In the Chesapeake Bay, 61% of P inputs orig- inate from agricultural nonpoint sources, while they contribute 79% of P in Finnish coastal wa- ters (Chesapeake Bay Program 1995, Rekolai- nen et al. 1997). In response, the Finnish gov- ernment decided in 1988 that agricultural P in- puts to freshwaters should be reduced 30% by 1995 (Ministry of the Environment 1988). Sim- ilarly, a 40% reduction in P inputs to Chesapeake Bay has been mandated by the year 2000 (Ches- apeake Bay Program 1995). Greater than expect- ed reductions in P discharges from wastewater treatment plants have occurred over the last 15 years (Chesapeake Bay Program 1995,Rekolai- nen et al. 1992). Even so, water quality problems remained. As further point source controls be- come less cost-effective, more attention is be- ing directed towards implementing nutrient man- agement plans and farm conservation practices to reduce P inputs from upstream sources. Wholesale change to current systems could have severe socio-economic impact in rural areas. Therefore, measures are needed which maxim- ise environmental benefit while minimising eco- nomic hardship to farmers and the wider com- munity. Thus, there is a need to understand the controlling processes by which P gets from its source in a catchment to water, and the impact of land management on these processes, in or- der to design, target and implement effective re- medial strategies. Background Sources The rapid growth and intensification of the live- stock industry in certain areas of the USA, UK and Europe, has created national and regional imbalances in system inputs and outputs of P (Kronvang and Svendsen 1991, Isermann 1991, Withers 1996). On average, only 30% of the fer- tiliserand feed input to farming systems is out- put in crop and livestock produce. Thus, when averaged over the total utilisable agricultural land area in the USA, an annual P surplus of 26 kg ha 1 exists (National Research Council 1993). The annual P surplus in the UK is around 10 kg ha 1 (Withers 1998). During the 1980s, there was an annual net P input in Finland of about 25 kg ha' 1 (Rekolainen et al. 1992). Although fertiliser P applications have declined in the 19905, there has been a substantial accumulation of P in the fields of many farms (Rekolainen 1997). Prior to World War 11, farming communities tended to be self-sufficient in that enough feed was produced locally and recycled to meet live- stock requirements. As a result, a sustainable food chain tended to exist. After World War 11, increased fertiliser use in crop production frag- mented farming systems, creating specialised crop and livestock operations that efficiently coexist in different regions within and among countries. By 1995, over half the corn grain pro- duced in the USA cornbelt was exported as ani- mal feed, while states in the eastern USA im- ported 83% of their grain for confined livestock operations (Lanyon and Thompson 1996). In fact, less than 30% of the grain produced on farms today is fed on the farm where it is grown (USDA 1989). The addition of more P to an area than is re- moved in crop harvest for several years can in- crease soil test P (Fig. 2). As manure applica- tion rate recommendations are routinely based on their N content and crop N requirement to minimise the purchase of commercial fertilizer N and risk of nitrate leaching, the mainresult of this imbalance has been an increase in soil test P. In 1989, several state soil test laboratories re- ported the majority of soils analysed had soil test P levels in the high or very high categories which require little or no P fertilisation (Fig. 3). How- ever, within states, distinct areas of general P deficit and surplus can also exist. For example, 299 Vol. 7(1998): 297-314. AGRICULTURAL AND FOOD SCIENCE IN FINLAND soil test summaries for Delaware and Pennsyl- vania indicate the magnitude and localisation of high soil test P levels that can occur in areas dominated by intensive livestock production (Fig. 3). In Lancaster County Pennsylvania, where agriculture is dominated by livestock and poultry production, 77% of soils were rated as optimum or above (>sl mg P kg 1 ; as Mehlich-3 test P) in 1996; nearby Adams County, with or- chard and crop production, was dominated (70%) by low and medium soil test P (2OO 1 2.34 86 26 118 14 404 16 44 9 30 2 8,92 222 99 166 7 788 43 13 6 38 3 4.70 106 52 199 21 449 6 16 39 39 4 23.58 332 262 141 10 775 41 9 21 29 Total 39.54 746 439 168 7 788 34 14 19 33 Phosphorus analyses Dissolved P was determined on filtered (0.45 |im) stream water samples by the molybdenum- blue method of Murphy and Riley (1962). The same method was used for TP following diges- tion of unfiltered surface runoff water with a semimicro Kjeldahl procedure (Bremner and Mulvaney 1982). Algal-available P was deter- mined using Fe-oxide impregnated strips (Shar- pley 1993). Five mL of unfiltered surface runoff (made up to 50 mL with distilled water) and one Fe-oxide strip were shaken end-over-end for 16 h at 4°C. The strip was removed, rinsed free of soil particles, and shaken end-over-end for 1 h in 1 M HCI to remove AAP Mehlich-3 soil P concentration was deter- mined by extraction of 1 g soil with 10 mL of 0.2 M CH,COOH, 0.25 M NH4N03 , 0.015 M NH 4F, 0.013 M HN0 3 and 0.001 M EDTA for 5 min (Mehlich 1984). Phosphorus in all filtered and neutralised extracts was determined by the method of Murphy and Riley (1962). Results and discussion Soil P distribution On a 30-m grid over the catchment, Mehlich-3 P ranged from 7 to 788 mg kg' 1 (Table 1). The Mehlich-3 soil P values were grouped into four categories based on agronomic and environmen- tal factors: <3O mg kg" 1, crops require addition- al P for optimum growth; between 30 and 100 mg kg ', there will generally be a crop response to P application but little enrichment of P in sur- face runoff(probable crop response decreases as Mehlich-3 P increases from 50 to 100 mg kg '); between 100 and 200 mg kg 1 , there will be no response to applied P while some enrichment of P in surface runoff may occur; >2OO mg kg ', levels are considered excessive in terms of crop requirements and enrichment ofP in surface run- off can be expected (Beegle 1996, Sharpley et al. 1996). The pattern of Mehlich-3 P values over FD- -36 is generally a function of land use and field boundaries within the catchment (Fig. 7). Soils in wooded areas have low values of Mehlich-3 P (<3O mg kg '), grazed pastures have values be- tween 100 and 200 mg kg 1 , and cropped fields receiving manure and fertiliser applications are, in most cases, above 200 mg kg" 1 . Based on the grid sampling, 52% of the soils on FD-36 have Mehlich-3 P concentrations in excess of levels sufficient for optimum crop growth (>lOO mg kg 1), with 33% above 200 mg kg ' (Table 1). Of the remaining 48% of soils, P application would be recommended on only 14% for optimum crop production (30-100 mg kg" 1 ) as the other 34% are mostly wooded (<3O mg kg 1 ) (Table 1). Streamflow P Average flow-weighted DP, AAP and TP con- centrations in streamflow leaving each of the four catchment segments were determined for each 305 Vol. 7(1998): 297-314. AGRICULTURAL AND FOOD SCIENCE IN FINLAND storm event from August to the beginning of November 1996 (Table 2). For all events, aver- age P concentration decreased downstream from segment 4 to segment 1 (the catchment outlet). On average, DP concentration decreasedby 60%, AAP by 56% and TP by 59%. Also, DP com- prised 54% and 60% ofAAP at segments 4 and 1, respectively, while AAP was 53% of TP at segment 4 and 49% at segment 1. Although the concentration decrease and distribution of DP and AAP were similar between segment 4 and the catchment outlet, the relative importance of controlling hydrological or chemical processes will likely vary along the stream channel. These processes may include dilution by input of sub- surface flow to the stream channel, deposition and resuspension of particulate material and as- sociated P, sorption of DP by suspended sedi- ment and channel bank/bed material, and a dif- ferential contribution ofP in surface runoff from spatially variable areas of surfacerunoff produc- tion and high soil P. From the above analysis, it is apparent that the distribution of P forms (DP, AAP and TP) in streamflow changed little during transport along the channel. Dissolved P averaged 29% of TP and AAP 50% of TP at each segment flume (Ta- ble 2). Also, the decline in P concentration from segment 4 to segment 1 (watershed outlet) was similar for DP, AAP and TP (56% to 60%). This suggests that channel processes may be relatively unimportant compared with variations in source area input among segments. While we are continuing this investigation of the controls on processes ofP loss, comparison of stormflow P concentration and soil P distri- bution patterns over the catchment may provide insight into the linkages between high P soils and surface runoff-producing areas. Estimated widths of saturated areas adjacent to the stream channel ranged from <1 to 62 m and showed a general increase downstream from segment 4 to 1 (P2OO mg kg’ 1 Mehlich-3 P (29% to 39%, Table 1 and Fig. 8). This is the Mehlich-3 P category that is expected to result in enrichment of DP in sur- face runoff. However, on a near-stream basis, the areal distribution of these high P soils decreased from 50% in segment 4to 8% in segment 1. Thus, the trend of decreasing stormflow DP concen- tration downstream was more closely related to 307 Vol. 7(1998): 297-314. AGRICULTURAL AND FOOD SCIENCE IN FINLAND Table 3. Saturated distance from the stream channel for catchment segments during each flow event in 1996. Flow Catchment segment Event 12 3 4 m 9 August 10.43.2 0.61.9 6 September 1.40.2 0.10.3 7 September 5.40.9 0.51.9 13 September 5.01.0 0.61.0 16 September 12.94.1 10.75.7 17 September 17.65.4 14.65.6 28 September 20.14.2 4.73.8 9 October 2.20.9 0.60.5 18 October 54.636.6 62.225.9 8 November 17.038.7 35.920.4 Average 14.79.5 13.06.7 the near-stream distribution of high P soils in each catchment segment than to the whole catch- ment (Fig. 8). This integration of hydrological processes and chemical properties of catchment soils suggests that near-stream soil P concentra- tion has a greater influence on P export from the catchment than does soil P concentration at the whole-catchment scale. These findings have important implications for catchment management of P from fertiliser or manure applications. For instance, current thinking may set P management goals based sole- ly on Mehlich-3 P concentrations for soils over the entire catchment (Sharpley et al. 1996). In this case, nearly 80% of the cropped and pasture soils overFD-36 are sufficiently high in P (>IOO mg kg' 1) that there would be no crop-yield re- sponse to further P applications. An environmen- tal soil test P level of 200 mg kg ' Mehlich-3 P has been proposed by several states in the USA as a threshold level above which P enrichment of surface runoffand increase in P export is like- Fig. 8. Distribution of soils with Mehlich-3 P >2OO mg kg ' on whole-catchment and near stream (>6O m) basis and mean flow- weighted dissolved P concentra- tion in streamflow from each seg- ment for August to November 1996. 308 Seminar in honour of the 100th anniversary ofMTT AGRICULTURAL AND FOOD SCIENCE IN FINLAND ly, indicating P applications shouldbe more care- fully managed in these areas (Sharpley et al. 1996). Based on this, application of P to 63% of the cropped area of FD-36 would be limited or restricted. Clearly, this would adversely impact those farmers having confined swine and poul- try operations on the catchment where produced manures are presently applied. Alternatively, delineation of surface runoff- producing areas and recognition of the similari- ty between patterns of P concentration in stream- flow and P concentration of near-stream soils suggests that P management goals should focus on the near-stream areas rather than the whole catchment. With this approach, accounting for the interactions among soil P, land use and hy- drological characteristics of the catchment, it is possible to better target remedial programs to critical P source areas of the catchment. Implications for remediation Phosphorus export from agriculture may be min- imised with source and transport management strategies. Although we know how, and have generally been able to reduce P transport from tilled land in surface runoff and erosion, less at- tention has been directed toward other land uses (e.g. grassland) and source management. For example, it is clear from the extent of soils with P in excess of levels sufficient for optimum crop yields, as in FD-36, that more attention should be paid to avoiding soil P build-up via P-source management. General remedial measures that minimise P export from agricultural catchments are presented, with reference to results from FD- -36, where appropriate. Source management Manures Manipulation of dietary P intake by livestock may help reduce regional surpluses of P. Morse et al. (1992) recorded a 17% reduction in P ex- cretion when dairy cows reduced their daily P intake from 82 to 60 g day'. In the Netherlands, reductions in concentrate P are now being im- plemented to help reduce the amounts ofP ex- creted to land (Wadman et al. 1987). Enzyme additives for livestock feed that increase P ab- sorption efficiency during digestion and weight gain are also being tested. One example is the use of phytase, an enzyme that enhances the ef- ficiency of P recovery from phytin in grains fed to poultry. This has the potential to reduce P concentration in poultry manures and litters. Commercially available manure amend- ments, such as slaked lime or alum, can reduce NH, volatilisation and P solubility ofpoultry lit- ter by several orders of magnitude (Moore and Miller 1994). Also, the DP concentration of sur- face runoff from fescue treated with alum- amended litter (11 mg L') was much lower than from fescue treated with unamended litter (83 mg L 1; Shreve et al. 1995). Perhaps the most important benefit of manure amendments (for both air and water quality), however, will be an increase in the N;P ratio of manure via reduced N loss from manure by NH, volatilisation. An increased N:P ratio of manure would better match crop N and P requirements. Thus, addi- tions of manure based on crop N requirements would reduce the P excess added, thereby mini- mising potential soil P accumulations. Localised surpluses of P are exacerbated by the fact that manures are rarely transported more than 20 km from where they are produced. How- ever, mandatory transport of manure from sur- plus areas to nearby farms where the nutrients are needed faces several significant obstacles. First, it must be shown that the current location is unsuitable, based on soil properties, crop nu- trient requirements, topography and hydrology. From European experiences this may be diffi- cult to justify scientifically due to the large tem- poral and spatial variability in the factors con- trolling N and P mobility in soils and transport to ground or surface waters. Second, in many areas there is no clearly defined legal basis for requiring farmers in one physiographical area to 309 Voi 7 (1998): 297-314. AGRICULTURAL AND FOOD SCIENCE IN FINLAND perform management practices that are not re- quired on neighbouring farms. More success with re-distribution of manures is likely to occur when consumers, local governments, the farm commu- nity and livestock industry are all involved in setting regional policies. Soils In parts of the world, regional authorities are considering development of recommendations for P applications based on the potential for P loss in surface runoff, as well as on crop P re- quirements. A major difficulty in development of these recommendations has been the identifi- cation of threshold levels of soil P that are like- ly to result in unacceptable losses of P in sur- face runoff. Establishing these levels is a con- troversial process for two reasons. First, the data base relating soil P levels to surface runoff P concentration is limited to a few soils and crops, and there is a reluctance to extrapolate data of this type to other regions. Second, the economic implications of establishing soil test P levels which may limit manure applications are signif- icant. In many areas dominated by animal-based agriculture, there simply is no economically vi- able alternative to land application. Thus, there is a need to assess the validity of using soil test P values as indicators of P loss in surface run- off. In FD-36, for example, manure application to over 60% of the catchment would be limited by a soil test P threshold of 200 mg kg '. Another approach developed in the Nether- lands and applicable to subsurface pathways of P transport, determines the potential for DP movement in drainage water by estimating soil P saturation as the percentage of P sorption ca- pacity as extractable soil P (Breeuwsma and Sil- va 1992). This approach is based on the fact that more P is released from soil to matrix flow or leaching water as P saturation or amount of P sorbed increases with P additions. Soil P satura- tion is used in the Netherlands, where farm rec- ommendations for manure management are de- signed to limit the loss ofP in surface and ground waters. For Dutch soils, a critical P saturation of 25% has been established as the threshold value above which the potential for P movement in surface and ground waters becomes unaccepta- ble (Breeuwsma and Silva 1992). Transport management Once water and sediment begin to move over the land surface, taking with them the nutrients orig- inally applied as fertiliser and/or manure, the quantities which reach the stream can be reduced by any feature which slows flow and/or encour- ages infiltration or sediment trapping. Such measures include terracing, contour tillage, cover crops, buffer strips, riparian zones, and impound- ments or small reservoirs. These practices are generally more efficient at reducing particulate P rather than DP. However, such approaches only work where subsurface pathways of P loss are unimportant. Furthermore, by encouraging infil- trationof surface runoff, which may be enriched with P, the problem is simply translated from surface delivery to subsurface delivery. While uptake by plant roots and adsorption onto soil particles may delay the delivery of P to surface waters, such mechanisms may be ineffective in soils with a high hydraulic conductivity (e.g. sands) or where macropore or drainflow is im- portant (Heathwaite 1997). Usually, farm N inputs can be more easily balanced with plant uptake than can P, particu- larly where confined animal operations exist. In the past, separate strategies for N and P have been developed and implemented at farm or catchment scales. Because of differing chemistry and flow pathways of N and P in soil and through the catchment, these narrowly targeted strategies often are in conflict and lead to compromised water quality remediations. For example, basing manure application on crop N requirements to minimise nitrate leaching to ground water in- creases soil P and enhances potential P surface runoff losses. In contrast, reducing surface run- off losses of P via conservation tillage can en- hance nitrate leaching. For P, a primary strategy is to minimise sur- face runoff and particulate transport. In most 310 Seminar in honour of the 100th anniversary ofMTT AGRICULTURAL AND FOOD SCIENCE IN FINLAND cases the necessary measures - soil cover as plants or crop residues, cultivation along con- tours, and buffer zones - have a neutral or bene- ficial impact on nitrate loss. An exception is ploughing, which if carried out in early autumn tends to increase leaching if the soil is not fro- zen. Another exception follows the conversion of conventional to no till practices. In the USA, where no till is commonly recommended as a conservation measure to reduce erosion, conver- sion to no till was followed by a decrease in soil, total N and total P loss in surface runoff but an increase in nitrate leaching and AAP transport (Sharpley and Smith 1994). Nitrogen and P management strategies may differ because N losses can occur from any lo- cation in a catchment, while areas prone to sur- face runoff contribute most to P loss. Hence, for N, remedial strategies may be applied to the whole catchment, whereas the most effective P strategy would be to apply simple measures to the whole catchment to avoid excessive nutrient buildup, and thereby limit losses in subsurface flow, and more stringent measures to the most vulnerable sites to minimise loss of P in surface runoff. These positive and negative impacts of con- servation practices on resultant water quality should be considered in the development of sound remedial measures. Clearly, a technically sound framework must be developed that in- cludes critical sources of N and P export from agricultural catchments so that optimal strate- gies at farm and catchment scales can be imple- mented to best manage both N and P. Conclusions Issues facing agronomic and environmental P management in agricultural systems are similar in most developed countries. Specialised farm- ing systems within and between these countries have tended to dismantle natural P cycles, re- sulting in an imbalanced flow of P from areas of fertiliser manufacture and grain production to areas of intensive crop and livestock operations. As a result, localised areas of high soil P can occur near areas of low soil P fertility. In less- developed countries, however, socio-economic constraints generally limit P use such that many soils are still deficient in P with respect to that needed for crop production. Many farm plans addressing P management assume that if erosion is controlled through soil conservation measures, so will P losses. Less attention has been directed to a source-based management of P at field, farm or catchment scales. As a result, soil P has generally increased in localised areas of intensive crop and livestock production, and increased losses of P in surface runoff and subsurface flow water are more fre- quently noted. Although the relationship between soil and mobilised P has not been quantified over wide areas, it is clear that the potential for P loss in surface runoff and subsurface flow, and thereby, accelerated eutrophication, increases as soil P accumulates. Unfortunately, soil P reduction via crop removal is slow; levels will be elevated for several years after application has ceased. Also, chemical amendments such as alum, fly ash, gypsum and iron compounds reduce the solubil- ity of soil P, not total amounts, and are thus only temporary measures. To a certain extent, these concerns have not been addressed because man- aging agricultural inputs and outputs of P is of- ten much more costly and restrictive to a farmer than is general N management. As a result, N continues to drive manure management decisions and exacerbates the build-up of soil P. It is often too simplistic to use threshold or change-point soil P levels as the sole criterion to guide P management and P applications. These values will have little meaning unless they are used in conjunction with an assessment of a site’s potential to mobilise P in surface runoff, erosion and subsurface flow. Thus, preventing P loss should take on the added dimension of defining, targeting and remediating source areas of P that combine high soil P levels with high erosion and surface runoff potentials. As a result, differing 311 Vol. 7(1998): 297-314. AGRICULTURAL AND FOOD SCIENCE IN FINLAND levels of management may be suggested for dif- ferent areas of a catchment, an approach to land management which will have to be addressed by action agencies. Without incorporation of source area perspectives to target application of P fer- tility, surface runoff and erosion control tech- nology, conventionally applied remediations may not produce the desired results and may prove to be inefficient and non-cost effective. Efforts to increase our understanding of P cycling in terrestrial ecosystems and develop technically sound, defensible remedial strategies that minimise P loss from agricultural land will require interdisciplinary research involving soil scientists, hydrologists, agronomists, limnolo- gists and animal scientists. As importantly, de- velopment of guidelines to implement such strat- egies will also require consideration of the so- cio-economic and political impacts of any man- agement change on both rural and urban com- munities, and the mechanisms by which change can be achieved in a diverse and dispersed com- munity of land-users. References Barber, S.A. 1979. 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MAFF Environmental protection Newsletter 3: 7. Yli-Halla, M., Hartikainen, H., Ekholm, P., Turtola, E., Puustinen, M. & Kallio, K. 1995. Assessment of solu- ble phosphorus load in surface runoff by soil analy- ses. Agricultural Ecosystems Environment 56: 53- 62. SELOSTUS Maatalous, fosfori ja veden laatu: alkuperä, kulkeutuminen ja vesistökuormituksen hallinta Andrew Sharpley, William Gburek ja Louise Heathwaite USDA-ARS, Pasture Systems and Watershed Management Research Laboratory, USA ja University ofSheffield, Englanti Veteen joutuvan fosforin määrä säätelee makeiden vesien rehevöitymistä. Maatalousvaltaiselta valuma- alueelta tulevan fosforin alkuperää selvitettiin maan- käytöltään vaihtelevalla alueella Pennsylvaniassa, USA:ssa tutkimalla valuma-alueen hydrologiaa ja maan fosforipitoisuutta. Alueen 39,5 ha:sta 30 % oli metsää, 50 % peltoa ja 20 % laidunta. Mehlich-3 -menetelmällä määritetty maan fosforipitoisuus oli 7 788 mg kg '. Metsäalueiden maan fosforipitoisuus oli pieni (<3O mg kg 1 ), laitumilla mitatut arvot vaihte- livat 100-200 mg kg 1 ja väkilannoitteita ja lantaa saa- neilla vilja- ja soijapelloilla fosforipitoisuudet ylitti- vät usein 200 mg kg 1. Alle 60 metriä laskuojasta ole- van maan fosforipitoisuus näytti säätelevän ojaveden fosforipitoisuutta enemmän kuin koko valuma-alueen maan fosforipitoisuus. Viljelytekniset vesiensuojelu- toimet tulisikin kohdentaa valuma-alueen tähän osaan, jolla on suurin vaikutus alueelta lähtevän va- lumaveden fosforipitoisuuteen. Maatalousmaalta tu- levaa fosforikuormitusta voidaan säädellä rajoittamal- la maahan lisättävän fosforin määrää ja vähentämäl- lä eroosiota ja pintavaluntaa. 314 Seminar in honour of the 100th anniversary ofMTT AGRICULTURAL AND FOOD SCIENCE IN FINLAND