View of Agricultural phosphorus and water quality: sources, transport and management

Agricultural phosphorus ^{and water} quality; ^sources,

transport and management

Andrew Sharpley, William Gburek

USDA-ARS, Pasture Systemsand WatershedManagement^ResearchLaboratory,^CurtinRoad, UniversityPark, Pennsylvania 16802-3702,USA, e-mail:ans3@psu.edu

Louise HealhwaiLe

DepartmentofGeography, Universityof^Sheffield, Winter Street, Sheffield, SlO^2TN, UnitedKingdom

Freshwatereutrophicationisusually controlledby inputsofphosphorus (P). Toidentifycriticalsources ofPexport fromagriculturalcatchmentsweinvestigated hydrological and chemical factors control- ling Pexport froma^{mixed land}use(30% wooded, 50% cultivated, 20% pasture) 39.5-ha catchment ineast-centralPennsylvania, USA.Mehlich-3 extractable soil^P, determinedon a30-mgridover^the catchment,ranged from^7to788mgkg'1^.Generally,soilsⁱⁿwoodedareas ^{had low}Mehlich-3P (<3O mgkg' 1^),grazedpasture had Mehlich-3Pvalues between 100and200 mgkg'1^{, and croppedfields}

receiving manureand fertiliserapplicationswere^{in most}cases^{above 200}mgkg ^1.Average Pcon- centrations for ^{ten storms} during 1996 decreased 50% downstream from segment 4 to segment 1 (catchment outlet). Flow-weighted streamflowPconcentrationswere morecloselyrelated to thenear- stream (within 60m) than whole catchment distribution ofhigh-P soils. This suggests thatnear- stream surface runoff and soilP are controlling P export from the catchment. Remedial measures should betargetedto these criticalP^sourceareasin^acatchment. Measures include ^source(fertiliser andmanure application) and transport management(reduce surface runoff anderosion).

Key words: animal manure,catchments,criticalsourceareas,drainflow,erosion, fertiliser,leaching, macropores,nonpointsourcepollution, remediation,surface runoff, subsurface flow

ntroduction

Agricultural management and catchment char- acteristics in the Chesapeake Bay Basin (on the northeastern USAcoast) are similarto those in Finland and justify their jointassessmentin this paper. Compared with other enclosed ^seas,the

ChesapeakeBay hasanappreciably largercatch- ment area relative to water stored in the Bay (2410km²km ^3;Fig. 1).The Gulfof Finland and Bothnian Bay, which both receive drainage from Finland, have the next highest ratios (380 and

180 km²km'³,respectively).

Fertiliser use in both the Chesapeake Bay Basin and Finland has declined in the last 5^years,

© Agriculturaland Food ScienceinFinland ManuscriptreceivedFebruary 1998

Vol.7 (1998): 297-

but the growth and concentration of livestock operations has the potential toproduce large amounts ofmanureand excreted nutrients in lo- calised areas. Inmanyareas, there is aninsuffi- cient land base available for efficient utilisation of nutrients inmanure,resulting in large local- ised nutrient surpluses. In bothareas, themore intensive agriculture tends^to be located near main tributaries and water bodies, with forests generally occupying the^outerperimeters of the catchments. Thus, agricultural management has the potentialto havealarge impact on the qual- ity ofwatersassociated with the Chesapeake Bay Basin and Finland.

Eutrophication has been identifiedas the main problem in surfacewatershaving 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 dueto the increased growth of undesirable algae and aquatic weeds and ox- ygen shortages caused by theirsenescence and decomposition. Associated periodic surface blooms of cyanobacteriaoccur in drinking _wa- tersupplies throughout the world and may pose a serious health hazardtolivestock 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 marinewatersand coral reefs of tropicalcoasts.

Recent outbreaks of the dinoflagellate

Pfieste-

riapiscicida in the^easternUSA, in Chesapeake Bay tributaries in particular, have been linkedto excess nutrients in affected waters. Neurologi- cal damage in people exposed^tothe highly ^tox- ic volatile chemical produced by this dinoflag- ellate has dramatically increased public aware- nessof eutrophication and the need for solutions.

This need iseven greater whenonerealises that by the time these impacts are manifest,remedi- al strategiesare often difficult and expensiveto implement, they cross political and regional boundaries, and itcan be several years before an improvement in waterquality occurs.

Eutrophication ofmostfreshwater around the world is accelerated by P inputs (Kauppi etal.

1993, Sharpley^etal. 1994, Schindler 1977).Al- though nitrogen (N) and carbon(C) are essen- tial to the growth of aquatic biota, most atten- tion has focused _onP 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 itscon- 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 areato volume of waterinseveral major lakes,bays andseasof the world.

298

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mentcontrolling aquatic productivity. This istrue for both the Chesapeake Bay and Finnishcoast- alwaters,where P tends^tobe the limiting nutri- entin the upper fresh and brackish waterreach- 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 coastalwa- ters(Chesapeake Bay Program 1995, Rekolai- nen ^etal. 1997).In response, the Finnish gov- ernmentdecided in 1988 that agricultural P in- puts to freshwaters should be reduced 30% by

1995 (Ministry of the Environment 1988). Sim- ilarly,a40% reduction in P inputstoChesapeake Bay has been mandated by the year 2000^(Ches- apeake Bay Program 1995). Greater thanexpect- ed reductions in P discharges fromwastewater treatmentplants have occurred _overthe last 15 years (Chesapeake Bay Program 1995, Rekolai- nenetal. 1992).Evenso,waterquality problems remained. As further point source controls be- come less cost-effective, more attention is be- ing directed towards implementing nutrientman- agementplans and farm conservation practices to reduce P inputs from upstream sources.

Wholesale changeto currentsystemscould have severe socio-economic impact in rural areas.

Therefore, measures are needed which maxim- ise environmental benefit while minimisingeco- nomic hardship tofarmers and the widercom- munity. Thus, there is a needtounderstand the controlling processes by which P gets from its source in a catchment to water,and the impact of landmanagement onthese processes, in or- dertodesign,targetand implement effectivere- medial strategies.

Background

Sources

The rapid growth and intensification of the live- stock industry in certain areas of theUSA, 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- tiliser and feed inputtofarming systemsis out- put in crop and livestock produce. Thus, when averagedover the total utilisable agricultural landareain theUSA, an annualPsurplus of 26 kg ha¹exists (National Research Council 1993).

The annual P surplus in the UK is around 10 kg ha

1

^(Withers 1998). During the 1980

s,

therewas anannualnetP input in Finland of about 25 kg ha'

1

(Rekolainen_etal. 1992). Although fertiliser P applications have declined in the 19905, there has beenasubstantial accumulation of P in the fields of many farms (Rekolainen 1997).

PriortoWorld War 11, farming communities tendedto be self-sufficient in that enough feed wasproduced locally and recycledto meet live- stock requirements. As a result, a sustainable food chain tendedtoexist. 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,overhalf thecorngrain pro- duced in the USA cornbeltwasexported _asani- 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 onthe farm where it is grown (USDA 1989).

The addition ofmorePtoan area than is re- moved in crop harvest for several yearscan in- crease soil testP (Fig. 2). As manure applica- tion^rate recommendations areroutinely based on their N contentand crop N requirement to minimise the purchase of commercial fertilizer N and risk of nitrate leaching, the mainresult of this imbalance has beenanincrease in soiltestP.

In 1989, several state soil test laboratories re- ported the majority of soils analysed had soiltest

P levels in the high orveryhigh categories which require little_{or no}P fertilisation (Fig.3). How- ever, within states, distinct areas of general P deficit and surplus canalso exist. For example, Vol. 7(1998): 297-314.

soil test summaries for Delaware and Pennsyl- vania indicate the magnitude and localisation of high soil test P levels thatcan 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 soilswere rated as optimumorabove(>sl mgP kg ^1;asMehlich-3 test P) in 1996; nearby Adams County, withor- chard and crop production,wasdominated (70%) by low and medium soil testP (<SO mg P kg ^1; Fig. 3).

Fig. 2. Bray-1 extractableP contentof the surface (0- 15cm) ofaRaub silt loam inIndianaas afunction of the difference betweenPinputasfertiliser and outputinhar- vested crop (adapted from Barber 1979).

Fig. 3Percent of soilstesting highorabove forP in 1989for the Northeast USA. Also shown is percent of soils rated aslow, medium,optimumorhigh from1995soil test summaries for Delaware (DE; Mehlich-1) andPennsylvania(PA; Mehlich-3) counties with little animalproduction(New Castle and Adams Co.) and with concentration of livestock production(Sussex and Lancaster Co.).

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Transport

The loss of P in surface runoffoccurs in sedi- ment-bound and dissolved forms (Fig. 4). Sedi- mentP includes P associated with soil particles and organic mattereroded during floweventsand constitutes the major proportion of Ptransport- ed frommost cultivated land(60-90%; Pietiläi- nenand Rekolainen 1991, Sharpleyetal. 1994).

Surface runoff from grass, forest landor non- erosive soils carries little sediment andis,there- fore,generally dominated by dissolved P (DP), although Ptransportattachedto colloidalmate- rial may be important (Haygarth and Jarvis 1997, Heathwaiteetal. in press). While DP is,for the mostpart, immediately available for biological uptake, sediment Pcanbealong-termsourceof P for aquatic biota (Ekholm 1994, Krogstad and Lovstad 1991, Sharpley 1993).The bioavailabil-

ity of sediment P ranges from 5% to 90% de- pending on the ^nature of the eroding soil and receivingwaters (Boström etal. 1988, Rekolai- nenetal. 1997).

Loss of P from land surfaceto streamiscon- trolled primarily by the interaction of P “source”

factors (functions ofsoil,crop andmanagement) with its “transport” factors (surface runoff, ero-

sion, subsurface flow and channel processes) (Fig. 4). As thesourcesofparticulate^Pinstreams include eroding surface soil, streambanks and channelbeds, processesdetermining soil erosion also control particulate P transport.The excep- tion is particulate Ptransportin macropores and drainflow where colloidal P transport may be important (Dilsand Heathwaite 1996).In gen- eral,the P contentand reactivity of eroded par- ticulate materialare greaterthan those ofsource soil,due topreferential transportof finermate- rial(<2fim). Thetransportof DP in surfacerun- off is initiated by the desorption, dissolution and extraction of P from soil and plant material.

These processes occur as a portion of rainfall interacts withathin layer of surface soil(1 to5cm) before leaving the field as surface runoff (Sharpley 1985).Although this depth is difficult toquantify in thefield,it is expectedtobe high- ly dynamicdue to variations in rainfall intensi- ty,soil tilth and vegetativecover.

Several studies have reported that the loss of DP in surface runoff is dependent on the soil P contentof surface soil (Fig. 5), but the specific DP^- soil P relationship varies with_management and soiltype(Sharpleyetal. 1996, Sibbesen and Sharpley 1997, Yli-Hallaetal. 1995). Regres-

Fig.4.Inputs,outputsandprocesses important to transportof Ptosurface watersinagricultural ecosystems.

Voi7(1998): 297-314.

sion slopes tendtobe lower forgrass than for cultivated land, but values are too variable to allowuse ofasingleoraveragerelationship for recommending P amendments based on water quality criteria. Clearly, several soil and land managementfactors will influence the relation- ship between DP in surface runoff and soil P.

The P content ofwaterpercolating through the soil profile is generally lower than for sur- face runoff, and will decrease as the degree of

soil^- water contact increases duetosorption of Pby P-deficient subsoils. While this generalisa- tion is true for matrix flow through soils, ma- croporeor bypass flow, together with P trans- port in artificialdrains, may showpatterns and magnitudes of P loss moresimilartothat ofsur- face runoff (Oils and Heathwaite 1996, Heathwaiteetal. in press). Some soil types are susceptible ^to P transport in matrix flow. For example, organic or peaty soils, where organic mattermay accelerate the downwardmovement of P together with organicacids,Fe and Al.

Phosphorus ismoresusceptibleto movement through sandy soils with low P sorption capaci- ties and also through soils that have become

waterlogged. In total though, the loss of P in

subsurface flow aswell as in surface runoff, is linked to soil P concentration (Sharpley et al.

1977),although thenature of the relationship is not always clear owingto the complexity of P transportpathways (Heathwaiteetal. in press).

Heckrathetal.(1995)found that aboveanOlsen P of 60 mg kg

1

in the plough layer ofasiltloam, the DP concentration in drainagewaterincreased dramatically (from0.15 to2.75 mg L 1;Fig. 5).

They postulated that this level, which is well above that needed by major crops for optimum yield (Ministry of Agriculture, Food and Fisher- ies 1994), is a critical “change point” above which the potential for P movement in land drains greatly increases. Similar studiessuggest that this change point can vary threefold as a function of site hydrology, relative drainage vol- umes and soil P sorption-desorption character- istics.

Management

Todate,research and implementation have iden- tified agricultural management practices that minimise P losses in surface runoff byseparate- Fig. 5. Effect of soilP ondissolvedPconcentration of surface runoff from several pasture catchments(adapted from

Sharpleyetal. 1996) and subsurfacedrainagefrom arable Broadbalk fields atRothamsted,UK(adaptedfrom Heckrath et al. 1995).

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ly addressing sourceandtransportfactors (Bot- tcher etal. 1995, Sharpley etal. 1994). These practices include applying P basedonsoiltestP recommendations and reducing surface runoff and erosion withcover crops, contour plough- ing and conservation tillage. However, imple- mentation of thesemeasures overbroad_areasof

acatchment hasnotresulted in expected reduc- tions in P export (Meals 1993, Sharpley and Rekolainen 1997).This is duetothe fact that in humidclimates,surface runoffproduction isusu- ally generated during limited times from limit- edsource areas within upland catchments. The source areasexpand and contractrapidly during a storm as a function of rainfall (intensity and duration) and site characteristics(soilmoisture, topography, groundwater level) of the catchment (Gburek _et al. 1996). For example, more than 75% of annual surface runoff from catchments in Ohio (Edwards and Owens 1991) and Okla- homa(Smithetal. 1991) occurred inone ortwo severe storms.Further, these eventscontributed over 90% of annual total P^(TP)export(0.2 and 5.0 kg ha'

1

^yr1,respectively). Also,about 90%

of annual algal-available P ^(AAP) loss from catchments in Pennsylvania occurred from only 10% of the landarea during_arelatively few large storms (Pionkeetal. 1997). As a result, overall P management strategies will reduce P export most effectively when targeted to the critical source-areas withina catchment that are most vulnerabletoPloss in surface runoff(Heathwaite and Johnes 1996, Heatwoleetal. 1987, Prato and Wu 1991).

Consequently,preventing P loss is nowtak- ing _onthe added dimensions of defining, target- ing and remediatingatthe scale of the critical P source areas, i.e., areas within the catchment where high soil P levelsarecoincident with high surface runoff and erosion potentials. Thus,in- formation is neededonthe hydrological controls linking spatially variablesourcesand transport processes that determineP loss from a catch- ment.This paperpresents the results ofastudy of hydrological and chemical processes defin- ing critical source areasand controlling ^P ex- port froma small,upland, agricultural catchment

in east-central Pennsylvania by examining flow and P concentrations in streamflow in light of soil P distribution over the catchment and po- tentialsource areas ofstormrunoff.

Material and methods

Study area

The study wasconductedon a39.5-ha subcatch- ment (FD-36) of Mahantango Creek which is tributarytothe Susquehanna River and ultimate- ly the Chesapeake Bay (Fig.6).FD-36 is typical of upland agricultural catchments within the nonglaciated, folded and faulted Appalachian Valley and Ridge Physiographic Province. Soils are mostly Berks (Typic Dystrochrepts), Calvin (Typic Dystrochrepts), Hartleton (Typic Hapud- ults)and Watson (Typic Fragiudults) channery silt loams,with slopes ranging from 1^%to 20%.

Climate is _temperate and humid,average rain- fall is approximately 1100 mm yr^'and _stream- flow about450 mm yr

1

^.

The catchment is of mixed landuse,with50%

in soyabean, wheator corn,20%as pasture,and 30% wooded. In the last 5 years, cropped land north of the FD-36^streamchannel received about 60

m 3 ha

^{'yr '}pig slurry in spring and no ferti- liser P. This^{amounts to}about 100 kg P ha^"'yr

1

^,

assuming a slurry P ^contentof 1.6 g L' ^(Gil- bertson etal. 1979).South of the stream chan- nel, approximately 5 Mg ha"' yr

1

^{poultrymanure}

was added each spring. This amounts toabout 85 kg P ha'

1

^yr

1

^{, assuming a manurePcontent}

of 16.9 g kg ^'(Gilbertson etal. 1979).As these application ^rates were obtained from annual farmer interviews, and the P content of slurry and manure can be variable (Eck and Stewart 1995),estimated manurial inputs of P^toFD-36 areapproximate.However, the values allow rel- ative comparison of inputs and stream flowex- port of P for FD-36.

FD-36 wasdivided into four_segmentsbased on topography and drainage patterns derived Vol. 7(1998):297-314.

from a detailed topographic survey and visual reconnaissance (Fig.6).Beginning in May 1996, streamflow below each segment was continuous- ly monitored using recording H-flumes, and

water samples for P analysis were taken auto- matically during storm hydrographs at 5- to

120-min intervals using programmable stage- activated samplers. Baseflow samples weretak- enateach flumeatmonthly intervals for subse- quent P analysis. All sampleswere refrigerated at4%C from collection until analysis.

In July 1996, soil samples (0 to5-cm depth) were collected on a 30-m grid overthe catch-

ment. The samples were air-dried and sieved (2 mm),and the Mehlich-3 soil P concentration

wasdetermined.

Hydrograph analysis

Streamflow hydrographs were separated into baseflow and stormflow components using a

semi-log technique (Hall 1968).The width of the near-stream surface runoff-producing zone for eachsegmentandeventwasestimated from flow increase within each segment using the follow- ing procedure. Incremental stormflow volumes were calculated and summed for the total hy- drographtoobtain total stormflow volume pro- duced from each^{event at}each flume. Beginning with themostdownstreamsegment (catchment outlet), total stormflow for the ^next upstream segment was subtracted from the total volume atthe flume to obtain stormflow volume pro- duced within each catchmentsegment.Segment flow volumeswerethen divided by total rainfall depth and stream length within the segmentto

approximate thenear-streamsurface runoff-pro- ducing width. This calculation assumes that stormflowcomesstrictly from rainfall fallingon the saturatedareasof eachsegmentand that the saturatedareas do notexpand during the storm event.It wasalso assumed that the saturatedar- eas were distributed symmetrically about the channel in each catchmentsegment.

Fig. ^6,Location,topography and instrumentation of catchmentFD- -36,Pennsylvania.

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Table 1.Area of each catchment segment, number of soilsamplescollected on a30-m gridand Mehlich-3Pcontentsfor FD-36.

Catchment Area,ha Channel Number of Mehlich3P,mg kg 1 ^Percentineach category, ^%

segment length,m samples Mean ^Min, Max. <3O 30-100 100-200 >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 watersamples by the molybdenum- blue method of Murphy and Riley (1962). The same methodwasused for^TPfollowing 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 upto50 mL with distilled^water)andone Fe-oxide strip^were shaken end-over-end for 16 h at4°C. The stripwas removed, rinsed free of soil particles, and shaken end-over-end for ^{1 h} in 1 M HCI toremove 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 NH}4N0₃, 0.015 M NH₄F,0.013 M HN0₃and0.001 M EDTA for 5 min (Mehlich 1984). Phosphorus in all filtered and neutralised^extracts wasdetermined by the method of Murphy and Riley ^(1962).

Results and discussion

Soil ^P distribution

On a 30-m grid overthe catchment, Mehlich-3 P ranged from 7 to 788 mg kg'

1

^{(Table 1).The}

Mehlich-3 soil^Pvalues were grouped into four categories basedon agronomic and environmen-

tal factors: <3O mg kg"

1

^{, crops}require addition- al P for optimum growth; between 30 and 100 mgkg ^', there will generally beacrop response toP application but little enrichment of P in sur- face runoff(probable crop response decreasesas Mehlich-3 P increases from 50^to 100 mg kg^');

between 100 and 200 mg kg

1

^, there will be no response^toappliedP whilesomeenrichment of P in surface runoff mayoccur; >2OO mg kg^', levelsareconsidered excessive interms of crop requirements and enrichment ofP in surfacerun- offcanbe expected (Beegle 1996, Sharpley et al. 1996).

Thepatternof Mehlich-3 P valuesoverFD- -36 is generally afunction of land_use and field boundaries within the catchment (Fig.7). Soils in woodedareashave 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 manureand fertiliser applications^are, in mostcases,above200 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% ofsoils,P application would be recommendedononly 14% for optimum crop production (30-100 mgkg"¹⁾ as the other 34%

aremostly wooded(<3Omgkg

1

^{)(Table 1).}

Streamflow ^P

Average flow-weighted DP, ^AAPand ^TP con- centrations in streamflow leaving each of the four catchment segments were determined for each Vol. 7(1998):297-314.

storm eventfrom August ^to the beginning of November 1996 (Table 2). For allevents, aver- ageP concentration decreased downstream from segment 4to segment 1 (the catchment outlet).

On average, DP concentration decreased by 60%, AAP by 56% and TP by 59%. Also, DP com- prised 54% and 60% ofAAP atsegments 4 and

1, respectively, while AAP was 53% of TP at segment 4 and 49%atsegment

1.

Although the concentration decrease and distribution of DP and AAP were similar between segment 4 and the catchmentoutlet,the relative importance of controlling hydrological or chemical processes will likely vary along thestreamchannel. These processes may include dilution by input of sub- surface flow to the stream channel, deposition and resuspension of particulate material andas- sociatedP, sorption of DP by suspended sedi-

mentand channel bank/bedmaterial,and adif- ferential contribution ofP in surface runoff from spatially variableareasof surface runoff produc- tion and high soil P.

From the above analysis, it is apparent that the distribution of P forms (DP,AAP andTP) in streamflow changed little during transportalong the channel. Dissolved P averaged 29% of TP and AAP 50% of TPateach segmentflume (Ta- ble2).Also,the decline in P concentration from segment 4tosegment 1 (watershed outlet)was similar for^DP,AAP and TP(56% to 60%).This suggeststhat channel processes may be relatively unimportant compared with variations in source areainput amongsegments.

Whilewe arecontinuing this investigation of the controlsonprocesses of^P loss,comparison of stormflow P concentration and soil P distri- butionpatterns overthe catchment may provide insight into the linkages between high P soils and surface runoff-producing areas. Estimated widths of saturatedareasadjacent tothe stream channel ranged from <1 ^to 62 m and showeda general increase downstream fromsegment4to

1 ^(P<o.s,Table3).

Assuming most of the stormflow increase Fig. 7. Mehlich-3 Pdistribution overFD-36;flume locations and segment numbersarealso shown.

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Table2.Meanflow-weightedconcentration ofdissolved,algal-availableand totalP instreamflowleaving each segment during ten flow eventsin 1996.

Flow Catchment segment

Event 12 3 4

DissolvedP,mgL^l

9 August 0.019 0.036 0.050 0.097

6 September 0.027 0.038 0,070 0.088

7 September 0.020 0.024 0.039 0.046

13 September 0.060 0.073 0.119 0.148

16 September 0.104 0.137 0.163 0.202

17 September 0.074 0.080 0.099 0.111

28 September 0.066 0.082 0.103 0.119

9October 0.065 0,123 0.172 0.206

18October 0.448 0.511 0.743 0.731

8November 0.195 0.253 0.322 0.392

Average 0.046 0.062 0.088 0.116

Algal-availableP,mgL'

9August 0.044 0.060 0.088 0.126

6 September ^0,050 0.071 0.113 0.175

7 September 0.043 0.057 0.122 0.144

13September 0.122 0.136 0.175 0.182

16 September 0.166 0.236 0.259 0,333

17 September 0.105 0.122 0.156 0.165

28 September 0.142 0.170 0.204 0.223

9October 0.109 0.163 0.248 0.299

18October 0.608 0.670 0.980 0.923

8November 0.230 0.284 0.378 0.447

Average 0.085 0.112 0.151 0.192

TotalP,mgL^‘

9August 0.089 0.109 0.147 0.181

6 September 0.129 0.262 0,473 0.543

17 September 0.114 0.156 0.268 0.334

13 September 0.200 0.232 0.324 0.334

16 September 0.266 0.315 0,434 0.580

17 September 0.198 0.230 0.312 0.347

28September 0.392 0.478 0.495 0.660

9October 0.204 0.261 0.591 0.761

18October 0.708 0.776 0.998 1.238

8November ^0,759 0.838 0.990 1.015

Average 0.160 0.215 0.329 0.394

withinsegmentsoriginatesassurface runoff from the near-stream area, the distribution of high Mehlich-3 soil P in thisareaand the whole catch- mentwerecompared (Fig.8).Onawhole catch- mentbasis,there_waslittle difference among the four catchment_segments in thepercentof soils

>2OO mg kg’

1

^{Mehlich-3P (29% to39%, Table 1}

and Fig. 8).This is the Mehlich-3Pcategory that is expected^toresult in enrichment of DP insur- face runoff.However, on anear-streambasis, the areal distribution of these high^Psoils decreased from50% insegment4to 8% insegment

1.

^Thus,

the trend of decreasing stormflow DP concen- tration downstreamwas moreclosely relatedto Vol. 7(1998): 297-314.

Table 3. Saturated distance from the stream channel for catchment segments duringeach flow eventin 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

9October 2.20.9 0.60.5

18October 54.636.6 62.225.9

8November 17.038.7 35.920.4

Average 14.79.5 13.06.7

the near-stream distribution of high P soils in each catchmentsegmentthantothe whole catch- ment (Fig. 8). This integration of hydrological processes and chemical properties of^catchment soils suggeststhatnear-streamsoil P concentra- tion has_{a greater}influenceonP_exportfrom the catchment than does soilPconcentrationatthe whole-catchment scale.

These findings have important implications for catchment management of P from fertiliser or manure applications. For instance, _current

thinking maysetPmanagementgoals based sole- lyon Mehlich-3 P concentrations for soilsover the entire catchment (Sharpley _etal. 1996). In

this case,nearly 80%of thecropped^andpasture soilsoverFD-36 aresufficiently high in P(>IOO mg kg'1) that there would be no crop-yield re-

sponsetofurtherPapplications. 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 Pexportis like-

Fig. 8. Distribution of soils with Mehlich-3 P >2OO mg kg ^{' on} whole-catchment andnearstream (>6O m) basis and mean flow- weighteddissolvedPconcentra- tioninstreamflow from each seg- ment for August to November 1996.

Seminar in honour

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ly, indicating P applications should be^{more care-} fully managed in theseareas (Sharpley et al.

1996).Based on this,application of Pto63% of the cropped area of FD-36 would be limitedor restricted. Clearly, this would adversely impact those farmers having confined swine and poul- tryoperations_onthe catchment where produced manures are presently applied.

Alternatively, delineation of surface runoff- producing _areasand recognition of the similari- tybetween_patterns of P concentration instream- flow and P concentration ofnear-stream soils suggeststhat P ^managementgoals should focus on the near-streamareas rather than the whole catchment. With this approach, accounting for the interactions among soil P, land use and hy- drological characteristics of thecatchment,it is possible _to better _targetremedial programs to critical^Psource areas of the catchment.

Implications ^for remediation

Phosphorus _exportfrom 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 anderosion, lessat-

tention has been directed toward other landuses (e.g. grassland) and source management. For example, it is clear from the^extentof soils with P in excessof levels sufficient for optimum crop yields, as in FD-36, that moreattention should be paidto avoiding soil P build-up via P-source

management. General remedial measures that minimise Pexportfrom agricultural catchments arepresented, with reference_toresults from FD- -36, where appropriate.

Source management

Manures

Manipulation of dietary P intake by livestock

may help reduce regional surpluses of P. Morse

etal. (1992)recorded a 17% reduction in Pex- cretion when dairy cowsreduced their daily P intake from82to60 g day'. In the Netherlands, reductions in concentrate P are now being im- plementedto help reduce theamounts ofP ex- cretedto 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 useof phytase, anenzyme that enhances the ef- ficiency of P recovery from phytin in grains fed to poultry. This has the potential toreduce P concentration in poultry manuresand litters.

Commercially available manure amend- ments, such as slaked limeor alum, canreduce NH,volatilisation and P solubility ofpoultry lit- terby several orders of magnitude (Moore and Miller1994).Also,the DP concentration ofsur- face runoff from fescue treated with alum- amended litter (11 mg L') wasmuch lower than from fescue treated with unamended litter(83 mg L 1; Shreve et al. 1995). Perhaps the ^most important benefit ofmanure amendments (for both air and waterquality), however, will be _an increase in the^N;Pratio ofmanurevia reduced N loss from manure by NH, volatilisation. An increased N:P ratio of manure would better match crop^Nand ^Prequirements. Thus, addi- tions ofmanure based on cropN requirements would reduce the Pexcess added, thereby mini- mising potential soil P accumulations.

Localised surpluses of Pareexacerbated by the fact thatmanures arerarely transportedmore than20 km from where theyareproduced. How- ever, mandatorytransport ofmanure from sur- plus areas to nearby farms where the nutrients

are needed faces several significant obstacles.

First, it mustbe shown that the currentlocation isunsuitable, based on soil properties, cropnu- trient requirements, topography and hydrology.

From European experiences this may be diffi- culttojustify scientifically duetothe largetem- poral and spatial variability in the factors con- trollingNandP mobility in soils and_transport to ground or surface waters. Second, in many areas there is no clearly defined legal basis for requiring farmers inonephysiographical areato Voi7(1998):297-314.

perform management practices that _are not re- quiredonneighbouring farms. Moresuccesswith re-distribution of_manuresis likelytooccurwhen consumers, localgovernments,the farmcommu- nity and livestock industry areall 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 cropP re- quirements. A major difficulty in development of these recommendations has been the identifi- cation of threshold levels of soil P thatarelike- ly to result in unacceptable losses of P in sur- face runoff. Establishing these levels is a con- troversial process fortworeasons. First, the data base relating soil P levels to surface runoff P concentration is limited^toafew soils and crops, and there is areluctance toextrapolate data of thistypetoother regions. Second,the economic implications of establishing soil test P levels which may limitmanureapplications aresignif- icant. In manyareasdominated by animal-based agriculture, there simply isnoeconomically vi- able alternativetoland application.Thus, there is aneedtoassessthe validity of using soiltest P values asindicators of P loss in surface run- off. InFD-36, for example,manure application toover60% of the catchment would be limited byasoil test P threshold of 200 mg kg^'.

Another approach developed in the Nether- lands and applicableto subsurface pathways of P transport, determines the potential for DP movement in drainage _water by estimating soil P saturationas thepercentage of^Psorption ca- pacity_asextractable soil P(Breeuwsmaand Sil- va ^1992).This approach is basedon 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 theNetherlands, where farmrec- ommendations for _manure managementarede- signed_tolimit the loss ofP in surface and ground waters.For Dutchsoils, acritical P saturation of 25% has been establishedasthe threshold value

above which the potential for P movement in surface and groundwaters becomes unaccepta- ble(Breeuwsma and Silva 1992).

Transport management

Oncewaterand sediment beginto move overthe landsurface, taking with them the nutrients orig- inally applied as fertiliser and/or manure, the quantities which reach thestreamcanbe reduced by any feature which slows flow and/orencour- ages infiltration or sediment trapping. Such measuresinclude terracing,contourtillage,cover crops, bufferstrips, riparian zones,and impound- ments or small reservoirs. These practices are generallymore efficientatreducing particulate P rather than DP.However,such approaches only work where subsurface pathways of P loss are unimportant.Furthermore, by encouraging infil- tration of surfacerunoff, 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 Ptosurface waters,such mechanisms may be ineffective in soils with a high hydraulic conductivity (e.g.

sands)or where macropore ordrainflow is im-

portant (Heathwaite 1997).

Usually, farm N inputs can be more easily balanced with plant uptake thancan P, particu-

larly where confined animal operations exist. In thepast, separate strategies for N and P have been developed and implementedatfarmorcatchment 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 _tocompromised waterquality 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.^Incontrast, reducing surfacerun- off losses of P via conservation tillage can en- hance nitrate leaching.

ForP, aprimary strategy is tominimisesur- face runoff and particulate transport. In most Seminar in honour

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cases the necessary measures ^- soil cover as plants or crop residues, cultivation along con- tours,and bufferzones^- have _aneutral_orbene- ficial impact on nitrate loss. An exception is ploughing, which if carried outin early autumn tendsto increase leaching if the soil is not fro- zen.Another exception follows the conversion of conventionalto notill practices.^In theUSA, where no till is commonly recommended as a conservationmeasuretoreduceerosion, conver- siontonotillwasfollowed byadecrease insoil, 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 occurfrom any lo- cation ina catchment, while_areas prone^tosur- face runoff contribute ^{most to}P loss.Hence,for N, remedial strategies may be applied ^to the wholecatchment, whereas the most effective P strategy would be to apply simple measures to the whole catchmenttoavoid excessive nutrient buildup, and thereby limit losses in subsurface flow,and more stringent measures tothe most vulnerable sitesto minimise loss of P in surface runoff.

These positive and negative impacts ofcon- servation practices onresultant water quality should be considered in the development of sound remedial measures.Clearly,atechnically sound framework mustbe developed that in- cludes critical sources of N and P export from agricultural catchments so that optimal strate- gies^atfarm and catchment scalescanbe imple- mentedtobest manage both N and P.

Conclusions

Issues facing agronomic and environmental P management in agriculturalsystems aresimilar in mostdeveloped countries. Specialised farm- ing systemswithin and between these countries have tendedto dismantle natural P cycles, re- sulting in animbalanced flow of P fromareasof

fertiliser manufacture and grain production to areasof 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 Puse such that many soils are still deficient in P withrespect tothat 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 atfield, farm _or catchment scales. As _{a result,}soil P has generally increased in localisedareasof intensive crop and livestock production, and increased losses of P in surface runoff and subsurface flow water are morefre- quently noted.

Although the relationship between soil and mobilised P has not been quantifiedover 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 isslow; levels will be elevated for several years after application has ceased. Also, chemical amendments such as alum, fly ash, gypsum and ironcompounds reduce the solubil- ity of soilP,nottotalamounts,andarethus only temporary measures. To acertainextent, these concernshave notbeen addressed becauseman- aging agricultural inputs andoutputsof P is of- tenmuch morecostly and restrictivetoafarmer than is general N management. As a result, N continuestodrivemanure managementdecisions and exacerbates the build-up of soil P.

It is often^toosimplistic to use thresholdor change-point soil P levelsas the sole criterion toguide Pmanagementand P applications. These values will have little meaning unless they are used in conjunction withanassessmentofasite’s potentialtomobilise P in surfacerunoff, erosion and subsurface flow. Thus, preventing P loss should takeonthe added dimension of defining, targeting and remediatingsource areasof P that combine high soil P levels with high erosion and surface runoff potentials. As a result, differing Vol.7(1998): 297-314.

levels ofmanagement may besuggested for dif- ferentareasofa catchment, anapproachtoland managementwhich will have^tobe addressed by action agencies. Without incorporation ofsource areaperspectives totarget application of P fer- tility, surface runoff and erosion control tech- nology, conventionally applied remediations may not produce the desired results and may prove tobe inefficient and _non-costeffective.

Efforts to increase our understanding of P cycling in terrestrial ecosystems and develop technicallysound,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 guidelinestoimplement suchstrat- egies will also require consideration of theso- cio-economic and political impacts of anyman- agement change on both rural and urbancom- munities, and the mechanisms by which change canbe achieved inadiverse and dispersedcom- munity of land-users.

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