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Riparian pasture retirement effects onsediment, phosphorus, and nitrogenin channellised surface run‐off frompasturesChristine M. Smith aa Division of Water Sciences Department of Scientific andIndustrial Research , Water Quality Centre , P.O. Box 11 115,Hamilton, New ZealandPublished online: 30 Mar 2010.

To cite this article: Christine M. Smith (1989) Riparian pasture retirement effects on sediment,phosphorus, and nitrogen in channellised surface run‐off from pastures, New Zealand Journal ofMarine and Freshwater Research, 23:1, 139-146, DOI: 10.1080/00288330.1989.9516349

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New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23: 139-1460028-8330/89/2301-0139$2.50/0 © Crown copyright 1989

139

Riparian pasture retirement effects on sediment, phosphorus, andnitrogen in channellised surface run-off from pastures

CHRISTINE M. SMITHWater Quality Centre, Division of Water SciencesDepartment of Scientific and Industrial ResearchP.O. Box 11 115, Hamilton, New Zealand

Abstract Riparian pasture retirement effects onthe chemistry of channellised surface run-off from 2moderately steep hillslopes was examined in a 22-month study. Seventy-one surface run-off eventsoccurred. They varied in size by about 2 orders ofmagnitude. Sediment, phosphorus, particulate- andnitrate-nitrogen concentrations in the run-off (ineach event) at retired riparian pasture sites weresignificantly (P < 0.001) and substantially lowerthan concentrations in run-off at grazed riparianpasture sites. Concentrations also varied lessextensively at retired sites (P < 0.001). Riparianpasture retirement impacts on total loads wereexamined by comparing the average concentrationsin run-off in 22 months at retired and grazed sites.These event-flow-weighted total and volatilesuspended solids, particulate P and N, dissolved P,and nitrate-nitrogen means were <87%, <84%,< 80%, < 85%, < 55%, and < 67%, respectively,lower at the retired sites. It was concluded thatriparian pasture retirement is an effective means ofreducing surface run-off pollutant loads to waterwaysin the short term, but long-term impacts need to beexamined.

Keywords riparian management; surface run-off;nitrogen; phosphorus; sediment loads

Received 23 November 1987; accepted 22 February 1988

INTRODUCTION

Diffuse run-off from agricultural land is an importantsource of nutrients to waterways and lakes in NewZealand (White 1982; Wilcock 1986). A largefraction of the nitrogen andphosphorus is transportedin surface run-off in particulate forms (Sharpley &Syers 1979; Cooke 1988; Cooke & Cooperl988)andclearly, any technique which substantially lowersthese surface run-offloads will reduce diffuse sourcepollution. Riparian zone retirement is a frequentlyrecommended technique (e.g., Williams & Brickell1983) because a dense, vegetative ground coverfilters sediments from run-off (e.g., Wilson 1967).For this and other reasons, such as reducedstreambank erosion and the protection of aquatichabitats, a policy of lake- and stream-riparian zoneretirement has been implemented in many areas ofNew Zealand. Although these schemes are expensive(e.g., streambank erosion control measures in theTaupo Catchment Control scheme are estimated tocostNZ$8 million), no studies have been undertakento evaluate the specific effects of riparian bufferstrips on inputs in diffuse run-off from pastures.McColl & Hughes (1981) concluded that there wasan urgent need for research in this field if riparianzone retirement is to be seen as a scientificallydefensible management option.

In 1983 an investigation began into the effects ofriparian pasture retirement on sediment, N, and P insurface run-off. In a preliminary 20-month study(Smith 1987), run-off chemistry was monitored atsites at the base of both a north- and south-facinghillslope while sheep and cattle were allowed tograze both slopes. Run-off chemistry did not differsignificantly between the south-facing sites, orbetween north-facing sites that intercepted channel-lised run-off. However, there was a significant "slopeaspect" effect (Smith 1987).

The results of a subsequent study when grazinganimals were denied access to riparian pastures atduplicate sites on each slope are presented in thispaper. The effects of retirement are assessed bycomparing channellised run-off chemistry at theseretired sites with run-off chemistry at 1 or 2 control(i.e., grazed) sites on the same slope.

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140 New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23

Fig. 1 A map of the Tauwharecatchment, showing the topo-graphically defined hillslopecatchment areas and location of thesurface run-off collectors and retiredriparian strips (inset).

Catchment boundary

I ~l Hillslope run-off source area/r2> Slump • — ^ ^ Perennial stream

I I Retired pasture strip J. Collector

Table 1 Site details, estimated total run-off at each site during the study, and number of events occurring at each site.

Length of retired strip in m and as a percentageof total hillslope

Length of the run-off intercepting trough (m)Estimated total surface run-off (m')Number of events

SITE DESCRIPTION

1

2.42752

South-facing sites

2 3

- 13 (11%)

2.4 1.5332 34

57 47

METHODS

4

13(11%)

1.5442

65

5

10

2.014764

North-facing sites

6

(6%) 10 (6%)

2.08134

7

2.06042

The study was undertaken on 2 hillslopes in a 16 haheadwater catchment at Tauwhare (NZMS 260 mapT14 gridref. 305761),near Hamilton.New Zealand.One hillslope is a 15°, 230 m long south-facingslope; the other a 20°, 94 m long north-facing slope(Fig. 1). They both are part of a high-producingsheep and cattle farm carrying 19 stock units perhectare. The pastures were predominantly perennialryegrass (Lolium perenne L.) and white clover(Trifoliwn repens L.). t

The Tauwhare silt loam, a gley soil, occupiesabout 68 % of the total area on both slopes. It has a siltloam topsoil over a prominently mottled, gleyedsilty-clay loam subsoil (McDonald 1985). Theseareas are major catchment surface run-off sourceareas (Cooke & Dons 1988).

The moderately well-drained Pakaroa hill soil, asilt loam (yellow-brown earth), covers the remainingarea as pockets near the base and on the upperconvex section of the south-facing slope, and overthe upper convex section of the north-facing slope(McDonald 1985).

Surface run-off samplingSamples were collected between 1 March 1985 and31 December 1986. Surface run-off from the 2hillslopes was interceptedin all run-off events duringthis 22-month interval. There were 7 collectors;each collector was sited within 5 m of the streambank(Fig. 1). Collector 1 and 2 abutted, as did Collector3 and 4.

PVC guttering set into the ground at right anglesto the slope intercepted therun-of f. These interceptingtroughs varied in length between sites (Table 1)because of the local topography. Perspex sheets,inserted about 30 mm into the soil upslope at a depthof about 10 mm, directed run-off into the gutteringwhile run-off drained from the guttering via PVCpipes in to a flow-proportional subdividing unit. Thisunit, illustrated in Smith (1987), discharged 12.5%of the run-off into a tipping bucket connected to aflow data logger. A further 1.25-12.5% fraction(varying from site to site) was discharged into aseries of 50-litre polyethylene storage containers.

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Smith—Riparian pasture retirement and run-off 141

No preservative was placed in the 50-litre storagecontainers (dissolved and total P and N in 4 "test"run-off samples changed by < 3% within 24 hstorage). Samples were taken from each containerfor chemical analyses at daily intervals, or asnecessary when run-off occurred infrequently.

This samplingprogramme generally gave a singleflow-weighted sample for each site in each run-offevent. However, when a single run-off eventcontinued over several days (refer Smith (1987) foradetailed definition of event), nutrient concentrationsin the daily samples were flow-weighted to obtain arepresentative chemistry for the event. Conversely,when several small events occurred during a single24-h period only 1 sample, with chemistry repre-senting the flow-weighted mean for all events in the24-hour period, was obtained.

Chemical analysesFiltration was used to distinguish between dissolvedand participate nutrient fractions. Filtered (0.45 \xmcellulose acetate) and unfiltered run-off sampleswere frozen until analysed for total phosphorus (TP)and total dissolved phosphorus (TDP) (vanSchouwenberg & Walinga 1967); total Kjeldahlnitrogen (TKN) and dissolved Kjeldahl nitrogen(DKN) (Glowa 1974; Technicon Corporation 1976)and NO3-N (Downes 1978). Run-off samples werealso analysed for suspended solids (SS) and volatilesuspended solids (APHA1980). Volatile suspendedsolids (VSS) were determined at 500°C because atthe higher temperature recommended by APHA(1980) inconsistent weight losses of glass fibre filteroccurred.

Pasture managementPasture strips extending 10-13 m immediatelyupslope of Collectors 3,4,5, and 6 (Table 1, Fig. 1)were fenced off to grazing stock on 1 March 1985.The retired pasture within these strips grew slowly inautumn and winter 1985. At 10 sites in the north- andsouth-facing strips it had reached an average heightof only 0.3 m and 0.25 m, respectively, by mid July1985. However by mid September, whilst verypatchy, it stood about 0.5 m high. Clovers hadbecome very sparse. Subsequent spring rains andwind severely flattened the sward and in summerthese windblown grasses went to seed. The sparsestanding vegetation comprised ryegrass, fescue(Festuca arundinacea S.), andpaspalum (Paspalumdilatatum P.). By late January 1986 bare ground inthe retired strips was visually estimated (at 8 sites) torange from 35 to 60% of the total soil surface area.

However, this was immediately overlain by a mat ofdecaying vegetation which became increasinglydense through late summer and autumn. In autumn,white clover re-emerged initially and perennialryegrass subsequently through the decaying mat.White clover dominated the standing swardthroughout 1986. By 1 September, the pastureaveraged (at 10 sites on each slope) 0.4 m in height,and remained standing through spring and summer1986.

Undesirable plant species did not invade theretired areas during this 22-month study. However,weeds such as gorse (Ulex europaeus L.), dock(Rumex sp.), blackberry (Rubus sp.), and ragwort(Senecio jacobaea L.) were notably absent from allpastures in the catchment over this period.

All pastures on both experimental hillslopeswere topdressed by plane on 1 May 1985 at anominal rate of 45 kg P ha-1. No fertiliser wasapplied in 1986.

RainfallRainfall was measured using a Fisher & Porter eventrecorder and manual check gauges sited c. 100 maway from the run-off collectors.

RESULTS

Surface run-off hydrologyNormal annual catchment rainfall is 1401 mm (Smith1987), whereas rainfall in 1985 and 1986 was 1447mm and 1165 mm, respectively. Rain was unevenlydistributed during each year. It produced surfacerun-off on 71 separate occasions (i.e., 71 "events"occurred) during the study, although run-off did notoccur at all sites in all events (Table 1).

The total run-off from all sites was 1123 m3. Thisrun-off was unevenly distributed between the 7sites, and most occurred in winter and early spring(Fig. 2). Although small and medium-sized eventswere numerous (Table 2), 49% of the total run-offwas produced in only 5 events. The surface run-offflow record was too short to compute return periodsfor run-off events. However, surface run-off is themajor contributor to stream stormflows in thecatchment (Cooke & Donsl988), so approximatereturn periods were obtained by computingrecurrence intervals for stormflows, in stormsgenerating surface run-off. For the 6 largest surfacerun-off events, these recurrence intervals (LogPearson type 3 distribution; Bobee 1975) were 1.5-16years, suggesting relatively large, infrequent surfacerun-off events occurred during the study period.

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142 New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23

300

200

c

CD

1£100

Fig. 2 Estimated total monthlysurface run-off (m3) from all 7 sites.

M A M J J A S O N1985

D J FI

M o n t h

M A M J J1986

A S O N D

Run-off chemistryExtreme and median values are given for eachdeterminand at each site in Table 3, includingparticulate phosphorus (PP, i.e., TP minus TDP) andparticulate nitrogen (PN, i.e., TKN minus DKN)values.

The median values suggest that riparian pastureretirement reduced sediment, phosphorus, pani-culate- and nitrate-nitrogen concentrations in run-off at sites on both slopes, and also that run-offchemistry varied with "aspect". Twenty-five and 75percentile values (e.g., Fig. 3A-D) indicate thatconcentrations varied less extensively between eventsat retired than at non-retired, and at south- than atnorth-facing sites.

The significance of these differences wasexamined using distribution-free tests because formost determinands, arithmetic- and logarithmic-transformed concentration density functions werenot normal. Concentrations in all events at theretired or grazed site(s) on each slope were combinedand the treatment density functions obtainedcompared using the squared rank test for variances(Conover 1980) and the Mann-Whitney U-test. Theseanalyses indicated significantly less variance betweenevents (P < 0.001) and lower median (P < 0.001)concentrations of all determinands except DKN atretired than at non-retired sites on both slopes. Themedian DKN concentration was significantly lower(P < 0.001) at retired than at non-retired sites on thesouth-facing slope only.

Table 2 Frequency of surface run-off events of varyingsize, and total run-off intercepted by the 7 collectors inthese events in m1, and as a percentage of total run-off in22 months.

Event size(m>)

0-11-55-1010-2020-3030-6060-100

100-150150-200

Number ofevents

1321111443311

Totalvolume

6.345.089.0

205.196.1

133.3246.3122.7178.9

% totalrun-off

0.54.07.9

18.38.6

11.921.910.915.9

Also, all determinands except DKN varied to asignificantly (P < 0.001) lesser extent at south-facing than at north-facing replicate (i.e., grazed)site(s) and all median concentrations were lower(P < 0.001) at the grazed south-facing than at north-facing site(s).

The type of event in which pasture retirementmost effectively reduced concentrations can bededuced from Table 3, and the percentile valuesgiven in Fig. 3 A-D. Suspended sediment minimumconcentrations (Table 3) were only slightly lower atthe retired than at the grazed site(s) on either slope.Twenty-five percentile values at non-retired andretired sites on the south-facing slope were also

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Smith—Riparian pasture retirement and run-off 143

rather similar, indicating that in a few events—whenSS concentrations in run-off entering the retiredstrips were probably relatively low—retirement hadlittle impact on sediment concentrations. On thenorth-facing slope retirement seems to have affectedconcentrations in these events to a greater extent,possibly becauseconcentrations were initially higher.However, retirement had an extremely large effecton concentrations in 75% of all surface run-offevents on both slopes. Median SS concentrationswere typically 50% lower at retired sites, and 75percentile and maximum concentrations only10-33 % of those values at grazed sites. It seems that,at comparatively high initial concentrations, retiredpasture strips removed a substantial and often majorfraction of the suspended sediment. Retirementinfluenced PP (not shown) and PN (Fig. 3B)concentrations in a similar manner.

Riparian pasture retirement also reduced NO3-N,TDP (Fig. 3C, D), and DKN (at south-facing sites)concentrations in almost all events. The overalleffect on N03-N concentrations was large.

Retirement impacts on loads

Because run-off volumes varied extremely betweenevents, concentrations in a few large events would

have had a large impact on total loads. The generallysubstantially lower median concentrations in run-off from retired sites may not therefore have beenmatched by relatively low sediment, N, and P loads.Loads could not be directly compared to assessoverall retirement impacts because volumes variedwidely between sites (Table 1). However, flow-weighted (across events) means accurately reflectthe relative importance of concentrations in different-sized events on total loads.

These means are given in Table 4. They werecalculated for each site from the combined load in all48 events with a complete chemistry record, dividedby the total run-off in these events. This included 6of the largest 10 events. Although these means couldnot be statistically compared because of the incom-plete factorial design, they and the large differencesbetween average treatment means (Table 5) indicateretirement reduced surface run-off loadssubstantially. The greater differences between allparticulate means at north-facing sites cannot besolely ascribed to greaterretirement effects, becauseof the relatively long retired pasture strips (i.e.,values of ratio high retired strip: hillslope length) onthis slope. Other modifying factors (e.g., slope,sediment concentrations in run-off entering retiredstrips on the 2 slopes) also differed.

Table 3 Median concentration and concentration range (in parenthesis) of constituents in run-off samples. P and Nconcentrations are in mg nr3, and sediment concentrations in g m"3. R, retired site.

Samples

TP

TDP

PP

TKN

DKN

PN

NO3-N

SS

vss

1

39

469(40-2036)303(137-1711)113(40-824)2157(910-8770)1280(790-4373)640(95^397)48(13-1982)30(8.6-535)9.7(2.3-84

South-facing sites

2

39

495(45-1152)352(160-977)100(45-614)2200(1095-8685)1505(635-5035)545(95-6885)24(9^1997)28(7.4-496)9.5(2.0-160)

3R

44

223(15-2180)171(73-1948)59(15-232)1409(535-12555)1060(420-11112)330(115-1453)13(3-3318)16(3.7-134)6.5(1.8-98)

4R

42

216(17-572)164(60-479)50(17-133)1535(655-2720)1125(510-1970)363(100-1350)11(3-773)14(2.1-42)5.2(t-28)

North-facing sites

5R

44

543(28-2313)398(170-1879)123(28-^3)2758(775-12230)2045(705-9720)698(70-2605)38(6-4908)33(6.7-203)11.2(t-66)

6R

28

429(32-1031)299(156-702)109(32^153)2019(925-6370)1385(750-2585)508(130-3785)33(8-1712)29(7.7-347)8.5(1.7-66)

7

29

1011(59-5348)682(236-2137)243(59-4179)3135(1735-26768)1740(930-5782)1415(555-24310)106(9-3935)64(5.8-1083)18.1(3.9-428)

tbelow detection level of 1 mg m 3.

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144 New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23

A. SUSPENDED SEDIMENT B. PARTICULATE NITROGEN 7

600 r

a>

100

South-facing .North-facing ,sites i sites "T

500 - d l G r a z e d s i te

0 Retired site

400

300

200

3 4

6 000-

5 000 -

4 000 -

D)

3 000

2 000

1 000 _ 1

B

5 6m

•U J-J.

C. NITRATE-NITROGEN

400

200

o>

100

0

3 4

1 000

200

, 600E

100

200

0

D. TOTAL DISSOLVED PHOSPHORUS7

3 4

Fig. 3 Boxplots with the solid horizontal lines showing (in order of increasing concentration) the 25, 50, and 75percentile concentration at each site. The site number is given immediately above each boxplot.

DISCUSSION

In this study riparian pasture retirement effects onthe chemistry of surface run-off from conventionallymanaged New Zealand pastures was examined. Theresults demonstrate both aspect and pasture retire-ment influenced run-off chemistry. The aspect effectscannot be explained by major differences in soil typeand/or distribution. However, the higher S S concen-

trations in run-off from the north-facing slopesuggests a basic difference in soil stability. Such adifference is also indicated by numerous massmovement scars on north- but not south-facing slopesin the study catchment.

The retirement effects on surface run-offpaniculate chemistry is explained by the sediment-filtering action of a dense vegetative ground cover.Grass-filters spread or diffuse run-off, thereby

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Smith—Riparian pasture retirement and run-off 145

reducing its velocity and sediment carrying capacity(e.g., Hayes et al. 1979). However, the pastureretirement effects on dissolved P and Nconcentrations were unexpected. Retired pasturescould have influenced concentrations by either (1)removing nutrients from or not (2) supplying nutrientsto run-off passing through or derived from within thestrip. With dissolved nutrients, the secondexplanation seems more realistic. In grazed pastures,there is apool of NandP actively cycling through thesoil-plant-animal system. ThisNandPis susceptibleto transport in surface run-off when returned to thesoil in excreta (e.g., McColl & Gibson 1979) orwhen present in the surface soil. However, at allstages during the study, an unusually large fractionof this N and P was in the abundant standingvegetation at retired sites and, consequently, was notsusceptible to transport in surface run-off.Furthermore, infiltration and consequently leachingrates were probably higher in the retired strips,thereby further depleting the pool of dissolved P andN available to surface run-off.

Table 4 Flow-weighted mean sediment, P, and Nconcentrations in surface run-off. Sediment concentrationsare in g m"3, and N and P in mg m~3. R, retired site.

South-facing sites North-facing sites

7~1 3R 4R 5R 6R

TP 519TDP 389PP 130TKN 1790DKN 1173PN 617N O - N 51SSVSS

477

741 295582 213160 81

2242 11731308 783934 38944 2868 3719 11

302 721226 63176 90

1500 16651003 1230497 435

15 5331 36

8 11

482 1043400 602

83 4411594 46351103 1611491 3024

33 13035 268

7 58

I am unaware of any earlier studies into riparianretirement effects on the chemistry of surface run-off from grazed pastures. However, grass and cropresidue filter strip effects on run-off from fallow(Alberts et al. 1981) and cropped land (Weston et al.1986), andfrom livestock feedlots (e.g., Young etal.1980; Dickey & Vanderholm 1981; Edwards et al.1983) have been examined. Reductions in sediment(i.e., 60-98%) and total or paniculate P and N (i.e.,70-78%) concentrations and/or loads were generallygreater than those obtained in the present study.However, in these earlier studies filter strip effectswere measured on gentle slopes; filter strip lengthswere high relative to the volume of run-off treated;and/or run-off was not channellised. In the presentstudy, impacts were evaluated under conditionslikely to minimise retirement effects; but low retiredstrip : hillslope length ratios and steep slopes aretypical of extensive New Zealand riparian retirementschemes (e.g., riparian retirement strips in the UpperKaituna Catchment), and so it was appropriate thateffects be investigated under these conditions.

Given the significant and substantially lowermedian concentrations at retired sites, and also thesubstantially lower flow-weighted means, riparianpasture retirement is potentially an effective methodof reducing surface-derived pollutant inputs tosensitive waterways. However, long-term impactsalso need to be investigated.

ACKNOWLEDGMENTS

1 am grateful to D. S. and K. D. Hoult who permitted meto undertake the study on their property. I thank L.O'Donoghue, K. Costley, and D. Jensen of the WaterQuality Centre for the chemical analyses; and D. Allenand L. O'Donoghue for their able assistance with the fieldwork.

Table 5 Average flow-weighted mean paniculate anddissolved P and N, and sediment concentrations at retiredsites as a percentage of average means at grazed sites.

South-facing slope(6% retired pasture)

North-facing slope(11% retired pasture)

TDPPPDKNPNNO.-Nss3VSS

45547257465977

86207215331316

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