recover ftom winter pugging damage more quickly than soil physical properties (Betteridge .... Soil physical data were analysed by analysis of variance using the.
SHORT SHORT TERM TERM RECOVERY RECOVERY OF OF SOIL SOIL PHYSICAL PHYSICAL PROPERTIES PROPERTIES AFTER AFTER WINTER WINTER GRAZING GRAZING IN IN W WAIKA AIKA TO: TO: IMPLICATIONS IMPLICATIONS FOR FOR ENVIRONMENTAL ENVIRONMENTAL MONITORING MONITORING JJ JJ Drewry1, Drewry!, PPLL SingJeton2, Sing]eton2,M M Boyes3, Boyes3,AA Judge3 Judge3and andD DWbeele.-3 Wheele~ 11A¥Research, Invermay, Private Bag 50034, Mosgie/ AfResearch,Invermay,Private Bag 50034,Mosgie/ Environment Waikato, PO 4010,Hamilton Environment Waikato, POBox Box 3 AgResearch, Ruakura. Private Bag4010, 3123,Hamilton Hamilton. 3 AgResearch,Ruakura.Private Bag 3123,Hamilton. Abstract Soil recovery ftom compaction is becoming an important issue for both monitoring soil compaction on-farm, and for monitoring soil conditions for nitrous oxide emissions.This paperreports natural recovery of somesoil physical parametersat 2, 6, 10, 14 and 21 weeks after a winter grazing event on wet soils for 3 trials in the Waikato region. Three trial sites were on Gley soils, and one on an Allophanic (volcanic) soil. Severalsoil physical properties showed some recovery over the period, while soil macroporosity and saturatedhydraulic conductivity showedthe greatestrecovery after the winter grazing event. Saturatedhydraulic conductivity acrossall sites at 0-5 cm depth almost fully recoveredwithin 6 weeks of the winter grazing event. The effect of the winter treadingdamageon subsequentpastuie growth, and the environmentalmonitoring implications are also discussed.For field trials, such as nitrous oxide emissiontrials, where soil physical measurements are used for interpretationof results, measurementsshould be taken at intervals during the trial to aid interpretation,to ~commodate changesin soil physicalproperties. Introduction The effects of soil and pasture disturbanceand deformation by grazing animals under wet winter conditions,(pugging), has beenwell documentedin severalNew Zealandand overseas trials (e.g. Scholetield& Hall 1985; Nie et al. 2001; Pandeet al 2002; Ward & Greenwood 2002). While pugging or poaching,of soil predominatesaroundthe hoofwhen the soil is very wet (Scholefield & Hall 1985), becauseof unevendistnoution of water in the soil profile, compaction in deeper layers may also occur. Pugging conditions show a very visual and measurableloss in production (e.g., Menneeret al. 2001). Suchconditionsare commonin the Waikato region during winter block-grazingof dairy cows, where cows are confined to small areas,and generally causemore soil and plant damageat this time than in spring lactation when stocking intensity is lower. It hasbeen suggestedthat pasturecover and tiller number recoverftom winter pugging damagemore quickly than soil physical properties(Betteridgeet at. 2002). For these intensive winter grazing situations, it would be useful for interpretation of soil : For these intensive winter grazing situations, it would be useful for interpretation of soj physical physical health heahh to to determine determinethe the extent extent of of recovery recovery of of soil soil physical physical quality quality indicators. indicatofi Improvements Improvementsto to soil soil structure structurecan can occur occur by by natural naturalprocesses processesincluding including fteeze freezeand andthaw thav cycles, wetting and drying cycles, and earthworm and root activity, while reduced grazing cycles, wetting and drying cycles, and earthwonn and root activity, while reducedgrazinj intensity intensity or or stock stock exclusion exclusion can can also also improve improve soil soil physical physical condition condition (Greenwood (Greenwood&cS McKenzie McKenzie 2001). 2001). Several Several studies studies have have shown shown soil soil structural structural improvement improvement after after stock' stocJ exclusion, over a period of several years (Stephenson & Veigel 1987; Greenwood et aL 1998). exclusion,shorter over a term period ofphysical severalyears (Stephenson & Veigel 1987;Greenwoodet aI. 1998) 'f. However, soil recovery is alsolikely to occur. However, shorterterm soil physical recovery is also likely to occur. 194 194
~
'~,
,
As soil pugging damageand subsequentrecoveryaffe.ctssoil drainageand aeration,it is also lilcely to affect nitrous oxide emissions ftom soil. Nitrous oxide is a greenhousegas contributing about 16% (on a CO2 equivalent basis) of New Zealand's greenhousegas emissions(Saggar et al. 2002). Severalstudieshave found indicators of soil water content (water-filled pore space)to be importantwhen measuringnitrous oxide emissions(e.g. Saggar et aI. 2002). Ruser et aI. (1998) showednitrous oxide fluxes were greatestat high (85%) water-filled pore spacein compactedsoils, with reducedmacroporespace.They also reported higher WFPS, but they statedthat someof their bulk density assumptionsdid not allow for . the transientincreasein porosity during &ost. Increased nitrous oxide emissionswere reported trom compact soil under pastoral conditions (e.g., Carranet at. 1995; Bhandral et al. 2003), while it hasbeen suggestedthat improvementin soil structurecould help mitigate emissions (de Klein & Clark 2002). In warmer climates,soil cracking or natural soil recoverycould be an important processin both the movementof nitrous oxide to atmosphere,and reduction in emissionsthrough improved soil structureandremovalof anaerobicconditions. There are very few published paperson recovery of soil physical propertiesin dairy pastoral systems.The primary purpose of this paper is to report the extent of short term recovery of soil physical properties after a winter pugging event, over a period of up to five monthson 3 Wailcatodairy fanns and 2 soil types. The paperalso discussesthe implications of using soil physicalmeasurements to interpret field trial and monitoring ofsoi! structuralcondition.
I
Methods and materials Site details, soil characteristicsand treatments Trial siteswere located on dally farms in the Waikato regio~ New Zealand.The Ruakura 1 and 2 siteswere at Dexcel No 1 Dairy, RuakuraResearchCentre,Hamilton, while the other siteswere on commercialdairy farms.The Orini site was 20 km north of Hamilton, while the Kiwitahi site was in the Toenepicatchment,20 kIn north eastof Hamilton. Siteswere chosen for apparentlack of recent puggmgdamage.Sites,soil classification(Hewitt 1998),treatment and sampling details are listed m Table 1. The Ruakuraand Orini sites on a Te Kowhai silt loam (a Gley soil), are poorly dramedand susceptibleto puggmg,while the Kiwitahi site on an Allophanic (volcanic) Kereoneyellow brown loam is consideredto be well structuredand resistantto treadmgdamage. .. . Treatmentswere (i) "pugged" (grazed in wet conditions), where the Te Kowhai soil plots were grazedwith the equivalent of 350 cows/ha for four hours, and the Kereonesoil plots grazed for 24 hours to get equivalenttreading damage>or (ii) ~'control"where plots were ungrazedduring the "grazing" event. A trial area of up to 600 m2 was fenced off and then cows grazedappropriateareasfor the puggedtreatment.Animals were excludedftom all the trial areafor up to 21 weeks after the grazingtreatmentswere applied. A penetrometerdevelopedat AgResearchwas usedto determinesusceptibilityto puggingby simulating hoof pressureat 350 kPa (Betteridgeet aI. 2003). When 30% or more, of at least ten penetrationmeasurementsreach2 cm depth or more, it hasbeensuggestedthat the soil is susceptibleto pugging damage(Betteridgeet a1.2003). Ten penetrationmeasurements were takenper plot, with site averagesrecordedin Table 1. Surfaceroughness,indicatingpugging severity,was measuredusing the 1 metre chaintechnique(Saleh 1993;Table 1). This index is the percentagereductionin straight-linelength dueto the chain following surfacecontours.
195
Table1. Soil classification (Hewitt 1998),grazingtreatmentsand samplingdates anddetails for eachsite. Waikato sites
Ruakura 1&2
Soil type
Te Kowhai loam
silt
Orini Te Kowhai loam
Kiwitahi silt
Kereone yellow brown loam
Typic Orthic Grey
Typic Orthic Gley
Typic Orthic Allophanic
24/0611999
7/08/2000
22/08/2001
Stocking rate (cows ha-I)
350
350
350
Period of grazing (hours)
4
4
24
53%
70%
79%
1.6 (0-3)
3 (1-5)
2 (1-4)
11.6
11.4
8.2'
3.5
4
3
48
48
26
Pre grazing
10/06/1999
3/08/2000
8/08/2001
0 weeks 2 weeks
24/0611999 8/07/1999
7/08/2000 21/0812000
23/08/2001 5/09/2001
6 weeks
4/08/1999
18/09/2000
3/10/2001
10 weeks
28/09/1999
16/10/2000
2/11/2001
14 weeks 21 weeks
15/11/1999 5/01/2000
13/11/2000 4/01/2001
22/11/2001
Soil classification Date of grazing/puggingevent
Gravimetric soil water content at grazing (% kg kg-I) Penetrationdepth (em) at grazing. (Rangein brackets) Sutfaceroughnessindex after pugging treatment Surfaeeroughnessindex (control) Bare groundimmediately after pugging (%) Soil physical samplings:
Physical measurements:smallporosity cores Small stainlesssteel rings (4.8 cm diameter, and 2 cm height) were used to measuretotal porosiiy, bulk density, and macroporosity(volumetric percentageoflarge soil pores,i.e., >30 IJ.m).Rings were carefully insertedvertically in the soil at nominal depthsof 0-5 cm and 5-10 cm). For the 0-5 cm nominal depth, a level benchwas cut at about 1 cm depth and the ring carefully insertedinto the soil, using a spacerring to push the ring into the soil to a depthof 1.5-3.5 cm. The ring and soil was then excavated,lightly trimmed and put into a plastic bag for transportto the laboratory.A similar methodwas usedto sampleat the nominal depthof 5-1,0 cm, namely 6.5-8.5 cm depth. There were 2 suhsamplecorestaken per depthper plot. D~ta ftom 1296cores,collectedat 6 or 7 samplingdateswere usedin the analysis.' This sampling technique was then later further developedfor on-farm commercial use by using three 4.8 cm diameter stainlesssteelrings comprising a central 2 cm deepsamplering. and two 1.5 cm deep spacerrings, which fitted above and below the samplering. All three rings were held togetherby tape. Pressingthe metal cylinders into the soil collected samples over the 0-5 cm depth,with the intact soil in the middle ring later usedfor analysis. In the laboratory, any worms were removed using a 1% formalin solution. The middle sampling ring was removed for analysis, i.e., measuringat 1.5-3.5 cm soil depth, (at the 196
~
nominal depth of 0-5 cm), or 6.5-8.5 cm soil depth, (at the nominal depth of 5-10 cm). The coreswere equilibratedto -10 kPa using pressureplatesand establishedtechniquesfor water release(Smith & Mullins 1991) to measurethe volume of macroporeslarger than 30 j.!m. Bulk density was determined on the same cores using standard oven drying techniques (McLaren & Cameron1996).Particle density,used in the calculations,was determinedusing pycnometry(modified version of Head 1992).For the volcanic soil, field-moist sampleswere used to determineparticle density to avoid changesin density that can occur on drying of allophanicsoil (Singleton et at 2000). Physical measurements:large hydraulic conductivity cores Other largercores(7.6 cm deep; 10 cm diameter)for measuringhydraulic conductivity (water flow in large soil pores) were field sampledas describedin Singleton & Addison (1999). These cores allowed some height above the surface of the 5 cm deep soil core to enable laboratorymeasurements to be made,andwere smearedwith petroleumjelly prior to insertion into soil, to avoid edge flow effects. Coresfor hydraulic conductivity were trimmed and the baseof eachcore was "peeled". Earthwormswere removedusing fonnaldehyde.Full details .
of core preparationare given in Singleton & Addison (1999). Saturatedhydraulic conductivity (Ksat)was then measuredafter core saturationin the laboratory (Singleton & Addison 1999). Pasture measurements Pastureyield ftom all treatmentswas measuredusmg a rotary lawn mower to harvestpasture ftom plots cut to about 3 cm height, and a sub-samplecollected for dry matter determmation by oven drying (Sinclair et al. 1996). Pasturewas harvestedftom 2.3 m x 1.1 m areasat Ruakura,to 2.5 m x 2 m at the Orini site. 1 Statistical designand analyses Eachtreatmentwas replicatedsix times in eachof the Orini and Kiwitahi trials, and at eachof two Ruakura sites, with a pairing of plots in blocks except for the Kiwitahi site. For the Kiwitahi site an artificial blocking structurewas used in the analysisto balancethe design. This was checkedagainsta cOlTesponding residualmaximumlikelihood (REML) analysisand found to be robust. Soil physical data were analysedby analysis of variance using the statistical packageGenStat version 6 (GenStatCommittee 2002). The block structure'Was given by depth withfu subsamplewithin plot within block, and the treatment structure by treatment,depthand their interaction.Yield datawere analysedin the plot stratum,by fitting treatment.Hydraulic conductivity datawere analysedafter a 10g1O transformation. Results Soil physical measurements Macroporosity, bulk density, porosity, and Ksat measurementsall showed evidence of compactionat pugging, followed by recoveryover the duration of the trial, with the greatest improvement shownin Ksatmeasurements.The control treatmentswere quite stableover time, in. that changesin control treatment macroporosity,bulk density, porosity and hydraulic conductivity, both ftom time to time, and ftom pre-treatmentto each sampletime, did not differ significantly. Macroporosity The treatmentmain effect for macroporositywas not significantly different at the pre-grazing sampling,but was significantly different at 0, 2, 6, 10 (P < 0.001) and 14 and 21 (P < 0.01) weeks after grazing. The pattern of changein macroporosityat 0-5 cm over time for the 197
pugged treatment is shown for all sites in Figure lA, and showed an increase (P < 0.001) :1 _!L_- ~--, A ' "', ., . " . "' .. .. .-.
averageaover SItestor 14 and 21 weeksrelative to week O.Controlsare not shown in Figure lA, as they were quite stableover time, as discussedabove. Therewere similar patternsfor 5-10 cm. Differencesin macroporosityvalues at 5-10 cm betweenvarious samplingsandthe pugging sampling were not significant (P> 0.05), nor was the site by treatment interaction significant for any case.
A
-~
--.~
I/)
~ 0 Co
g ~
Weeks after grazing
B 1C
~ E .§. ~1 '>
~ ::J
"t:I c:
8 .5:2 "3 !!! 1)1 e .a
~
Weeks after grazing
Figure 1. Pugged treatment means for each site for (A) macroporosity (%), and (B) saturatedhydraulic conductivity (rnmJh)at 0-5 cm soil depth, following pugging in week 0 at eachsite. V erticallines representSED. 198
Saturatedhydraulic conductivity Saturatedhydraulic conductivity was depressedmore during pugging and recovered more after pugging, than the other soil physical properties. Changesin Ksatacross all the sites, seemedto have almost fully recoveredby 6 weeksafter pugging (Figure IB). For the pugged treatmentat 0-5 cm (Figure IB), valuesincreasedafter the immediatepost treatmentdrop, to higher levels at 2 and 6 weeksat different sites,andmaintainedlevels similar to theseweek 6
levelsat the remainingsamplings.Controlsfor ~at arenot shownin FigureIB, astheywere quite stable over time, as discussedpreviously. Saturatedhydraulic conductivity, showed significant differencesbetweenthe control and puggedtreatmentsat 0 weeks(P < 0.001), 10 weeks(P < 0.01) and2,6 and 14 (P < 0.05) weeksafter grazing. At the Kiwitahi site (Figure 2), Ksatincreasedafter pugging, showing recoveryparticularly at 0-5 cm, but also at 5-10 CtD.At the Orini site>Ksatat 0-5 cm for the pugged treatment improved from 60 to 647 mm/h, while at .5-10 cm Ksatfor the pugged treatment improved from 20 to 467 mmlh (data not shown).
-E .J: -
E '-'
Z;-
O> 13 ~ "C t:: 8 .~ ~
~
"C
>-
.J:;
"C CD
~
~ 1U tJ)
Weeks after grazing
Figure 2. Treatmentmeansat 0-5 cm and 5-10 cm, following puggingin week 0, for saturatedhydraulicconductivity(mmlh) at the Kiwitahi site (Allophanicsoil). V erticallines representSED. Bulk density showed and totalevidence porosityof mcreasedcompactionimmediatelypost pugging (P < 0.01),
"
, ,
but no significant recovery subsequentto this (data not shown). The overall mean for the Kiwitahi site was 0.75 M§ m-3, comparedwith 0.86 Mg m-3 for Orini, 1.00 Mg m-3 for Rualcura 1 and 1.03,were. Mg mcOD.sistent for Ruakura sites(P < 0.001). Despite large differences, patterns of ~ha~ge over2 sites. For example, bulk these de~sity over ti~e .for~:
,It 'II
II It
i:: '
f
Ruakura 1 Site 18shown m Figure 3. Patternsof changefor total porosity followed similar
trends(datanot shown).
'
it!
;;~
i 'J
199i;~1 " . -
,
j~I~~;{
-
'"E -en ~
-
~
°0 I:: G) "tJ ~
:; a:I
Weeks after grazing
Figure 3. Treatmentmeansat 0--5 cm and 5-10 em, following pugging in week 0, for bulk density (Mg/m3) at the Ruakura I site (Gley soil). Verticallines representSED.
Pastureyield measurements There were significant reductionsin pastureyield in the puggedtreatmentcomparedwith the control treatment.The effect was significant only for the first harvestafter pugging for the Kiwitahi (23 days)and Qrini (39 days) sites,and for the first two harvestsat the Ruakurasites (Table 2). Pugging at the Kiwitahi site reducedpastureproduction by 38% (312 kg DM/ha) comparedwith the control for the first harvest. Pugging at the Qrini site reducedpasture production by 54% (421 kg DM/ha) comparedwith the control for the first harvest.For the two Ruakurasiteshowever,the reductionin yield causedby pugging over the two harvestsup to 90 dayspost pugging, was 290~(699 kg DM/ha) and 24% (737 kg DM/ha) for Ruakura 1 andRuakura2 sites,respectively.
200
Table 2.
Site Kiwitahi
Orini
Ruakura1
Pastureyield (kg DM/ha) for the grazingtreatmentsat eachsite. *, P < 0.05, **, P < 0.01; ***, P < 0.001;ns. not significant. Days since pugging
Control
23
868
Pugged 512 1479 1108
sed 57.6 152.1 123.5
824
47
1249
69 39
774
353
67
1185
1052
66.6 88.7 84.0 78.2
95
1338
1299
149
2033
2167
540
230
1813
1424
47 90 112 127 152
/
1689
1589
464
501
1382
1371
45.1 111.7 114.1 33.3 105.1
P ***
ns ns
*** ns ns ns *.... ",* "
ns ns ns
Discussion This study showedthe detrimentaleffectson soil physicalpropertiesand pastureyield after a single winter block-grazing pugging event. The trials also showed that many of the soil physicalpropertiesrecoveredthrough natural processesduring the 5 month period following pugging. How~ver,the Gley soils were only grazedfor 3 hours, suchas in an on-off grazing situation,so it is very likely that soil physical damagewould have beengreaterif cows were grazed for longer, as is common practice in the region. Such work is currently being undertakenin the Waikato (K. Zegwaard,peTs.com.).It would also be useful to determinethe extent of soil recovery in the presenceof grazing cows during lactation. Of interest in these trials, is that much of the recovery occurred in winter or early spring. Winter recovery occuned for the Ruakura siteswhich were grazedin early June.This contrastswith several South1andtrials where recovery occurred mainly during summerand autumn (AgResearch, unpublisheddata). However,during the Southlandtrial, cows were grazedat intervals during lactation, while in this series.of Waikato trials cows were excluded during the recovery period. Several other studies have indicated natural improvements in soil physical condition, particularly in the 0-5 cm depth (Drewry & Paton 2000). Greenwoodet at. (1998) showed improvementsin bulk density and unsaturatedhydraulic conductivity m ungrazedplots when sheepwere excludedfor 2.5 years. They showedthe differenceswere largely limited to the surface5 cm depthof soil, with the greatestdifferencesoccurring at 0-2 cm depth.However,
201
oJ
I the current study shows considerablerecovery at both 0-5 cm and 5-10 cm, particularly for Ksat.
The improvements in physicalmeasurements may haveresultedfrom worm activity,
root growth and soil crackmgand drying during the trial period. Warrenet a1.(1986) found 30 days of stock exclusion after grazing events was insufficient to allow full recovery of infiltration, while Elliot et a1.(2002) reported recovery of soil infiltration took at least six monthsor longer. The effect of pugging on pastureyield is also clearly evid~nt.However,the decreasesin yield were less than reported for somestudies.Menneeret al (2001) reportedannualpastureyield reductions of up to 45%, and annual clover reductions of up to 65% following severe pugging,on a Te Kowhai soil. However,we observedthat pugging severityof the currenttrial was lessthan that ofMenneer et at (2001). Soil condition in that trial was closeto satmation, and animalswere stockedat a higher rate (450 cows/ha),than in the current trials. Although pasturegrowth at the two Ruakurasitestook longer to recover than at the other two sites,it shouldbe noted that the grazing event at Ruakurawas in early June,comparedwith early to ale AUgustIOrme omer snes,so n ISDOtsurpnsmgtM1 regrowtll WOUldwas slower. late August for the other sites,so it is not surprisingthat regrowthwould was slower. ~hanging soil physical conditions during the year should be taken into account when monning both routine monitoring of soil physical condition, and when consideringtrials ~here soil physical measurementsare used for calculations or interpretationof filed trial 'esults.An example is where changesin soil physical condition occur when animals are ,xcluded,such as in nitrous oxide emissiontrials. Although total porosity and bulk density lid not display large changes over the trial period, Ksatand macroporosity did. Early
mprovements following puggingdamagein macroporosityand ~at suggestthat rapid air novementmay occur and that these connectedpathways allow rapid exchangeof gasses. ~hangesin macroporosity will consequentlyaffect values of soil water content at field :apacityand hencewater-filled pore space.Researchersshould thereforeconsidertaking soil )hysicalmeasurementsat intervals,rather than at a single point in time. Researchersshould tlso considerwhether the trial area has had stock excluded or not, as this would affect soil ecovery. The wider application of macroporosity measurements for environmental lssessment was shownby McDowell et aI. (2003). ~hangesin macroporositywhen monitoring soil physical status, or the extent of recovery Changes in macroporosity-when monitoring soil physical status, or the extent of recover) Lfter - pugging, should also - be . taken . into account when interpreting soil physical - data where after pugging, should also be taken into accountwhen interpreting soil physical data where; vinter block-grazing is practised.For interpretation of commercial soil testing of physical winter block-grazing is practised.For interpretation of commercial soil testing of physica: :haracteristicson-farm, we recommendrecording the recentgrazinghistory of a paddock.For characteristicson-farm, we recommendrecording the recentgrazinghistory of a paddock.FO1 lJorthIsland dairy fanns, it is likely to be worthwhile monitoring soil structureat a consistent North Island dairy fanns, it is likely to be worthwhile monitoring soil structureat a consisten~ ime in relation to winter block grazing. For example, sampling is recommendedin late time inprior relation to winter grazing. Foras example, recommended in late lutumn to winter block block grazing, or as long possiblesampling after blockisgrazing in late winter, autumnprior to winter block grazing, or as long aspossibleafter block grazing in late winter, 0 that long term trends in soil physical condition canbe monitored. so that long term trends in soil physical condition canbe monitored. :onclusioDS Conclusions ~gging had a significant effect on many soil physical propertiesand on pastureyield. This Pugging had a significant effect on many soil physical propertiesand on pastureyield. This tudy indicated that several soil physical properties show some recovery over the 21 week study following indicated that soil physical properties show recovery overconductivity the 21 week .eriod soil several damage, with soil macroporosity and some saturated hydraulic period following soil damage,with soil macroporosityand saturatedhydraulic conductivity howing the greatestrecoveryrate. Soil recoverywas apparentfor somepropertiesat both 0greatest recovery rate. Soil recovery for in some properties at both 0jshowing em andthe 5-10 em depths. These issues shouldwas be apparent considered field trials where soil 5 cm and 5-10 cm depths. These issues should be considered in field trials where soil Ihysical measurementsare used for interpretationor calculation of results. Consequently, physical measurementsare used for interpretation or calculation of results. Consequently, neasurements shouldbe takenat intervalswhen samplingafter a pugging event,ratherthanat measurements shouldbe taken at intervalswhen samplingafter a pugging event,ratherthan at , singlepoint in time during suchexperiments. a singlepoint in time during suchexperiments. 202 202
