Is there a link between elevated atmospheric carbon dioxide concentration, soil water repellency and soil carbon mineralization?
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Agriculture, Ecosystems and Environment 139 (2010) 98109
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Is there arsoil wa zat
Karin Ma The New Zeal Centreb PFR, Private Bc AgResearch Lt
a r t i c l
Article history:Received 13 NReceived in reAccepted 9 JulAvailable onlin
Keywords:Free Air CarboexperimentCarbon mineralizationHydrophobicityOrganic carbonElevated atmospheric carbon dioxideInltration
spherilwaton. Se Airre hyof SW
een tn bet
importance of SWR for water redistribution. At the meso-scale of disc inltrometry, inltration rateswere reduced by SWR, and were higher under ambient than under elevated [CO2]. This corroborates thetendency of reduced SWR under elevated [CO2] observed at the micro-scale. SWR showed a spatial struc-ture, exhibiting short ranges. SOMandMRR showed no spatial pattern at the scale analyzed, emphasizingthat SWR did not contribute to an increase of the long-term terrestrial C sink in response to increased
Soil watspontaneousoil propertical soil waoccur moredroughts hachange (Wathe ecologic
We hypofeature of creduces soisions from tis governedbioavailabilwater contMoncrieff, 2For exampland root tu
0167-8809/$ doi:10.1016/j.atmospheric [CO2]. 2010 Elsevier B.V. All rights reserved.
er repellency (SWR) is when a soil does not wet upsly when water is applied to its surface. It is a transienty and will occur whenever soils dry out below a crit-ter content (Dekker and Ritsema, 1994), which mightoften given the extent to which climatic extremes andve been forecast formost regions in thewake of climatetson, 2001; Meehl et al., 2007). A true understanding ofal signicance of SWR is still very limited.thesized that elevated atmospheric [CO2] as a typical
limate change promotes the occurrence of SWR whichl carbon mineralization and thus decreases CO2 emis-he soil. The mineralization of soil organic carbon (SOC)by the quality of soil organic matter (SOM), viz. the
ity of SOC to microorganisms, soil temperature and soilent (Kirschbaum, 1995; Leiros et al., 1999; Fang and005). All three factors are affected by climate change.
e, it is known that elevated [CO2] enhances root growthrnover rates (Canadell et al., 1996; Allard et al., 2005).
ding author. Tel.: +64 7 959 4555; fax: +64 7 959 4430.ress: email@example.com (K. Mller).
This may stimulate soil carbon mineralization rates (Sowerby etal., 2000; Paterson et al., 2008). But also, no consistent effect of ele-vated [CO2] on the turnover rates of the soil carbon pools has beenreported (Pendall andKing, 2007). Elevated [CO2] could alsomodifythe quality of SOM, which in turn would change the soils wetta-bility. SWR is considered a material property of SOM (Ellerbrocket al., 2005), and has been shown to be correlated with differentfractions of OM (McKissock et al., 2003; Doerr et al., 2005; Morleyet al., 2005), and is thought to be linked to microbial activities(Hallett and Young, 1999) and root or fungal exudates (Wallis andHorne, 1992). The effects of SWR on carbon mineralization are onlyrarely investigated (Borken and Matzner, 2009). CO2 losses froman agricultural soil were reduced by 30% after adding hydrophobicsubstances to the soil (Piccolo and Mbagwu, 1999). Carbon miner-alization of a silty agricultural soil, silt loam grassland and arablesoils, and a sandy forest soil declined with increasing SWR (Goebelet al., 2005, 2007). Goebel et al. (2005) showed thatwater-repellentsoil surfaces inhibited aggregates from imbibing water. The result-ing lower water content of the aggregates reduced the microbialdecomposition of the SOM located in the aggregates.
The site of this study has been exposed for 10 years to ele-vated atmospheric [CO2] in a Free Air Carbon Dioxide Enrichment(FACE) experiment, at Bulls, New Zealand. Ambient atmospheric[CO2] levels are expected to reach the elevated [CO2] of 475L L1
see front matter 2010 Elsevier B.V. All rights reserved.agee.2010.07.005a link between elevated atmospheric cter repellency and soil carbon minerali
llera,, Markus Deurerb, Paul C.D. Newtonc
and Institute for Plant & Food Research Limited (PFR), Private Bag 3123, Waikato Mailag 11030, Manawatu Mail Centre, Palmerston North 4442, New Zealandd., Grasslands Research Centre, Palmerston North 4442, New Zealand
e i n f o
ovember 2009vised form 7 July 2010y 2010e 4 August 2010
n Dioxide Enrichment
a b s t r a c t
We hypothesized that elevated atmochange promotes the occurrence of soand thus increases carbon sequestratiatmospheric [CO2] in a long-term Frehigh spatial resolution. All samples wedifferences in degree and persistencerates (MRR) and water content betwwith SOM or MRR. A strong correlatiom/locate /agee
bon dioxide concentration,ion?
, Hamilton 3240, New Zealand
ic carbon dioxide concentration [CO2] as a feature of climateer repellency (SWR)which reduces soil carbonmineralizationoil surface transects under elevated (475L L1) and ambientCarbon Dioxide Enrichment experiment were sampled at adrophobic at the time of the sampling. At the micro-scale, theR, soil organic matter (SOM) content, microbial respiration
he treatments were not signicant. SWR was not correlatedween water contents and SWR parameters demonstrated the
K. Mller et al. / Agriculture, Ecosystems and Environment 139 (2010) 98109 99
simulated in this experiment around 2030. The soil at the site is asandy soil prone to drying out and SWR (Wallis et al., 1993;Newtonet al., 2003). The elevated atmospheric [CO2] and soil water con-ditions mimic a future scenario anticipated for many regions asa consequeatmospheriet al. (2003sistence ofof exposurethe degreechange in rknowledgevated [CO2]
How climwater dynaet al., 2004SWR is an iin dry soilsto climate csoil water cpreferentiapattern ofimportant f(Birch, 1958
It is cleationsbetweTo date, moon subcritic(Goebel et a2009). Hydrthe objectivelevated atmcarbon minsoil carbonsoils.
The stuatmospheriexperimentexperimental., 2001). Ital areas ofduring thesame time.manent fenestablishedincluding leC3andC4grsis L., CynoL., Hypochaperiodicallyment area d(19451995annual tem
The soil2006) withfew distincthas a weakboundary tocal differenunder ambi
and 861%; silt, 71 and 91%; and clay, 62 and 51% (pers.communication Des Ross). Soil tests showed soil pH(water) to be 5.8,and an Olsen-P of 20gmL soil1 (Newton et al., 2006).
ture0.07grou50gs beftal h
tereachof a rsed o. Thesealter w
quane and onsh sor andred iDPTrpla
recor993his the recpersmeans sev, sligR (6
tremh), cs diden tl-drof thel soln deturrethe
respn andion rnce of climate change. We hypothesized that elevatedc [CO2] would affect the occurrence of SWR. Newton) showed for the same experimental site that the per-the potential SWR signicantly decreased after 5 yearsto elevated atmospheric [CO2]. They did not analyse
of SWR. The mechanism responsible for the observedepellency has not been identied. To the best of ourthis is the only published study on the impact of ele-on SWR.ate change affects soil physical properties such as soil
mics (Niklaus et al., 1998; Nelson et al., 2004; Nowak) is even less well understood. We hypothesized thatmportant mechanism for the local water redistributionand that it is part of a terrestrial feedback mechanismshange. SWR considerably slows down the increase inontents following rain, exacerbates the occurrence ofl ow pathways, which results in an inhomogeneoussoil moisture. The local redistribution of soil water isor soil microbial activities and carbon mineralizationa,b).
r that we require a better understanding of the interac-ensoil biologyand the resultingphysical soil properties.st studies investigating the occurrence of SWR focusedallywater-repellent soilswith contact angles below90
l., 2004, 2005, 2007;Wocheet al., 2005; Lamparter et al.,ophobic soils have been rarely investigated. Therefore,es of our studywere to assess in hydrophobic soils (1) ifospheric [CO2] increased SWR; (2) if SWR inuenced
eralization; and (3) if SWR could indirectly inuencemineralization by governing soil water distribution in
ls and methods
tion of study site
dy site has been exposed for 10 years to elevatedc CO2 in a Free Air Carbon Dioxide Enrichment (FACE)at 4014S and 17516E, Bulls, New Zealand. The FACEhas been described in detail elsewhere (Newton et
n brief, since October 1997, three circular experimen-12m diameter have been enriched to 475L L1 CO2photoperiod. Three control rings were installed at theThe rings are in a 2.5-ha eld and contained with per-ced areas of 25m25m (Fig. 1). The site carries anpasture that contains a range of about 25 plant speciesgumes (Trifolium repens L., Trifolium subterraneum L.),asses (Agrostis capillaris L., Loliumperenne L.,Poapraten-
don dactylon L.) and dicotyledons (Leontodon saxatiliseeris radicata L.). The permanent pasture of all rings isgrazed by sheep that are enclosed within the treat-
uring the grazing period. The long-termaverage rainfall) at the experimental site was 874mm and the meanperature is 12.9 C.at the site is classied as a Mollic Psammaquent (FAO,0.2m black loamy ne-sand topsoil characterized byreddish mottles in the lower part of the horizon, whichly developed nutty structure and leads with a sharp0.2m grey single grained sand. There was no statisti-
ce in the soil texture of the two treatments, with valuesent and elevated [CO2] being, respectively: sand, 872
TheSWR isdiameacrosscentrebias ba(Fig. 1)tightlyand lit
Wesistencassessethe fre(Dekkemeasutest (Wofwatesoil iset al. (1soils. Ting. Wfor theto theguisheclass 1tent SWand ex4 (13sample60 s. Thethanoform oethanotest cathe occsion ofto thethe consoilswat roomet al., 2
herbage in eight randomly placed rectangles8m) in each ring was harvested by cutting 20mmnd level just before grazing events. A sub-sample offrom the bulked herbage in each ring was sorted intoore drying for 24h at 60 C to determine dry weight oferbage and its botanical components (Edwards et al.,
samples were collected at the end of summer whents peak. On 8 March 2008, we took soil cores (0.025m0.020m deep) at 0.1-m intervals along a 4-m transectring (41 samples per ring). All transects started at theing but their orientation was randomly chosen to avoidn the distribution of vegetation and micro-topographysoils were placed immediately in plastic bags thatwereed to minimise evaporative losses. Plant material, rootsere removed gently by hand before the soil analyses.
ination of the persistence and degree of SWR
tied the topsoils wettability by determining the per-d the degree of SWR: The persistence of SWR can be
eld-moist samples for the actual SWR present inil material or on dried samples for the potential SWRRitsema, 1994). The persistence of the actual SWR was
n the laboratory using the water drop penetration time) (King, 1981). In essence, the time it takes for a dropletcedonto the soil surface to inltrate completely into theded. We used the threshold of 5 s proposed by Bisdom) to differentiate between wettable and water-repellentreshold is arbitrarily chosen and has no physicalmean-
orded WDPT values up to 6h and applied the categoriesistence of SWR dened by Dekker and Jungerius (1990)of three repetitions per sample. This denition distin-
en classes for the persistence of SWR: class 0, wettable;htly persistent SWR (560 s); class 2, strongly persis-0600 s); class 3, severely persistent SWR (600 s to 1h);ely persistent SWR (>1h), further subdivided into classlass 5 (36h), and class 6 (>6h). Only for 20% of thetriplicate drops have a standard deviation larger than
he degree of SWR was determined using the molarity ofplet (MED) test and the results were quantied in thecontact angles (CA) between the drop of the aqueous
ution and the soil surface (Roy and McGill, 2002). Theermine a CA larger than 90, which is the threshold fornce of hydrophobicity. In the MED test the surface ten-wetting liquid, an aqueous ethanol solution, is variedt where the soil spontaneously adsorbs the liquid, andangle between the soil surface and the liquid is 90. Theven dried at 65 C for 48h and then equilibrated for 24hperature before conducting the MED test (Kawamoto
ination of microbial respiration rates
iration includes the contribution of root-associated res-mineralization of organic matter. We determined CO2
ates of fresh soil samples sieved to 2mm and thus only
100 K. Mller et al. / Agriculture, Ecosystems and Environment 139 (2010) 98109
Fig. 1. Photos E) expThe orientatio egeta
consideredsieved soil22 C after aThe CO2Cmeasured agas into anDevelopmeevolution rvolumes of
The gravmined by dat 105. Aftesamples forence betwecontent, bepH of 5.8.
Inltratidisc inltroducted withThe impactexperimentabilities k othe fact thatand hydrop2000).
We meadisc inltrorate of eth(R=32.5mmat two tensconductivittration rateh2 =10mmliquids, thesethanol byowintountion (Wood
q(h) = K(h)
R issorpttiony et
ing thetering Rtivitn pr
Ksat. Theo estmeas on
s ratiof the experimental site. (a) A ring of the Free Air Carbon Dioxide Enrichment (FACn of the transects was randomly chosen to avoid bias based on the distribution of v
microbial respiration rates (MRR). Analiquot of 5 g freshmaterial was incubated for 4h in airtight containers at24-h pre-incubation period at the same temperature.
concentration of the headspace of each container wasfter 1 and 4h by injecting a sub-sample (0.11ml) ofInfra-Red Gas Analyzer (ADC 225 MK3, The Analyticalnt Co. Ltd., Hoddesdon, UK). We assume that the CO2ate is linear during the measurement period. Variablegas had no effect on the accuracy of the analysis.
ination of total organic matter
imetric soil water content of all soil samples was deter-rying a sub-sample of about 5g for 24h in the ovenrwards the SOM content was measured by igniting the5h at 550 C. Following Tumer et al. (2005), the differ-en the dried and ignited samples was taken as the SOMcause the samples were non-calcareous with a topsoil
cation of the impact of SWR on water inltration
on was measured in the eld at the meso-scale of ameter (R=32.5mm), while all other analyses were con-a disturbed soil sample of about 5g in the laboratory.of the soils wettability on water dynamics at the
al sitewasquantiedbycomparing the intrinsicperme-f water and ethanol. This approach takes advantage ofa fullywetting liquid likeethanolwetsbothhydrophilichobic soils at a contact angle equal to zero (Letey et al.,
a = ln
wheresolvedused tsic perdependis owits pro
sured the inltration rate of water with a tensionmeter (R=100mm) and subsequently the inltrationanol at the same location with a glass inltrometer) on4 January 2008. The inltration ratewasmeasured
ions h1, h2 in order to derive the respective hydraulicies K1(h1), K2(h2) (cms1) from the steady-state inl-s. For water, the tension was set to h1 =20mm and. To compare the intrinsic inltration rates of the twoe tensions were converted into equivalent tensions fordividing them by 2.5 (Jarvis et al., 2008). Steady-statesaturatedsoil canbedescribedwith the followingequa-ing, 1968):
[1 + 4
by SWR. Intof the inltsame whethfor a wetta10 would ifold by SWRand ethanoinltration.was deriveinltrationthe inltratdiscussed ietitions of tarea underwere condueriment, (b) a 4-m soil sampling transect starting at the rings centre.tion and micro-topography.
the radius (m) of the inltration surface (m2) and *ive number of the soil (mm1). From the steady-staterates q1(h1) and q2(h2), the parameter * can be derivedal., 1991):
at * is constant for the chosen range of tensions. The* is then used to solve Eq. (1) for an average tensioneynolds and Elrick (1991). With the derived hydraulicy (K(h1, h2)), the unsaturated hydraulic conductivityoposed by Gardner (1958):
is the hydraulic saturated conductivity (h=0), wasconductivities for ethanol Ke(h) and water Kw(h) wereimate the intrinsic permeabilities k (m2). The intrin-bility describes the part of the conductivity K(h) thatly on the properties of the soil through which a liquidhile the conductivity is a function of both the soil and
es and of the properties of the liquid itself (Bear, 1972):
the dynamic viscosity (N sm2), which is 0.0010 for.0012 for 95% ethanol at 20 C. The ratio of the intrinsicies was calculated as
o R(k) reects how much the permeability is reduced
rinsic permeabilities account for the specic propertiesrating liquid and should, in a hydrophilic soil, be theer determined with ethanol or water. This means that
ble soil R(k) would have a value of 1. An R(k)-value ofndicate that the intrinsic permeability is reduced ten-. Thus, comparing the intrinsic permeabilities of water
l allows us to quantify by howmuch SWR reduceswaterLike SWR, this ratiowould be timedependent. Our ratiod for the inltration of water after 90min and for theof ethanol at steady state. The importance of relatingion of water in soils with SWR to a particular time isn detail elsewhere (Lamparter et al., 2010). Three rep-he inltration experiment within a randomly selectedthe elevated and ambient atmospheric CO2 treatmentscted. The values here reported are mean values.
K. Mller et al. / Agriculture, Ecosystems and Environment 139 (2010) 98109 101
Fig. 2. Spatialthe microbialatmospheric [
The Shawere normto meet thsoftware Gguish if thand ambieent (P
102 K. Mller et al. / Agriculture, Ecosystems and Environment 139 (2010) 98109
Table 1Mean (standard error) and coefcient of variation CV for the soil properties contact angle (CA), water drop penetration time (WDPT), microbial respiration rate (MRR), totalorganic matter content (SOM), gravimetric water content (WC), determined under elevated and ambient [CO2] along three transects of 4m (n=123). Mean values in thesame row are signicantly different if followed by different letters (P90 ranging between 93.8 and 104.5 (Fig. 2a).ces in the degree of SWR between the rings with ele-mbient [CO2] were small and non-signicant (Table 1).f all sampleswerewettablewithin a second (Fig. 3). It isheWDPT reects only the rstwetting step of the outer. The persistence of SWR was highly variable (Table 1).s often varied between smaller than 5 s and larger thansingle ring over short distances of a few centimetres
eWDPT datawere not normally distributed. To normal-
Theindicatniedbynot a rdegreeby amprapidlysignictensioninltramatedthan th(P
K. Mller et al. / Agriculture, Ecosystems and Environment 139 (2010) 98109 103
Fig. 4. (a) Intrinsic permeabilities of ethanol and water measured at two tensions(water: 10 and 20mm water; ethanol: 4 and 8mm) under ambient andelevated atmospheric [CO2] (475L L1), respectively, on 04 January 2008. (b) Cal-culated ratio of intrinsic permeabilities. Tension 1 was 10 and 8mm, and tension2 was 20 and 8mm for water and ethanol, respectively.
found under ambient atmospheric conditions (0.570.05m). Thesamewas true for thepersistence of SWR (log(WDPT)),with a rangea of 0.270.07m for the elevated [CO2] compared with a range of0.500.06m for the ambient atmospheric conditions.
3.2. Impact of elevated atmospheric [CO2] on pasture growth
For both periods, 19992003 and 20032008, the differencesbetween the annual average total pastoral dry matter productionunder ambient and elevated [CO2] were not signicant (Table 2).Of the four functional groups analyzed, C3 grasses, C4 grasses,dicots and legumes, only the average annual dry matter yield ofthe legumes was increased signicantly under the elevated [CO2]during both time periods. For the dicots, this was only true duringthe early stages of the FACE experiment (Table 2).
3.3. Impact of elevated atmospheric [CO2] on MRR and SOM
The topsoil samples under ambient and elevated [CO2]had similar microbial respiration rates (6.082.24 and6.122.74gCO2 g1 dry matter 24h1, respectively). TheSOM contents of the topsoil samples ranged between 7.9 and14.6%. The average SOM content was 10.81.0 and 111.2% forthe topsoil samples exposed to elevated and ambient atmospheric[CO2], respectively. The difference was not signicant (Table 1).
The spaWDPT, SOMThis procedferent (TabSWR characeter a expreffects Cn wpersistencescale variabThe range orespectivelpattern wa(Fig. 6). Thea spatial pa
Fig. 5. (a) Exponentialmodeltted to theexperimental semivariogramsof thedegreeof soilwater repelln=123) and ambient (n=123) atmospheric [CO2]. (b) Exponential model tted to the experimental semfor the topsoil samples collected under elevated (n=123) and ambient (n=123) atmospheric [CO2].of SWR on carbon mineralization
tial structure of the log-transformed parameters CA,and MRR were analyzed for both treatments together.
ure is justied, as the means were not signicantly dif-le 1). Increasing the number of observations for the twoteristics decreased the uncertainty of the range param-essed by its lower standard error (Fig. 6). The nuggetere 8 and 40% of the total variance for the degree andof SWR, respectively. This indicates either high short-ility and/or can be attributed to themeasurement error.f 0.46 and 0.36m for the degree and persistence of SWR,y suggests a short spatial dependence of SWR.No spatials detected for the parameters log(SOM) and log(MRR)transect length might have been too short for revealingttern for these parameters.
ency (CA) for the topsoil samples collectedunder elevated (475L L1;ivariograms of the persistence of soil water repellency (log(WDPT))
104 K. Mller et al. / Agriculture, Ecosystems and Environment 139 (2010) 98109
Table 2Mean (standard error) annual average dry matter production (total and by functional groups) from pasture at ambient or elevated atmospheric [CO2]. Data are for twoperiods, 19992003 and 20032008 inclusive. P values are from a repeated measures analysis of variance for the 5 years of data.
Period 19992003 20032008
TotalC3 grassesLegumesDicotsC4 grasses
Fig. 6. Best mmicrobial respbest, with the
Table 3Parameter valupenetration tim
648 (67) 712 (125) 0.394 5473 (68) 473 (105) 0.998 5
71 (7) 126 (7) 0.02721 (2) 43 (2) 0.00711 (3) 11 (1) 0.928
odel tted to the experimental semivariograms of (a) the degree of soil water repellencyiration rate (log(MRR)), and (d) the soil organic matter content (log(SOM)) for all topsoilexception of the soil respiration rate, where the linear model gave the best t.
es, their standard errors and the coefcient of determination (R2) of the multiple linear re (log(WDPT)) and contact angles (CA) for all topsoil samples (n=246).
estimating WC Standard error for the values
109CA 0.0813 0239 log(WDPT) 0.0022 028CA0.0204 log(WDPT) 0.084 0011CAe 0.023 log(WDPTe) 0.0985 041CAa0.0181 log(WDPTa) 0.0161 0mbientgdryweightm2)
93 (85) 674 (49) 0.24980 (88) 684 (44) 0.21451 (6) 92 (5) 0.008
73 (20) 74 (4) 0.9388 (3) 4 (1) 0.223
(CA), (b) the persistence of soil water repellency (log(WDPT)), (c) thesamples (n=246). In all cases, the exponential model tted the data
egressions between soil water content (WC) and values of water drop
.0008 0.420.0011 0.61
.0015 0.0009 0.67
.0009 0.0017 0.73
.0016 0.0029 0.56
K. Mller et al. / Agriculture, Ecosystems and Environment 139 (2010) 98109 105
Fig. 7. Relatioand SOM contand MRR (n=2
Correlattionships bof SWR (Pbetween th(P
106 K. Mller et al. / Agriculture, Ecosystems and Environment 139 (2010) 98109
Fig. 8. Measured and estimated soil water contents for all topsoil samples (collected under the elevatedof the degree and persistence of soil water repellency. (a) Separate regressions for the two treatments(n=246). The solid line is the line of equal values.
al., 2004). No signicant differences in the persistence and degreeof SWR were found. Our results on the persistence of SWR con-trasted thethe same spersistent i(Newton eticant differerepellency csome obviothe same exlonger in ouenrichment
Fig. 9. Relatioof soil water rThe transition
the soil watprior to our
eviouwere.8 gdrm acede atn-siglisheas st cogrou3) omeasurements of a previous sampling in March 2003 atite, which had shown that SWR was signicantly lessn the soil under elevated than under ambient [CO2]al., 2003). Like Newton et al. (2003), we found a signif-nce in the distribution of soil samples in the soil waterlassesbetweenambient andelevated [CO2]. Therewereus differences between the two samplings conducted atperimental site: the duration of CO2 enrichment wasr study than the previous study, being 10 years of CO2compared with 6 years. Furthermore, at our sampling
the pryieldsand32long-teinuenherbagbut nounpubyield wambienabove-al. (200nship between the gravimetric soil water content and the persistenceepellency of all topsoil samples (n=246) collected on 8 March 2008.zone for the criticalwater content ranges between0.15 and0.24gg1.
comparisonsomewhatstudy the olyzed.
In accorvated [CO2]rates. Zak et[CO2] on soies found trtwo of the 2changes inet al. (2008glasshouseincreased Srates. It is ifactors potetemperaturstudy. Simithe previourateswere cOur mean mreported byThe relativement differand ambient [CO2] (n=246). The soil water contents were a function(n=123); (b) all topsoil samples were pooled into a single regression
er content was much higher because of a rainfall eventsampling, leading to a lower actual SWRcomparedwithsly determined actual SWR. The above-ground plantcomparable at harvest on 18 March 2008, with 35.9
mm2 under ambient andelevated [CO2]. Similarly, theverage annual dry matter yields were not signicantlyby elevated [CO2]. The proportion of legumes in thethis particular sampling date was increased markedlynicantly under elevated atmospheric [CO2] (Newton,d data), while the long-term average annual legumeignicantly higher under elevated [CO2] than undernditions. The summers drought might have maskednd responses of legumes to elevated [CO2]. Newton etnly determined the persistence of SWR. Therefore, the
of previous measurements of SWR with our results is
restricted. To the best of our knowledge, in no otherccurrence of SWR under elevated [CO2] has been ana-
dance with the effect of elevated [CO2] on SWR, ele-alsohadno signicant impact on carbonmineralizational. (2000) reviewed the impact of elevated atmospheric
il microorganisms. While they reported that most stud-ends of more rapid soil and microbial respiration, only1 rates measured were signicantly higher. Signicantsoil respiration rates have been observed by Paterson), Ross et al. (2004) and Sowerby et al. (2000). In astudy of 64 days, Paterson et al. (2008) found bothOM mineralization and increased plant root respirationmportant, however, to note that other environmentalntially limiting plant and microbial activities, such aseandwater availability,wereoptimized throughout thelarly, in the long-term New Zealand FACE experiment,sly reported signicant differences in soil respirationonned towet summer and autumns (Ross et al., 2004).icrobial respiration rates were in the range of thoseZak et al. (2000) for soils under different grass species.lyhighsamplevariability, andnon-signicanceof treat-ences found in the here reported carbon mineralization
K. Mller et al. / Agriculture, Ecosystems and Environment 139 (2010) 98109 107
ratesmay be partly attributable to the uneven return of sheep dungand urine to the grazed pasture (Aslam et al., 2009).
Similarly to the MRR, SOM contents were not signicantlyaffectedby theelevated [CO2]. This is in accordwithpreviousobser-vations for t[CO2] (Rossexperiment(2005), howthe generalcan be explapplying alitter/root pwith the tothus hypothof response
SWR is aThe within-is importanow in hydrhydrophobion micro-tostatus of ththe soil mavide prefereand Ritsemthe micro-sdo not provthe eld untial resolutRegalado aparametersstatistics. Tnot randomlar under amSWR are alsexponentiaand McBratacteristicswwas supporbetween thtionship be1981; Dekkand Ritter,content (KiThe correlamuch shortsuch as orgaand McBratspatial pattization rateMRR and thand explaintents and Mbetween SOTumer et afrom a singresults chalSOM and SWples are tak
We delishortly afteto examineIn contraststrong imptents. SWRtopsoil. The
soilwater distribution in the topsoil,withpatches of lowandhighersoil water contents. Our results emphasize that SWR is a transientsoil property. If SWRwere a long-termstrategy for the conservationof SOM (Goebel et al., 2005), then the lower availability of water in
chesnd wowee spancesn andcal cal fate breper the
onlye cathe sotheand
wasanspf spaann,le isriodseed beathen thee ofl rainencehe thc maTumorticGer
is naeratuc inuick
thand (FRhangpiran he
. Incrated Cand tuotal carbon from this study site after 5 years of elevatedet al., 2004), and with most results from elevated [CO2]s (Hungate et al., 1997; Gill et al., 2002). Jastrow et al.ever, challenged these former results by showing thatlack of signicant changes in SOM contents observed
ained by the low statistical power of most experimentsmeta-analysis technique. In our study, the increases inroduction under elevated [CO2] were small comparedtal SOM pool of 10% already present in the topsoil. Weesized that this also might have contributed to the lackof carbon mineralization rates under elevated [CO2].spatially very variable property (Shakesby et al., 2000).site distribution of water repellent and wettable areast for the occurrence of runoff, erosion and preferentialophobic soils at the catchment scale.Water ponding onc soil surfacesmay be laterally redistributed dependingpography. As a consequence of the water repellencye neighbouring surface, the water will inltrate intotrix or run off the surface. Macropores can also pro-ntial pathways for water in hydrophobic soils (Dekkera, 1994). Measurements for SWR are usually made atcale with soil samples in the laboratory. These testside insight into the distribution of hydrophobicity inless the soil samples are collected with a high spa-
ion (Ritsema and Dekker, 1998; Tumer et al., 2005;nd Ritter, 2008). We analyzed the distribution of theof SWR at ambient and elevated [CO2] applying geo-
his revealed that persistence and degree of SWR werebut followed short-ranging patterns, which were simi-bient and elevated [CO2], suggesting that the causes ofo not randomly distributed. The range parameter a forl models is identical to the correlation length (Mullaney, 2002). The correlation lengths for both SWR char-ere similar, indicating that theywere interrelated. This
ted by the weak but signicant linear correlation founde two SWR parameters. Others have analysed the rela-tween WDPT and CA, with inconsistent results (King,er and Ritsema, 1994; Lamparter et al., 2006; Regalado2008). The non-linear behaviour of SWR with waterng, 1981) might partly explain the different outcomes.tion lengths found for the two parameters of SWR wereer than those reported for other basic soil parametersnic matter content, pH and hydraulic properties (Mullaney, 2002). In our study at the scale of the transects, noernwas detected for SOM contents and carbonmineral-s. Furthermore, while the relationships between SOM,e parameters of SWR were signicant, they were weaked at most 14% of the observed variability of SOM con-RR. In contrast, a strong signicant positive correlationM and SWR was found by Lamparter et al. (2009) andl. (2005). In both studies, the soil analysed was derivedle site comparable to the study presented here. Ourlenge the hypothesis that a strong relationship betweenR is typical for a small-scale investigation where sam-
en in close proximity (Doerr et al., 2006).berately determined the actual persistence of SWRr a rainfall event because one objective of the study wasthe impact of SWR on the redistribution of soil water.to all other relationships that we analyzed, we found aact of SWR on the local distribution of soil water con-was the driving factor of the soil water contents in thepersistence and degree of SWR led to a mosaic of the
the patrates ation. Hthat thsubstaing rainew lochemicintegrawater-furtheSWR.
Theysis arthus, tthat indegreerangeand trkind oBachm
Littdry pethe spwet wlight osistencnaturaoccurrcal to torgani2005).for a HBerlin,studythe litthe disished qof intrshowntake beresultsthe tembetterdegradincreasatmosp
WeZealanmate Cthe resgelman
Allard, V2005elevratewith a higher SWR would have reduced soil respirationould have had a positive feedback on carbon sequestra-ver, we did not nd such correlations, thus we assumetial pattern of SWR is an ephemeral phenomenon. Thecausing SWR seemed to be readily washed out dur-then rebuilt in different patterns as a response to the
onditions. A true understanding of the biological andctors of SWR is still missing. Future research shouldiodiversity aspects with a chemical characterization ofllent soil materials, vegetation patterns, and investigatetemporal nature of the short-ranged spatial pattern of
other studies on SWR employing a geostatistical anal-chment studies (Regalado and Ritter, 2006, 2008) andil samplings were conducted at a different scale frompresent study. However, the authors also detected thatpersistence of SWR followed a spatial structure. Thelarger than 200m (Regalado and Ritter, 2006). Flowort models valid for water-repellent soils require thistial information on the occurrence of SWR (Deurer and2007).known about the thresholds of soil moisture, length ofnecessary for the development of hydrophobicity andy which hydrophilic conditions develop in periods ofr (Shakesby et al., 2000). Our experiments shed somepersistence of SWR. Under both treatments, the per-
SWR of the hydrophobic soil was rather low after thefall event. In our study, the upper thresholds for theof SWR were between 0.23 and 0.24gg1 and identi-reshold determined for a Typic Psammaquent with antter content of 18% in the Netherlands (Dekker et al.,er et al. (2005) found an upper threshold of 0.18gg1
Andosol on a former wastewater inltration site nearmany. The transition zone of 0.09gg1 found in ourrrow compared with the transition zones reported inre (Doerr and Thomas, 2003; Tumer et al., 2005). Inltrometer experiments, the persistence of SWR dimin-ly, as reected in the difference between the two ratiospermeabilities under high and low tension. It has beenthe actual wetting process of hydrophobic soils mayn2 and3weeks (Hurrass and Schaumann, 2007). These
her emphasize that more knowledge is required aboutal and spatial occurrence of SWR, which might lead to arstanding of themechanismsdriving this important soilprocess. We conclude that SWR does not contribute tohe long-term terrestrial C sink in response to elevatedc [CO2], a feature of climate change.
k the Foundation for Research and Technology NewST) for the nancial support of this project (SLURI, Cli-e). The authors thank Michael Trolove for conducting
tion rate measurements. Tehseen Aslam and Eva Klin-lped with the soil sampling.
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Is there a link between elevated atmospheric carbon dioxide concentration, soil water repellency and soil carbon mineraliz...IntroductionMaterials and methodsDescription of study sitePasture samplingSoil samplingDetermination of the persistence and degree of SWRDetermination of microbial respiration ratesDetermination of total organic matterQuantification of the impact of SWR on water infiltrationStatistical analysisGeostatistical analysis
ResultsImpact of elevated atmospheric [CO2] on SWRSpatial distribution of the persistence and degree of SWR
Impact of elevated atmospheric [CO2] on pasture growthImpact of elevated atmospheric [CO2] on MRR and SOMImpact of SWR on carbon mineralizationImpact of elevated atmospheric [CO2] on soil water content