effect of compaction, tillage and climate change on soil water balance of arable luvisols in...

Soil & Tillage Research 124 (2012) 211–218

Effect of compaction, tillage and climate change on soil water balance ofArable Luvisols in Northwest Germany

P. Hartmann a,b,*, A. Zink a, H. Fleige a, R. Horn a

a Institute of Plant Nutrition and Soil Science, Christian-Albrechts-University zu Kiel, Hermann-Rodewald-Str. 2, 24118 Kiel, Germanyb Department of Soils and Environment, Forest Research Institute Baden-Wuerttemberg, Wonnhaldestr. 4, 79100 Freiburg, Germany

A R T I C L E I N F O

Article history:

Received 23 February 2012

Received in revised form 18 June 2012

Accepted 19 June 2012

Keywords:

Soil compaction

Climate change

Soil water balance modeling

Tillage systems

Luvisol

A B S T R A C T

In this study we determined wheeling (external loads of 6.3 Mg by 10 times wheeling) and tillage effects

(conventional and conservation tillage) on the soil hydraulic properties of Stagnic Luvisols in Northwest

Germany and modeled the soil water balance’s reaction on both loading and changing climatic

conditions.

Due to the mechanical stress applied by loading, physical properties changed distinctly in the top Ap-

horizons and the subsequent Eg-horizons at both tillage systems. Especially pore size distributions and

soil hydraulic conductivities were affected. The Btg horizons did not show changes due to loading.

Soil water balance was measured with soil tensiometers during one growing period and the following

autumn and was modeled with Hydrus 1D for loaded and unloaded conditions under winter wheat for

three different periods (1991–2000; 2051–2060; 2091–2100) based on a regional A1B climate scenario.

At the loaded sites we found an increase of actual transpiration rates in the growing period. As a

consequence of stronger drying and changed hydraulic properties, rewetting in autumn and winter was

retarded and less complete on average. Furthermore, simulations indicated an increase of the variability

of matric potentials. Consequently, compaction might result in a higher drought risk and a higher

susceptibility for water logging in spring, which may result in less favorable soil conditions and plant

growth.

Reactions of soil water balance on changing climatic conditions were comparable for all loading

variants and tillage systems. Predicted changes in precipitation (in general: summer �, winter +) and

temperature (+) would result in a reduction of transpiration rates in the growing period while the

climatic water balance in autumn and winter would increase distinctly.

� 2012 Elsevier B.V. All rights reserved.

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1. Introduction

Climate in the North West German Plain is expected to changeaccording to the IPCC-scenario A1B in the 21st century (Spekat et al.,2007). In the last 30 years of the 21st century, mean temperature ispredicted to increase by 2.5 K, summer precipitation shoulddecrease by 10–20% while winter precipitation should increase by15–25%. However, the uncertainty of predictions especiallyconcerning rainfall intensity and extreme events has to beconsidered as being very high (Bloschl and Montanari, 2010). Otherpossible multicausal relationships than the role of CO2 regardingclimate change might be over- or underestimated (Kutılek, 2011) inthe IPCC-scenarios. Nevertheless, more pronounced summery

* Corresponding author at: Department of Soils and Environment, Forest

Research Institute Baden-Wuerttemberg, Wonnhaldestr. 4, 79100 Freiburg,

Germany. Tel.: +49 0761 4018215.

E-mail address: [email protected] (P. Hartmann).

0167-1987/$ – see front matter � 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.still.2012.06.004

droughts, heavy precipitation and floods are forecasted with greatereffects on society than derived from mean climate changes. Moreextreme rainfall regimes are expected to increase the duration andseverity of soil water stress in mesic ecosystems (Knapp et al., 2008).In the context of climate variability on the regional scale land–atmosphere interaction is an important mechanism. Especially soilmoisture plays an important role as there are distinct feedbacks totemperature and precipitation upon evapotranspiration (Senevir-atne et al., 2006).

It is well described that besides actual climatic conditions, soilproperties control the actual plant available water. However, thesesoil properties are subjected to a dramatic degradation due tohuman impact, e.g. about 32% of the subsoils in Europe are highlydegraded and 18% are moderately vulnerable to compaction(Fraters, 1996, cited in Van den Akker, 2002). In addition, if themechanical strength in the top- and subsoil are exceeded, soils arefurthermore deformed, which also effects crop yield and groundwater recharge, increases soil loss by water and wind erosion andincreases greenhouse gas emissions from soil (e.g. Horn et al.,


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P. Hartmann et al. / Soil & Tillage Research 124 (2012) 211–218212

2000; Pagliai and Jones, 2002). These coupled mechanical andhydraulic processes are described in detail in Horn et al. (2009).How far compaction in the context of a changing climate affects thewater availability for plants and soil water balance is mostlyunknown. Aims of this investigation are (1) to quantify changes ofsoil physical properties (pore size distribution, hydraulic conduc-tivity) due to compaction under consideration of different tillagesystems (conventional–conservation), (2) to simulate the soilwater balance of the investigated soils continuously to derive theeffects of compaction and climate change on soil water balance.

2. Materials and methods

2.1. Site description

Field measurements and samplings took place on Stagnic Luvisolsunder two soil tillage treatments (conservation (cons), conventional(conv)). Horizon designation is the same for both sites with a tilledAp-horizon (at the conservative site a relict Ap), followed by an Eg(loss of silicate clay, stagnic conditions) and a clay enriched Btg-horizon, also with stagnic conditions (according to FAO, 2006). Thesites are located in the federal state of North Rhine-Westphalia,Germany with soils derived from periglacial aeolian deposits (loess).The investigated soils are characterized by very high silt and low sandcontents. Clay contents increased with depth due to lessivation(Table 1). Carbon contents were highest in the Ap-horizons wherebythe ‘‘cons’’ site had slightly higher carbon contents. The sites arecharacterized by a distinct secondary pore system due to aggregationand biopores (Zink et al., 2010). Underneath 1.2 m soil depthfollowed an impermeable clayey marl layer and drainage wasinstalled at a depth of 0.8 m with 12 m spacing. The ‘‘cons’’ site hasbeen tilled since 1993 with a chisel plough to 0.1 m depth. The ‘‘conv’’site has been mouldboard ploughed to 0.28 m depth. The five-yearcrop rotation included two years of winter wheat and annual winterbarley, winter rye and sugar beet. Initial soil properties are describedin detail by Zink et al. (2010).

2.2. Experimental setup, soil sampling, laboratory methods and field

measurements

Wheeling experiments were carried out in early spring at soilmoisture contents close to field capacity (�6 kPa). A tractor towedsingle wheel (650/75 R32) load frame was used to apply theexternal loads of 6.3 Mg. Slip and smearing were avoided. Allexperimental plots were sampled at comparable horizons bytaking undisturbed soil cores (10�4 m3) and disturbed compositesamples. Unloaded reference profiles (ref) and loaded wheel tracks(comp) were sampled at three depths (0.2–0.25 m; 0.4–0.45 m;0.65–0.70 m) and pore size distribution as well as saturatedhydraulic conductivity were determined. For this purpose,undisturbed soil cores were completely saturated and desiccatedat defined pressure heads (Hartge and Horn, 2009): �0.5, �1, and

Table 1Mean values of texture classes sand, silt, clay (g kg�1) and the organic carbon

content C (g kg�1) of the three investigated horizons of two Stagnic Luvisols derived

from loess at the differently tilled sites (conv = conventional, cons = conservation).

Horizon Depth in m Tillage Sand Silt Clay C

Ap 0.00–0.30 conv 30 840 130 19

cons 40 810 150 23

Eg 0.30–0.60 conv 20 850 130 5

cons 30 810 160 8

Btg 0.60–1.00 conv 20 820 160 5

cons 30 770 200 3

�3 kPa (on a laboratory sand-bath with a hanging water column);�6, �15, �30 and �50 kPa (on a ceramic plate) and in a pressurecell (positive air pressure at 1500 kPa) (n = 5). The following poreclasses were calculated: wide coarse pores wCP (air filled porosityat �6 kPa), narrow coarse pores nCP (water content between�6 kPa and �30 kPa), meso pores MP (water content between�30 kPa and �1500 kPa), fine pores FP (water content at�1500 kPa) and total pore volume TPV. Furthermore, the sampleswere dried at 105 8C to determine bulk density dB. Saturatedhydraulic conductivity Ks was determined using a falling-headmethod (n = 7) (Hartge and Horn, 2009). Mean values werecalculated and the comparable variants (loaded vs. unloaded) weretested for significant differences with a one way analyses ofvariances (ANOVA) at the significance level of P < 0.05.

Tensiometers were installed in the three sampled depth(�0.25 m, �0.45 m, �0.70 m) (n = 3) after seedbed preparationand soil matric potential was measured once per week at anunloaded and at a loaded field site from March till November 2007.

2.3. Modeling

In order to model the effects of compaction on soil waterbalance we simulated the one-dimensional water flow in a variablysaturated soil by HYDRUS 1D (Simunek and van Genuchten, 2005).The program solves numerically the Richards’ equation usingGalerkin-type linear finite element schemes. Selected spatialresolution was 0.01 m, time resolution was limited to 10�5 days.Water uptake by plant roots was included and atmosphericconditions and drainage taken as variable boundary conditions.The used soil hydraulic parameters of the water retention curveand unsaturated hydraulic conductivity Ku for each investigatedsoil depth were parameterized using the software RETC (vanGenuchten et al., 1994). The water retention curve was fitted byusing the equation according to van Genuchten (1978):

uðhÞ ¼ ur þ ðus � urÞ � ½1 þ ða � hÞn��m(1)

with u(h) the actual water content, ur the residual water content, us

the water content at water saturation, a the negative reciprocalvalue of air entry and n and m parameter concerning thesmoothness of the water retention curve (m = 1 � 1/n). Theunsaturated hydraulic conductivity Ku was estimated from thewater retention curve according to the model described byMualem (1976):

Ku ¼ Ks �½1 � ða � hÞm�n � ð1 þ ða � hÞnÞ�m�

2

½1 þ ða � hÞn�m�l(2)

with Ks the measured saturated hydraulic conductivity and l thepore-connectivity parameter.

The used root water uptake model of wheat refers to the waterstress response function described by Feddes et al. (1978), theused parameters are implemented in HYDRUS 1D according toWesseling et al. (1991) with a maximum transpiration ratebetween �0.1 and �50 kPa at a potential transpiration rate of5 � 10�3 m/day (�90 kPa at a potential transpiration rate of10�4 m/day, interpolation implemented in HYDRUS 1D) and areduced transpiration between 0 and �0.1 kPa and between�50 kPa (�90 kPa respectively) and �1600 kPa. At pressure headshigher 0 kPa and lower than �1600 kPa, transpiration ceases. Thegrowing season for wheat was implemented as follows: continu-ous growing of crop and roots (initial crop height 0.10 m, rootingdepth 0.05 m) from April 01 until July 15 (final crop height 0.6 m,rooting depth 0.6 m); fallow after harvest from July 15 untilAugust 15; afterwards a continuous growing until November 01(crop height from 0.01 to 0.10 m and rooting depth from 0.01 to0.05 m). Maximum rooting depth was restricted to 0.6 m soil

273

283

293

303

0

20

40

60

80

100

120

271

275

279

283

287

Σ monthly precipitation mm / mean max. temperature K/ mean min. temperature K/

month

1991-20002051-20602091-2100

1991-20002051-20602091-2100

1991-20002051-20602091-2100

fj am jm aj os dn

month

fj am jm aj os dn

month

fj am jm aj os dn

Fig. 1. Monthly mean values of precipitation (sum) and temperature (mean of maximum and minimum) for the periods 1991–2000, 2051–2060 and 2091–2100 based on the

WETTREG simulations (SRES A1B) at the meteorological station in Guetersloh (Enke and Kreienkamp, 2006).

P. Hartmann et al. / Soil & Tillage Research 124 (2012) 211–218 213

depth, as there is a dense horizon with stagnic conditionsunderneath (Btg). Effects of compaction on root developmentwere not considered. The impervious layer below 1.2 m and thedrainage spacing (12 m) were also included.

At the soil surface, an atmospheric boundary condition wasimposed using daily data of precipitation, potential evaporation,and transpiration based on meteorological data of the meteoro-logical station in Guetersloh, which is located close to the studysite. The ratio between potential transpiration and soil evaporationwas based on radiation partitioning as a function of the leaf-areaindex (Ritchie, 1972). In order to implement future climatescenarios on daily basis, data of the same meteorological stationbased on the WETTREG simulation using the ECHAM5/OM IPCCcontrol and scenario run 1 (SRES A1B) were used for the decades1991–2000, 2051–2060 and 2091–2100 (Enke and Kreienkamp,2006) (Fig. 1).

3. Results

3.1. Effects of loading on soil physical properties

The ploughed and unloaded Ap horizon of the conventionallytilled site (Ap conv ref) showed the highest value of TPV and wCP(Table 2), which were both significantly reduced due to compac-tion. At the ‘‘cons’’ site, loading led to a reduction of TPV and wCP,too, but less pronounced. While nCP were reduced due to loadingat the ‘‘conv’’ site, they increased slightly at the ‘‘cons’’ site. MPdominated the pore size distribution because of the silty texture atall sites. Bulk density dB was lowest and Ks highest at the ‘‘conv ref’’Ap horizon. With loading, Ks was significantly reduced afterloading with 6.3 Mg under both tillage variants.

Table 2Mean values of pore size distribution (TPV = total pore volume, wCP = wide coarse por

hydraulic conductivities Ks (m/day), bulk densities dB (Mg m�3) of the three investi

(conv = conventional, cons = conservation, ref = unloaded reference site, comp = loaded s

Horizon Tillage Variant TPV wCP

Ap conv ref 0.49* 0.12*

comp 0.39 0.03

cons ref 0.46 0.05*

comp 0.43 0.03

Eg conv ref 0.40 0.05

comp 0.41 0.03

cons ref 0.42 0.08*

comp 0.41 0.03

Btg conv ref 0.39* 0.05

comp 0.40 0.03

cons ref 0.41 0.04*

comp 0.40 0.03

* A significant different between unloaded reference site and the compacted site wit

Stress propagation in the subsequent Eg horizons was lesspronounced and resulted in fewer changes in pore size distributionthan in the Ap horizons. While TPV was not affected, coarse poreswere partially significantly reduced and MP increased under bothtillage systems. Aggregation and a high amount of biopores causedpreferential flow paths and thus high values of Ks, which weredistinctly reduced by loading. Bulk densities dB did not differbetween the variants.

The subsoil Btg horizons showed a slightly lower TPV than theoverlaying horizons, while FP increased due to clay enrichment.MP dominated the pore size distribution but did not show distinctdifferences as a result of compaction. Ks as well as dB values werehigh at all sites and differed significantly irrespective loading.

3.2. Hydraulic properties used for modeling

Compaction led to modifications of the pore size distribution inthe Ap and the Eg horizons at both tillage systems which can bealso derived from the Van Genuchten parameters (Fig. 2 and Table3). While us was reduced by compaction in the top soil horizons, awas reduced by one order of magnitude only for the ‘‘conv’’ site’sAp horizon. The subsequent Eg horizons showed reductions of adue to compaction, too. The subsoil Btg horizons did not showdistinct differences.

In combination with a reduction of Ks, Ku at pressure heads ofbetween �1 and �6 kPa was reduced by two orders of magnitudedue to the lower value of wCP in the ‘‘conv’’ Ap-horizon butincreased below pressure heads of �6 kPa slightly relative to thereference site. In the ‘‘cons’’ Ap-horizons compaction led to adistinct reduction of Ku values. In the Eg-horizons, Ks was alsoreduced and the amount of MP increased with the consequence

es, nCP = narrow coarse pores, MP = meso pores, FP = fine pores) (vol.%), saturated

gated horizons of two Stagnic Luvisols at the differently tilled and loaded sites

ite).

nCP MP FP Ks dB

0.05* 0.22 0.10 2.7* 1.42*

0.02 0.23 0.11 0.01 1.56

0.01* 0.27* 0.12 0.63* 1.50

0.04 0.24 0.12 0.02 1.58

0.04 0.19* 0.12 1.03* 1.55

0.04 0.23 0.11 0.45 1.57

0.01 0.18* 0.16 5.81* 1.53

0.03 0.22 0.14 0.38 1.56

0.03 0.17 0.14* 0.95 1.58

0.04 0.20 0.13 0.98 1.59

0.01 0.17 0.19* 0.65* 1.57

0.03 0.17 0.17 0.54 1.59

hin one variant with P < 0.05.

Ku(m

d)-1

100

10-4

10-8

10-12

10-16

100

10-4

10-8

10-12

10-16

10 0

10-4

10-8

10

10

-12

-16

Ku(m

d)-1

100

10-4

10-8

10-12

10-16

100

10-4

10-8

10-12

10-16

100

10-4

10-8

10-12

10-16

0.6

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.5

0.4

0.3

0.2

0.1

0.0

pressure head / -kPa

1 10 103

10 5

00.6

0.5

0.4

0.3

0.2

0.1

0.0

pressure head / -kPa

0

0.6

0.5

0.4

0.3

0.2

0.1

0.0

pressure head / -kP a

0

RETC ref measured ref

RETC comp measured comp

conventional

conservation

Ap BtgEg

pressure head / -kPa pressure head / -kPa pressure head / -kP a

Ap BtgEg

1 10 103

10 5

1 10 103

10 5

1 10 103

10 5

0 0 01 10 103

10 5

1 10 103

10 5

1 10 103

10 5

0 0 01 10 103

10 5

1 10 103

10 5

1 10 103

10 5

0 0 01 10 103

10 5

1 10 103

10 5

WC(m

3m)

-3WC(m

3m)

-3

Fig. 2. Effect of compaction on the water retention and the hydraulic conductivity (measured values: circles with standard deviation; lines: fitting based on RETC). Shown are

the Ap- and subsoil Eg- and Btg-horizons of Stagnic Luvisols under conventional (top) and the conservation soil management (bottom). Black circles and continuous lines refer

to the unloaded reference site (ref), white filled circles and dashed lines to the loaded site (comp).


that the Ku-function was modified with almost continuouslyhigher Ku-values than the reference Eg-horizons.

3.3. Field measurements and validation of modeling

The simulations of matric potentials as a function of time arein agreement with the measured values (unloaded site in Fig. 3).It should be mentioned, that the measured period was a

distinctly wet period resulting in high soil matric potentialvalues. The direct comparison of simulated and measured valuesshowed, that drying in the vegetation period as well asrewetting due to precipitation was well described for the Apand Eg horizons. Root mean square errors, which indicate theabsolute values of deviation, range between 0.4 and 5 kPa andcan mainly be ascribed to a slight overestimation of dryingduring the vegetation period. Thus, the established models were

Table 3van Genuchten parameters ur (=residual water content Vol.%), us (=water content at

saturation vol.%), a (=negative reciprocal value of air entry 1/0.01 m) and n

(=parameter concerning the smoothness of the water retention curve) of the three

investigated horizons of two Stagnic Luvisols at the differently tilled and loaded

sites (conv = conventional, cons = conservation, ref = unloaded reference site,

comp = loaded site).

Horizon Tillage Variant ur us a n

Ap conv ref 0 0.485 0.0461 1.195

comp 0 0.375 0.0014 1.393

cons ref 0 0.438 0.0036 1.305

comp 0 0.411 0.0025 1.333

Eg conv ref 0 0.393 0.0144 1.205

comp 0 0.395 0.0025 1.336

cons ref 0 0.419 0.0613 1.117

comp 0 0.396 0.0022 1.294

Btg conv ref 0 0.352 0.0040 1.225

comp 0 0.390 0.0036 1.265

cons ref 0 0.391 0.0071 1.150

comp 0 0.382 0.0032 1.211


used for further analyses of the effects of soil compaction andclimate change.

3.4. Effect of compaction, tillage and climate change on soil water

balance

The simulated water balance for all sites resulted in just slightdifferences between the tillage variants in the Ap- and Eg-horizons(0.25 m and 0.45 m soil depth) for the decade 1991–2000 (Fig. 4).While the topsoil (0.25 m) of the ‘‘conv’’ site reacts faster on dryingin May and June, rewetting was less retarded from Septemberonwards. On the other hand, the subsoil (0.45 m) showed astronger drying at the ‘‘cons’’ site due to less nCP and MP and thus

Fig. 3. Measured and simulated matric potentials at the conventionally tilled and unloade

simulated matric potential (m.p.) are in agreement with the measured matric potentia

less available water. Consequently, cumulative transpiration ratesand climatic water balance were almost identical for both tillagevariants (Table 4).

Compaction resulted in more distinct differences indepen-dent from tillage. From January to April the median values at thecompacted variants show a tendency to drier conditions with ahigher variability, especially in 0.25 m depth. From May to Julydrying proceeds to pressure heads of �1000 kPa in 0.25 m with ahigh variability remaining at all sites. In 0.45 m depth, theloaded sites show a distinctly higher degree of drying. Therewetting from September to December (February) proceedsdistinctly retarded and less completely at the compacted sites inboth soil depths. Additionally, a higher variability of pressureheads was simulated during this period. The reason for thestronger drying at the compacted sites is attributed to theincrease of transpiration rates in the vegetation period (Table 4)due to changed pore size distributions and hydraulic conductiv-ities, whereby a better supply with water could be maintained inthis period.

The effect of a changing climate was estimated by thesimulation and comparison of the water balances in three decades(1991–2000, 2051–2060, 2091–2100) for each tillage treatmentand loading situation. Mean annual precipitation is slightlyreduced with a shift to distinctly reduced precipitation in theperiod April–July and a less pronounced increase of precipitationin August–March (Table 4). Changed climate conditions affect thetranspiration and evaporation rates. At all sites, actual transpira-tion decreases during the vegetation period April–July, while theevaporation rate remains more or less the same. In the periodAugust–March, transpiration does not change distinctly, butactual evaporation increases slightly at all sites due to highertemperatures. This results in a reduction of the climatic waterbalance in the vegetation period and an increase in the periodAugust–March.

d Stagnic Luvisols during the vegetation period and the following autumn (left); the

ls in the Ap and Eg-horizons (right).

Table 410-Years means of precipitation and results of the HYDRUS-1D modeling of the actual transpiration, actual evaporation and the resulting climatic water balance

(=precipitation � actual transpiration � actual evaporation) for the periods April–July (growing period) and August–March in the decades 1991–2000, 2051–2060 and 2091–

2100 based on the WETTREG simulations (SRES A1B) (Enke and Kreienkamp, 2006) at the differently tilled and loaded sites (conv = conventional, cons = conservation,

ref = unloaded reference site, comp = loaded site).

Decade 1991–2000 2051–2060 2091–2100

Period April–July August–March April–July August–March April–July August–March

Precipitation/mm 287 491 261 508 232 529

conv ref Actual transpiration/mm 354 126 336 115 317 121

Actual evaporation/mm 22 90 23 94 24 98

Climatic water balance/mm �90 276 �98 299 �109 310

conv comp Actual transpiration/mm 412 125 395 115 376 117



cons ref Actual transpiration/mm 359 121 338 112 318 118



cons comp Actual transpiration/mm 408 126 387 115 370 117



conv ref

conv compcons ref

cons comp

0.25 m soil depth

0.45 m soil depth

Jan Feb Mar Apr May Jun Jul Oct Nov DecAug Sep

Jan Feb Mar Apr May Jun Jul Oct Nov DecAug

1

100

1000

J M M J S NF A J A O D

conv ref

cons refcons comp

conv comp

10

Sepmonth

month

median

1

10

100

1000

J M M J S NF A J A O D

conv ref

cons refcons comp

conv compmedian

conv ref

conv compcons ref

cons comp

0.1

pressurehead/-kPa

1

10

100

1000

0.1

pressurehead/-kPa

1

10

100

Fig. 4. Simulated pressure heads (�kPa) in 0.25 m and 0.45 m soil depth summarized as box and whisker plots and median values (small graphs) for each month for the period

1991–2000 based on the WETTREG simulations (SRES A1B) (Enke and Kreienkamp, 2006) for the tillage systems conv = conventional and cons = conservation and for each

wheeling situation ref = unloaded reference site and comp = loaded site.



4. Discussion

In our experiments, loading with 6.3 Mg resulted in modifica-tions of pore size distributions and hydraulic conductivities onboth the conventional and the conservation site. Conservationtillage practices should result in a better trafficability, as there is ahigher structural regeneration in the topsoils than at convention-ally tilled sites (e.g. Alakukku et al., 2003; Schjonning et al., 2006;Wiermann et al., 2000). Using lower wheel loads, conservationtillage showed a distinctly higher resistance against harmful soilcompaction due to advanced aggregation in the topsoil of Luvisols(Zink et al., 2010). Furthermore, they found out that these effectsare restricted to stresses smaller than the precompression stresses,while exceeding the internal soil strength results in virgincompression behavior and comparable hydraulic properties andfunctions. Hence, our results led to the conclusion that wheel loadsof 6.3 Mg exceed the precompression stress and are thus too highfor both investigated tillage systems and caused soil structurechanges.

The results of laboratory measurements on the water retentioncharacteristics as well as the satisfying correlations betweenmeasured and simulated matric potentials in the field support ourused simulation approach, where we used a well-establishedporosity (van Genuchten, 1978) and hydraulic conductivity model(Mualem, 1976). The measured and predicted hydraulic propertiescoincide with findings of Richard et al. (2001) at compacted siltysoils, who determined an increased unsaturated hydraulicconductivity at tensions lower than �15 kPa and a likewiseincreased available water capacity. Further improvements couldbe expected by the use of more complex porosity models (e.g.Gerke and van Genuchten, 1993; Matthews et al., 2010; Simuneket al., 2008), which might describe the pore properties (distribu-tion, modality, connectivity) better (Alaoui et al., 2011).

Nonetheless, our simulation approach distinctly pointed outthat these changes in soil hydraulic properties as a result ofcompaction influenced water supply and consumption by transpi-ration and thus affected soil water balance. Transpiration ratesdepend on Ku and increased especially at tensions below �1 kPa. Incombination with drier soil states, rewetting in autumn and winterwas distinctly retarded and less complete. Furthermore, theremight be a higher risk of increased water logging situations atStagnic Luvisols. The decrease of wCP and the destruction ofcontinuous pores is a common problem of compacted soils andresults in aeration problems in wet periods (Horn and Fleige,2009). This degradation problem was indicated by the increasedoccurrence of soil water conditions near saturation (extremevalues in Fig. 4) especially in the Ap and Eg horizons of thecompacted variants.

Variable climatic conditions were reflected by an increase ofprecipitation in winter, which results in an increase of the waterbalance (higher seepage rates), while predicted summer periodswere characterized by both a reduction of periods with idealtranspiration conditions (actual transpiration = potential tran-spiration) as a result of the decrease of precipitation and theincrease of temperatures. The results of our simulation approachcoincide with the responses summarized by Varallyay (2010),who points to a reduction of transpiration with increasing meantemperatures and an increased probability of extreme moistureregimes (water logging) in wet periods. On the other hand, latterauthor outlines that the influence of climate change on soilphysical (e.g. texture differentiation, aggregation) and chemicalproperties (e.g. organic matter) can hardly be estimated becauseof the strong interdependence of a changing hydrological cycle,influences of the vegetation and the land use pattern. Conse-quently, an integration of changing soil physical properties as apossible effect of climate change into simulations would be

speculative, thus we varied those conditions just as a result ofmeasured compaction.

Furthermore, possible effects of climate change and soilcompaction on plant physiology were not implemented in thesimulations as we solely focused on the soil physical effects. We arecertainly aware of the temperature effects on plant physiology,which furthermore affects crop yield. Vincent and Gregory (1989)found out, that total root length and root dry weight of winterwheat increased exponentially with thermal time. As a result ofhigher mean temperatures, the vegetation period might beprolonged and start earlier with a stronger root growth in spring.In contrary to that, it is possible that compaction reduces rootingdensities and depths with consequences for plant growth, actualtranspiration and yield. However, Ehlers et al. (2000) found out forwinter wheat grown on northwest German soils from loess(comparable to our study site) that compaction just slightlyaffected maximum rooting depth and did not significantly reduceyield. On the other hand, a possible restriction of root growth ofwinter wheat in dry periods might be compensated due to higherCO2 concentrations in the atmosphere (Chaudhuri et al., 1990).

Taking into account all these findings of our study and therestrictions concerning circumstances in modeling (goodness ofpredicted climate change, improvements by the use of complexporosity models, effects of changing climatic conditions on soilproperties and plant reactions), further research is needed includingthe long term interaction between root growth, mass flow anddiffusion and altered thermal conditions under less aerated and gasas well as water conducting flow and accessibility conditions.

5. Conclusions

Loading with 6.3 Mg led to compaction induced changes of poresize distributions and hydraulic conductivities at both tillagevariants in top and subsoil. On the average there were seeminglyadvantageous plant transpiration conditions due to higheramounts of plant available water. However, the simulations alsoshowed that compaction might result in an increase of soil watervariability. Thus, the risk of drought and water logging mightincrease at the compacted sites in spring. Furthermore, as moreextreme precipitation regimes are expected in future as a result ofclimate change, there might also be an increase of drought periodsthat might even intensify the higher variability of extreme waterstates at compacted sites and may result in less predictable cropyield and plant growth. Thus, an increase of flexibility of land use,crop production and tillage systems is needed to handle a changingclimate (Olesen and Bindi, 2002).

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