Occurrence and spatial pattern of water repellency in a beech forest subsoil

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1 J. Hydol. Hydomech., 64, 16,, 11 DOI: /johh-16-5 Occuence and spatial patten of wate epellency in a beech foest subsoil Jög Bachmann 1*, Jiem Kuege 1, Mac-O. Goebel 1, Stefanie Heinze 1 Leibniz Univesität Hannove, Institute of Soil Science, Heenhäuse St., 3419 Hannove, Gemany. Geogaphisches Institut, Ruh-Univesität Bochum, Univesitätsstaße 1, 4478 Bochum, Gemany. * Coesponding autho. bachmann@ifbk.uni-hannove.de Abstact: Most ecent studies on soil wate epellency (WR) wee limited to the humous topsoil o to shallow subsoil layes slightly below the main oot zone to appoximately.5 m depth. Hence, the main objective of the pesent study was to investigate the wettability patten of a foest soil including the deepe subsoil. The selected site was a yeas old beech foest on a well-dained sandy Cambisol in nothen Gemany which showed modeate to patly extended acidification. Results obtained fom thee sampling tansects (3 m length, m depth; sampling gid 8 8 samples pe tansect; minimum distance of sampling locations to neaest tee about.5 m) show that contact angles (CA) wee always in the subcitical WR ange ( < CA < 9 ). Significant impact of the tee distance on WR was not obseved fo any of the tansects. A pominent featue of two tansects was the minimum WR level (CA < 1 ) fo samples with soil oganic cabon (SOC) contents aound.5.4%. Fo the topsoils it was obseved that CA inceased with SOC content fom that minimum to a maximum CA of 6 75 fo tansects 1 and with mean values < 3.5. Fo tansect 3 with slightly highe aveage close to 4., aveage CA of samples wee always < 1 and showed no tend to incease with inceasing SOC content o othe soil paametes like N content o C/N atio. Subsoil samples, howeve, behave diffeently with espect to SOC: fo these samples, geneally low in SOC, the CA incease with deceasing SOC occued at all tansects fo appoximately % of the samples but did not show any clea tendencies with espect to futhe paametes like textue, o N content. We conclude that the SOC content is the most pominent paamete detemining wettability, eithe positively coelated with WR fo topsoils o negatively coelated fo subsoil samples vey low in SOC. We finally conclude fo modeately acid beech foest stands that emeging WR stats in the A hoizon afte eaching a lowe than 3.5, wheeas subsoil WR might appea aleady at highe values. Even SOC contents of.1.% tuned out to be vey effective in inceasing the CA up to 7, which points out clealy the impotance of small amounts of soil oganic matte in affecting subsoil wettability. With espect to site hydology we conclude that ongoing acidification as well as pedicted highe fequencies of extended doughts due to climate change will pomote the occuence of WR with coesponding implications fo site and catchment hydology. Keywods: Beech foest; Soil oganic cabon; Subsoil; Wate epellency. INTRODUCTION Soil oganic matte (SOM) affects many physical, chemical o biological soil popeties. Inceasing SOM content of mineal soils incease the stoage capacity fo wate and nutients and impove soil stuctue and aggegate stability. Howeve, eseach of the last decades evealed also some negative effects associated with the amount and composition of SOM like soil wate epellency (WR), which may lead to advese effects with espect to soil hydaulic functions. Bachmann et al. (8) showed that the availability and distibution of wate in the soil matix depends on soil paticle wettability. WR occus in soils of diffeent textue, land use and unde a vaiety of climatic conditions (DeBano, 1981; Doe et al., ; Woche et al., 5). Mineal sufaces ae geneally well wettable by wate when they ae fee of any adsobed oganic substances. In contast to pue mineal sufaces, soil mineals neve expose clean sufaces since soil solution caies molecules o colloids of dissolved oganic compounds which ae easily adsobed onto mineal sufaces due to the soption sites and the pemanent contact with soil solution. Depending on the type of oganic compound, the wetting popeties of mineal sufaces may change dastically (Hone and McIntosh, 3) even afte adsoption of only a few molecula layes of oganic compounds (Šolc et al., 15; Zisman, 1964). Fomely wettable mineal o glass sufaces may be endeed stongly wate epellent by eithe specific o non-specific adsoption of hydophobic substances (Ellebock et al., 5; Lampate et al., 14; Šolc et al., 15). It is well known that tee canopy affects the infiltation patten of pecipitation in foest stands, leading to inhomogeneous infiltation of nutients, acids and pollutants into the foest topsoil (Böttche and Stebel, 1988; Böttche et al., 1997). With espect to beech tees, highe leaching ates in the vicinity of tee stems may cause enhanced acidification, which might affect the wetting popeties of soil oganic components like humic o fulvic acids (Tschapek, 1984). Coespondingly, it can be expected that the amount and composition of SOM in the topsoil vaies accoding to site-specific infiltation pattens. Small scale heteogeneity in WR has been identified fo neasuface soils (Dekke and Ritsema, 1994; Hallett et al., 4) but almost nothing is known about lage scale vaiability of the wetting popeties of foest soils such as the systematical impact of tee canopy o tee oots. Some studies addessed mechanisms to couple and CA and showed contovesial esults. The addition of lime (Rope, 5) and sodium hydoxide (Kanok et al., 1993) as a tool to incease soil was documented to impove soil wettability. The opposite behaviou was obseved fo humic acid when the ambient was deceased (Teashima et al., 4). Baye and Schaumann (7) investigated the effect of modifications on WR by adding diffeent concentations of aqueous NaOH and HNO 3 to wate epellent and wettable soil with a field between 4. and 4.6. They found the maximum WR above the oiginal, contadicting the assumption that inceasing Beeitgestellt von Technische Infomationsbibliothek (TIB) Heuntegeladen am :59

2 Occuence and spatial patten of wate epellency in a beech foest subsoil will always incease wettability. A succeeding study (Diehl et al., 1) developed a new expeimental appoach by exposing the samples to gaseous HCl and NH 3, thus avoiding the addition of wate and a potential dissolution of oganic compounds. The authos explained diffeent WR tends with with a soil specific impact of on paticle suface chage. The maximum WR occued usually fo soils whee the specific suface chage density was at minimum o aound zeo (point of zeo chage). Accodingly, WR deceased as inceased while negative suface chage density inceased due to dissociation of acidic functional goups (Diehl et al., 1). In conjunction with depotonation of the suface moe pola sites become available to enable the inteaction with wate molecules via hydogen bidges. On the othe side, inceasing WR with deceasing may aise fom potonation of basic functional goups such as amine goups. Incease in hizosphee of about one unit was obseved fo wheat and alfalfa, pobably caused by pefeed nitogen uptake as nitate. The incease of was accompanied by deceasing WR which suppots some obsevations on the invese elationship between CA and (Hassan et al., 14). A study of Lebon et al. (1) on topical foest sites showed that the topsoil of pine plantations had a high degee of WR and pesistency and that WR was moe homogeneously distibuted compaed with an adjacent native foest. Results showed futhe that maximum WR occued between and was not obseved fo > 5.. Obseved changes of in the hizosphee suggested that the elease of eithe H + o OH and HCO 3 acts as chage compensation fo the uptake of nutient anions o cations like NO 3 o NH 4 +, which may led to vaiations in the hizosphee (Dakoa and Phillips, ; Nye, 1981). With espect to soil hydaulic pocesses, oganic coatings can educe adhesion foces at the wate solid inteface compaed to the cohesion foces in bulk wate, leading to educed wettability and capillaity of the patly satuated medium (Lampate et al., 14). Fo wate to spead along solid sufaces, adhesive foces at the wetting font must exceed the cohesive foces within the bulk wate (Adamson, 199). Stong cohesion between the wate molecules finally foces the liquid in capillay systems to each a static minimum liquid ai and liquid solid intefacial aea countebalancing capillay and gavity foces (Adamson, 199). Soils showing high levels of pesistent WR fom thee-phase systems with educed solid liquid intefacial aea and educed connectivity of the liquid domains (Goebel et al., 7; Muehl et al., 1). Macoscopically, WR educes the infiltation of wate (King, 1981), the available wate stoage capacity in soils (Deue and Bachmann, 7), and causes pefeential flow in the unsatuated soil (Ritsema et al., 1993; Wessolek et al., 9). At the catchment scale, wate epellent soils pomote oveland flow (Imeson et al., 199) and apid stomflow (Doe et al., 3). Findings of Buczko et al. (7) suggest that changes in WR can be tiggeed within shot time scales (appox. thee days), and moeove, that WR can be affected even by modeate changes in pecipitation and slightly elevated tempeatues. Paticulaly unde elatively dy conditions, small changes in wate content can esult in stong changes in WR (Liu et al., 1). In ode to model the hydaulic popeties of wate epellent soils, infomation on the spatial occuence and patten of WR is necessay. In one of the few studies dealing with geostatistical analysis of WR pattens, Hallett et al. (4) suggested that educed wate infiltation may be linked with small scale micobial and/o chemical pocesses that cause the spatial vaiability of WR. They concluded that small-scale infiltation measuements ae needed to detect wate soptivity pattens. Recent studies confimed fo pine foest soil an extended vaiability of small-scale WR on the cm-scale, which was evaluated by wate dop penetation time o contact angle (CA) measuements (Lampate et al., 1; Ofanus et al., 8). In this context, Ganz et al. (13) confimed via in-situ tace infiltation expeiments analyzed by electic esistivity tomogaphy (ERT) and hydaulic modeling (HYDRUS-3D) the elevance of WR fo subsoil hydology. Subsoil WR enhances inhomogeneous infiltating wetting fonts which incease the tendency of subsoil pefeential flow as indicated by the satuation oveshoot phenomenon (Ganz et al., 13). That study pointed out that subsoil WR is lowe than in topsoils in tems of CA but showed a highe level of pesistence (stability in time). This may pomote the popagation of initially developed topsoil pefeential flow pathways into the subsoil, thus showing the elevance of subsoil WR fo hydaulic pocesses in deepe soil. Summaized, soil WR shows seveal levels of spatial vaiability, anging fom mm to dm and even lage and might be affected by seveal envionmental impact factos. The main objective of the pesent study was to investigate the impact of tees on the wettability patten in foest top- and subsoils on an intemediate (m) scale. To unavel the systematic effect of tee abundance we extended the analysis to the deepe subsoil to depths of about m. We chose a foest site with a modeate WR and soil development level to avoid a stong WR backgound as often obseved fo acid soils like podzolic soils unde conifeous o heath vegetation (Woche et al., 5). MATERIAL AND METHODS Study Site The sampling site is located 4 km noth-west of Hannove, nea Nienbug/Wese, Lowe Saxony, Gemany ( Noth, East). The beech (Fagus sylvatica L.) foest stand Gindewald, established in 1916, developed on Distic Cambisol (IUSS Woking Goup WRB, 14) on pleistocene fluvial and aeolian sandy deposits (Saale glaciation). The site is well dained with a goundwate level of >1 m depth. Mean annual pecipitation (1981 1; Souce: WoldClim; is 718 mm, mean annual tempeatue is 8.7 C. Tensiomete and TDR measuements wee evaluated fom thee soil hydaulic stations located in the vicinity of the thee tansects (distance < 3 m). Fo 14, each station showed that the depth of the hoizontal wateshed (lowest depth with pemanent downwad diected hydaulic gadients duing the yea) is localized between 1 and 18 cm. Tansects and expeimental design The tansect soil sampling design was chosen to quantify the effect of beech tees on top- and subsoil WR with thee independent eplicates (T1, T, T3). A photogaph of a 3 m m tench (defined as tansect T) is shown in Fig. 1. All tansects wee diected towads an individual main tee; the closest distance of the tench to the base of the tee stem was appoximately cm at the soil suface. The distance to the neaest neighboing tee was geneally > 1 m. All tansects wee chaacteized by elatively homogeneous sandy substates in the top fist mete of the pofile. Moe heteogeneous conditions wee found in the subsoil with thin loamy-silty lenses embedded in a sandy matix which also contain gavel and stones at some locations. Table 1 shows data and the typical soil hoizons fom a soil pit in the vicinity of the tansect locations whee soil type Beeitgestellt von Technische Infomationsbibliothek (TIB) Heuntegeladen am :59 11

3 Jög Bachmann, Jiem Kuege, Mac-O. Goebel, Stefanie Heinze Fig. 1. Image of Tansect (left). Lines show the hoizontal position of sampling locations A to H (Δx = 45 cm); depth incements Δz = 1, 35, 6, 85, 11, 135, 16, 185 cm. The image on the ight shows the view diectly fom the adjacent tee. Table 1. Chaacteization of the podzolic Cambisol at site Gindewald. Soil hoizons accoding to FAO-WRB (IUSS Woking Goup WRB, 14). In backets: soil hoizons accoding to the Geman Classification Manual, KA5 (Ad-hoc-Abeitsguppe Boden, 5). Soil hoizon AE (Ahe) Bsw (Bsv) Bw (Bv) BwC (Bv/C) C (C) C (IIC) Cg (IICg) 3C (IIIC) Depth (cm) > 18 (CaCl ) SOC (%) <.1 Sand Silt Clay (%) and soil hoizons wee typecast. The oganic laye of the foest soil was chaacteized as mode (L hoizon:.5 cm, F hoizon:.5.7 cm, and H hoizon:.7 5 cm in thickness). Soil samples wee taken on the 1 th and 11 th of June 13. Steel coes (8.5 cm in diamete, 6 cm in height with a volume of 48.5 cm 3 ) wee taken at each gid intesect. Soil mateial was sampled along all thee eplicate tansects (T1 T3); gid dimension was cm in vetical and 315 cm in hoizontal diection. Pobed distances wee 1 cm, 35 cm, 6 cm, 85 cm, 11 cm, 135 cm, 16 cm, and 185 cm in vetical diection and cm (A), 45 cm (B), 9 cm (C), 135 cm (D), 18 cm (E), 5 cm (F), 7 cm (G), and 315 cm (I) in hoizontal diection. Accuacy of coodinates was about 1 cm. Afte sampling, soil was stoed in plastic bags until sieving < mm. Samples wee aidied and stoed at oom tempeatue. Analytical methods Bulk analyses Soil textue was analyzed by an Analysette (Fitsch, Ida- Obestein, Gemany), and (in CaCl ) was measued with a soil to solution atio of 1:.5. Bulk oganic C and N contents wee detemined by dy combustion with a Vaio EL-Analyse (Elementa, Hanau, Gemany). 1 Contact angle measuement The CA was detemined with the sessile dop method (SDM) as descibed by Goebel et al. (13). A micoscope (OCA 15, DataPhysics, Fildestadt, Gemany) equipped with a video camea (appoximately 3 fames pe second) was used to measue the CA. Ideally, a one-gain laye of homogenized soil mateial (thickness and oughness depending on aveage textue of the sample) was fixed on a glass slide in contact with double-sided adhesive tape to avoid movement of the paticles duing measuement. Doplets (3 µl) of distilled wate wee placed on the hoizontally attached sample. Afte 3 ms the initial CA of each dop was detemined with the Softwae SCA (DataPhysics). The mean value of 6 dops (1 eadings in total) was used fo futhe analysis. SDM was chosen due to shot measuing times, high epoducibility and epeatability combined with the unlimited measuing ange of CA between and 18 (Bachmann et al., 3). It should be noted, howeve, that soil wettability can only be measued in tems of an appaent (phenomenological) CA fo seveal easons: The esults depend on the size and shape of the paticles, on the combined influence of oughness and homogeneity of oganic film coatings that cove the soil paticles, and on timedependent pocesses on paticle sufaces like eaangement of molecula stuctue of the intephase duing contact with wate Beeitgestellt von Technische Infomationsbibliothek (TIB) Heuntegeladen am :59

4 Occuence and spatial patten of wate epellency in a beech foest subsoil (Schaumann et al., 13). Theefoe, esults fo diffeent soils ae only compaable when methods ae pecisely standadized (Bachmann et al., 3). Statistical data evaluation Classical statistical paametes wee calculated accoding to standad methods (e.g. Snedeco and Cochan, 198). Regession analyses wee made with SigmaPlot 11. (Systat Softwae GmbH, Ekath, Gemany). RESULTS Table summaizes the aithmetic means of basic physical and chemical popeties fo T1 T3. Data ae computed using the entie data set of 64 samples pe tansect. Mean values suggest a sandy textue with diffeences in sand content of about 1%. Aveage soil oganic cabon (SOC) contents wee between. and.3%, and aveage values wee between 3.8 and 4.1. Aveage CA wee between 5 and 13, indicating a low level of subcitical WR. Summaized, the means of the selected key popeties suggest quite simila conditions fo all thee tansects with some systematic vaiations fom T1 to T3. Standad deviations fo T1 T3 wee also simila, suggesting a compaable level of spatial vaiability fo the thee tansects. Coefficient of vaiation was lagest fo the CA with moe than % which was consistent fo all tansects. It indicates the highest level of vaiation fo soil wettability compaed with textue,, and SOC. Table. Mean and standad deviation (± 1 SD) of sand content, oganic cabon content (SOC),, and contact angle of the thee tansects (n = 64 samples pe tansect). Tansect 1 (T1) Tansect (T) Tansect 3 (T3) Sand content (%) 6.6 ± ± ± 4. SOC (%).33 ±.45.6 ± ±.3 (CaCl ) 3.8 ±.3 4. ± ±. Contact angle ( ) 1.5 ± ± ± 11. Table 3. Mean and standad deviation (± 1 SD) of oganic cabon content (SOC),, and contact angle of the A hoizons of the thee tansects (n = 8 samples pe hoizon). Tansect 1 (T1) Tansect (T) Tansect 3 (T3) SOC (%) 1.49 ± ± ±.4 (CaCl ) 3.1 ± ± ±.3 Contact angle ( ) 47.7 ± ± 5.1. ± 17.7 Figue displays CA (a), SOC content (b) and (c) fo T as contou plots (side view of the tansect). Side of the tansect adjacent to the tee (distance.5 m) is maked by x =. Consistent with T1 and T3, we obseved fo the mapped CA lage wettable domains in the subsoil and also patly in the topsoil. CA geate than 6 obseved in T wee eithe located in the A hoizon close to the mineal soil suface o in thin layes deepe than 1 cm. In compaison, T3 had a lowe SOC content in the A hoizon and vey low maximum CA (< 1 ) in the entie A hoizon, while T1 with a highe SOC level in the A than T3 showed CA fom 3 to 6 along the tansect (Table 3). Regading A-hoizon wettability, tansect T showed an intemediate chaacteistic between T1 and T3 with the highest level of vaiation. Only the side away fom the tee (x > 1 cm) showed CA > 1 fo the A hoizon (Fig. a), which was not eflected by coesponding SOC o vaiations along the tansect in the A hoizon at T. As indicated in Table 3, CA of T1 wee geate (anging fom 33 to 77 ; Fig. 3), wheeas most CA of T3 wee aound, confiming the intemediate level of T compaed with T1 and T3. Also indicated in Fig. a, some subsoil samples with a low SOC content, i.e. <.3%, showed subcitical WR (CA ange: 6 ) at oughly 1.5 m depth. Those domains wee located fo T at subsoil positions chaacteized oughly by < 4 (Fig. c). Simila dependencies wee not found in the topsoil because fo values < 4 the coesponding CA wee in the entie ange fom to > 6. It is woth to note that highe WR in small subsoil domains wee also obseved fo T1 and T3 (Fig. 4). The location of these WR spots, howeve, was not clealy elated to tee position o depth as shown exemplay fo T, whee the spot occued at a position most emote fom the tee (x = 3 cm). Fo T3, a simila spot was oughly located at the same position with espect to distance and depth, wheeas fo T1 such a domain in the subsoil was smalle, close to the tee ( m) and close to the suface (1. m depth). Figue 3 shows the CA vs. SOC and N content fo T1 T3. At SOC contents of appoximately aound.5% all tansects exhibited small CA, i.e. T1 and T showed a minimum with CA 1. Fo lowe SOC contents all tansects showed again a CA incease, while fo SOC >.4% the CA inceased clealy fo T1, patly fo T, but not fo T3, whee all CA emained on the low level obseved fo SOC contents aound.4%. As mentioned, T3 with the lowest aveage SOC content in the A hoizon and highest aveage showed no futhe CA incease with SOC in the fist two sampling depths. The gaphs with N content as independent vaiable (Fig. 3, ight) geneally also show an incease in CA with N content fo T1 and patly fo T but not fo T3. The majo diffeence to the elationship between SOC and CA is appaently that fo N the minimum obseved fo SOC is missing. The occuence of a minimum fo the SOC vs. CA elationship was fist descibed by Ellebock et al. (5) fo thee foest soils in Gemany. That study epoted a CA minimum at a slightly highe SOC content of.9-1.% fo the fine textued soils. Figue 4 shows depth functions of mean and standad deviation fo all investigated paametes. SOC content (Fig. 4, ow b) deceased monotonically with depth fo all tansects. In contast, N content inceased again in the subsoil fo T1 and T at geate depth, expessed by a still deceasing C/N atio (Fig. 4, ow d) which indicates a continuous tansition of SOM composition with depth. Due to the low SOC content in the subsoil of T3 coesponding values ae missing hee. The CA depth elationship (Fig. 4, ow a) indicates fo T1 and T deceasing CA fo the fist two depths to a minimum at about 6 cm. Figue 4 eveals that the CA decease was not accompanied by systematic textual changes (ow f) fo all tansects but depended fo T1 and T fo the fist thee sampling depths appaently on the (ow e), i.e. incease lowe than one unit fom 6 cm depth changes CA fom aound (T1) o 5 (T) to almost zeo. It is inteesting to note that the CA incease in the A hoizon and the uppe B hoizon was not obseved fo T3 which is chaacteized by a mean of about.7 units highe in the A hoizon. In the deepe subsoil a CA incease was obseved at 1 cm (T1) o Beeitgestellt von Technische Infomationsbibliothek (TIB) Heuntegeladen am :59 13

5 Jög Bachmann, Jiem Kuege, Mac-O. Goebel, Stefanie Heinze Tansect T Length (cm) Depth (cm) 3 1 CA [ ] Length (cm) 1 SOC [%] Depth (cm) Length (cm) 1 (CaCl ) Depth (cm) Fig.. Contou plots of contact angle (CA), SOC content and fo tansect T. Hoizontal value of indicates the end of the tansect diected to the tee. 14 Beeitgestellt von Technische Infomationsbibliothek (TIB) Heuntegeladen am :59

6 Occuence and spatial patten of wate epellency in a beech foest subsoil Fig. 3. Contact angle (CA) vs. oganic cabon content and nitogen (SOC) fo tansects T1 T3. deepe at 1 cm depth (T, T3). On the othe hand, the CA incease in the subsoil was moe ponounced fo T and T3 compaed with T1 with the geatest CA incease in both tansects aound 1 cm depth. Depth functions of the sand content of T and T3 show no significant changes in the pofile to a depth > cm, indicating, simila to T1, that textue, i.e. the specific suface aea, is not a majo vaiable detemining WR vaiation with depth. Summaized, Figs. 3 and 4 indicate that the most significant paametes in detemining WR ae the SOC and N contents. A coesponding geneal elationship of CA with could not be found. Fo example, summaizing the CA vs. data fom T1 T3 into one plot, the value shows a less specific impact on the CA (Fig. 5). A geneal tend to smalle CA was indicated fo > 4, wheeas fo smalle values a wide ange fom to almost 8 was evident. Statistically, the multiple linea egession analysis which involved the independent vaiables SOC, N, and and the dependent vaiable CA showed ambiguous esults. Accoding to the elationship between SOC and CA shown in Fig. 3, we split the data into two sets with SOC >.5% and SOC <.5% (Table 4). The most significant egession was obtained fo T1 (SOC >.5%) with =.86 on the P <.1 significance level. Less significant was the egession fo tansect T with values <.15 fo both SOC domains, expessing the CA chaacteistic of still inceasing CA with SOC but also showing the occuence of samples with CA of. Less significant egessions wee also obtained fo T3 and T1 fo samples with SOC contents <.5%, indicating that neithe the no the SOC content is able to explain the spectum of obseved CA in the low SOC domain. With espect to the egession paametes displayed in Beeitgestellt von Technische Infomationsbibliothek (TIB) Heuntegeladen am :59 15

7 Jög Bachmann, Jiem Kuege, Mac-O. Goebel, Stefanie Heinze Fig. 4. Contact angle (CA), oganic cabon content (SOC), nitogen content (N), C/N atio,, and sand content as a function of pofile depth fo tansects T1 T3. Missing values indicate samples below detection limit fo N analysis. Table 4, no systematic tend can be obseved, pobably due to two easons: A elatively small vaiation of key vaiables fo the majoity of the subsoil samples and appaently the occuence of high CA and zeo CA samples fo the same SOC content shown fo T at SOC >.5. Summaizing, data fo all tansects in the low and high SOC domain yielded also low values between.31 and.6 (Table 4). It is finally inteesting to note that an adsobed SOC content of <.1% tuned out to be effective in inceasing the CA fom 3 to > 7, which points out clealy the impotance of only small amounts of SOM in affecting subsoil wettability. DISCUSSION Ellebock et al. (5) epoted that the degee of soil WR depends not only on the total amount of SOM. Simila to the esults of this study they found fo thee sandy and loamy foest 16 sites a minimum in WR at low SOC contents. To explain the existence of a CA minimum with espect to SOC content they poposed the following mechanisms: In the case of low SOC content in elation to the numbe of soption sites of the initially uncoated mineal suface, the hydophilic goups of the amphiphilic oganic molecules ae diected towads the mineal suface to fom bindings with the mineal inteface, e.g. via cation bidges. Afte adsoption, hydophobic moieties like cabohydate backbone chains ae diected towads the poe space making the sufaces moe wate epellent (stage 1). Continued adsoption leads then fom a flat to a moe upight position of the oute molecules with an inceasing numbe of hydophilic sites exposed to the solid liquid inteface (stage, maximum wettability afte adsoption of SOM), while even highe amounts of adsobed oganic molecules led to a moe complex bundle-like stuctue at the inteface (stage 3). This complex Beeitgestellt von Technische Infomationsbibliothek (TIB) Heuntegeladen am :59

8 Occuence and spatial patten of wate epellency in a beech foest subsoil Table 4. Linea multiple egession analysis of the elationship between CA vs. SOC,, and N content. C >.5 % T1: CA = C N 3.7 T : CA = C N 4.8 T3: CA = C 7.4 N +.34 T1: CA = C 6. T: CA = 48, C 8.4 T3: CA = C C <.5 % T1: CA = C N T: CA = C +. N 7 64 T3: CA = C 6.5 N T1: CA = C T: CA = C T3: CA = C C >.5 % T1 T3: CA = C +.3 C <.5 % T1 T3: CA = C 3.74 * =.86 P.1 =.9 P =.794 =.15 P =.614 =.84 P.1 =.4 P =.753 =.8 P =.643 =.46 P =. =.11 P =.755 =.46 P =.6 =.6 P =.3 =.1 P =.87 =.1 P =.779 =.31 P=. =.59 P=.5 * * * * Significant at the 95% level. Fig. 5. Contact angle (CA) vs.. Summaized data set fo data fom tansects T1 T3. stuctue is able to poduce vaying states of wettability depending on the envionmental conditions. A possible impact facto influencing WR in stage 3 is the spatial macomolecula stuctue, modified i) by polycation bidges, ii) molecula eaangement, and iii) coss-linking effects (Schaumann et al., 13), tuning eithe pola o non-pola molecula stuctues of amphiphilic oganic molecules (Tschapek, 1984) towads the suface. Stage 3 might be attibuted to the SOC domain >.5%. The diffeences in CA in the high SOC domain between tansects T1 and T3 might be explained by diffeences in the A hoizon. Laboatoy studies evealed that the type of exchangeable cations adsobed at exchange sites of extacted humic acids significantly affects thei wettability, wheeas adsoption of eithe H + o Al 3+ poduced a maximum in WR (Tschapek et al., 1984). Howeve, in case of moe complex systems such as unfactionated soil, coesponding effects could not be clealy veified. In a ecent study by Diehl et al. (14), whee nonfactionated peat and Spodosol mateial was teated with diffeently chaged cations, i.e. H +, Na +, Ca +, Al 3+, a maximum of WR fo H + - and Al 3+ -teated samples could not be epoduced. On the othe hand, in a study by Woche et al. (5), analyzing seveal aable and foest soils anging fom 3. to 6.8, a significant invese coelation between CA and was found which is in ageement with the geneal tendency found fo the topsoils of this study. Accoding to the model of Ellebock et al. (5) subsoil samples (attibutable to stage 1) should be teated diffeently. Inceasing CA with deceasing SOC content suggests that the effectiveness of SOM to ende wettable mineal sufaces wate epellent does not pimaily depend on the amount of SOM. It athe seems that the elative amount of effective wate epellent molecula stuctues in the subsoil SOM wee highe compaed to the topsoil, whee highe amounts of SOM ae needed to each o exceed the level of WR measued fo subsoil samples. Ellebock et al. (5) epoted the CA minimum fo SOC contents aound 1.% fo foest soils with textues anging fom sand to loam. Coesponding FT-IR specta which eflect the intefacial popeties down to 1 µm depth (Feguson and Whitesides, 199) evealed futhe that the occuence of hydophilic pola functional goups was invesely elated to the Beeitgestellt von Technische Infomationsbibliothek (TIB) Heuntegeladen am :59 17

9 Jög Bachmann, Jiem Kuege, Mac-O. Goebel, Stefanie Heinze CA, i.e. highe fo wettable samples, wheeas a eduction in wettability was obseved with inceasing abundance of hydophobic (non-pola) molecula stuctues, leading to a CA incease fom 5 to 88 (Ellebock et al., 5). This esult suggested a diect elationship between the spectoscopic signal of the oganic mateial and wettability. The impotance of the outemost molecula aangement fo the wettability of glass sufaces was ecently shown by Šolc et al. (15). Using molecula foce field simulations they demonstated that suface hydophilicity of ulta-thin siloxane coatings is diectly elated to the concentation of pola hydoxyl goups and thei accessibility fo the wate molecules. Šolc et al. (15) futhe showed that deceasing concentation of hydoxyl goups leads to inceasing hydophobicity and lage CA in the same way as pedicted by the FT-IR study by Ellebock et al. (5) fo moe complex soil paticle sufaces. Diehl et al. (14) showed peliminay esults to elate suface elemental composition in the inteface of about 1 nm depth with WR using X-ay photoelecton spectoscopy (XPS). Results show that the element atio of O to C was elated to WR, hence the chemical composition of the thin inteface govens paticle wettability. Regading the diffeences in wettability obseved fo topand subsoil, diffeent mechanisms of SOM enichment ae easonable. With espect to the subsoil, the potential of adsobed wate-soluble oganic compounds in modifying paticle wettability has been epoted by Lampate et al. (14). Wettability of washed quatz sand was found to decease significantly afte soption of dissolved oganic matte components. It is still unclea, howeve, if the composition of the adsobed oganic components is depending on the depth in the soil pofile. With espect to soil hydology at the Gindewald site it is evident that the wate epellent subsoil domains in the low SOC domain aound.5% ae in tendency located below the hoizontal wateshed (defined as soil depth whee the hydaulic gadient is diected downwad ove the entie yea). This hydaulic bounday was identified to be located between m depth by pessue tansduce data measued continuously fom 13 to 14 at thee independent soil hydaulic field stations which wee located within a adius < 3 m fom the tansects. Some final emaks should be mentioned hee: Not consideed in this study is the fundamental effect of solutes on soil WR (Andeson et al., 1995; Hatge, 1958). Inceasing liquid suface tension may lead to geate CA, wheeas a eduction in liquid suface tension may decease the CA. Liming of foest soils could incease the liquid suface tension of soil solution which inceases potentially the CA. Howeve, at pesent no futhe infomation is available on the popeties of the soil solution at the Gindewald site. Analyzing the simultaneous effect of both mechanisms with espect to soil WR is subject of futhe investigations. CONCLUSIONS Soil wettability in the field depends on the distibution of SOM and its composition which usually vaies consideably in the unsatuated zone. Commonly it is epoted that wettability inceased with deceasing SOC content, especially in subsoils accoding to the SOC depth gadient (Dekke and Ritsema, 3). The testing site used fo the pesent investigation is not expected to show a high level of WR. Coespondingly, CA wee in the subcitical ange ( < CA < 9 ) and become smalle with depth and deceasing SOC content until the minimum at SOC aound.5%. Howeve, samples fom depths > 1 m evealed inceasing CA with deceasing SOC content. A 18 geneal featue of all tansects is that minimum CA wee measued at SOC contents anging fom.5%, wheeas at smalle o lage SOC contents, the CA incease in case of two tansects, chaacteized by slightly lowe as compaed to the thid tansect. In the low SOC domain <.5%, WR did not show clea tendencies with espect to single paametes like textue,, o N content, except the geneal tend of inceasing CA with deceasing SOC content fo some samples. Fo the A hoizon (high SOC domain) the impact of has additionally to be consideed. In ou study a of 3.5 seems to be a theshold value in a sense that topsoil samples with > 3.5 showed geneally good wettability. Significant impact of the tee distance on WR could not be obseved fo the topsoil, wheeas fo the subsoil WR tends to occu in tansect domains fae away fom the tee. We conclude fom ou study that two easons might be esponsible fo the elatively weak coelation between the obseved WR and the investigated basic soil paametes. Concening the sample volume it might be pobable that WR vaiability, especially fo the subsoil samples, is smalle than sample volume and spatial diffeences in WR and SOC ae equalized. Fo foest soils, Lampate et al. (1) found ponounced smallscale vaiability of CA at the cm scale, which was consideably smalle than the sample volume of about.5 L content used in the pesent study. A second eason is elated to spatial esolution: bulk soil CA analysis does not necessaily conside the wettability of the intefaces. Adjusted techniques with espect to suface popeties ae equied to link physicochemical popeties of the inteface like suface chage, suface enegy o the chemical composition of the intefase with wettability. Geneally, inceasing WR pomotes on-site pefeential flow pathways which might be impotant on the catchment scale with espect to unoff and flood events. Fo ou testing site, the level of subsoil WR is low fo lage pats of the soil, except of some isolated domains in the deepe subsoil. Fo the futue, howeve, pedicted lage fequencies of extended doughts due to climate change suggest a highe pobability fo the occuence of WR in deciduous o mixed conifeous foests stands in cental Euope. Acknowledgements.The authos like to thank the Deutsche Foschungsgemeinschaft DFG fo financial suppot within the Reseach Goup SUBSOM, BA 1359/13-1 and BA 1359/14-1. We also thank Matin Volkmann fo technical assistance. REFERENCES Adamson, A.W., 199. Physical Chemisty of Sufaces. 5 th ed. John Wiley & Sons, New Yok. Ad-hoc-Abeitsguppe Boden, 5. Bodenkundliche Katieanleitung. [Soil Mapping Manual]. 5 th ed. Fedeal Institute fo Geosciences and Natual Resouces in coopeation with the Geological Sevices of the Fedeal States, Hannove. (In Geman.) Andeson, M.A., Hung, A.Y.C., Mills, D., Scott, M.S., Factos affecting the suface tension of soil solutions and solutions of humic acids. Soil Sci., 16, Bachmann, J., Woche, S.K., Goebel, M.O., Kikham, M.B., Hoton, R., 3. 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10 Occuence and spatial patten of wate epellency in a beech foest subsoil nisms unde special consideation of soil wettability. J. Plant Nut. Soil Sci., 171, Baye, J., Schaumann, G.E., 7. Development of soil wate epellency in the couse of isothemal dying and upon changes in two uban soils. Hydol. Pocess., 1, Blackwell, P.S., 3. Management of wate epellency in Austalia. In: Ritsema, C.J., Dekke, L.W. (Eds): Soil wate epellency Occuence, Consequences, and Amelioation. Elsevie, Amstedam, pp Böttche, J., Stebel, O., Spatial vaiability of goundwate solute concentations at the wate table unde aable land and conifeous foest. Pat 1: Methods fo quantifying spatial vaiability (geostatistics, time seies analysis, Fouie tansfom smoothing). J. Plant Nut. Soil Sci., 151, Böttche, J., Laue, S., Stebel, O., Puhlmann, M., Spatial vaiability of canopy thoughfall and goundwate sulfate concentations unde a pine stand. J. Envion. Qual., 6, Buczko, U., Bens, O., Hüttl, R.F., 7. 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11 Jög Bachmann, Jiem Kuege, Mac-O. Goebel, Stefanie Heinze Ofanus, T., Bedna, Z., Lichne, L., Hallett, P.D., Knava, K., Sebin, M., 8. Spatial vaiability of wate epellency in a pine foest soil. Soil Wate Res., 3, Ritsema, C.J., Dekke, L.W., Hendickx, J.M.H., Hamminga, W., Pefeential flow mechanism in a wate epellent sandy soil. Wate Resou. Res., 9, Rope, M.M., 5. Managing soils to enhance the potential fo bioemediation of wate epellency. Aust. J. Soil Res., 43, Schaumann, G.E., Diehl, D., Betme, M., Jaege, A., Conte, P., Alonzo, G., Bachmann, J., 13. Combined poton NMR wideline and NMR elaxomety to study SOM wate inteactions of cation-teated soils. J. Hydol. Hydomech., 61, 63. Snedeco, G.W., Cochan, W.G., 198. Statistical Methods. 7 th ed. The Iowa State Univesity Pess, Ames Iowa. Šolc, R., Tunega, D., Gezabek, M.H., Woche, S.K., Bachmann, J., 15. Wettability of oganically coated tidymite suface Molecula dynamics study. Pue Appl. Chem., 87, Teashima, M., Fukushima, M., Tanaka, S., 4. Influence of on the suface activity of humic acid: micelle-like aggegate fomation and intefacial adsoption. Colloids Suf. A, 47, Tschapek, M., Citeia fo detemining the hydophilicity hydophobicity of soils. Z. Pflanzenen. Bodenk., 147, Wessolek, G., Stoffegen, H., Täume, K., 9. Pesistency of flow pattens in a wate epellent sandy soil Conclusions of TDR eadings and a time-delayed double tace expeiment. J. Hydol., 375, Woche, S.K., Goebel, M.O., Kikham, M.B., Hoton, R., van de Ploeg, R.R., Bachmann, J., 5. Contact angle of soils as affected by depth, textue, and land management. Eu. J. Soil Sci., 56, WoldClim Global Climate Data, Model e: Zisman, W.A., Relation of equilibium contact angle to liquid and solid constuction. In: Gould, R.F. (Ed.): Contact Angle, Wettability and Adhesion. Advan. Chem. Seies, 43, Ame. Chem. Soc., Washington, D.C., pp Received 14 July 15 Accepted 4 Novembe Beeitgestellt von Technische Infomationsbibliothek (TIB) Heuntegeladen am :59