geosciences Article The Influence of Soil Compaction on Runoff Formation. A Case Study Focusing on Skid Trails at Forested Andosol Sites Julian J. Zemke * , Michel Enderling, Alexander Klein and Marc Skubski Department of Geography, Institute for Integrated Natural Sciences, University Koblenz-Landau, Campus Koblenz, 1, 56070 Koblenz, Germany; [email protected] (M.E.); [email protected] (A.K.); [email protected] (M.S.) * Correspondence: [email protected]; Tel.: +49-261-287-2282 Received: 11 March 2019; Accepted: 6 May 2019; Published: 8 May 2019 Abstract: This study discusses the influence of soil compaction on runoff generation with a special focus on forested Andosol sites. Because of their typical soil physical characteristics (low bulk density, high pore volumes) and the existent land use, these areas are expected to show low to no measurable overland flow during heavy rainfall events. However, due to heavy machinery traffic in the course of forestry actions and pumice excavations, skid trails have been established. Here, a distinct shift of soil dry bulk density (DBD) was observable, using a detailed soil mapping and data interpolation in order to generate in-depth DBD-cross profiles. Additionally, infiltration measurements and rainfall simulations (I = 45 mm h 1, t = 30 min) were conducted to evaluate effects · − of observed soil compaction on infiltration rates and overland flow formation. Results show that soil compaction was increased by 21% on average in skid trail wheel ruts. As a consequence, observed runoff was 8.5-times higher on skid trails, while saturated hydraulic conductivity was diminished by 36%. These findings show, that soil compaction leads to a higher possibility of runoff formation during heavy rainfall events, especially at sites which showed initial conditions with presumably low tendencies of runoff formation. Keywords: rainfall simulation; soil compaction; overland flow; forest hydrology; Andosol; soil hydrology; infiltration 1. Introduction Wide areas of northern Rhineland-Palatinate (southwest-Germany) are covered by Laacher See Tephra (LST), originated from the last major eruption of the Laacher See volcano 12,916 yr B.P. Mostly pumice lapilli and ashes were deposited; initial base condition for an Andosol-pedogenesis in these areas [1–3]. Andosols feature soil physical properties that prevent a quick formation of overland flow during heavy rainfall events: The rather coarse pumice lapilli lead to characteristically low dry bulk densities (DBD) and high pore volumes [4–7]. This leads typically to a low tendency of runoff formation [8–13]. However, observed sites are subject to two different anthropogenic impacts on pedosphere and hydrosphere: First of all, forestry actions taking place condition the establishment of a skid trail network in order to increase trafficability. Secondly, pumice is being excavated leading not only to a total conversion of soil physical properties—which has to be discussed in future studies – but also to increased heavy machinery traffic in the course of these operations. Soil hydrological and soil physical consequences of machinery traffic on unprotected forest soil are widely discussed. Studies clearly show that, due to soil compaction, an increase of overland Geosciences 2019, 9, 204; doi:10.3390/geosciences9050204 www.mdpi.com/journal/geosciences Geosciences 2019, 9, 204 2 of 14 flow during heavy rainfall events is expectable [14–20]. That is why traffic induced compaction can compromiseGeosciences 2019 beneficial, 9, x FOR PEER effects REVIEW of forest soils regarding decentral water retention. However, studies2 of 15 that discuss these effects for Andosols and their typical soil physical properties are scarce when it comescomes to to forestryforestry sitessites in humid oceanic oceanic climate climate [21] [21. ].Yet, Yet, detailed detailed results results were were publicized publicized in [22] in [for22] forthe the Inland Inland-northwest-northwest USA. USA. Here, Here, all all of of the the basic basic assumptions assumptions are are affirmed: affirmed: Soils Soils originated originated from from volcanicvolcanic ashes ashes are are described described asas low-densitylow-density soils with a high porosity porosity,, which are are easily easily affected affected by by machinerymachinery tra traffic.ffic. Therefore,Therefore,Andosols Andosols havehave toto bebe observedobserved eveneven moremore carefullycarefully because of two reasons: 1 1)) The The shiftshift between between undisturbed undisturbed and and disturbeddisturbed conditionsconditions isis presumably quite significant, significant, as as the the initial initial soil soil physicalphysical conditions conditions should should lead lead to comparativelyto comparatively high high infiltration infiltration rates. rates. 2) Moreover, 2) Moreover, andic andic soils showsoils theshow tendency the tendency that “( that::: “(…)) soil soil material material changes, changes, under under pressure pressure or or by by rubbing, rubbing, fromfrom a plastic solid solid intointo a a liquefied liquefied stagestage andand back into into the the solid solid condition.” condition.” [2 [323] (pp] (pp.. 146 146–147).–147). This This means, means, that thatthey theyare areexceptionally exceptionally well well formable formable when when it itcomes comes to to pressure pressure-induced-induced impacts impacts,, leading leading to to presumably presumably noteworthynoteworthy changes changes caused caused by by tra trafficffic load. load. ThisThis studystudy concentratesconcentrates on a site were were a a complete complete deforestation deforestation was was conducted, conducted, allowing allowing a asubsequent subsequent pumice pumice excavation. excavation. Because Because of the of theintensive intensive machinery machinery traffic tra duringffic during deforestation, deforestation, new newskid skid trails trails were were established. established. This This allowed allowed a detailed a detailed analysis analysis of ofalterations alterations made made by by traffic. traffic. The The aim aim isis to to investigate investigate if if there there isis aa notablenotable changechange inin soilsoil DBDDBD duedue toto tratrafficffic load leading to to diminished diminished infiltrationinfiltration rates, rates and, and if if there there is is a measurablea measurable shift shift in runoin runoffff formation formation during during heavy heavy rainfall rainfall events events that that could possibly trigger side-effects like flash-flooding or soil erosion processes. could possibly trigger side-effects like flash-flooding or soil erosion processes. 2.2. Materials Materials and and Methods Methods 2.1.2.1. Study Study Area Area StudiesStudies took took place place in inthe the forestforest districtdistrict Bendorf,Bendorf, located approximately 20 km E Eastast of of the the Laacher Laacher SeeSee in in northern northern Rhineland-Palatinate Rhineland-Palatinate(Figure (Figure1 ).1). Figure 1. Location of the study area. Figure 1. Location of the study area. The forest district is located on the plateau of the Rhenish Massif, representing the peneplain of the Variscan basal complex, which drops off steeply towards river Rhine about 3 km E of the study area. Because of the rather large elevation difference (study area 260 m.a.s.l., Rhine 60 m.a.s.l.), all Geosciences 2019, 9, 204 3 of 14 The forest district is located on the plateau of the Rhenish Massif, representing the peneplain of theGeosciences Variscan 2019 basal, 9, x FOR complex, PEER REVIEW which drops off steeply towards river Rhine about 3 km E of the study3 of area. 15 Because of the rather large elevation difference (study area 260 m.a.s.l., Rhine 60 m.a.s.l.), all tributaries intributaries the forest in district the forest are deeply district cut are into deeply the surroundingcut into the surrounding landscape, leading landscape to partially, leading steep to partially slopes. steepSoil slopes compaction. and runoff formation were investigated along a newly established skid trail featuringSoil distinctcompaction wheel and ruts, runoff which formation was built were parallel investigated to the line along of inclinationa newly established (Figure2a). skid Pumice trail deposits,featuring according distinct wheel to a historicalruts, which geological was built mapping parallel to conducted the line of by inclination the Geological (Figure State 2a). OPumiceffice of Rhineland-Palatinatedeposits, according to (LGB a historical RLP), are geological shown in mapping Figure2b. conducted by the Geological State Office of Rhineland-Palatinate (LGB RLP), are shown in Figure 2b. FigureFigure 2. 2.( (aa)) AerialAerial viewview ofof thethe studystudy site, sampled skid trail trail indicated indicated by by arrow; arrow; ( (bb)) p pumiceumice deposits deposits accordingaccording to to Geological Geological StateState OOfficeffice ofof Rhineland-PalatinateRhineland-Palatinate (LGB(LGB RLP)RLP) mapping. TheThe skid skid trailtrail spansspans aa formerformer beechbeech standstand which was def deforestedorested because because of of subsequent subsequent pumice pumice excavation.excavation. Overall,Overall, 1 144 cross section sectionalal plots plots were were sampled. sampled. Even Even though though the the study study site site lies lies within within a arather rather flat flat-angled-angled secondary secondary valley valley and and features features a amean mean sloping sloping of of αα == 5°,5◦ ,steep steep slope slope sections sections are are existentexistent at at the the lower lower slope,slope,