Acta agriculturae Slovenica, 118/1, 1–9, Ljubljana 2022 doi:10.14720/aas.2022.118.1.2415 Original research article / izvirni znanstveni članek The effects of temporary occupation of agricultural land by gravel depos- its and construction on selected soil properties Marko ZUPAN 1, 2, Vesna ZUPANC 1, Helena GRČMAN 1 Received November 09, 2021; accepted January 22, 2022. Delo je prispelo 9. novembra 2021, sprejeto 22. januarja 2022 1 University of Ljubljana, Biotechnical Faculty, Agronomy Department, Ljubljana, Slovenia 2 Corresponding author, e-mail: marko.zupan@bf.uni-lj.si The effects of temporary occupation of agricultural land by gravel deposits and construction on selected soil properties Abstract: We addressed the condition of restored soil on alluvial plain in the south-eastern Slovenia after they have been given for the gravel deposit easement during construction. According to pre-investigation using soil probes, two soil pro- file pits were dug: Profile 1 on the area where excavated soils were deposited over original soils; and Profile 2 on the area where topsoil had been removed before gravel deposition and reapplied after the easement. Undisturbed and disturbed soil samples were collected and analyzed for physical and chemi- cal properties. The results show that chemical properties were generally not the limiting factor for soil fertility. Compaction of the soil reduced hydraulic conductivity and resulted in wa- ter stagnation. The bulk density on the area where the material was deposited directly on the soil surface ranged from 1.41 to 1.77 g cm-3. The hydraulic conductivity of the saturated soil was practically impermeable at depths of 10, 20, and 30 cm, indicat- ing compaction due to high mechanical load. At the area where topsoil was removed before deposition and restored after ease- ment the hydraulic conductivity of the saturated soil was low to moderate. Removal of the topsoil before construction began was an appropriate action, but reclamation measures are also required. Key words: fluvisols; soil degradation; soil restoration; soil physical properties; soil chemical properties Vpliv začasne zasedbe kmetijskih zemljišč z deponijo gramo- za ob gradbenih posegih na lastnosti tal Izvleček: Namen raziskave je bil preveriti lastnosti tal na območju spodnje Save po rekultivaciji zaradi začasne zasede- nosti zemljišč za deponijo gramoza. Po pregledu območja (son- diranje) smo na dveh mestih izkopali talna profila; profil 1 na delu, kjer je bila na obstoječa tla odložena odstranjena rodovit- na zemljina; profil 2 na delu, kjer je bil vrhnji sloj tal pred de- poniranjem gramoza odstranjen in nato ponovno vzpostavljen. Odvzeli smo neporušene in porušene talne vzorce za merjenje fizikalnih in kemijskih lastnosti tal. Izmerili smo teksturo, vo- lumsko gostoto tal, nasičeno hidravlično prevodnost, pH, vseb- nost organske snovi, parametre kationske izmenjalne kapacite- te in rastlinam dostopna hranila. Ugotovili smo, da kemijske lastnosti v splošnem niso ovirale rodovitnosti tal. Zbitost tal je omejevala hidravlično prevodnost in povzročila zastajanja vode. Na območju, kjer je bil deponiran material neposredno na površino tal, je bila gostota tal od 1,41 do 1,77 g cm-3. Tla so bila na tem delu praktično neprepustna na globinah 10, 20 in 30 cm, kar kaže na veliko zbitost zaradi mehanskih obremeni- tev. Tla, na območju, kjer je bila vrhnja plast tal odstranjena in po odstraniti začasne deponije ponovno nanesena, so bila manj zbita. Odstranitev zgornje plasti tal pred deponiranjem gramo- za je bil ustrezen ukrep, vendar so potrebni tudi melioracijski ukrepi po zaključku gradbenih del. Ključne besede: obrečna tla; degradacija tal; rekultivacija tal; fizikalne lastnosti tal; kemijske lastnosti tal Acta agriculturae Slovenica, 118/1 – 20222 M. ZUPAN et al. 1 INTRODUCTION Alluvial plains are important agricultural areas due to favourable soil properties, topography and the vicin- ity of water resources. Even though several soil types occur, Fluvisols and Cambisols are the most important. Fluvisols are young soils formed by frequent deposi- tion of sediments along river courses and streams. In the upper reaches of the channel, the sediments are usu- ally dominated by large boulders and angular stones, but downstream the particles increase in roundness, become smaller and represent a good basis for soil development. Soils of alluvial plains (Fluvisols, Cambisols) of the lower reaches of rivers or along streams are mostly under agri- cultural land use, less commonly under forest. The sedi- ments are fine-grained (silty or clayey), and the epipedon may be thick and rich in humus (Vidic et al., 2015, Vrščaj et al., 2017). The lower part of a soil profile may contain gravel and sand; if finer, we usually find reductimor- phic features as evidence of gleying, such as grey-brown mottling, which is a consequence of a changing levels of groundwater table and alternating reduction and oxida- tion processes. Alternation of these processes results from alternating wet and dry phases in the soils, which are as- sociated with a seasonal distribution of precipitation. In Slovenia, wet autumn and spring periods lead to stagnant water in the soil profile alternating with dry winter and summer periods when soil pores fill with air and cause oxidation of Fe substances on the walls of pores and sur- face of soil aggregates. However, permanent water stag- nation in Fluvisols and Cambisol is rare and occurs only in lower soil horizons, which can express predominantly grey color. The alluvial plains are typical for their distinct hydraulic properties (e.g. by higher hydraulic conductivi- ties ranging from 2 to 180 m day-1 in the subsoil) com- pared to upper laying parts of the watershed (Miller et al., 2016; Šípek et al., 2019). Soils are usually enriched with nutrients and characterized by high vertical and horizontal heterogeneity, which is explained with the varying characteristics of alluvial sediments, regime of deposition, age of formation (distance to the river), and land use (Kercheva et al., 2017). In some cases, Fluvisols may be subjected to contamination of deposits (Antić et al., 2006; Schwartz et al., 2006; Mabit et al., 2012). The predominant land use of alluvial plains is agri- cultural, where high quality arable land for intensive crop and vegetable production (Maršić et al., 2012; Vrščaj et al., 2017) alternates with grassland (for livestock). The latter is more often found in areas with clayey soils and stagnating water. Fluvisols are of high importance be- cause of their broad ecosystem functions, not only for agricultural production but also for their role in soil wa- ter (Zupanc et al., 2011; Zupanc et al., 2012, Zupanc et al., 2020) and flood water retention (Glavan et al., 2020; Bezak et al., 2021). The agronomic significance provokes long-standing interest in determining and mapping of soil physical and chemical properties for designing ei- ther drainage or irrigation system (Kercheva et al., 2017, Matičič and Steinman, 2007). Alluvial plains are very often the subject of differ- ent interests of land use planners (Zupanc et al., 2011). Beside agricultural land use, the construction of urban and industrial infrastructure pose negative effects on soil resources (Grčman and Zupanc, 2018), not only directly with soil sealing but also due to the indirect influence of construction work on nearby land and siting of meliora- tive measures necessary for compensating natural habi- tats (e.g. flood protection measures, Bezak et al., 2021). As Fluvisols are young soils, soil morphological proper- ties, namely soil structure aggregates are unstable and weakly expressed. Such soils are susceptible to compac- tion and their structure is not easily re-established after the disturbance (Zupanc et al., 2016, Schomburg et al., 2019), which leads to water logging and hampers soil till- age (Grčman and Zupanc, 2018). As the areas of Fluvisols are very limited in Slovenia (5 % of Slovenian territory; Vrščaj et al, 2017), we have to pay attention to soil sealing and other degradation pro- cesses caused by construction works, which often require easement of the surrounding area. The aim of this study was to evaluate the soil properties on the alluvial plain of the lower Sava River, to assess its possible degradation af- ter the construction of a hydropower plant, for which an easement for gravel deposits was required. We evaluated chemical and physical parameters crucial for soil fertil- ity to establish possible degradation and causes of water stagnation. 2 MATERIALS AND METHODS The study area is located in the alluvial plains of the lower Sava and Krka rivers (Figure 1). The area was affected by the construction of the Brežice hydropower plant, as part of the agricultural land was used for gravel deposition during the construction works. After the con- struction works were completed, the gravel deposits were removed and the land was returned to agricultural use (Figure 1). However, stagnant water was seen in some parts of the area, raising questions about the quality of the earthworks used to restore the land. After detailed surface inspection and soil probing, two sites were selected for excavation of the soil profile pits (Fig. 1). One on the area where the excavated fertile topsoil was deposited directly on the agricultural land (Profile 1), and the other on an area where the fertile top- Acta agriculturae Slovenica, 118/1 – 2022 3 The effects of temporary occupation of agricultural land by gravel deposits and construction on selected soil properties soil was removed before the gravel was deposited up to the height of 2 - 6 meters and later restored (Profile 2). The description of morphological properties was done according to the Guidelines for soil description (FAO, 2006) and disturbed soil samples were taken from each recognized horizon. Undisturbed soil samples (V = 100 cm3) were taken in 10 cm increments. The soil samples were analysed for soil physical properties, i.e., texture, soil bulk density, and saturated hydraulic con- ductivity, as well as chemical properties, i.e. pH, plant available nutrients organic matter content and param- eters of cation exchange capacity. Texture was measured by sedimentation pipette method (SIST ISO 11277), bulk density of soil was determined gravimetrically (ISO 11272, 1993). Saturated hydraulic conductivity was measured using a Darcy apparatus. Five measurements of water flow under saturated conditions were made for each sample and the average was calculated. Results for saturated hydraulic conductivity were interpreted using Bear’s (1972) permeability classes (< 0.001 m day-1 prac- tically impermeable, 0.001–0.01 very low permeability, 0.01–1m day-1 low permeability and from 1m day-1 per- meable soils). Organic matter content was measured by SIST ISO 14235 – modified method after Walkely-Black, total nitrogen after dry combustion (ISO 13878), cation exchange capacity according to Soil survey laboratory methods manual (1992), pH in extraction with CaCl2 af- ter SIST ISO 10390, and plant available phosphorous and potassium after ÖNORM L 1087 – modification – amon- lactate extraction. Figure 1: The land use on alluvial plain between the Sava River and the Krka River before, during and after hydropower plant construction; the location of two soil profile pits are marked on the right picture Slika 1: Raba tal na aluvijalni ravnini med Savo in Krko pred, med in po izgradnji hidroelektrarne; na desni sliki sta označeni lokaciji profilov 1 in 2 Acta agriculturae Slovenica, 118/1 – 20224 M. ZUPAN et al. 3 RESULTS AND DISCUSSION Both soil profiles were deep and had an anthro- pogenic influence. The soils on the western part of the formerly occupied land (Profile 1) have a sequence of horizons typical of the Fluvisols of the lower Sava River (Prus, 2000; Prus et al. 2015; Vidic et al., 2015; Vrščaj et al., 2017). Textural differences between the soil horizons were typical of sedimentation processes, but the struc- tural aggregates which were angular-blocky in shape and weak in grade indicating that pedogenetic processes had already started, leading to the development of eutric brown soils (Eutric Cambisols). Morphological evidence of stagnant water, i.e. grey-brown mottling, was found throughout soil profile 1 and in two layers of profile 2, although to a small extent. No water occurred at the bot- tom of the profiles, although sampling was conducted several days after heavy rain, suggesting that textural dis- continuities and soil compaction may be affecting water movement through the soil profile (Figure 2, Table 1). Soil properties vary horizontally and vertically. These differences occur mainly in texture, which is a re- sult of the different alluvial sediments. Variation in chem- ical properties was less pronounced (Tables 2 and 3). In =Profile 1, the soil texture varies, silt particles are the predominant soil texture fraction, after which the sand fraction increases to over 80 %. Cation exchange capacity reflects clay content and decreases with depth. Base satu- ration is high, greater than 90 %, with calcium being the predominant cation. The pH is high, ranging from 7.6 to 7.9. Organic matter decreases with depth, which is typi- cal of undisturbed automorphic soil profiles. The soils are poor in plant-available phosphorus and potassium, indicating that the soils were extensively farmed in the past without the use of fertilizers. In Profile 2 greater textural variability through the depth was measured. Organic matter content is much higher compared to Profile 1, with concentrations greater than 4 % to a depth of 89 cm. The texture, organic mat- ter content, and soil color indicate that approximately 90 cm layer was removed and later reapplied as topsoil. The cation exchange capacity reflects the clay and organic Figure 2: Two soil profile pits were dug;Pprofile 1 on the area where the excavated fertile topsoil was deposited directly on the ag- ricultural land (left); Profile 2 on the area where the fertile topsoil was removed before the gravel was deposited and later restored (right) Slika 2: Izkopana sta bila dva pedološka profila; profil 1 na območju kjer je bila začasno deponirana izkopana zemljina neposredno na površino kmetijskih tal (levo); profil 2, na kasneje rekultiviranem območju, kjer so pred začasno deponijo gramoza odstranili zgornji sloj tal (desno) Acta agriculturae Slovenica, 118/1 – 2022 5 The effects of temporary occupation of agricultural land by gravel deposits and construction on selected soil properties matter content and decreases with depth. Base saturation is high, greater than 90 %, with calcium being the pre- dominant cation. The pH is high, ranging from 7.5 to 7.8. Similar to profile 1, the soils are poor in plant-available phosphorus and potassium. The results show that chemical properties are gener- ally not the limiting factor for soil fertility, especially high base saturation, high pH and high organic matter content were favorable characteristics. However, nutrient content could be increased by intensive fertilization. The bulk density of the soil in Profile 1 (area, where topsoil has not been removed) ranged from 1.41 to 1.77 g cm−3. Notable is a large difference between the soil den- sity of the uppermost 30 cm and the depth from 40 cm, where soil bulk density was from 1.41 to 1.54  g  cm−3) (Figure 3). Soil bulk density of the upper 30 cm exceeds Area/Profile Hori-zon* Soil depth (cm) Colour*** Structure Consistency when moist Roots Pedogenetic forms Topsoil was not removed Profile 1 Ap 0 - 22 2.5Y 4/2 Angular blocky Very firm very few few mottles II 22 - 46 10YR 4/4 Angular blocky Firm very few few mottles III 46 - 82 10YR 4/3 Angular blocky Firm/Friable very few few mottles IV 82-102 10YR 4/4 Angular blocky Friable very few few mottles V 102-138 10YR 5/3 Angular blocky Friable/Loose very few few mottles VI 138-179 10YR 6/4 Single grain Loose no - Topsoil was removed and later restored Profile 2 I** 0-45 2.5Y 3/3 Angular blocky Friable few no II** 45-89 2.5Y 3/2 Angular blocky Firm/Friable few few mottles III 89-119 10YR 5/3 Angular blocky Firm/Friable very few few mottles IV 119-160 10YR 5/4 Angular blocky Friable no no V 160-175 10YR 6/6 Single grain Loose no no Table 1: Morphological characteristics of soil Preglednica 1: Morfološke lastnosti tal *according to Slovenian national classification, horizons of Fluvisols and Technosols are marked with roman number (Prus et al., 2015) **replaced layers ***soil colour was identified using Munsell soil colour chart Area/ Profile Hori-zon* Soil depth Sand Silt Clay Texture pH Org. matter C N C/N P2O5 K2O cm % % mg/100 g Topsoil was not removed Profile 1 A 0-22 16.0 62.6 21.4 SL 7.6 2.6 1.5 0.16 9.4 0.7 7.9 II 22-46 5.5 71.5 23.0 SL 7.7 1.8 1.0 0.13 7.7 < 0.5 5.5 III 46-82 6.4 70.9 22.7 SL 7.8 1.3 0.8 0.09 8.9 < 0.5 4.7 IV 82-102 20.6 64.5 14.9 SL 7.8 0.9 0.5 0.06 8.3 < 0.5 3.5 V 102-138 62.1 30.2 7.7 Sl 7.8 0.5 0.3 0.02 15.0 < 0.5 2.1 VI 138-179 87.4 8.0 4.6 S 7.9 0.2 0.1 0.01 10.0 0.6 1.6 Topsoil was removed and later restored Profile 2 I** 0-45 50.8 34.5 14.7 L 7.5 4.5 2.6 0.14 18.6 1.4 6.1 II** 45-89 43.0 41.4 15.6 L 7.6 4.2 2.4 0.14 17.1 1.4 6.4 III 89-119 17.4 67.1 15.5 SL 7.7 1.2 0.7 0.07 10 < 0.5 4.1 IV 119-160 20.9 64.6 14.5 SL 7.8 0.9 0.5 0.06 8.3 < 0.5 3.4 V 160-175 62.1 30.7 7.2 PL 7.8 0.3 0.2 0.02 10 < 0.5 2.7 Table 2: Soil texture and chemical soil characteristics Preglednica 2: Tekstura in kemijske lastnosti tal *According to Slovenian national classification, horizons of Fluvisols and Technosols are marked with roman number (Prus et al., 2015) ** replaced layers Acta agriculturae Slovenica, 118/1 – 20226 M. ZUPAN et al. values, commonly found in the soils of alluvial plains (Kercheva et al., 2017). These results confirm the findings from the field, namely that the uppermost soil layer is highly compacted, hindering the flow of water to depth. The bulk density in profile 2 (area with removed and re- stored soil) ranged from 1.47 to 1.37 g cm−3 (Table 3). In addition to the removal of topsoil prior to the placement of gravel, soil texture could also influence the bulk den- sity. Soils with higher sand content are less susceptible to compaction. Since hydraulic conductivity below 0.001 m  day−1 indicates practically impermeable soils (Bear, 1972), the top 30 cm layer was practically impermeable (Figure 4). This could also imply that no water would be infiltrating and percolating vertically and replenishing water storage below the root zone without meliorative measures (deep plowing, plant cover) potentially indefinitely. In the area where topsoil was removed before depo- sition and later soils were restored (Profile 2), there were differences in the hydraulic conductivity of the saturated soil within individual soil layer and between soil layers (e.g. 10, 30 and 40 cm depth, Fig. 4). When the hydraulic conductivity of the upper layer is much larger compared to the hydraulic conductivity of the lower layer (factor 10 or larger), the effect of impervious layer occurs. This may cause water stagnation even between more permeable layers. However, large differences were observed between soil profiles at different depths, most likely due to the dif- ferent soil texture typical for Fluvisols and heterogeneous consolidation of soil mass after soil restoration. After construction and restoration works are com- pleted, the soil must be rehabilitated to improve the physical properties of the soil (Krümmelbein et al., 2010; Krümmelbein et al., 2012). Generally, restoration can- not be done with construction measures alone (Krüm- melbein et al., 2010; Zupanc et al., 2016), necessary time for soil rehabilitation depending on the extent of distur- bance to the soil profile (Grčman and Zupanc, 2018). The reasonable approach is to leave the last phase to land users (farmers), who are better able to adapt to weather conditions and optimal soil moisture and consistency than construction companies (Zupanc et al., 2016). A high value of bulk density and poor hydraulic conditions expressed as stagnant water at the soil surface indicate that meliorative measures need to be taken to accelerate soil aggregation and thus improve soil structure. This can best be achieved with a suitable plant cover (e.g. Med- icago sativa L.), where the roots of the plants can help to structure and loosen the compacted layers (Schom- burg et al., 2019). It is important to establish plant cover as soon as possible, and we recommend that protective plants for reclamation remain for at least three years. Area/Profile Hori-zon* Soil depth Ca Mg K Na H CEC Base saturat. cm mmolC 100 g -1 % Topsoil was not removed Profile 1 Ap 0-22 25.07 2.01 0.19 0.07 1.40 28.7 95.1 II 22-46 27.14 2.17 0.12 0.12 1.05 30.6 96.6 III 46-82 27.00 1.99 0.11 0.10 0.85 30.1 97.2 IV 82-102 23.22 1.33 0.07 0.06 0.10 24.8 99.6 V 102-138 18.89 0.70 0.04 0.04 0.10 19.8 99.5 VI 138-179 17.60 1.41 0.03 0.03 0.10 19.2 99.5 Topsoil was removed and later restored Profile 2 I** 0-45 27.25 1.44 0.15 0.06 1.85 30.8 94.0 II** 45-89 25.58 1.41 0.15 0.05 1.65 28.8 94.3 III 89-119 23.29 1.06 0.09 0.07 0.10 24.6 99.6 IV 119-160 23.92 1.15 0.07 0.08 0.10 25.3 99.6 V 160-175 19.71 0.68 0.05 0.05 0.10 20.6 99.5 Table 3: Parameters of cation exchange capacity Preglednica 3: Izmenljivi bazični kationi in kationska izmenjalna kapaciteta *According to Slovenian national classification, horizons of Fluvisols and Technosols are marked with roman number (Prus et al., 2015) ** replaced layers Acta agriculturae Slovenica, 118/1 – 2022 7 The effects of temporary occupation of agricultural land by gravel deposits and construction on selected soil properties Figure 3: Average soil bulk density (g cm-3) for soil Profile 1 and soil Profile 2 Slika 3: Povprečna volumska gostota tal (g cm-3) v talnem profilu 1 in v talnem profilu 2 Figure 4: Saturated hydraulic conductivity (m day−1) for soil Profile 1 and soil Profile 2 (three replicates) Slika 4: Hidravlična prevodnost nasičenih tal (m dan-1) v talnem profilih 1 in 2 (tri ponovitve) Acta agriculturae Slovenica, 118/1 – 20228 M. 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