1. Introduction
Contrary to agricultural land, a forest is capable of retaining water in the landscape, thus stabilizing runoff from a catchment (e.g., [
1]). Although showing higher total evapotranspiration [
2], forests contribute more to groundwater recharge (e.g., [
3]) and the maintenance of groundwater storage [
4], due to the minimization of surface runoff.
Based on that information, the impact of forest management on both the volume and quality of surface water [
5], subsurface water [
6] and groundwater have been frequently discussed (e.g., [
2,
7,
8,
9,
10,
11]). Negative consequences are particularly apparent in connection with clear-cutting. The main points are as follows: how much water is retained and accumulated in a forest and also how groundwater recharge rates are influenced by many factors, such as climate, meteorology, topography, properties of percolating water and soil, geology and vegetation [
9].
The removal of living above-ground biomass due to clear-cutting reduces evapotranspiration and nutrient pools in an ecosystem [
12]. Soil surface exposed to direct solar radiation changes with respect to topsoil temperature and moisture content, thus increasing the breakdown rate of forest-floor humus layers [
13,
14], to which both ground vegetation and soil biota respond [
15,
16]. Therefore, an impact on the volume and quantity of subsurface percolating water and groundwater can be expected.
Clear-cutting is a common practice in nutrient-poor pine forests. The removal of a stand increases the amount of water that falls onto the forest floor. Water with dissolved substances readily percolates through deep sandy soils, which have low ability to retain water [
17]; these are then expected to enter deeper soil layers, potentially having an impact on groundwater properties.
Based on the monitoring of precipitation and percolation water in a forest stand and on clear-cuts, the objective of this study was to estimate the impact of clear-cutting using whole-tree and stem-only harvesting on the previously mentioned water balance components. The volume and water properties, such as pH, total organic carbon (TOC), nitrates, ammonium nitrogen and phosphates, were analyzed and we considered whether clearcutting threatened the groundwater as a theoretical source of drinking water [
18].
2. Materials and Methods
2.1. Study Site
The experimental site was established in the municipal forests of Hradec Králové city in spring 2017 (50.18 N, 15.96 E). It was designed as three non-replicated research plots, comprising a forest stand left on site without any felling (Control) and two clear-cut stands (Clear-cut 1 and 2). The forest site is a “Scots pine with sessile oak on nutrient-poor soils” [
19] at an altitude of 255 m (
Figure 1). The age of the treatments ranged from 85 to 98 years; all stands were dominated by Scots pine, other tree species were Norway spruce growing mostly in lower-storey and also rarely Weymouth pine [
19]. Scots pine forms from 91 to 93% of the stand basal area, which was 44, 42 and 42 m
2 in Clear-cut 1, 2 and Control, respectively. The average height of Scots pine was from 27 to 28 m at the research plots [
20]. The forest soils of the study site included unconsolidated fluvial sandy gravel sediments of a Pleistocene origin, below which Cretaceous sediments existed [
21,
22].
Each of the three treatments was equipped with ten 0.1 m
2 pan lysimeters made of pan sheet that were placed into the intact soil profile at a depth of 70 cm in the middle of the plot. Besides these, three shallow-placed lysimeters (at a depth of about 10 cm) with total area of 0.3 m
2 were added to catch water percolating through the forest floor below the Control treatment (see [
23]). The square lysimeters with ca 5-cm-high rims were made from zinc-plated iron sheet and have cone-shaped bottom draining water caught within its area to front lower spot from which hoses collected water into barrels. Furthermore, one rain gauge was installed within the adjacent clear-cut treatment and two rain gauges were placed below the canopy of the Control stand.
According to particular geomorphology, three exploratory wells were drilled at the lowest altitudes of the research plots where the highest response of groundwater properties to above-ground conditions of the respective plot was expected. The casing inner diameter of the three wells was 12.3 cm; each well was equipped with a submersible pump. The groundwater table levels were at 4 m in wells GW_1, GW_3 and at 8 m in well GW_2 (
Figure 2). The deeper drilling in GW_2 was due to the convex surface of the terrain.
In November 2017, both clear-cut treatments were established. All trees were harvested in the area of 1.1 ha and 0.6 ha in the Clear-cut 1 and Clear-cut 2 treatments, respectively and the stand was left on site on the Control plot. Logging residue removal and soil disking was conducted at the Clear-cut 1 site and windrows of logging residues were left at the Clear-cut 2 site (
Figure 3). In spring 2018, both Clear-cut sites were equipped with shallow lysimeters below the forest floor at a depth of 10 cm (see
Table 1). In the Clear-cut 1 treatment, the lysimeters were installed to catch percolating water below both the furrow and undisturbed topsoil, which was additionally covered with an overturned slice of forest-floor humus after disking (see [
23]).
2.2. Sampling and Storage of the Samples
The sums of precipitation were measured and the samples of precipitation water, lysimeter water, and also groundwater samples were taken at three-week intervals. Sampling started in May 2017. Groundwater was taken using a portable battery to power the pumps, and the water was collected after 10 min of draining (running the pump; according to ČSN EN ISO 5667-1 (757051)). In the laboratory, the lysimetric samples were filtered to reduce the amount of raw material using multi-layered filter paper. The samples were kept under dark and cold conditions (4 °C) prior to the analyses, following ČSN EN ISO 5667-3 (757051).
2.3. Chemical Analyses
pH values were measured potentiometrically using an HQ 30d multimeter (Hach Lange s.r.o., Prague, Czech Republic) with a combined electrode following ČSN ISO 10523 (757365). Total organic carbon (TOC) was measured using a Formacs TOC Analyzer (Skalar Analytical B.V., Breda, The Netherlands) with an infrared detector and 50 μL was injected according to ČSN EN 1484 (757515). The determination of ammoniacal nitrogen was performed using a UV/VIS DR 6000 spectrophotometer (Hach Lange s.r.o., Prague, Czech Republic) at 655 nm following ČSN ISO 7150-1 (75 7451). A coloring agent (4 mL) and sodium dichloroisocyanate (4 mL) were added to 40 mL of the sample and the volume was increased to 50 mL by adding deionized water. After standing for one hour, the mixture was transferred to a 4 cm cuvette and the concentration of ammoniacal nitrogen was calculated using a calibration curve.
Ion chromatography was performed via a Compact IC Flex Oven/SeS/Deg (Metrohm, Switzerland) with the following parameters: Metrosep A Supp 5 chromatographic column (150 × 4.0 mm; 5 μm), a flow rate of 0.7 mL/min, injection volume of 20 μL, and column temperature at 30 °C. The isocratic elution was set with a total run time of 20 min, where the mobile phase was deionized water containing 1 mmol/L sodium bicarbonate and 3.2 mmol/L sodium carbonate. The method was developed by Metrohm (Herisau, Switzerland). The ion chromatography system was coupled to a conductivity detector and data were acquired using Magic Net 3.2 software (Metrohm, Herisau, Switzerland). The concentrations of nitrate ions and phosphorus contained in the phosphates were determined using calibration curves that were constructed based on the area of the chromatographic peaks.
2.4. Methods of Data Analyses
Precipitation/throughfall and seepage waters on each measuring date were compared with one another to verify the functionality of the equipment and to capture any eventual disproportions. To stabilize the vegetation and humus layers in the topsoil lysimeters, seepage data from the −10 cm layer were included from the third quarter of 2018 only. The amount of precipitation in both gauges was averaged. Subsequently, the precipitation and seepage sums of each measured feature per individual quarter year were counted. The sums and their shares are presented.
Based on the exploratory analysis, the consistency of the data of the analyzed chemical parameters and their distribution was evaluated. Considering the low number of samples, significant outliers were excluded, including 2 extremely high TOC values and 1 extremely low value of NH
4+ in the precipitation data; 1 high TOC value in the water percolating through forest-floor humus (L-10) data; 1 low pH, 1 high TOC and 2 high NH
4+ in the water percolating at the 70-cm depth (L-70) data; and 3 high TOC in the groundwater data. The chemical data obtained in individual quarter years were compared. In prior analyses, the TOC, NO
3− and NH
4+ data were logarithmically transformed to improve their properties. The period declared as the post-harvesting period started in the third quarter (Q3) of 2018 to exclude the time of unbalanced conditions after clear-cutting. The data were analyzed using t-test or ANOVA (in case of heteroscedasticity with Welch’s correction) with a subsequent Tukey test, respectively. The analyses were performed using software such as MS Excel and R 4.0.3 [
24]. The results were considered to be significant at
p ≤ 0.05.
4. Discussion
The clear-cutting of forest stands significantly impacts nutrient cycles [
25,
26,
27], as well as changes soil properties [
28], especially if it is accompanied with total above-ground biomass removal when whole-tree harvesting is used (for data from the same study site, see [
20]). The nutrient cycle change in a forest ecosystem is, however, also attributable to natural processes, e.g., for nitrates see [
29]. For example, air-polluted [
30,
31], dormant [
32,
33] and bark beetle-infested forests [
34] showed a diminished consumption of nitrates, which can be reflected in increased concentrations of nitrates in surface streams [
31,
34]. The deliberate harvesting of a stand ceases the function of “nutrient pump”, which also leads to increased nitrates in groundwater following clear-cutting [
7,
35,
36]. This can be shown even over a few years following clear-cutting [
37,
38] and corresponds with our findings.
The different concentrations of nitrates observed in the wells of the Control and Clear-cut 1 treatments, which were close by, already from the beginning of the monitoring, were noteworthy. Both the drilling depth and geological layering of the two wells were almost the same (
Figure 2). However, the presence of some rifts in the bottom Cretaceous layer with weathered marvel drilled in GW_1 was likely, which could have connected aquifers of a different origin with higher nitrate concentrations. This is, however, a question for further verification.
Precipitation acidity is a consequence of emissions and chemical processes in the atmosphere [
39], whereas throughfall pH is additionally influenced by contact with the tree crown layer. Pierret et al. [
40] showed that the structure and persistence of needles (they investigated spruce) enhanced the capture of particles in water, accentuating the acidity of the deposition, thus leading to the intensification of acidification processes and nutrient leaching in soils [
40]. A similar impact on soil solutions was observed at L-10 from Q1-2019 in our study, at least with respect to Clear-cut 1 (
Table 3).
In addition, other substance contents can increase as precipitation falling through the canopy washes out dry deposition from surfaces [
41,
42]. This effect can be observed on TOC and also NO
3− concentrations in throughfall in our study, which also obviously influenced their concentrations in soil water solutions. The differences in TOC of throughfall and precipitation were the highest in 2018, when low precipitation sums probably led to higher concentrations of the solution. A similar situation in groundwater could be observed after about a half-year delay. The outcomes correspond to the analyses performed in a Douglas-fir stand [
26]. On the other hand, broadleaves can reduce the acidity of throughfall and the acidification of soil leachates under broadleaved species would be lower compared to that below conifers [
43].
As was shown in our study, clear-cutting combined with felling residue removal and soil surface ploughing can mitigate acidification of the upper soil layer to some extent. It is likely affected by furrows’ surfaces, where the infiltrating water was minimally affected by the forest floor humus. As water penetrated the deeper soil, pH levels became comparable among all treatments. On the other hand, groundwater pH dropped below both clear-cut treatments compared to the control stand. The acidification rate seems to correspond with the size of the clear-cut. Considering the different methods of harvesting and soil preparation, as well as the ambiguous progress of the pH levels at L-10 and L-70, this hypothesis can be hardly proved. Similarly, whole-profile nitrates were affected by clear-cutting; on the other hand, the trend was not so obvious for TOC. In addition, both clear-cut wells differed as they showed different layers within the profile (
Figure 2). As mentioned above, neighboring GW_3 and GW_1 were similar in their profiles and differed only in the immediate roof and underlying rock of a marl layer. The more distant GW_2 differed in alternating layers of fine-grain and middle-grain slightly loamy sand. From depths of 5.6–6.8 m, the sand contained more loam compared to the overlying and underlying layers around the GW_2 well. These variations could also explain the different filtering effects of the soil profiles (compare [
44]). For example, water flow moves downwards quickly through gravely layers at deeper parts of an unsaturated zone [
45].
Both the acidity and the content of nitrates are important parameters of drinking water quality. Clear-cutting was found to increase acidity and nitrate concentrations due to increased nitrification [
46,
47]. Drinking water pH limits [
18] are set between 6.5 and 9.5. The groundwater pH in our study was already near the low limit before clear-cutting; afterwards, the values slightly decreased on both clear-cuts.
Long-term acidification and/or higher nitrate concentrations in groundwater can pose a rise in increased drinking water treatment costs. The limits for nitrates (50 mg·L
−1) were, however, exceeded only in infiltrating sub-surface water, whereas groundwater quality was not threatened. Based on the reported trends following clear-cutting (see e.g., [
37,
38]), one can estimate the restoration of soil water and groundwater properties, and the differences among the treatments will also diminish with new forest stand establishment and growth on the study site. In addition, ground vegetation, which progressively developed on the analyzed clear-cuts, contributes to the process [
48]. Subsurface water is cleaned of nitrates and ammonium nitrogen as it passes through the unsaturated zones of soil [
49]. Different soils, however, have different capabilities of filtering these pollutants, as was reported by Jaber et al. [
50] for calcareous and sandy soils in Florida.
The average TOC values in each quarter fluctuated significantly; subsurface water showed a downward trend, whereas the groundwater TOC increased in the last period. This was likely to be attributable to carbon released from decomposing organic matter, as well as the activities of microorganisms [
51] and fungi [
52].
The concentrations of substances dissolved in percolating soil water are affected by the intensity and frequency of precipitation events. Organic matter accumulates, is transformed, released, and later washed out. In certain circumstances, the amounts are many times higher compared to periods with “regular” precipitation. On the other hand, sandy layers function as a filter (see e.g., [
53]), an effect that was documented as subsurface water contained much more humic substances of a soil biota origin [
52], changing its color to brown, compared to the clear water caught at the deeper-70 cm lysimeters installed on our study site. One of the evolution lines of groundwater properties is that it is transformed from polluted to clean water [
54,
55].
It should be also pointed out here again that our findings are based on the non-replicated study-site design in soil-environmental conditions typical for nutrient-poor pine forests. Single-site design limits generalization of the outcomes and their management implications. Soil and weather conditions (especially precipitation regime) impact vegetation and soil biota and can differ rates of nutrient cycle components [
56,
57] from the conditions of our experimental site. As compared to the analyzed sandy soils, higher participation of fine particles in soil would increase e.g., the reduction in the nutrients in seepage water through the adsorption in soil due to ion effects [
58]. One of the reasons is presumably the fact that sandy soils have lower abilities to retain water [
17], which more readily percolates into deeper soil layers. Therefore, the rates of the observed trends in the analyzed parameters of our study should be taken as specific to the site conditions.
5. Conclusions
This study, by monitoring precipitation and water percolation at the non-replicated treatments such as Scots pine stand and two clear-cuts located on deep sandy-gravel soils, showed that clear-cutting and the method of soil preparation represented only a limited extent of the changes in the soil water and groundwater chemistry. It was particularly due to the relatively small increase in the amounts of water percolating to the groundwater below the clear-cut areas. As compared to throughfall, clear-cut precipitation was less acidic with lower TOC, nitrate, and ammonium nitrogen contents. The lower acidity of percolating water was observed two years after felling only in the sub-surface layer of the clear-cut area with ploughed soil. Contrarily, groundwater pH after felling decreased. The lower input of TOC in clear-cut precipitation also gradually influenced its concentration in percolating water; however, its concentrations in groundwater remained apparently independent of the changes that occurred above. Nitrate concentrations in the percolating water of clear-cut areas significantly increased with greater decomposition. This increase was also observed in groundwater. On the other hand, concentrations of both ammonium nitrogen and phosphorus showed no relation to clear-cutting.
Considering the potential of groundwater as a source of drinking water, three years after clear-cutting, groundwater acidity was threatened, making it harder for it to be used as a source of drinking water. However, the other evaluated parameters remained far below the permitted limits. No negative impact of soil ploughing was observed. The subsequent growth of new forest stands is expected to slowly restore the water cycle processes in the forest. Due to the non-replicated design of the experiment, generalization of the conclusions is limited. Application of the observed changes outside the conditions of the study site is a question for further research.