The condition of forest soils in central and northern Poland in the decade 2007–2017 based on selected indicators

Soil is an important component of the forest ecosystem (Prusinkiewicz 1970; Gatica-Saavedra et al. 2023), and chemical indices describing its condition provide information on the relationships between vegetation and soil. In the study of soil indices, Kowalkowski (2002), Pavlů et al. (2021) and Gatica-Saavedra et al. (2023) showed that the most frequently assessed soil parameters are pH and the content of total organic carbon (TOC) and total nitrogen (TN). According to Greinert and Greinert (1999), an approximate assessment of the soil’s resistance to chemical degradation can be made based on the soil’s sorption capacity for cations. The chemical quality of forest soils is related to the ratio of acids to bases, which is represented by the acidity of the soil (pH), cation saturation and their ratio to aluminium (Ca : Al or SBC : Al) (Cronan and Grigal 1995). The content of cations in forest soils is an important determinant for the interactions in the soil-soil-solution system (Matschonat and Vogt 1998) and the possible susceptibility of the soil to degradation processes (Siuta 1987). Sorption capacity (CEC) and base saturation (BS) determine the availability of nutrients for plants (Augusto et al. 2002; Ampong et al. 2022; Gmitrowicz-Iwan et al. 2023) and the toxicity of aluminium and heavy metals (Medyńska-Juraszek and Ćwielag-Piasecka 2020). One of the most important factors determining soil fertility is its pH value. Excessive acidification of the soil can contribute to an increase in the hydrogen ion content (H⁺) and the toxicity of aluminium ions (Al³⁺) for plants. Knowledge of soil quality is an important criterion for rational forest management and the protection of forest ecosystems from harmful processes caused by global climate change. Excessive soil acidification leads to the loss of base cations (K⁺, Na⁺, Ca²⁺, Mg²⁺), which are essential for the proper functioning of forest ecosystems. The risks associated with aluminium toxicity in soil can be predicted by assessing the ratio of the sum of base cations (SBC) to the aluminium content, that is, SBC : Al. The molar ratio of the sum of base cations, that is, Ca2+, Mg2+, K+ and Al3+ in the soil solution, is a critical chemical parameter, and the assumption of SBC : Al >1.0 can protect forest ecosystems from damage by aluminium ions (Brunner and Sperisen 2013). Excess aluminium in soil poses a serious threat to forest health and can accelerate its decline (Eggenberger and Kurz 2000; Rout et al. 2001). The assessment of soil quality is complicated due to the complexity and variability of its properties and composition (Greinert and Greinert 1999; Kowalik 2001) and the influence of different tree species (Galka et al. 2014; Gruba and Mulder 2015; Mulder and Stein 1994). Knowledge of soil quality is an important criterion for rational forest management and the protection of forest ecosystems from harmful processes caused by global climate change.

The aim of the study was to assess the condition of forest soils in central and northern Poland using selected chemical indicators.

Selected chemical parameters of forest soils in central and northern Poland were assessed in 2007 and 2017. The forest districts were located on different soils and forest sites, which affected their physical and chemical properties and the relationship between selected chemical indicators (Fig. 3A–F).

It was found that soil acidity, TOC, TN and C : N ratio did not change significantly after a decade, while the other parameters related to soil cation richness, that is, SBC, CEC, BS and aluminium toxicity indices (SBC : Al and Ca : Al), were subject to changes depending on the location of the studied forest districts.

The study hypothesised that high acidification and low organic carbon content could be the cause of soil chemical degradation and the deterioration of living conditions for forest ecosystems.

Soil acidity is one of the most important indicators of soil health, as it influences the development of soil microflora and the availability of nutrients for vegetation and determines soil-forming processes (Bednarek 1991; Christ and David 1996; Badora 2002; Yano et al. 2002; Wang et al. 2006). The reaction values in the forest soils of central and northern Poland ranged from extremely acidic (pH_CaCl2 2.75) under Pinus sylvestris L. conifer stands in the Krucz Forest District to strongly acidic in the Gdańsk, Łąck and Strzałowo Forest Districts (pH_CaCl2 4.50–4.60) under Fagus sylvatica L, Quercus L. and Pinus sylvestris L., The obtained pH values are confirmed in the works of many authors (Brożek and Zwydak 2010; Gruba 2009; Janowska 2001; Jonczak 2012, 2015; Degórski et al 2013; Jonczak et al. 2013). Some studies (Augusto et al. 1998; Błońska et al. 2016) indicate that coniferous species have a stronger acidifying effect than deciduous species. Augusto et al. (2002) ranked tree species according to their effect on soil acidification: lime, ash, hornbeam and maple ≤ birch, oak, beech ≤ conifers. The results obtained in the study show the acidifying effect of Pinus sylvestris L. stands on forest soils, which was also confirmed in the work of Schulze (2000).

The acidified state of the soils is due to the properties of the sedimentary rocks deposited by the Scandinavian glaciers (Filipek 2001). The observed lack of pH differences in the study area between 2007 and 2017 could be an indication of the stability of the physical properties of the studied forest soils (Kabala et al. 2021) and the lack of impact of anthropogenic activities on the soils.

The pH of the investigated forest soils influenced the content of selected chemical parameters in the soils. The correlations between the 2007 and 2017 results show that the increase in soil acidity is accompanied by a decrease in TOC (r = –609***), TN (r = –0.599***) and CEC (r = –0.424**) content. On the other hand, an increase in pH was positively influenced by the percentage increase in BS base saturation of the sorption complex in the soil, as evidenced by the addition of the values of the non-parametric Spearman coefficient (r = –0.602***). In addition, the reduction in soil acidity had a positive effect on the risk of aluminium toxicity, as evidenced by the positive values of the correlation coefficients between pH and the content of SBC : Al (r = 0.570***) and Ca : Al (r = 0.506***).

Studies by Borowiec (2005) and Uziak et al. (2005) confirm the existence of a relationship between the sorption capacity of the soil and the organic matter content. Soils with a higher sorption capacity (CEC) tend to have a higher content of organic matter, clay and clay fractions (McCauley et al. 2009). The results obtained agree with those of numerous researchers (Chojnicki et al. 2010; Jonczak and Parzych 2015; Jonczak et al. 2022; Isa et al. 2023). The fractions of the granulometric composition of the soil influence various parameters of the investigated forest soils. It was found that the content of the sand fraction reduced the accumulation in the soil: TOC (r = –0.333*), nitrogen (r = –0.396*) and SBC (r = –0.702***). It was shown that soils rich in fine fractions (clay and silt) have a greater capacity to retain alkaline cations. This trend is illustrated by the positive values of the non-parametric Spearman correlation coefficients for SBC (silt: r = 0.523***; clay: r = 0.727***).

Soil organic carbon content is an important qualitative parameter that determines the content of forest soils in the basic cations required for forest ecosystems. The study showed that the top litter layer (114.3–353.73 g TOC kg^-1) contained significantly more organic carbon than the mineral layer (<25.0 g TOC kg^-1). The stratified distribution of TOC and TN is due to the fact that both components are supplied to the soil mainly with the precipitation of the assimilation apparatus (Kuznicki et al. 1978; Jankowska 2001; Błońska and Januszek 2010; Brożek and Zwydak 2010; Osman 2013). The differences between the TOC and TN content on the plots in the Strzałowo and Krotoszyn Forest Districts are due to the different habitat conditions and species of the predominant stand. The indicated trend is confirmed by studies of Dziadowiec et al. (2007), Jonczak (2011, 2013), Woś et al. (2022) and Hu et al. (2024). Pinus sylvestris L. dominated in the Strzałowo Forest District, while Quercus L. was found in the Krotoszyn Forest District.

The C : N ratio is considered one of the most important indicators of the biological activity of soils and the quality of the forest habitat (Wawrzoniak et al. 2004; Brożek and Zwydak, 2010). The C : N value in the organic soil layer was higher under coniferous stands than under deciduous stands in almost all forest districts, with the exception of the Suwałki Forest District. The indicated trend is most likely due to the content of wood and cones in the plant exudates (Jonczak 2011). The C : N values determined in this study were typical for Polish soils (Dziadowiec 1990; GetinoÁlvarez et al. 2023). The lack of differences in the C : N ratio between 2007 and 2017 can be explained by the too short observation period (Berg and Matzner 1997; Chao et al. 2023) and the low anthropogenic load (Sien-kiewicz et al. 2018). Positive Spearman rank correlation coefficients were found between C : N and soil TOC (r = 0.416**) and TN (r = 0.316*) and negative ones with pH (r = –0.620***) and SBC (r = –0.455***) and CEC (r = –0.608***). The observed trends could indicate that soil acidification is associated with a slowdown in the decomposition of plant residues (increase in the C : N ratio) and thus with a decrease in the saturation of the sorption complex with nutrients important for forest ecosystems.

The sum of base cations (SBC) and the sorption capacity (CEC) are among the key properties of soils, as they have a significant influence on the functioning of terrestrial ecosystems. The results obtained in this work for SBC and CEC show a large variation depending on soil depth, which is also confirmed by the studies of other authors (Kuznicki et al. 1974; Janowska 2001; Hendershot et al. 2008; Chojnicki et al. 2010; Jonczak 2015). According to the studies by Glinski (1985) and Olszowska (2018), the abundance of SBC and CEC in the soil is significantly influenced by the type of vegetation on a given area.

The degree of saturation of the sorption complex with bases (BS) in the organic layer was 37.87–93.30% in 2007, and in 2017, the lower range increased to 62.60–88.59%. In contrast, mineral soils had lower values of 10.67–84.18% (2007) and 12.08–63.29% (2017), respectively. Similar ranges of this indicator in forest soils were also shown by other authors (Kuznicki et al. 1974; Blaszczyk 1998; Konecka-Betley et al. 1999; Brożek and Zwydak 2010; Wanic and Błońska 2011). An increase in the proportion of BS in organic horizons may indicate improved soil properties, that is, water retention, better nutrient availability and less damage (Roussos et al. 2017; Paradelo et al. 2024). According to studies by Bennett et al. (2021) and Pérez-Harguindeguy et al. (2013), increasing BS values in mineral horizons may be due to agroecological practices, and a decrease in this parameter below 50% may indicate soil degradation.

The ratio of SBC : Al and Ca : Al in forest soils is an important chemical indicator of the potential toxicity of aluminium ions to forest ecosystems. It is assumed that a value of both ratios below <1.0 may indicate the toxicity of this element in the environment. The study shows that the value of this ratio is significantly higher than the limit value for the organic soil layer. A positive Spearman rank coefficient was found between the TOC content and the SBC : Al ratio (r = 0.618***). This relationship shows the important role of organic matter in protecting forest ecosystems from the toxic effects of aluminium. The described trend is also confirmed by the studies of other authors (Brümmer and Hermus 1983; Neagoe et al. 2012; Stefanowicz et al. 2020). The large values of the ratios of SBC : Al and Ca : Al in the organic layers indicate that they are dominated by basic cations. Similar results were obtained by Kopáček et al. (2001). On the other hand, a significant decrease in SBC : Al values was found in the soils of the Strzelowo Forest District, which could indicate changes in the mineralogical composition of forest soils. If the downward trend of these indices continues in the long term, this could lead to increased aluminium toxicity in the soil and poor health of the forest stands. This risk was present in most of the soils examined and remained at a similar level after a decade 2007 and 2017 (Tab. 5).

To reduce the risk of forest soil degradation due to the loss of basic cations and the increase in the proportion of acidic cations in the soil sorption complex, as well as the increase in toxic aluminium in the area under study, regular forest soil testing should be carried out.

The forest soils in central and northern Poland were characterised by a highly acidic pH_CaCl2 value of 2.75–4.60, with the lowest value measured under Pinus sylvestris L. stands in the Krucz forest area. The influence of soil pH on some properties of forest soils was demonstrated, which was confirmed by negative correlation coefficients between pH and TOC (r = –609***), TN (r = –0.599***) and CEC (r = –0.424**). Positive correlations were also found between pH and parameters such as SBC : Al (r = 0.570***), Ca : Al (r = 0.506***) and BS (r = 0.602***). The positive Spearman rank coefficient between TOC content and the SBC : Al ratio (r = 0.618***) may indicate the important role of organic matter in protecting forest ecosystems from the toxic effects of aluminium on forest habitats and stands. To maintain appropriate soil chemical indicators, soil organic matter should be monitored at least once per decade.