Changes of vegetation effects in soil properties in the post-agriculture landscapes (south-eastern Poland)

Plant succession is a sequence, or orderly corollary of plant communities, both in space and in time (Falińska 1997). This theory was formulated by Clements (1928) at the beginning of the 20th century, and was significantly developed and refined by other researchers (Trojan 1977; Odum 1982).

According to Clements (1928), plant succession is a change of species combination on an area that was not previously occupied by vegetation (a primary succession), or on an area that was previously settled by vegetation whose structure was demolished or destroyed (a secondary succession). There are three main stages of succession. In the first, a free space appears, which initiates the processes of a primary succession (e.g. dunes) or secondary succession (e.g. arable lands). The second stage is mainly environmental conditions being modified by the growth of vegetation. At this stage, the observed changes appear first and foremost in the chemical and physical properties of soils (Li et al. 2013). The last stage is determined as a final stadium or “climax”. The community stabilises, which in certain climatic conditions can take time, and it also expands (eds. Glenn-Lewin et al. 1992).

Plant succession is not only a process of change in species combination, but also one that modifies soil. This is related to a “closing” of the circulation of both matter and energy in the landscape (Richling & Solon 2011). This is manifested by a clear increase in total biomass, which then decomposes and leads to changes in the geochemical properties of soils (Sosnowska 2012).

Secondary plant succession is a fairly common occurrence in all regions of the world (Kennard 2002; Ruprecht 2005; Cojzer et al. 2014; Derroire 2016; Hahn et al. 2017). It is a consequence of long-term intensive land use and appears both in urban (Gantes et al. 2014) and agricultural areas (Vesterdal et al. 2002). Due to intensifying economic and demographic processes, plant succession phenomenon will certainly increase in the near future (Piussi & Pettenela 2000).

Ecosystems that have been transformed by humans as a result of intensive agricultural use and then abandoned tend to change towards natural forest or grassland communities. Initially, in the first stage of succession, species of herbaceous (segetal plants) appear (Kinhal & Parthasarathy 2008; Sojneková & Chitrý 2015), and over time are displaced by other species, while trees arrive at the end (Prach et al. 2014). In Central Europe, the entry of trees onto fallow lands is a long-term phenomenon. Forest returns to abandoned dunes within about 60 years (Rahmonov 2007), while for post-agricultural lands this may take from 140 to 290 years (Faliński 1991). However, the first tree stands are formed after around 20 years. They are usually composed of pioneer tree species (e.g. silver birch tree) that prepare the soil for the entry of appropriate species (Jõgste at al. 2003; Bijak et al. 2014).

The intensive development of vegetation leads to the transformation of the physical and chemical properties of soils (Kosmas et al. 2000, Harasimiuk 2008, Hall et al. 2017). The mechanisms by which soils are transformed, especially poorly fertile soils, have been quite well established and investigated in the literature (Smal et al. 2004; Sosnowska 2013). Fertile loess soils are rarely abandoned, hence studies relating to these in the literature are fewer, and mainly come from China (Zhang et al. 2012; Li et al. 2013; Wang et al. 2018).

The secondary succession process on agricultural areas in Poland is very common and has been observed for a long time (Szwagrzyk 2004). It can be seen in an increase in the area of ​abandoned lands, especially at the turn of the 20th century, and Poland therefore seems to be a representative area for conducting research on soil transformation as a result of abandonment. The increase in post-agricultural lands in Poland was caused by: a collapse of the state sector of agriculture; extensive crop production; the diversification of farms in terms of level of development, size and direction of production; and a steep fall in cattle and sheep populations (Węcławowicz et al. 2006). At the beginning of the 21st century, abandoned arable lands occupied 1,289,000 ha, i.e. almost one tenth of Poland’s agricultural land (Statistical Yearbook of Agriculture 2010). After Poland’s accession to the European Union, a lot of wasteland was re-developed, which contributed to a decline in the area of ​set-aside and fallow lands. In recent years these leveled out at around 500,000 ha (Statistical Yearbook of Agriculture 2013), but decreased further in 2016 (Statistical Yearbook of Agriculture 2016).

The aim of this article is to determine changes occurring in vegetation – and what impact those changes have on the transformation of the properties of loess soils after arable lands are abandoned. The additional goal is to indicate changes in soils properties as the vegetation develops, and to conclude whether, 20 years after abandonment, soils are more similar to those under forest than those under arable land.

The research area was located in south-eastern Poland, in the Lublin Upland (Kondracki 2009). This area is east of the Wieprz river valley and the town of Krasnystaw, near the village of Chełmiec. Geologically, it is a hummock of approx. 1,283 ha built mainly of Upper Cretaceous limestone rocks and marls. These rocks are covered with loess of varied thickness (6–12 m) (Jahn 1956). Luvisols and Cambisols (brown soils) dominate here. They cover about 35% of this region (Turski et al. 1993).

Due to the ages of human activity, the vegetation of this area has been thoroughly transformed. Forest and xerothermic grasslands have been replaced by fields and meadows, and segetal and ruderal plants became dominant. Forest vegetation covers only a few percent of the area, mainly on steep slopes and in gorges (Baran-Zgłobicka et al. 2001).

Four polygons of different land use were selected for detailed research: an arable land, two post-agricultural lands at various stages of succession (five and twenty years after abandonment of agricultural use), and one forest (Fig. 1). All research areas are located very close to each other (Fig. 2).

Figure 1

In each research area, the most frequent plant species were determined based on the Braun-Blanquet method (1951). According to the methodological assumptions, plant species that cover from 75% to 100% of the area have been ascribed a frequency of 5, while those that occur on less than 5% have been ascribed a value of 1.

During field works, soil cover was also examined. Twelve soil pits were excavated to a depth of 150–180 cm – three for each type of land use. All soil pits were located along the line passing through the central part of each research area. The schematic location of soil pits is presented in Fig. 3. Soil samples were taken from all soil horizons, then dried, pulped and sieved in laboratory. Sampling, preparation for laboratory analyses and proper laboratory analyses were carried out in accordance with methodological guidelines (Bednarek et al. 2005; Jahn et al. 2006).

Figure 2

Figure 3

The laboratory analyses of soils include:

Soil type was determined based on the Polish Soil Classification (2011) and World reference base for soil resources (2015), while the particle-size distribution was determined according to Granular Classification 2008 (annex 3 in Polish Soil Classification).

According to the obtained results, the average values of parameters (Table 3) were determined.

This paper focuses on the secondary succession of vegetation occurring on post-agricultural lands, as well as on soil transformation resulting from the development of vegetation.

The abandonment of arable land leads to natural plants communities overgrowing its surface already in the first year (Harmer et al. 2001; Wolski 2007). In the initial phase, very expansive species of segetal and ruderal plants spread (Falińska 1997; Lencová & Prach 2011). In the early stages of succession, the plants’ life strategy consists primarily of producing numerous, and easily and widely disseminated seeds, which enables rapid colonisation (Bernadzki 1990). The pioneer species include silver birch (Betula pendula Roth). It appears in dense stands twenty

years after abandonment (Bernadzki & Kowalski 1983; Hynynen at al. 2010). The roots of the birch can also penetrate through the plow sole, leading to a relatively quick improvement in soil conditions (Harasimiuk 2013).

The obtained results are consistent with other researchers’ described above. The polygons where the research work was carried out are in various stages of secondary succession. Arable land is devoid of natural vegetation and subject to strong anthropopressure. In this area, there is no natural development of vegetation. Both types of post-agricultural lands are in the second phase of succession. Despite the fact that a compact forest grows on post-agricultural lands abandoned twenty years ago, the tree species (pioneers) that build it can be considered temporary.

The development of vegetation contributes to the growth of biomass, which undergoes slow decomposition, mainly by two groups of microorganisms: fungi and bacteria (Richards 1979). In arable and post-agricultural soils, cellulose that is a product of herbaceous plants’ decomposition is further decomposed, and bacteria are responsible for its distribution. Meanwhile, in woodlands it is lignin that is to be decomposed, and fungi are responsible (Tuszyński 1990; Berg 2000). Microbes play a key role in the carbon cycle and the nutrient cycle. A well-formed soil fauna needs 20–45 years after abandonment to develop (Szujecki 1996). As the organic carbon content in post-agricultural soils decreases in the first years of subsidence and then grows, it can be concluded that the soil fauna syndrome is not yet fully formed. Twenty years of abandonment is too short a time to fully develop the appropriate microbial communities, hence the slow decomposition of biomass (Drewnik et al. 2010). Based on the research, it was found that after five years of abandonment the ectohumus level is about 1–2 cm thick and maintains this volume in the following years.

Long-term, intensive mechanised soil cultivation leads to the mixing of upper soil levels and the production of a plow layer. The top horizons are usually very compact, with limited biological activity (Szujecki 1996), while fertilisation of arable land increases the pH value (Stojek 2005), thus changing the chemical properties of soils (ed. Gorzelak 1999). It was discovered during the research that plant succession leads to the transformation of soil profiles, although this phenomenon is much slower than the change in land cover. Post-agricultural soils are characterised by features that are similar to arable soils in terms of the construction of soil profiles: there is not the fully developed humus level that is characteristic for forest soils.

Abandonment also affects the chemical properties of soils, especially organic carbon content. After five years, a slight decline was recorded, and after twenty years an increase. A decrease as a result of abandonment was found in studies by Strączyńska & Zawieja (2001) and Sosnowska (2013). According to Łętowska & Strączyńska (2001), the decline appears only in the initial phase of abandonment, and after 10 years a slow increase is observed. Żukowska et al. (2007) suggest that the decrease in organic carbon content in post-agricultural soils may be the result of long-term, extensive tillage.

Change in pH value is one of the characteristics of transformations in soils that is most commonly used in research (Ritter et al. 2003). This is a consequence of this parameter’s significant sensitivity to changes. It was found in the research that the abandonment of agricultural use and the development of natural vegetation lead to a drop in the pH value of post-agricultural soils. The results obtained are consistent with the data provided by other authors (Kosmas et al. 2000; Smal et al. 2004; Harasimiuk 2008).

As a result of the decomposition of organic matter to the soil, nutrients are released that affect the properties of the sorption complex. The percentage of ions in the sorption complex varies greatly depending on land use. Organic and mineral fertilisers are supplied to arable soils, which results in an increase in the saturation with alkaline cations (Wang et al. 2001). In the present research, acid cations were predominant in forest soils, while alkaline cations predominated in the agricultural soils. Abandonment of arable lands affects the change of proportions, leading to an increase in acid nutrients in the sorption complex. The increase in saturation with acid cations has been also confirmed by Łętowska and Strączyńska (2001), Strączyńska and Zawieja (2001), and Braun (2011). In turn, Miklaszewski (1990) observed a decrease in acidity in set-aside soils. Changes in saturation of the sorption complex are relatively quick although, according to Ritter et al. (2003), after 30 years the post-agricultural soils are still closer to agricultural soils than to forest soils. The authors also emphasise the slight impact of the vegetation type (deciduous or coniferous species) on the rate of change, emphasising the importance of the method and intensity of past management.

It can be concluded from the research that plant succession on post-agricultural lands in loess landscapes occurs relatively quickly, starting immediately after abandonment. After just five years, the vegetation covers about 90% of the area. A well-developed woodland develops after more-or-less twenty years, and is characterised by the pioneer species (silver birch) that it comprises.

As a result of decomposing vegetation on post-agricultural land, an ectohumus (O) layer is formed – something that does not occur in arable land soils. It usually consists of poorly or medium distributed organic matter.

Post-agricultural soils have a profile layout similar to that occurring in arable soils. The humus level (Ap) is mixed with the underlying level. Clear changes in post-agricultural soil

Decomposing organic matter affects soil properties. The fastest-occurring changes are seen in the drop in pH value. Abandonment also leads to a slow increase in organic carbon content and saturation of acid cations in the soil sorption complex.