*1.1. The Process of Setting Aside/Fallowing Land in the Political and Environmental Context*

The time of political transformations that took place in Poland in the 1980s and 1990s significantly contributed to changes in land use [1]. The process of agricultural land abandonment on large scale became visible then. Besides the economic effects resulting from abandoning agricultural activity, also structural and functional transformations of landscape units took place [2,3]. The first visible effect of land abandonment is regrowth of vegetation through natural secondary succession [4]. In many regions of the country, as a result of land use discontinuation and succession of natural vegetation, a significant part of the agricultural plots became permanently covered with trees and bushes. At the same time, regional nature of this process became apparent [5]. It is difficult to assess unequivocally whether it is a negative or a positive phenomenon, it depends mainly on the local environmental, political, or social conditions. In some cases, such a change may result in restoration of the old ecosystem or emergence of completely new landscape or utility functions [6], and in the case of mountain areas, it may also affect the functioning of valley ecosystems [7]. Currently, in order to identify abandoned areas, a number of

**Citation:** Kozak, M.; Pudełko, R. Impact Assessment of the Long-Term Fallowed Land on Agricultural Soils and the Possibility of Their Return to Agriculture. *Agriculture* **2021**, *11*, 148. https://doi.org/10.3390/ agriculture11020148

Academic Editor: Mariusz J. Stolarski

Received: 31 December 2020 Accepted: 8 February 2021 Published: 11 February 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

remote sensing methods are being developed, using satellite photos or Airborne Laser Scanning (ALS), which allows for precise identification of the size of areas covered with high vegetation and the dynamics of changes over time [8]. According to some researchers, tree stands resulting from spontaneous succession are much more abundant in elements of environmental value than conifer monoculture stands [3]. Natural succession also creates ecological corridors, prevents soil erosion thanks to vegetation cover, increases carbon sequestration in the soil, and can be a stronghold of biodiversity in intensively utilized agriculturally areas [9,10]. On the other hand, uncontrolled succession may pose a threat to the biodiversity in open areas and the species occurring there, e.g., by the invasive plants entering those areas, causing the loss of cultural landscapes [6,11]. Invasive plants have a negative effect on native species, not only by displacing them from their growing area, but also by modifying soil conditions [12]. For example, in Poland, we can observe how the goldenrod species (*Solidago L.*) enters after the segetal phase of natural succession as an invasive plant, creating dense fields [13]. Just as it is difficult to unequivocally assess the process of farmland abandonment, so are the decisions on how to manage it. Bell [14] identified three main categories of strategies that are undertaken in the process of abandoned land management in relation to its new functions:


The researchers also admit that implementing the most appropriate post-abandonment strategy will be based on a number of variables. The aforementioned production function related to the acquisition of biomass has recently gained particular importance in formulating bioeconomy development strategies. The research shows that the cultivation of perennial industrial plants on marginal lands can represent a significant potential in obtaining biomass [15–19].

In addition to the many challenges associated with this issue, some opportunities were also recognized for the sustainable management of fallow land to compensate for the consequences of climate change. This is because soils, in addition to the basic functions of food production and ensuring food security, also provide ecosystem services that are necessary for the functioning and resilience of the environment on Earth. The main nonagricultural functions are: (i) Storage of massive amounts of carbon, which helps to regulate CO2 emissions and shape climate processes; (ii) functioning as the largest water filter and storage tank on Earth, which ensures control of its circulation, retention, and quality of freshwater resources; and (iii) storage of nitrogen, phosphorus, and other essential nutrients [20].

#### *1.2. Testing Conditions of the Fallow Soil*

The possibility of carbon sequestration in soil was and still is widely discussed in many publications, also in the context of abandoned agricultural land. Among other things, the effect of converting agricultural land into other forms of land use (arable land to grassland, arable land to abandoned agricultural land, and arable land to afforested land), and the possibility of SOC sequestration in the topsoil were investigated. Kazlauskaite-Jadzevice et al. [21] showed, among others, that carbon sequestration in the Arenosol soil layer was positively influenced by long-term fallowing and transformation into grassland. Abandoned land or fertilized grassland accumulated significantly more CO2 (48% and 38%—respectively), compared to arable land. Whereas the potential of "mature" forest succession in terms of CO2 sequestration was confirmed by the studies conducted by Foote and Grogan [22], showing that the total contents of organic carbon and nitrogen in soil at a depth of 10 cm were lower in arable fields compared to forests with secondary succession, by about 32% and 18%, respectively.

Studies on the impact of the natural succession diversity on the storage capacity and rate of C accumulation in soil over a period of two decades were conducted by Yang et al. [23]. The authors concluded that the annual rate of carbon storage was higher in the second period of the study (years 13–22), in the same time suggesting that restoring high plant diversity could significantly increase carbon capture and storage on degraded and abandoned agricultural land. The impact of afforestation and deforestation on soil carbon content was also investigated [24]. The impact of land use change in Mediterranean areas on the processes of co-carbonization, decarbonization, and recarbonization was taken up by researchers Lozano-García et al. [25], anticipating at the same time the possibility of soil regeneration and climate change. The topic of reclamation of degraded agricultural land by changing the variety of vegetation and restoring organic matter was also discussed by researchers Zhang et al. [26].

In publications concerning fallow land, the influence of land use change on physical and chemical properties of soils is often discussed. The negative influence of relatively young (5–10 years) fallow land on the properties of the soil environment was found by Str ˛aczy ´nska et al. [27]. It was manifested in the unfavorable change in pH and reduction of humus resources in silty and clay soils, while in light soils, the humus content slightly increased. Moreover, Tomaszewicz and Chudecka [28] did not find any enrichment of fallow rusty soils with humus either. At the same time, fallow soil was characterized by a lower content of plant-available forms of magnesium, potassium, and phosphorus. On the other hand, the positive effect of setting aside on the properties of the soil environment was observed by Włodek et al. [29]. In their research, the authors observed that after several years of excluding the field from agricultural production, there was a significant increase in the content of carbon, phosphorus, potassium, and magnesium in the soil. In the study of the physicochemical properties of soils, a detailed analysis of changes in the quantitative and qualitative composition of humus compounds is of key importance. Based on their research results, Licznar et al. [30] concluded that the fractional composition of humus compounds in fallow soils shows strong relationships with their physicochemical properties. They also noticed that in set-aside light soils, the process of organic matter accumulation occurs, thus showing a lower degree of humification than in a cultivated soil.

The results presented in this paper were obtained as part of the BioMagic [31] project, whose main objective is to develop bioproducts from lignocellulosic biomass obtained from marginal soils to fill the gap in the national bioeconomy. On the selected fallow land, meeting the marginality criteria defined in the project, physicochemical tests of soil properties were carried out, the results of which were then compared with the neighboring utilized agricultural lands. The main aim of the study is to answer the question, whether it is possible to restore weak and marginal soils to biomass production after a long-term abandonment. The obtained results, within the BioMagic project, were also used to estimate the technical and economic potential of fallow soils to be used for the production of perennial industrial plants.

#### **2. Material and Methods**

#### *2.1. Main Assumptions*

The following research hypothesis was assumed: Fallowing process does not cause a significant deterioration of soil conditions, which would be a problem when they are returned to agricultural production. An alternative to the research hypothesis is to show that long-term set aside affects the soil in such a negative way that its restoration to agricultural production requires expensive agrotechnical treatments. The following properties were considered essential for soil fertility: Carbon content in the topsoil, soil pH, phosphorus, and potassium content. The nitrogen content in the soil was not analyzed, because this element is not stable (its content changes rapidly during vegetation period, especially in case of intensive agricultural production).

In this study, it was assumed that the necessity to use "expensive treatments" to restore the soil to agricultural production will occur if the tested soil parameters for fallowed land deteriorate so that the ranges adopted in the determination of soil fertility in Poland will be exceeded twice. In the case of soil carbon, it is a 1.7% decrease in its content (compared to the content determined for the sample from arable land). For pH, it is an increase in soil acidity by two units. In the case of phosphorus and potassium, it is a reduction of 10 mg per 100 g of soil (forms: P2O5 and K2O).

In order to assess the impact of fallowing on soil conditions, the following work was carried out:


#### *2.2. Area of Research*

The research was carried out in the area of Puławy municipality (gmina), Local Administrstive Unit (LAU code:1006061121409) according to the Eurostat nomenclature, located in the north-western part of the Lubelskie Voivodeship (NUTS-2: PL81). This municipality is directly adjacent to the city of Puławy and constitutes its suburban area, which is visible, among others, in employment the structure: In 97% of farms, at least 1 person has an additional non-agricultural source of income [32]. The economic situation and the large fragmentation of farms in this region determine the high percentage of fallow land in the agricultural landscape.

For soil sampling 17 sites were chosen. Research area and locations of sampling plots were shown on the Figure 1. The article's supplement includes a download link of the KLM file, which contains the location of the sampling sites, enabling their identification in the Google Maps/Earth application, which visualizes this region on high-resolution orthophotomaps (like in the right picture on Figure 1).

**Figure 1.** Research area—locations of sampling sites.

*2.3. Spatial Data Collection*

In the study, the following types of spatial data were used:


• Geo-referenced photos taken in-situ during visits at the selected sites (for examples, see Section 2.4).

**Figure 2.** Time sequence of aerial photographs, showing the approximate moment of the entry of natural succession (1997, 2006, 2010, after 2017). Source: GUGIK [33].

The above data fed the geographic information system, which was built in the QGIS (open source environment).

#### *2.4. Definition of Fallow Types*

Due to the diversity of natural succession within the selected plots, the investigated areas of fallow plots have been divided into:

• Grassland (FGL)—mostly newly abandoned agricultural land, possibly fallow land, with a predominance of grassland vegetation, with only a few plantings of later succession, e.g., goldenrod (*Solidago* L.) (Figure 3).

**Figure 3.** Example of photos on-site No. 2 (left side) and No. 5 (right side).

• Goldenrod (FG)—areas with a predominance of plants of later succession stages, mainly goldenrod (*Solidago* L.), tansy (*Tanacetum vulgare* L.). Criterion: Over 80% share of goldenrod or tansy in land cover (Figure 4).

**Figure 4.** Example of photos on-site No. 1 (left side) and No. 16 (right side).

• Bushy (FB)—areas where apart from ruderal plants, such as goldenrod (*Solidago* L.), there are bushes, e.g., in the form of blackberries (*Rubus* L.), blackthorn (*Prunus spinosa* L.) and single self-seeded trees. Criterion: Over 30% share of bushes in land cover (Figure 5).

**Figure 5.** Example of photo on-site No. 8.

• Wooded/afforested (FW)—areas with trees, dense shrubs, advanced succession. Criterion: Samples were taken in places where young forest covered at least 0.10 ha (Figure 6).

**Figure 6.** Example of photos on-site No. 10 (left side) and No. 14 (right side).
