The impact of cover crops on soil quality

One of the fundamental problems in modern agriculture is the deterioration of arable soil quality due to excessive compaction. Determining bulk density is a method for assessing soil compaction and, consequently, its quality (Blanco-Canqui, 2020). This parameter defines the ratio of soil mass to its total volume and is an important physical property used to determine soil porosity and water characteristics. Information on bulk density is also utilized as an input parameter in models for water and nutrient transport. Moreover, bulk density is essential for accurate estimates of soil organic carbon stocks. Soils with high bulk density are characterized by reduced pore volume, which consequently limits the flow of water and air (Lal, Shukla, 2004). This leads to reduced availability of water and oxygen for plant roots, thereby negatively impacting their development and growth. Lower bulk density, apart from creating optimal conditions for plant development, positively influences the growth of microorganisms in the soil, thus promoting the formation of a stable soil structure (McKenzie, 1996).

Although most scientific studies indicate a positive effect of cover crops on soil bulk density (Breland, 1995; Terzoudi et al., 2007; Wiesmeier et al., 2015; Cerdà, 2018; Demir, Işık, 2019a; Demir et al., 2019; Cerdà et al., 2022), there are also studies in the literature that have shown no such effect (Sartori et al., 2022). The impact of cover crops on soil properties is complex and depends on many environmental factors such as seasonal changes in moisture and temperature, the type of cover crops used, and soil type (Hu et al., 2012). Studies conducted by Wiesmeier et al. (2015), Breland et al. (1995), Cerdà et al. (2022), and Sartori et al. (2022) demonstrated the positive effects of cover crops on soil microbial activity due to increased organic matter content (Blanco-Canqui et al., 2015). Microorganisms decomposing plant residues produce organic acids and polysaccharides, which promote the formation of stable macropores and soil aggregates. This process helps to reduce soil bulk density (Rorick, Kladivko, 2017; Koudahe et al., 2022). On the other hand, studies by Hubbard et al. (2013), Adeli et al. (2020) and Koudahe et al. (2022) emphasize that soil bulk density depends on the type of crop used. Research by Harasim et al. (2020) found that cover crops significantly modified this parameter. Mixed legume (Vicia faba, V. sativa) cultivation resulted in lower bulk density in the 5–10 cm soil layer (on average by 5%) compared to control plots without cover crops. The bulk density in the 15–20 cm soil layer was also lower in plots with cover crops compared to the control plots. A statistically significant reduction in bulk density was achieved with the cultivation of a mix of broad beans (Vicia faba) and common vetch (Vicia sativa) as well as lacy phacelia (Phacelia tanacetifolia). This effect occurs because plants with deep root systems, such as phacelia, loosen the soil structure, thereby increasing pore space (Villamil et al., 2006; Blanco-Canqui et al., 2015). As a result, soils demonstrate a greater capacity for water infiltration and storage as well as nutrient retention, which improves fertility and overall condition while promoting plant growth (Gentsch et al., 2024). In other studies conducted in Italy bulk density was measured at different depths and analyzed the impact of various tillage systems combined with cover crop cultivation, including oilseed radish (Raphanus sativus) and winter wheat (Triticum aestivum). In this case, cover crops did not significantly affect bulk density in any of the tested combinations. The lack of significant impact may be due to the short duration of cover crop cultivation (Sartori et al., 2022). Short-term use of cover crops may not induce substantial changes; however, as highlighted by Blanco-Canqui et al. (2011), long-term application of cover crop can mitigate soil compaction effects, particularly in the upper soil layers, thereby improving conditions for crop growth. The absence of effects may also be related to environmental conditions, soil type, and the type of cover crops used. For instance, in heavy clay soils, the roots of cover crops may struggle to penetrate, limiting their influence on soil structure. Therefore, the selection of cover crop species should be adapted to soil type. For light soils, species such as yellow lupine (Lupinus luteus), serradella (Ornithopus sativus), and phacelia are recommended. On medium-textured soils, sunflower (Helianthus annuus), narrow-leafed lupine (Lupinus angustifolius), rapeseed (Brassica napus), buckwheat (Fagopyrum esculentum), oilseed radish, and stubble turnip (Brassica rapa) are advised. For heavy soils, broad beans (Vicia faba) and common vetch are more suitable (Pikuła; https://www.tygodnik...). Gentsch et al. (2024) found that plants like rye and clover, with finer root systems, have a greater capacity for creating pore spaces in the soil, which in turn increases soil resistance to erosion. In contrast, Blanco-Canqui et al. (2015) observed that the deep roots of many cover crops, such as oilseed radish and alfalfa, loosen compacted soil, which is particularly significant in heavy soils such as clays and silts. The selection of cover crop species with appropriate root traits is therefore critical for improving soil health.

Moreover, under conditions of very intensive agricultural exploitation, the beneficial effects of cover crops can be counteracted by other farming practices (Sartori et al., 2022).

In conclusion, cover crops have the potential to reduce soil bulk density and improve its structure, especially with long-term use. In contrast, their short-term application, as well as specific soil and climatic conditions where other factors dominate their effectiveness, may limit their potential impact.

Soil aggregates are the fundamental structural units of soil, formed during the decomposition of organic matter. In this process, microorganisms breaking down organic matter produce substances (mucus, polysaccharides, humic acids and proteins) that bind soil mineral particles, such as sand, silt, and clay, into larger structural units (Jouquet et al., 2006; Guhra et al., 2022). Structurally stable soil, with improved porosity, has a greater capacity for water absorption and retention, which increases its availability to plants while promoting air circulation, facilitating root respiration (Franzluebbers, 2002; An et al., 2010). Soil with stable aggregates is more resistant to disintegration caused by rainfall and surface runoff, reducing erosion risk. Furthermore, proper aggregation positively influences nutrient accumulation and microbial activity in soil (Sekaran et al., 2021). Soil structure is thus essential for both environmental protection and soil fertility (Obalum et al., 2013). As highlighted by authors, soil structure is critical to soil health and productivity, making the enhancement of aggregate stability an effective method for improving soil quality and addressing environmental issues resulting from soil degradation.

In recent decades, soil degradation has commonly been observed as a result of shifts in aggregate size distribution towards smaller aggregate classes, leading to reduction of their stability (Boix-Fayos et al., 2001). This degradation has been attributed to increased tillage intensity, monoculture practices, and mineral fertilization (Williams, Petticrew, 2009) which result in the loss of organic matter and deterioration of biological functions of the soil (Obalum et al., 2017). Restoring proper soil structure has therefore become a critical component of soil conservation strategies in sustainable agriculture (Williams, Petticrew, 2009; Lal, 2015; Obalum et al., 2017). The effect of cover crops on soil aggregate stability, particularly in the context of soil conservation, structural improvement, and increased erosion resistance, is a topic widely analyzed in scientific literature (Breland, 1995; Velykis et al., 2014; Wiesmeier et al., 2015; Cerdà et al., 2018; Garcia-Gonzalez et al., 2018; Harasim et al., 2020). Studies confirm that cover crops can be an effective tool for restoring healthy soil ecosystems (Williams, Petticrew, 2009; Lal, 2015; Obalum et al., 2017), which is crucial for sustainable soil management in the face of climate change (Six, Paustian, 2014).

The root systems of cover crops play a particularly important role in maintaining proper soil structure. Extensive and branched root systems penetrate deeply into the soil, creating a network that stabilizes soil structure. This process increases the soil’s microporosity and macroporosity, improving its ability to infiltrate and retain water while also facilitating air circulation (Blanco-Canqui et al., 2015; Gentsch et al., 2024). Additionally, cover crop roots release organic substances, acting as a “natural glue” that binds soil particles, thereby enhancing their stability. These root secretions also positively influence the development of microorganisms (bacteria and fungi), which further strengthen aggregates by forming organic connections between soil particles. Moreover, the decomposition of root residues after the growth period of cover crops adds organic matter to the soil, improving its structure over the long term (Kavdir, Smucker, 2005).

The positive impact of root systems on aggregate stability was confirmed by Hudek et al. (2022). In a greenhouse experiment, the authors studied the effects of root traits of seven different cover crop species (oats, rye, buckwheat, vetch, radish, mustard, and phacelia) on improving soil physical properties. The authors emphasized that the positive impact on macroporosity and aggregate stability during cover crop growth depended on the morphology of the individual plant root systems. The results showed that total root length and root surface area strongly correlated with aggregate stability and soil macroporosity. Buckwheat, mustard, and rye increased aggregate stability and microporosity at the interface with the compacted layer by 10, 8, and 7%, respectively, compared to the bare soil control. Moreover, average root diameter negatively correlated with soil macroporosity, indicating that cover crops with finer root systems are more beneficial for pore formation than those with thicker taproots.

Climate change has increased the frequency of extreme weather events, such as drought, which has become a critical factor limiting agricultural production (Choat et al., 2012; Bu et al., 2018; Sun et al., 2020). In this context, the ability of soil to retain water is essential not only for plant survival during water scarcity but also for improving nutrient use efficiency by plants and reducing erosion. Thus, soil water retention is a crucial factor influencing agricultural production and the sustainable use of agricultural lands (Blanco-Canqui, Ruis, 2020).

Literature reports indicate that cover crops can serve as a nature-based solution to enhance soil water retention capacity (Franzluebbers, 2002; Thorup-Kristensen et al., 2003; Blanco-Canqui et al., 2015; Garcia-Gonzalez, 2018; Bacq-Labreuil et al., 2019). Cover crops can improve soil field capacity by enhancing soil structure. Their root systems create a network of channels in the soil, increasing porosity and allowing better water infiltration into deeper soil layers (Franzluebbers, 2002; Basche, DeLonge, 2017). This also reduces surface runoff and water erosion (Basche et al., 2016). Consequently, soil water balance is maintained, which is particularly beneficial in drought-prone regions (Lal, 2015; Blanco-Canqui, Ruis, 2020). Studies by Garcia-Gonzalez et al. (2018) demonstrated that using vetch and oats as cover crops increased soil field capacity in the 0–25 cm layer by 8% and 13%, respectively. Research by Haruna and Nkongolo (2015) and Bacq-Labreuil et al. (2019) showed that cover crops such as white mustard and lacy phacelia significantly increased soil moisture in the topsoil layers (0–10 cm), which is particularly beneficial for cereal crops. Similarly, in experiments by Harasim et al. (2020), white mustard significantly increased soil moisture content (in volumetric terms) in the 5–10 cm layer compared to control plots. Other cover crops, such as oats and phacelia, caused only minor increases in soil moisture in the 5–10 cm layer. Additionally, crops like rye and oats were found to promote a more uniform distribution of water within the soil profile (Kaspar, Singer, 2011). Wang et al. (2015) emphasized that cover crops, particularly those with deep root systems, are capable of retaining water over extended periods. Other studies by Thorup-Kristensen et al. (2003) showed that introducing cover crops can significantly increase soil organic matter content. Elevated organic matter levels reduce the risk of soil drying, which is particularly important for light, permeable soils (Jiang et al., 2020). Harasim et al. (2020) also observed that leaving cover crop residues on the field as mulch increased soil moisture by reducing surface runoff, enhancing infiltration, and decreasing evaporation. In a study conducted in Serbia, Krstić et al. (2018) demonstrated that the cultivation of cover crops can lead to reduced soil moisture content during the spring period. In this research, cover crops such as common vetch (Vicia sativa) and triticale (× Triticosecale) decreased water availability for silage maize, which negatively affected its yield. The authors suggest that under conditions of limited precipitation, cover crops may compete with main crops for water resources, potentially leading to yield reductions.

In conclusion, the use of cover crops is an effective method for improving soil water retention, which is particularly relevant in the context of climate change and the need for sustainable water resource management in agriculture.

One of the primary indicators of soil quality that influences its productivity is pH (Havlin et al., 2014). Literature data indicate that pH affects the physical, chemical, and biological properties of soil. It influences soil structure through its effect on soil colloids and aggregates. In acidic soils, dominated by H⁺ and Al³⁺ ions, aggregates tend to break down (Oades, 1984). Soils with excessively low pH also exhibit reduced phosphorus availability to plants, as phosphorus forms insoluble compounds with aluminum and iron. Also in alkaline soils, phosphorus binds with calcium, similarly becoming less available (Penn, Camberato, 2019). Additionally, pH affects the conditions for soil microorganisms involved in organic matter decomposition. Soil Acidification may limit bacterial activity, altering microbial balance and influencing biogeochemical processes (Rousk et al., 2010).

Cover crops play an important role in soil quality management, including shaping its pH. However, this impact varies and depends on many factors, including the species of cover crops (Feng et al., 2021). In research conducted by Demir (2020), in which cover crop treatments significantly reduced soil pH from 7.48 (in the control plot) to 6.92 for hairy vetch (Vicia villosa) at a depth of 0–20 cm, and from 7.46 (control) to 6.90 in the case of white clover (Trifolium repens L.), also at a depth of 0–20 cm. Similarly, the use of cover crops significantly decreased soil pH (Demir, Işık, 2019b; Demir, Işık, 2019c; Demir et al., 2019), which has been attributed to the presence of acidic root exudates that may affect the availability of nutrients in the rhizosphere. According to information available in the literature, acidification of soil by cover crops usually results from an imbalance between the carbon and nitrogen cycles (Vanzolini et al., 2017). Additionally, the uptake of cations and anions by plants from the soil solution to meet growth demands alters ionic equilibrium. In most cases, plants absorb more cations than anions, leading to the release of protons into the soil to maintain charge balance. Furthermore, during the biological fixation of atmospheric nitrogen (N₂) by legumes, proton excretion by roots is associated with the accumulation of organic anions in plant tissues. The decomposition of these anions by soil microorganisms can lead to an decrease in soil pH. Some authors suggest that soil acidification may also result from the accumulation of fresh organic matter (Mengel, Steffens, 1982). The decomposition of plant residues from species producing large biomass quantities, such as rye or rapeseed, releases organic acids, which lower soil pH (Gentsch et al., 2024). Conversely, the incorporation of residues from cover crops like buckwheat, characterized by a high calcium content in their tissues, can neutralize or raise soil pH (Khan et al., 2021).

Soil organic matter (SOM) is a critical component for maintaining soil health. It influences various physical, chemical, and biological soil properties, such as structure, compaction, sorption capacity, nutrient availability for plants, pH, and microbial activity, all of which directly affect crop yields (Dexter, 2004; Van Groenigen et al., 2017). SOM has a high water-holding capacity, which is particularly important during drought periods. Additionally, SOM contributes to mitigating climate change by reducing carbon dioxide emissions into the atmosphere (Thorup-Kristensen et al., 2003).

The incorporation of cover crop biomass into the soil is an effective method for enhancing SOM in the long term (Lu et al., 2000). Cover crops supply fresh organic matter to the soil through the decomposition of root biomass and aboveground plant parts. These effects are often compared to the fertilizing effects of manure, with the impact of cover crop biomass on soil properties potentially lasting for several years (Reddy et al., 2003; Sarrantonio, Gallandt, 2003). Studies have shown that different cover crop species, such as rye and clover, can increase soil organic carbon (SOC) levels by stimulating microbial and fungal activity in the soil (Khan et al., 2021; Feng et al., 2021). The biomass serves as a substrate for microorganisms, which convert it into stable forms of organic carbon. Increased SOC content due to cover crop use has also been observed in studies by Demir et al., (2019); Ding et al. (2006), and Köpke, Nemecek (2010), Ramos et al. (2018), Cerdà et al. (2022). For instance, Demir et al. (2019) demonstrated that the application of hairy vetch (Vicia villosa) and Hungarian vetch (Vicia pannonica) increased the soil organic matter (SOM) content in clay soil from 14.9 g kg⁻¹ to 24.4 g kg⁻¹ in an apricot orchard located in the Malatya Province of Türkiye. In a study conducted in Spain, the use of vetch and oats as cover crops increased SOC content from 5.4 g kg⁻¹ (in bare soil) to 8.4 g kg⁻¹ and 9 g kg⁻¹, respectively (Ramos et al., 2018). Similarly, Cerdà et al. (2022) confirmed these findings, highlighting the effectiveness of vetch and oats in enhancing SOM. The impact of broad beans as a cover crop on SOM was discussed by Köpke and Nemecek (2010), who emphasized that broad bean residues, rich in nitrogen, enrich SOM upon decomposition, improving soil structure and fertility. Incorporating broad beans into crop rotations contributes to long-term SOM increases, positively impacting soil productivity. Positive outcomes of cover crop use were also reported in studies conducted on degraded soils in Moldova. The application of a mix of hairy vetch and winter wheat increased SOC in the topsoil layer from 11.3 to 11.6 kg m⁻² in experiments located in Orhei and from 10.2 to 10.5 kg m⁻² in experiments in Cahul. Researchers noted that using this mix as a cover crop and green manure resulted in carbon sequestration of 3 t C ha⁻¹ yr⁻¹, indicating the potential of cover crops to mitigate climate change (Wiesmeier et al., 2015). In contrast, studies by Yang et al. (2004) and Abdollahi and Munkholm (2014) showed no significant changes in organic matter content after the application of cover crops.

By increasing soil organic matter content, improving soil fertility and structure, and enhancing nutrient availability, cover crops contribute to higher yields in subsequent crops. Cover crop cultivation is an environmentally friendly practice that provides numerous benefits for both soil and agriculture.

Nitrogen (N) and phosphorus (P) are essential nutrients for plant growth and development, but their excessive use in intensive agriculture often leads to environmental degradation. The leaching of nitrogen and phosphorus from soils into groundwater and surface waters contributes to eutrophication, resulting in reduced biodiversity and impaired ecosystem functioning (Carpenter et al., 1998). Effective retention of these nutrients in the soil is therefore critical both for environmental protection and for improving soil fertility.

One effective strategy for nutrient retention is the implementation of appropriate agricultural practices. Cover crops play a significant role in nitrogen retention, particularly during fallow periods when the risk of nutrient leaching is high (Sapkota et al., 2012). They act as buffers, providing an effective solution to prevent nutrient loss (Askegaard, Eriksen, 2008). As such, they are considered an important element of sustainable agriculture. Cover crops are especially valued by organic farmers, who often use them as green manures to enhance nitrogen availability for subsequent crops (Komainda et al., 2016). By accumulating nitrogen in both aboveground and belowground biomass, cover crops reduce nutrient losses, thereby minimizing groundwater and surface water pollution (Tonitto et al., 2006; Cerdà et al., 2022). Leguminous cover crops, such as vetch and clover, are particularly noteworthy as they fix atmospheric nitrogen through symbiosis with rhizobia. After the growing cycle, the nitrogen accumulated in the biomass is released into the soil, increasing its availability for subsequent crops (Thorup-Kristensen et al., 2003). This was confirmed by Wiesmeier et al. (2015) in their studies on degraded steppe soils in Moldova. Their experiments demonstrated that hairy vetch (Vicia villosa), used as green manure, significantly enriched the soil with nitrogen. This effect was attributed to the ability of hairy vetch to fix atmospheric nitrogen, with up to 80% of the nitrogen introduced into the soil originating from this process. The increased nitrogen content improved soil fertility, leading to higher yields of subsequent crops: sunflower yields increased by 22% in Orhei, and maize yields by 18% in Cahul. In Poland, studies conducted by the Małopolska Agricultural Advisory Center in Karniowice found that the use of vetch as a cover crop in zucchini and tomato cultivation under covers supplied 252 kg N ha⁻¹. Pea crops provided 155 N ha⁻¹, while mixtures of vetch with rye and peas with oats accumulated 173 kg N ha⁻¹ and 136 kg N ha⁻¹ of organic nitrogen, respectively (Domagała-Świątkiewicz, Siwek; https://modr.pl/technologie...). Long-term field experiments in France by Constantin et al. (2010) evaluated various agricultural practices in terms of their impact on nitrogen balance and nitrate leaching. These studies confirmed that cover crops effectively reduced nitrate losses, contributing to improved nitrogen management in agricultural systems.

Phosphorus losses, on the other hand, are primarily caused by surface runoff and soil erosion. Cover crops, with their extensive root systems, improve soil structure, which aids in phosphorus retention and its efficient use by cash crops. The decomposition of cover crop biomass gradually releases phosphorus over time, integrating it into the nutrient cycle (Sarrantonio, Gallandt, 2003). Sartori et al. (2022) found that in conservation agriculture systems that combine the use of cover crops with reduced tillage intensity, phosphorus is retained more effectively in the soil. This improves the overall phosphorus balance in the soil and minimizes its losses to the environment, contributing to the protection of surface water quality.

Soil microorganisms are an integral component of a healthy soil ecosystem. Their diversity and activity are essential for soil fertility, plant growth, and environmental protection. Microorganisms contribute to the decomposition of organic matter, the formation of soil aggregates, nutrient availability for plants, and protection against pathogens. The interaction between microorganisms and higher plants maintains equilibrium in soil environments, and any changes in the physical and chemical properties of the soil can disrupt this balance (Hayat et al., 2010; Wang et al., 2024).

The level of enzymatic activity in the soil, particularly dehydrogenase activity, provides insights into the soil environment and serves as an indicator of its fertility and productivity. Factors such as soil pH, water-air relations, and organic matter content—shaped by farming systems and the type of fertilizers applied—play a crucial role in determining microbial populations and enzymatic activity in the soil. Supporting microbial biodiversity through sustainable agricultural practices is essential for the long-term productivity and health of soils (Bielińska, Mocek, 2003).

Although relatively few studies in Europe have investigated the effects of cover crops on microbial biodiversity, existing data indicate that sustainable farming practices, including the use of cover crops, play a significant role in ensuring the long-term productivity and health of soils. Research suggests that cover crops can substantially influence the diversity and activity of soil microorganisms, thereby enhancing soil quality. Cerdà et al. (2022) demonstrated that the use of vetch and oats as cover crops increased microbial and fungal activity, which positively affected soil organic carbon content. Cover crops enrich soils with organic matter, supporting nutrient cycling, particularly nitrogen, which, in turn, enhances microbial activity (Quintarelli et al., 2022; Schön et al., 2024). In a study conducted by Harasim et al. (2020), it was demonstrated that the use of white mustard (Sinapis alba) as a catch crop significantly increased the activity of dehydrogenase and urease enzymes in various soil layers. The control treatments (without a catch crop) exhibited the lowest enzymatic activity, both in the 5–10 cm and 15–20 cm soil layers. In turn, research by Pięta and Kęsik (2006) conducted in Poland revealed that the cultivation of spring rye (Secale cereale) and common vetch (Vicia sativa) as cover crops may lead to an increased presence of soil-borne pathogenic fungi, including Fusarium culmorum, F. oxysporum, F. solani, Penicillium verrucosum, and Pythium irregulare. These pathogens contributed to poor emergence and compromised seedling health of onion (Allium cepa).