The Role of Cover Crops in Agriculture and Their Environmental Significance
Summary and Keywords
Growing a cover crop between main crops imitates natural ecosystems where the soil is continuously covered with vegetation. This is an important management practice in preserving soil nutrient resources and reducing nitrogen (N) losses to waters. Cover crops also provide other functions that are important for the resilience and long-term stability of cropping systems, such as reduced erosion, increased soil fertility, carbon sequestration, increased soil phosphorus (P) availability, and suppression of weeds and pathogens.
Much is known about how to use cover crops to reduce N leaching, for climates where there is a water surplus outside the growing season. Non-legume cover crops reduce N leaching by 20%–80% and legumes reduce it by, on average, 23%. There are both synergies and possible conflicts between different environmental and production aspects that should be considered when developing efficient and multifunctional cover crop systems, but contradictions about different functions provided by cover crops can sometimes be overcome with site-specific adaptation of measures. One example is cover crop effects on P losses. Cover crops reduce losses of total P, but extract soil P to available forms and may increase losses of dissolved P. How to use this effect to increase soil P availability on subtropical soils needs further studies. Knowledge and examples of how to maximize the positive effects of cover crops on cropping systems are improving, thereby increasing the sustainability of agriculture. One example is combined weed suppression in order to reduce dependence on herbicides or intensive mechanical treatment.
A Need for Cover Crops in Cropping Systems
Modern agriculture is characterised by a high degree of specialisation. On arable farms, annual cash crops follow each other, in monocultures or in crop rotations with few different crops. Cropping systems dominated by annual spring-sown crops often mean that there is no vegetation cover over winter. In climates where precipitation exceeds evapotranspiration during autumn and winter, this in combination with soil tillage in autumn is a main cause of leaching from the soil of nitrate originating from residual fertiliser and mineralization after incorporation of crop residues (Hansen & Djurhuus, 1997; Stenberg, Aronsson, Lindén, Rydberg, & Gustafson, 1999). The specialization of agriculture not only means low crop diversity on farms but also concentration of livestock production to farms and regions where there is net import of nutrients with animal feed. The manure produced and used on these farms results in increased soil fertility. The resulting long-term surplus of nutrients increases the risk of nitrogen (N) and phosphorus (P) losses to the environment (Carpenter et al., 1998). In contrast, crop production areas with low inputs of organic matter have decreasing soil fertility (Poeplau et al., 2011).
Many marine and freshwater systems are severely affected by eutrophication. One example is the Baltic Sea, a shallow inland sea, where almost half the riverine load of N and P originates from diffuse sources, of which agriculture constitutes 60% and 90%, respectively (Helcom, 2011). Nitrate contamination of groundwater is also a worldwide problem, often arising from nitrate leaching from agricultural areas (Spalding & Exner, 1993).
Growing a so-called cover crop between main crops is a way to make annual crop rotations more like natural ecosystems, where the soil is continuously covered with vegetation, which is the most effective management practice for preserving soil nutrient resources and reducing N losses to waters. In intensive livestock production systems, cover crops can be important buffers against accumulation of mineral N in the soil in autumn due to manure application and mineralization (Thomsen, Hansen, Kjellerup, & Christensen, 1993; Torstensson & Aronsson, 2000). Thereby, N that would otherwise have been exposed to leaching over winter is preserved in cover crop biomass. In systems with low inputs of N and organic materials, cover crops can be important for increasing soil organic matter content, in addition to reducing N leaching (Steenwerth & Belina, 2008) and, if N-fixing, adding N to the system. Cropping systems with cover crops supply several functions that can contribute significantly to the long-term sustainability and profitability of modern agriculture (Schipanski et al., 2014) (see Figure 1).
Definition and Function of Cover Crops
As the word “cover” indicates, the main function of cover crops is to cover the soil and thereby protect it from damage, losses, or other types of negative impact. The cover crop concept can also include the green manure function (i.e., N-fixing crops for N fertilization). According to the Collins English Dictionary (2012), a cover crop is “a crop planted between main crops to prevent leaching or soil erosion or to provide green manure.” The definition in the American Heritage Dictionary (2016) is even more detailed: “a crop, such as winter rye or clover, planted between periods of regular crop production to prevent erosion and typically turned under before maturity to increase the soil’s organic matter and nitrogen content.”
These definitions indicate that the purpose of cover crops is mainly related to nutrients in the soil, more precisely to preserve nutrients by reducing erosion, enabling uptake into biomass and mobilising nutrients in the cover crop for the next crop. In a context where the main purpose of the cover crop is to prevent N leaching from the soil, “catch crop” is used as a synonym for “cover crop” (Thorup-Kristensen, Magid, & Stoumann Jensen, 2003). The Oxford English Dictionary (2017) gives a more general definition covering more than the nutrient aspect: “a crop grown for protection and enrichment of the soil.”
The aim of this article is to summarize knowledge about cover crops. The focus is on N and P management, especially reduced losses to waters, for climates where there is a water surplus outside the growing season. It also aims to highlight synergies and possible conflicts between different environmental and production aspects, which should be considered in order to develop efficient and multifunctional cover crop systems (overview in Table 1). Cover crop is defined here as a crop grown between two main crops, undersown in the first main crop or sown after it is harvested. It is thereby distinguished from green manure crops, which replace ordinary main crops, and also from living mulches, which are biennial or perennial cover crops growing under a main crop (Hartwig & Ammon, 2002).
From Green Manure Crops to Cover Crops
Reclamation of agricultural land and continuous crop production have resulted in a worldwide decline in soil fertility and crop yield (Don, Schumacher, & Freibauer, 2011; Poeplau et al., 2011). On the other hand, there is long experience of how to maintain soil fertility by crop rotation. In the late 1700s, introduction of legumes in order to improve soil fertility and increase yields was promoted in America and in several European countries, and crop rotations were developed (Groff, 2015). The interest in, and need for, legume green manure crops was maintained throughout the following centuries but decreased when mineral fertilizers were introduced after World War II. However, legumes continue to be an important component of crop production systems with low inputs of mineral fertilizers and in organic farming. What are referred to as “cover crops” during the early 21st century have evolved from the concept of green manure crops.
The interest in green manure crops decreased after the introduction of mineral fertilizers, but since the 1980s, the focus on cover crops to reduce N leaching has increased. There were some studies on using cover crops to reduce N leaching already during the 1940s (Chapman, Liebig, & Rayner, 1949; Jones, 1942; Morgan, Jacobson, & LeCompte, 1942). Although eutrophication of aquatic and marine ecosystems has been a well-known problem since the 1970s (Boesch, Brinsfield, & Magnien, 2001; di Toro, O’Connor, Mancini, & Thomann, 1973; Finni, Laurila, & Laakonen, 2001) or even earlier, research on cover crops did not accelerate until the 1980s, when restoration programmes for water bodies were first introduced. Studies during the 1980s and 1990s on cover crops (e.g., winter rye [Secale cereale] and different types of ryegrasses [Lolium spp.]), demonstrated considerable reductions in N leaching (e.g., Davies, Garwood, & Rochford, 1996; Hansen & Djurhuus, 1997; Meisinger, Hargrove, Mikkelsen, Williams, & Benson, 1991).
Cover crops have been extensively used in agriculture since the 1990s (e.g., in northern Europe [Aronsson et al., 2016] and eastern North America [Dabney et al., 2010], in order to reduce N and P losses from fields. This has been achieved through extensive advisory work, subsidy systems, and/or regulations. In EU countries, the Water Framework Directive and Nitrate Directive have been important drivers for implementation of cover crops.
The literature on cover crops is dominated by studies on the effects on N leaching and there are several excellent reviews about cover crops and N management, for example by Meisinger et al. (1991), Thorup-Kristensen et al. (2003) and Dabney et al. (2010). Studies of the effects on P leaching and on erosion are much less frequent, and studies simultaneously evaluating the effects on both N and P are rare.
Cover Crop Species for Multi-Functions
Although environmental issues such as reduced N and P losses are important from a societal point of view, the potential benefits for crop production relative to the extra costs are the main question for farmers when deciding whether to grow a cover crop and which type of cover crop to choose. Additional benefits of cover crops apart from reduced leaching (e.g., suppression of weeds and pests) improved N and P availability and improved soil structure, have increased interest among farmers (Snapp et al., 2005). There are numerous different species used as cover crops all over the world, in pure stands or in mixtures, undersown or sown after harvest of the main crop. Climate conditions, soil type, type of crop rotation, and other factors set limits on the species that can be used. Examples of cereal cover crops are winter rye and black oats (Avena strigosa). Many grasses are used (e.g., ryegrasses [Lolium spp.], timothy [Phleum pratense] and Sudan grass [Sorghum]). Among the legumes, different clovers (Trifolium spp.), hairy vetch (Vicia villosa), peas (Pisum spp.), lupins (Lupinus spp.), and lucerne (Medicago spp.) are common. Examples of non-legume herbs are honeyherb (Phacelia tanacetifolia), buckwheat (Fagopyrum esculentum), chichory (Cichorium intybus), and sunflower (Heliantus annuus). Cruciferous species (often called Brassica crops) such as mustards (e.g., Brassica juncea and Sinapis alba), oilseed rape (Brassica napus) and different radishes (Rhaphanus spp.) are popular. Radishes are especially interesting because of the root performance in the soil. Soil compaction has detrimental effects on crop growth on many soils. To counteract this, so-called tillage radishes have been tested and are used as cover crops, especially for loosening compacted soils. They have a deep, fast-growing taproot that is able to penetrate dense soil layers and form pathways that can be used by the roots of the following crop (Williams & Weil, 2004). Crucifers are also interesting for other reasons (e.g., their capability for fast growth). This makes them applicable for sowing after harvest in regions where the period available for cover crop growth during autumn is short (Dabney et al., 2010).
Crucifers have dual effects on soil pathogens. One problem is that they may preserve soil pathogens such as that causing clubroot disease (Plasmodiophora brassicae) if used often, or in combination with crucifer main crops. This is an increasing problem in some regions (Wallenhammar, Almquist, Söderström, & Jonsson, 2012). Cover crops in general are reported to have suppressive effects on soil-borne pathogens due to their positive impact on soil conditions in general and by other mechanisms, such as allelopathy (Abawi & Widmer, 2000). Crucifers, especially mustards (Sinapis alba and Brassica juncea), have been proven to have direct sanitation effects on soil-borne pathogens, as nematodes and different fungal diseases, due to their production of glucosinolates (Kirkegaard & Sarwar, 1999). These compounds form volatile isothiocyanates after incorporation into the soil, and this is used for “biofumigation” (Gardiner, Morra, Eberlein, Brown, & Borek, 1999).
The use of cover crop biomass for harvest (e.g., for production of materials for biogas digestion) means that cover crops can be directly profitable as a cash crop. However, this often requires fertilization with N, P, and potassium in order produce enough biomass, a practice that has been questioned in northern Europe. In fact, in Denmark and Sweden fertilization of cover crops is not permitted in the mandatory programs or for the subsidies developed during 1990s. This is mainly because the period for growth is short and sometimes unpredictable, and fertilization of cover crops thus poses an increased risk of nutrient losses. It also reflects that fact that in this region, cover crops are mainly used in agriculture solely as a means of reducing N leaching. However, in the Nordic countries there is during the beginning of the 21st century growing interest in the development of systems where cover crops (outside the subsidy system) are used for production of biomass for biogas digestion (Molinuevo-Salces, Larsen, Ahring, & Uellendahl, 2013).
There is a market for cover crop seed, and several Internet tools are available to assist farmers in selecting cover crop seed mixtures that provide one or several functions on different soils. This indicates that there is interest and awareness among farmers about the different benefits of cover crops but not necessarily that the underlying decision support is always solid. Understandably, there is also skepticism about the potential outcome of using cover crops, especially in regions where the climate constrains the use of many potential species. Moreover, including an additional crop in the rotation requires investment in money and time, which may discourage farmers from using cover crops despite subsidy programs (Aronsson et al., 2016). There can also be a risk involved in growing cover crops (e.g., yield depression of the main crop due to competition from the cover crop). The cover crop may also become a weed if parts of it survive in the next main crop or if seeds are produced. Obvious direct benefits such as reduced N leaching and recycling of the N taken up by the cover crop to the next main crop through mineralization. Less obvious effects (e.g., long-term changes of the soil microbial community, suppression of pathogens or more stable yields) are more difficult to quantify due to the complex processes involved (Kirkegaard, Christen, Krupinsky, & Layzell, 2008) and are not so easy to demonstrate as the direct benefits of cover crops.
Table 1. Cover Crop Functions and Management of Cover Crops, Related Risks of Negative Side Effects, and Factors to be Considered for Assessment or for Reduction of Negative Effects.
Cover Crop (CC) Function and Management
Potential Negative Side Effects
Factors to Consider to Asses or to Handle Negative Side Effects
Reduced N leaching and P erosion
Increased leaching of dissolved P from CC biomass
Soil texture, P sorption capacity, and transport pathways
Amount of biomass incorporated
Efficient soil N depletion during autumn
Reduced N availability next year due to preemptive competition
CC species in pure stand or mixtures (C/N ratio)
Time of herbicide treatment and/or incorporation
Increased plant availability of P due to extraction of soil P by the CC
Increased P leaching due to release of P from CC biomass
Climate, soil P content, P forms, and sorption capacity determines which effect will dominate
Weed suppression by the CC
The CC itself becomes a weed
CC seed production in autumn
Method for killing CC (herbicide, frost, mechanical)
Suppression of pathogens by the CC
Propagation of rotation diseases
CC species and their relationship with crops in the rotation
Use of legumes for addition of N to the system (N-fixation)
Less or no reduction of N leaching
Composition of legumes and non-legumes in CC mixture
N application for increased CC biomass
Leaching of residual fertilizer and increased mineralization and leaching
N application rate
Harvest of CC biomass
Undersown CC for efficient growth and N uptake
Reduced main crop yield due to competition from CC
CC species in combination with main crop
Time of sowing the CC
Perennial CC for long N uptake period during autumn
Increased need of herbicide or intensive tillage for incorporation of CC
Soil texture and transport pathways affect the risk of herbicide leaching in comparison with N leaching
Annual CC, which is killed by frost (less need of herbicide)
Rapid mineralization and leaching due to early frost
Frost tolerance of CC species
CC sown after harvest (no competition with the main crop)
Risk of reduced function due to short growth period and mineralization due to seedbed preparation
Time of sowing
Intensity of seedbed preparation
Cover Crops for Reduced N Leaching
The Ideal N Cover Crop
A cover crop reduces N leaching primarily by taking up mineral N from the soil, so-called soil N depletion (Thorup-Kristensen, 1993). The most important characteristics of the ideal cover crop thus relate to rapid growth (Karlsson-Strese, Umaerus, & Rydberg, 1996). The cover crop must grow sufficiently fast even during cold conditions in order to utilize the excessive amounts of N that may be present in the soil due to residual fertilizer, manure application, or mineralization. The cover crop must also be easy to establish, either undersown in the main crop or sown after harvest. If it is undersown it should not be too competitive with the main crop but competitive enough to develop a dense stand that is ready to grow after harvest of the main crop. If it is sown in autumn it should not need intensive seedbed preparation in order to avoid mineralization caused by tillage (Table 1). Rapid root development is important where vertical growth is preferable to horizontal (Thorup-Kristensen, 2001). A cover crop should be frost-tolerant if grown in regions with early autumn frost events in order to take up soil mineral N as long period as possible. Moreover, frost-killed material may rapidly release N and P which may be lost, both directly from damaged plant cells and as a result of degradation and mineralization processes.
Furthermore, for all types of cover crops, it is important that the cover crop does not survive or produce seeds that can become weeds in the following crop, and it must not host and amplify pathogens, which can be a potential risk in cropping systems without bare soil between crops. A cover crop should preferably also provide positive side effects, in order to compensate for costs, or even be profitable for the farmer (Snapp et al., 2005).
Reduction of N Leaching by Different Cover Crops
The degree of reduction in N leaching by a cover crop depends on its plant characteristics, in combination with growing conditions. The amount of precipitation and the soil type strongly influence the extent to which a cover crop is needed and the magnitude of the associated reduction in leaching, both in absolute and relative terms. The measured effect is also highly dependent on the treatment with which the cover crop is compared (i.e., if the cover crop effect is evaluated separately or if a comparison is made between different systems where a cover crop is one component). For example, if a system with undersown cover crops replaces a system where the soil would otherwise have been cultivated or plowed after harvest of the main crop, the net effect on N leaching is the added effects of no tillage and the cover crop itself. In wet climates where there is drainage from the soil roughly between November and May, cover crops are efficient, especially on soils with a low clay content which are often most prone to N leaching (e.g., Simmelsgaard, 1998).
In a review, Meisinger et al. (1991) compiled results from cover crop studies in the United States and Europe and also compared the effectiveness of different types of cover crops. They found that grasses and crucifers reduce N leaching by 20% to 80%, with average values around 70%. Their comparison between legumes and non-legumes showed that the relative reduction in N leaching by legume cover crops is much lower, only 23%. In a Scandinavian review, the mean reduction in N leaching brought about by undersown grasses was 48%, while red clover did not reduce N leaching at all (Aronsson et al., 2016). Legumes are not suitable as cover crops in pure stands due to less efficient soil mineral N depletion, as shown by Breland (1996), Ranells and Wagger (1997), and Möller and Reents (2009).
A non-legume cover crop does not add any N to the system, but the ultimate goal is to recycle N within the system without unwanted losses and without an increased need for fertilizer application. Therefore, the release of N after killing the cover crop (by frost, chemicals or soil incorporation) should be synchronized with the N demand of the following main crop in order to utilize the N and avoid increased leaching later. The optimal timing of N release from cover crops and green manure crops for the following crop has thoroughly been reviewed, for example, by Thorup-Kristensen et al. (2003). There is an intricate balance between achieving as great a reduction in N leaching as possible and the desired residual effect described by Thorup-Kristensen (1993). The preemptive effect through depletion of soil mineral N by the cover crop must be followed by remineralization in order to avoid resulting in reduced N availability or an increased need for fertilizer for the following crop (Table 1). Time of incorporation or chemical treatment should be adapted during autumn or spring in order to combine leaching reduction and residual effects, with the optimal timing depending on different factors such as soil type and amount of precipitation, composition of the plant material, and time available for growth (Thorup-Kristensen & Dressbøll, 2010). For example, in a wet region with a cover crop with a low C:N ratio, the cover crop should be grown over winter, if possible, while under dry conditions it should preferably be incorporated in late autumn in order to optimise N availability for the following crop. Cover crop N content and C:N ratio, where a C:N ratio below 25 results in net mineralization (Paul & Clark, 1996), are factors to consider in this regard. In addition to time of incorporation, mixtures of species and mowing or grazing during autumn can be used in order to influence crop composition and N dynamics after incorporation. The time of sowing of the cover crop and the choice of the following cash crop can be manipulated to better synchronize mineralization of cover crop N and crop demand. However, for practical reasons time of sowing of the cover crop after harvest of a main crop is often restricted to “as soon as possible,” to get a good effect on N leaching.
Many studies, for example the meta-analysis Tonitto, David, and Drinkwater (2006), report that residual effects on crop yield of non-legume cover crops are often zero or sometimes even negative, for which efficient soil N depletion in combination with slow remineralization is a plausible explanation. Mixing legumes with grasses (e.g., hairy vetch and rye) is suggested in several American studies in order to improve N management in cover crop systems by including a green manure effect (Dabney et al., 2010). Studies in Sweden have shown that mixtures of perennial ryegrass and red clover efficiently deplete soil mineral N (Bergkvist, Stenberg, Wetterlind, Båth, & Elfstrand, 2011; Neumann, Torstensson, & Aronsson, 2011) and that the residual effects are positive (Torstensson, 1998; Wallgren & Lindén, 1994).
The Ideal P Cover Crop and Effects on P Losses
Cover crop effects on P leaching to groundwater or tile drains are far less frequently studied than effects on N leaching. A Scandinavian review has shown that cover crop effects on P leaching vary between an increase of 43% and a decrease of 86% (Aronsson et al., 2016). Small effects are not surprising, since the amount of dissolved P in the soil matrix is often small in most soils (less than 1 kg/ha), and most P is bound to, for instance, clay particles or hydroxides of aluminium and iron (Pierzynski, 1991). The amount of dissolved P is 10- to 100-fold less than the amounts often found for mineral N. An important pathway for P losses is instead soil erosion caused by wind or water flows, where precipitation, soil cover, topography, and soil infiltration capacity, for example, are important factors. Even in relatively flat landscapes, P losses by erosion may constitute a considerable proportion of P losses from clay soils with a high P content and/or poor structure and low aggregate stability, for example, during snowmelt periods (Bechmann, Øgaard, Stålnacke, & Ulén, 2013; Ulén & Jacobsson, 2005).
A cover crop provides continuous ground cover, which protects the soil surface against forces exerted by wind and raindrops. In the long term, a cover crop also improves the soil structure by increasing the organic matter content. This increases the infiltration capacity of the soil and thereby reduces the risk of surface runoff. The main criterion for a good P cover crop against erosion is that it forms a dense ground cover. While best management of cover crops for N recycling may include killing or incorporation of the cover crop in late autumn, a cover crop for P must be grown over winter in order to protect the soil surface against erosion. This means that cover crops killed by frost are less effective and have even been shown to increase losses of dissolved P by surface runoff (Ulén, 1997), especially in conditions where they are exposed to repeated freezing/thawing cycles (Bechmann, Kleinman, Sharpley, & Saporito, 2005; Liu, Ulén, Bergkvist, & Aronsson, 2014; Øgaard, 2015; Sturite, Henriksen, & Breland, 2007).
As for N, the effect of cover crops on P losses is a dual effect of reduced/no tillage during autumn and the crop cover. The separate effect of no tillage during autumn on P erosion varies from zero to 90% under northern European conditions (Bechmann, 2012; Lundekvam & Skøien, 1998; Ulén, 1997; Ulén & Kalisky, 2005). Calculating a general mean value for the effect of reduced tillage and cover crops on P erosion is difficult, since the effect is highly dependent on soil, field slope, tillage, crops, and climate. In general, P losses occur episodically, related to, for example, extreme rainfall and snowmelt events and are highly dependent on site-specific conditions such as topography, soil structure, water pathways, and legacy P in the soil (Sharpley et al., 2015). This is a challenge when seeking to understand the mechanisms for losses and for planning mitigation strategies and measuring their effect. It can be questioned whether cover crops of say, for example, ryegrasses or oilseed radish could be assumed to further contribute to reduced P losses when considering both particulate and dissolved P, or even increase the risk of losses of dissolved P in surface runoff and in tile-drain flow, compared with reduced tillage without cover crops. However, a catchment study in a Norwegian monitoring program has shown that an increase in cover crop area coincides with reduced P losses (Bechmann et al., 2008).
Is There a Contradiction Between Effects of Cover Crops on N and P Losses?
While mitigation of N losses can be planned based on average water surplus, soil texture and crop management, assessment of the need for mitigation of P losses must include a thorough investigation of risks associated with the specific field and soil properties, as erosion risk and increased risk of P leaching due to legacy P or low P sorption capacity of the soil (Djodjic & Villa, 2015). There is a possible contradiction between using cover crops to reduce N and P losses, because while cover crops efficiently reduce N leaching and particle-bound P losses, they may enhance losses of dissolved P (Table 1). Laboratory and lysimeter studies have shown that cover crop plant material poses an increased risk of losses of dissolved P (Liu, Khalaf, Ulén, & Bergkvist, 2013; Liu et al., 2014; Riddle & Bergström, 2013). Thus, it could be assumed that this would give increased P losses not only when exposed to surface runoff but also in tile drains.
Some Swedish studies have included field measurements of both N and P leaching in cropping systems with cover crops on clay soils and on sand soils (e.g., Aronsson, Liu, Ekre, Torstensson, & Salomon, 2014). Field facilities with separately drained plots were used, where drainage flow was measured continuously and water samples were taken in proportion to water flow, in order to quantify total losses from each plot. For a Swedish clay soil, it was confirmed that a growing grass/clover ley, which efficiently reduced N leaching, increased leaching of dissolved P compared with a cereal crop with autumn plowing (Aronsson et al., 2014). Other studies on a clay soil have found increased P leaching in systems with incorporation of grass/clover and Lucerne green manure crops (Ulén, Aronsson, Torstensson, & Mattsson, 2005) but no effect on P losses of using a ryegrass cover crop (Aronsson, Stenberg, & Ulén, 2011). Moreover, studies in southern Sweden on sand soils with cover crops of grasses or grass/clover mixtures (Aronsson et al., 2011; Liu, Aronsson, Blombäck, Persson, & Bergström, 2012) and oilseed radish (Neumann, Torstensson, & Aronsson, 2012) have shown that P losses are not significantly affected by these cover crops. In one of the studies on a sand soil, increased P concentrations were observed in drainage water from treatments with cover crops (Aronsson, Ringselle, Andersson, & Bergkvist, 2015) (see Figure 2). However, P leaching losses were minimal overall in the Swedish experiments, especially on sand soils, due to high P sorption capacity in the subsoils. For the sand soils, N leaching was instead the main concern. On the other hand, clay soils were more prone to P leaching, while N leaching was less severe. The reason for increased P losses on the clay soils was probably preferential flow paths that can result in fast transport of any P released on the soil surface or in the topsoil (Andersson, Bergström, Djodjic, Ulén, & Kirchmann, 2013). The conclusion from these Swedish studies was that the effects of cover crops on N and P losses are not necessarily contradictory but that both flow pathways through the soil and the P sorption capacity of the subsoil should be considered in order to evaluate if cover crops could increase the risk of leaching of dissolved P.
Cover Crops in a Cold Climate
Southern Scandinavia, including Denmark and southern Finland, is a humid region where the period between harvest and winter often involves bare soil. This affects the risk of N leaching, which occurs from October until April. Cover crops have proven successful in reducing N leaching and have been widely implemented since the 1990s in Denmark and Sweden and to some extent in Norway and Finland. An extensive regulatory framework in Denmark and a subsidy system in Sweden—in both countries combined with advisory programs—have resulted in use of cover crops on 5% to 8% of arable land (2011), which have contributed to achieving national leaching reduction targets (Aronsson et al., 2016).
However, the Nordic climate places great demands on cover crop properties and management regime. The Nordic region is at the northern limit for use of cover crops as a measure to reduce N leaching, and the choice of species is restricted compared with in many other regions. Cereals, which occupy 22% to 45% of arable land, are harvested between July and September and the first frost event normally occurs in September to October in inland areas and in November to December in southern and coastal areas of the Nordic region. The most suitable cover crops are grasses or mixtures of grasses and clover (e.g., perennial ryegrass [Lolium perenne] and red clover [Trifolium pratense], used as undersown cover crops). Aronsson et al. (2016) is a review of 11 experimental sites in this region found a mean reduction in N leaching of 48% but with large variation (0–89%). Cover crops sown after harvest are restricted to main crops that are harvested early and cover crops need to be fast growing. Crucifers are fast growing but are not frost tolerant, so they can only be used in southern parts of the Nordic region.
The focus in cover crop research in the Nordic countries has been to find frost-tolerant crops that are able to grow vigorously during the autumn without negative residual effects on yield. Thus, there have been studies on management factors such as seed rate and time and method of undersowing in cereals (Ohlander et al., 1996) and time of incorporation (e.g., Wallgren & Lindén, 1994; Thorup-Kristensen & Dressbøll, 2010). Fertilization of cover crops has not been the focus, but their effect in intensive livestock systems, with manure application directly on cover crops, has been studied. It has been found that undersown grasses such as Italian ryegrass (Lolium multiflorum) and perennial ryegrass are able to buffer manure applications and high mineral N concentrations in the soil (Thomsen et al., 1993; Torstensson & Aronsson, 2000). Other grasses, such as timothy (Phleum pratense), red fescue (Festuca rubra) and meadow fescue (Festuca pratensis), have also been studied, for example, in Finland (Känkänen & Eriksson, 2007). The risk of negative effects on yield during the year of undersowing due to competition with the main crop, and lack of residual effects on N availability for the next crop, have resulted in low interest or scepticism about undersown grasses among farmers. Grasses can become a weed in the next crop if plant parts survive over winter or if seeds are produced. There are also concerns that they could increase pathogens, although this has not been confirmed. Moreover, practical matters such as time availability in spring and weather conditions during autumn increase resistance among farmers to using cover crops, although increased potential for use of cover crops has been identified in all four Nordic countries, based on the area used for cover crops in 2011 (Aronsson et al., 2016). The subsidy systems in Sweden, Denmark, and Norway and the obligation to grow cover crops in Denmark were the drivers for successful implementation on farms until then.
Herbicide Killing of Cover Crops Causes Risk of Water Contamination
Since tillage is important for control of weeds, systems with less tillage in general rely on chemical control (Melander et al., 2013). This is one of the reasons behind a worldwide increase in use of the herbicide glyphosate since the 1970s (Benbrook, 2016).
The use of cover crops is part of this problem in the Nordic countries and elsewhere. Grass cover crops and other cover crops that do not die over winter are often killed off with a herbicide before incorporation in late autumn or spring, in order to ensure that they do not become weeds in the next crop (Table 1). Thus, these systems may have low nutrient losses and instead carry an increased risk of glyphosate contamination of groundwater and surface waters, a side effect observed in agricultural monitoring (Byer, Struger, Klawunn, Todd, & Sverko, 2008; Stenrød et al., 2007). In a cold climate with a short, wet autumn, as in northern Europe, the window available for glyphosate killing of cover crops is narrow. Application in late autumn when the weather is cold and rainy implies a risk of impaired effects (or a need for increased doses of the chemical) and an increased risk of leaching. A Swedish study hypothesized that a ryegrass cover crop could be treated with glyphosate as early as September, in order to ensure the herbicide effect and reduce the risk of water contamination, but incorporated in November and still reduce N and P leaching satisfactorily (Aronsson et al., 2011). That study found that early herbicide killing of the cover crop resulted in more or less immediate release of N, which is similar to that caused by incorporation of a cover crop, as also found by, for example, Snapp and Borden (2005). The conclusion was that for sand soils, where cover crops are needed to reduce N leaching, early herbicide treatment is a bad compromise with respect to N management and that the cover crop should grow as long as possible during autumn. No traces of glyphosate were found in drainage water in that study and P leaching was low, due to high sorption capacity for both P and glyphosate in the soil. On the other hand, similar treatments on a heavy clay resulted in a recommendation not to grow cover crops if herbicide treatment is unavoidable. On that soil the risk of N leaching was not high, but the soil contained preferential flow paths where glyphosate was rapidly transported to tile drains, although in low concentrations. Thus, the results of that study highlight that different environmental aspects must be considered in order to make accurate decisions for sustainable development of cropping systems.
Cover Crops for Weed Control Reduce the Need for Herbicides
While use of glyphosate in cover crop systems results in a clear conflict between different types of environmental impacts, use of cover crops can also be a tool to reduce the need for chemical weed control (Teasdale et al., 2007). A confirmed benefit of cover crops, especially crucifers, is suppression of weeds, as reported in Schipanski et al. (2014). The Swedish study Aronsson et al. (2015) evaluated different methods whereby an undersown cover crop of perennial ryegrass and red clover was used for control of couch grass (Elymus repens), which is a severe perennial weed in temperate regions. The cover crop mixture of grass and clover and was chosen to exploit competition mechanisms for both light (clover) and N (grass) for control of this weed, which propagates by rhizomes. In the study, it was combined with hoeing between the rows or with cutting of the cover crop and weed biomass during autumn. The results showed that there was some control of the weed, as also found by, for example, Ringselle, Bergkvist, Aronsson, and Andersson (2015), although the effect was not consistent. The N leaching was considerably reduced in cover crop treatments, compared with conventional repeated disc cultivation during autumn (see Figure 2). The P losses were low, but significantly increased P concentrations in drainage water were observed in the treatment where the cover crop was mown, which probably derived from the cut biomass left on the soil surface. Another method, tested by Bergkvist, Ringselle, Magnuski, Mangerud, and Brandsaeter (2017) in a cover crop of white clover (Trifolium repens), was to fragment the couch grass rhizome in a grid system using a flat spade. These studies do not give final recommendations about how to control couch grass by using cover crops but rather show possible ways of developing sustainable methods that can be used to reduce the need for herbicides. This is in line with and highly required for adaptation to the concept of integrated pest management (IPM) (Melander et al., 2013). Combining cover crops and mechanical treatment with low N leaching is highly interesting (e.g., for organic farming). Intensive tillage of the soil during autumn for control of perennial weeds is the conventional approach; however, this approach also increases the risk of N leaching and is energy consuming.
Cover Crops for Increased P Availability on Sub-Tropical Soils
Studies on the effects of cover crops on P leaching (see, e.g., Aronsson et al., ) show that they can increase the amount of dissolved P available for leaching, due to P release, for example, after freezing and thawing events. The P taken up by the crop is transformed into forms and relocated to places that make it more exposed to losses. This is an environmental disadvantage in wet climates and on soils with labile P pools but is also regarded as a promising benefit to increase P availability on soils with low P availability, such as highly weathered sub-tropical soils (Horst, Kamh, Jibrin, & Chude, 2001). To succeed, the cover crop should be able to exploit soil P fractions that are not used by the main crop, as described by Horst et al. (2001) and Richardson et al. (2011), for example. Since the diffusion rate for P in the soil solution is low, the root distribution and active root surface are important for efficient P uptake. Roots also induce chemical changes in the rhizosphere that affect the bioavailability of P (Hinsinger, 2001), such as pH changes and excretion of organic acids and exudates. For example, lupins (Lupinus spp.) are known for high excretion of organic acids, which may enhance P uptake and thereby recycling in available forms (Le Bayon et al., 2006). Colonization by arbuscular mycorrhizal fungi can affect P availability in low-fertility soils. Schipanski et al. (2014) reported that colonization of crops by mycorrhizal fungi is faster after use of cover crops, which represents another ecosystem function provided by cover crops.
If cover crops could be used for solubilizing and recycling P in soils where P adsorption is a problem for crop production, this would achieve an important reduction in the need for P fertilizers. It would thereby increase the productivity and sustainability of cropping systems. Teles et al. (2017) tested the hypothesis that cover crops can utilize moderately labile P in soils and increase the labile P pool on a Brazilian subtropical clay soil that is rich in P, but where immobilization of applied fertilizers is a problem due to its high content of iron and aluminium. The different cover crop species tested moderately reduced the labile P concentration in the soil, due to crop uptake, but did not increase the labile pool of soil P. The authors concluded that in order to quantify cover crop effects on different soil P pools more precisely, longer-term studies are needed. Calegari et al. (2013) on the same type of soil (Oxisol) found that cover crop systems increase available P amounts in the system, probably due to uptake and recycling. This has also been found for a Nigerian soil with low P availability, where legume cover crops recycled P to a maize crop (Horst et al., 2001).
Cover Crops for Climate Adaptation and Against Climate Change
Based on existing knowledge about cover crops, there is no doubt that they provide functions that are important for adaptation of agriculture to climate change. Reduced nutrient losses and erosion control, better water management, buffering against weather extremes, maintained soil fertility, management of pests and stable yields are general requirements in climate adaptation of agriculture (Iglesias, Quiroga, Moneo, & Garrote, 2012). Cover crops can be part of the solution to several of these challenges (e.g., Schipanski et al., 2014). However, this will need further development of strategies and advice.
To assess the impact of cover crops in mitigating climate change by emissions of greenhouse gases, the net reduction in carbon dioxide due to accumulation of organic C must be balanced by, for example, how fluxes of nitrous oxide from the soil are affected by the cover crop. Cover crops affect emissions of nitrous oxide directly in the field during growth and afterward when killed or incorporated. Cover crops also indirectly reduce emissions of nitrous oxide elsewhere due to reduced nitrate leaching. Indirect emissions constitute a considerable part of the nitrous oxide emissions from agriculture, according to Mosier, Kroeze, Nevison, Oenema, Seitzinger, and van Cleemput (1998).
The question of whether cover crops increase or decrease direct emissions of nitrous oxide was examined in a meta-analysis by Basche, Miguez, Kaspar, and Castellano (2014), where 40% of the studies showed decreased emissions due to cover crops, and 60% showed increased emissions. Species, management, and climate affected the emissions. Higher emissions were reported: at sites with high and variable precipitation, for legume cover crops and when cover crops were incorporated. The meta-analysis showed that the whole cycle of the cover crop must be considered for this type of evaluation, since reduced emissions during the uptake period can almost be balanced out by the increased emissions afterward. In total, the net effect of cover crops on nitrous oxide emissions, considering both the N uptake period and the period after incorporation, was close to zero. Considering the smaller N load to water bodies, and thus lower indirect emissions, cover crops would have resulted in a net reduction in nitrous oxide emissions. However, reduced nitrous oxide emissions through use of cover crops was not included as an ecosystem service in the evaluation by Schipanski et al. (2014).
Increased soil organic matter means increased nutrient availability for crops, but increased mineralization also increases the long-term potential for N leaching from soil when cover crops are used repeatedly (Blombäck, Eckersten, Lewan, & Aronsson, 2003). However, cover crops also affect the soil microbial community, causing it to be more dominated by fungi. This alters the relationship between build-up and degradation of soil organic to result in less degradation (Six, Frey, Thiet, & Batten, 2006). Poeplau, Aronsson, Myrbeck, and Kätterer (2015) used data from Swedish and American long-term experiments (16 to 24 years), together with modeling of the carbon balance, to quantify the effect of ryegrass cover crops on carbon stocks. They showed that despite the growing period and aboveground biomass production being quite restricted at northern latitudes, cover crops act as a significant sink of carbon dioxide—with their considerable root biomass being an important reason. Carbon sequestration by cover crops is similar in other parts of the world according to a meta-analysis by Poeplau and Don (2015), which concluded that cover crops may be important in reducing the climate impact of agriculture on global level. Poeplau and Don (2015) estimated that if cover crops were used on 25% of the arable land in the world, carbon sequestration (emissions of nitrous oxide not considered) would correspond to 8% of the greenhouse gas emissions from agriculture.
Modern agriculture, including both crop and livestock production systems, poses environmental challenges such as N and P losses to waters, reduced soil fertility, toxic substances in the environment, and negative climate impacts. Cover crops fill the gap between main crops and buffer N surpluses in the soil during autumn, thereby combining reduced N leaching with preservation of N resources, long-term increases in soil organic matter and carbon sequestration.
Besides direct effects on N recycling in crop production, cover crops provide several short- and long-term positive effects as regarding increased sustainability and resilience of agricultural cropping systems (e.g., weed and pest suppression and increased mycorrhizal colonization), although these are seldom quantified and not fully explained. A better understanding of these complex processes is needed in order to develop strategies for exploiting them in crop production. For example, methods for utilizing the competition effect of cover crops on weeds look promising for reducing the need for both herbicides and intensive tillage, which otherwise have negative environmental effects.
Cover crops increase the availability of soil P. This is a negative side effect in wet regions owing to the increased risk of P leaching when cover crops are used to reduce N leaching. Since there are many studies of either N or P leaching, but not of both, this possible contradiction between functions is rarely examined or discussed in the literature. However, it is a benefit that should be further explored for sub-tropical soils, where P availability needs to be improved. Thus, in order to avoid negative side effects and utilize synergies for production and environment, cover crop systems must be adapted to actual soil and climate conditions. Methods and strategies for this are a challenge for research in collaboration with growers and are important for maintaining general interest in cover crops. The diversity of potential cover crop species gives many possibilities, but in cold, humid climates the management flexibility and suitable species are more restricted. However, cover crops will still be an important component of future agricultural cropping systems in cold humid regions.
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