Selection of compatible plant species

Agroecosystem benefits vary depending on the botanical family of the cover crop, as does its suitability for cultivation (Table 1). Differences in component species’ morphological and physiological factors have been identified as reliable predictors of final intercropping yields (R. W. Brooker et al., 2015; Li et al., 2021). Fast-growing species are desirable for adoption in the intercropping of cover crops (Alonso-Ayuso et al., 2020). The component crops preferred in cover crop intercropping combination should be different in their grand growth period; otherwise, there may be a chance of interspecies competition for necessary growth resources if it coincides (Banik et al., 2000; R. W. Brooker et al., 2015). Therefore, complementarity among the species is desirable to obtain the benefits of cover crop intercropping reflected by biomass productivity. Hence, differences in growth duration and morphology between crop species will result in interspecies complementarity. It may be stated that maize has been considered a suitable cereal species and treated as a dominant crop because of its economic value in smallholder continuous cropping systems. Its association with preferably legume species results in higher plant productivity which is indicated by an overyielding effect.

Table 1.

Cover Crop Species Selection Rationale for Agroecosystem Benefits in Tropical Areas.

However, a significant question surrounding intercropping cover crops in annual crop systems revolves around whether their benefits exceed the losses, particularly of scarce resources such as water (Thornton et al., 2018). Intercropped cover crops that provide a dense ground cover early in the growing season are most recommended and prevent weed establishment in annual crop systems (Kunz et al., 2016; Pouryousef et al., 2015). Although yearly and perennial cover crops should be selected according to different criteria for the agroecosystem services provided. Appreciation of perennial species is low in dryland cropping systems because of the fear of competition and limited water resources (Abad et al., 2021). Self-seeding annuals benefit from the rapid and complete establishment, which is particularly suitable in no-till strip crop systems and thus reduces the need for re-planting during the next growing season (Leoni et al., 2020). Termination of annual cover crops before seed set is a management activity vital in preventing volunteer plants from emerging in the next season and reducing competition.

Cover crop species are chosen based on their positive environmental impacts in an agroecosystem. Cover crops can help to reduce water pollution in agroecosystems where nitrogen (N) leaching is a problem by trapping or absorbing excess nitrogen in their biomass and later releasing it when residues decompose to improve soil health (Arruda et al., 2021; Dobbratz et al., 2019; Gaimaro et al., 2022; Saleem et al., 2020). Legumes are often selected for biological N₂ fixation. The legume species is beneficial, especially in cereal grain cropping systems, which may reduce N inputs required for the cash crop (Liang et al., 2014). However, in-season and post-season availability of fixed N to the component crop may vary (Sanders et al., 2017). Some crop species have taproots that can penetrate and loosen the soil profile, thereby improving the soil’s physical condition (Zhang & Peng, 2021). For example, brassicas are deep soil tillage crop species capable of producing large taproots that can penetrate up to 1.8 m to alleviate soil compaction (Thorup-Kristensen, 2006). Other research has shown that cover crop mixtures are more effective in maximizing the agroecological benefits than sole species (Couëdel et al., 2018; Gaimaro et al., 2022).

Appropriate planting and termination times

The selection of a suitable intercropping or termination time of cover crops is another vital management practice because they have been shown to interfere with the dominant crop by competing for plant growth resources (Antosh et al., 2022; Mhlanga et al., 2016). Late intercropping time, growth suppression, and earlier termination are all management techniques to control living mulch interference (Abdin et al., 2000; Afshar et al., 2018; Amossé et al., 2013; Wortman et al., 2012). Cover crops are generally recommended to be introduced when the cash crop is nearing maturity or has completed a significant period of growth to reduce the intensity and outcome of mulch-crop competition on yield losses (Amossé et al., 2013; Belfry & Van Eerd, 2016). However, Kunz et al. (2016) recommend that additional weed control measures be implemented before living mulch establishment when planting living mulches late in the cash crop growing season. The disadvantage of late intercropping cover crops is a lack of early weed control, which may result in no benefits during the growing season (Alonso-Ayuso et al., 2020). Earlier intercropping can result in better cover crop establishment and improved ground cover, which is desirable when living mulches are not highly competitive (Abdin et al., 2000; Brooker et al., 2020).

A delay in cover crop intercropping usually decreases weed control and the risk of cash crop yield losses. Complementary management practices such as living mulch suppression can be recommended to prevent yield losses (Abdin et al., 2000). Brooker et al. (2020) mentioned that delayed living mulch planting might minimize maize yield losses. For example, in Pennsylvania State, USA, rye biomass increased by 50% when planted in early September rather than mid-October (Mirsky et al., 2011). Lawson et al. (2015) also observed that delaying rye planting by 2.5 weeks reduced average winter ground cover by 65% and biomass by 50% in Washington State, USA.

Limiting the living mulch interference time by killing the living mulch during the growing season with either chemical or mechanical control techniques may also reduce mulch-crop competition (Brandsaeter et al., 1998). Wortman et al. (2012) studied the effect of cover crop termination methods like disking or undercutting on cash crop yield. Cover crop mixtures increased cash crop yield and profitability when paired with an undercutter for termination compared to a traditional no cover crop organic cropping system. In Montana, USA, Afshar et al. (2018) discovered that terminating living mulches at the sugarbeet V2 stage did not reduce sugarbeet (Beta vulgaris L.) yield. Glyphosate-resistant sugarbeet species were also utilized to allow for targeted chemical termination of the living mulch. Therefore, the optimum termination time of a living mulch can significantly improve cash crop yield. Early termination of living mulches, like late planting, is most likely a proper complementary management technique when mulch-crop competition is high. However, complementary approaches are sometimes applicable and limited, depending on the cash crop species, planting patterns and tillage system. Strip planting of living mulch is quickly killed, allows for cash crop planting and establishment, and can decrease interspecific competition. Cash crop herbicide-resistant varieties are ideal for chemical suppression or termination of living mulches. In one of 2 years, terminating 61 cm bands of kura clover with glyphosate and post-emergence dicamba resulted in higher maize yield than treating the entire field with glyphosate and post-emergence bromoxynil (Zemenchik et al., 2000). Strip tilling was used to manage Kentucky bluegrass in the fall, followed by applying paraquat and glyphosate in preparation for the planting of maize as the cash crop prevents maize grain loss (Wiggans et al., 2012).

Optimum planting density and pattern

Planting geometry and spacing of dominant and subordinate species in intercropping systems are modified or altered to achieve the desired plant stand (Campiglia et al., 2014; Maitra et al., 2021; Nurgi et al., 2023). Plant density, that is, the number of plants per unit area, is required to obtain optimal yield outputs from the living mulch and the cash crop. However, if the replacement series is chosen, the plant population of both the cash crop and the cover crop species will be reduced, whereas the cash crop will have a similar plant stand in the additive series (Maitra et al., 2021). A higher planting density of a subordinate crop or living mulch can influence its competition with the cash crop and its ability to provide the desired agroecosystem services (Dzvene et al., 2022a; Mohammadi, 2010; Nurgi et al., 2023). Previous research using different living mulch seeding rates has linked the interaction to excessive mulch-crop competition at high living mulch density where weed pressure is not severe (Kaneko et al., 2011; Mohammadi, 2010; Pouryousef et al., 2015). As a result, mulch-crop competition can result in three possible outcomes for cash crop yield: (i) decline with increasing living mulch density in resource-constrained environments (Pouryousef et al., 2015); (ii) increase with increasing living mulch density where weed pressure is severe (Kaneko et al., 2011); and (iii) no difference at either low and high living mulch densities, indicating that the competitive effects of living mulch on the cash crop are similar to the weeds (Mohammadi, 2010).

Furthermore, the paired-row geometry pattern of planting the cash crop is advantageous because more space is available for planting the living mulch at higher densities (Campiglia et al., 2014). A uniform crop plant arrangement pattern increases the chances of a single plant obtaining an equal share of the available resources (light, water, and nutrients), resulting in less intraspecific competition (Thorsted et al., 2006). A uniform spatial arrangement of crop plants increases weeds’ competitive ability because a more significant proportion of the weeds will be affected by crop competition (interspecific competition). Thorsted et al. (2006) found that increasing the width of the rototilled strip reduced early competition for light by leaving a narrower band of clover before sowing the wheat. Again, planting maize in paired rows is meant to facilitate complementary management of living mulch (i.e. mechanical or chemical suppression) and possibly reduce interspecific competition. To increase maize grain yield, Sanders et al. (2017) suggested planting maize in 90 cm rows on top of 20 cm herbicide bands applied to a white clover mulch. The planting pattern and method of living mulch can significantly impact cash crop yield more than the cash crop itself.

Water

Living mulches can significantly impact the soil-water relationship, with the magnitude varying depending on the species, cropping system, climate, and soil type. The importance of understory vegetation in agricultural systems for controlling soil erosion influenced by runoff water is now widely acknowledged. Intercropping cover crops by planting them between rows of an economic crop allows for a more diverse agroecosystem (Baldé et al., 2011). Conventional tillage practices leave bare soil between crop rows, making it less stable, prone to rainwater and wind erosion, and quickly degrading. The striking actions of raindrops can erode the bare or uncovered soil. Still, the coverage of soil by cover crop canopy and the leaves can intercept most of the rainfall, reducing the effects of rain splash (Blanco-Canqui et al., 2011). Therefore, residues and understory vegetation can increase flow resistance and slow flow velocities in living mulch systems, which is critical in effectively controlling soil erosion.

Furthermore, rainfall penetrating living mulch due to interception can increase precipitation retention and promote infiltration. Cover crops reduce soil compaction, improve soil aggregate stability and enhance water infiltration through the fibrous root with is extended by arbuscular mycorrhizal fungi system (Arruda et al., 2021; Blanco-Canqui et al., 2011). Living mulch soil cover improves soil water retention compared to bare soils, but the result is highly dependable on the water use of the living mulch species (Liedgens et al., 2004).

Intercropping increases yields on a given plot of land by utilizing water that would otherwise be lost as unproductive in monocropping. However, soil type significantly impacts how much water a soil can retain and how much water plant roots can extract at different soil water potentials. There is a death of information balancing the water costs used by intercropping crops and water conserved by the living cover crop mulch, particularly in dryland farming systems. Morris and Garrity (1993) argue that the difference in the water consumption of intercropping system during the growing season is lower than the weighted average value of the corresponding sole-cropping water consumption. Although the consumption of intercropping water is higher than that of monocultures, the water consumption of intercropping systems varies considerably due to environmental conditions and crop types (Morris & Garrity, 1993). Water-sufficient environments have a great potential for developing intercropping systems due to the ability to meet the water requirements of high-yield sole-cropping systems. However, under rainwater harvesting (RWH) systems in dryland areas, the water consumption of intercropping is not apparent. For example, in irrigated areas, intercropping water consumption is not a simple accumulation of the water consumption of component crops. It is lower than the accumulation value due to water competition and complementary utilization effects during the growing season (Yin et al., 2018).

There are conflicting reports on the impact of managing living mulches on soil water. According to some studies, living mulches may compete with plants for water, and thus mulched soils may have lower moisture content than bare soils (Qu et al., 2019). Other research has shown that living mulches can increase soil moisture content (Liedgens et al., 2004; Ni et al., 2016; Thorsted et al., 2006). Ni et al. (2016) observed that live mulching with turf grass increased soil moisture at the 0 to 5 cm depth but had no effect at the 5 to 10 cm depth. A separate study, however, found that living mulches increased plant transpiration, resulting in soil water competition at 10 to 20 and 20 to 40 cm depths (Qu et al., 2019). Living mulches increased soil water use in a wheat cropping system through increased canopy transpiration from cash crop and living mulch (Thorsted et al., 2006). Reduced soil water content (30–90 cm) was reported in maize production with living mulch [Italian ryegrass (Lolium multiflorum Lam.)] compared to the control bare treatment (Liedgens et al., 2004).

The fraction of transpirable soil water (FTSW) was higher at the end of the maize cycle when maize was the sole crop than in the living mulch system (Figure 1a and b), indicating that the living mulch systems used soil moisture and rainfall more efficiently (Baldé et al., 2011). However, other studies indicated that cover crops did not affect soil moisture (Nielsen et al., 2015; Payero et al., 2021). Payero et al. (2021) study found no differences in soil moisture due to the humid conditions of South Carolina, where 1400 mm of rainfall was received during the growing season in cotton (Gossypium hirsutum L.) with single and mixed cover crop species. Gravimetric soil water content in maize grown with kura clover as a living mulch showed no difference from a pure kura clover stand, which could be attributed to an abundant rainfall water supply (Zemenchik et al., 2000). Through water uptake and transpiration, live cover crops can reduce soil water content (Unger & Vigil, 1998); however, differences in soil moisture can be negligible during growing seasons with average or above-average rainfall (Wortman et al., 2012).

Figure 1.

The fraction of transpirable soil water (FTSW) in 0 to 150 cm soil depth during (a) 2007-2008 and (b) 2008-2009 growing seasons as affected by BS1: early sown Brachiaria as sole crop, PS1: early sown pigeon pea as sole crop, MB1: maize intercropped with early sown Brachiaria, MP1: maize intercropped with early sown pigeon pea, MS: maize as sole (Source: Baldé et al., 2011).

Light

The ability of intercropping to absorb, transmit, and reflect solar energy modifies soil temperature; thus, plant canopy shading affects soil temperature modification. Above-ground interactions in living mulches influence competition for light which is size asymmetric (Thorsted et al., 2006). Living mulch species with high legume biomass productivity have been shown to use solar radiation better while limiting the amount of light that reaches the ground when relay intercropped (Amossé et al., 2013). Living mulches should remain short to prevent excessive competition for light with the cash crop because competition for light is largely asymmetric (Leoni et al., 2020). If living mulches develop sufficiently early in the growing stage of the cash crop and their canopy reaches that of the cash crop leaf, there may be light competition (Carof et al., 2007). As a result, the late planting of living mulches into a cash standing crop implies that the cash crop takes precedence over the living mulch, suggesting that light competition negatively impacts the living mulch rather than the cash crop (Shili-Touzi et al., 2010). The benefit of relay planting living mulch into a standing cash crop is due to the senescence of older leaved cash crops at grain filling, which allows for an increased light transmittance to the cover crops. The available photosynthetically active radiation (PAR) at the soil surface and for living mulches planted at the R4 maize stage increased as the lower maize leaves began to senesce (Schmitt et al., 2021). They discovered a significant decrease in PAR reaching the living mulches by the third week after living mulch planting in their study (Figure 2a and b). Green cover from living mulches planted at the V7 maize growth stage decreased as PAR levels increased throughout the maize canopy, according to Schmitt et al. (2021). They concluded this was due to the significant stress imposed on plant development by the maize canopy’s low light intensity. Gallo et al. (1985) discovered that the percentage of incoming PAR absorbed by the maize canopy increases rapidly between the V5 and V12 stages of maize growth, from about 20 to 90%. They added that the canopy absorbs more than 80% of the incoming PAR from the V12 maize growth stage to the dough stage of grain fill (R4).

Figure 2.

The percentage of living green mulch cover planted at the V7 maize growth stage concerning PAR reaching the living mulch canopy within established maize at (a) Caseelton and Hickson in 2018 and (b) Hickson and Prosper in 2019 (Source; Schmitt et al., 2021).

Nitrogen

Soil nutrient content is not expected to change significantly during 1 year of live mulching, but long-term mulching may affect soil nutrient concentrations and availability. Cover crops impact soil functioning substantially by adding to the stock of SOM through both below-ground and above-ground production (Saleem et al., 2020). Cover crops encourage soil biota activity by providing a readily available food source and creating a more favorable soil habitat (Arruda et al., 2021). Few studies that used living mulches in perennial crops focused on the effects of living mulches on soil nutrient dynamics and availability (Carof et al., 2007). Cover crop species, management, and site-specific factors, the short-term impact on soil fertility is frequently influenced by cover crop species. However, the literature on legume-living mulches and nitrogen availability is contradictory.

Despite their ability to fix nitrogen (N) and ability to scavenge excess N, thereby reducing potential nutrient leaching (Mohammed et al., 2020), legume living mulches can reduce cash crop yields due to nitrogen competition (Mia et al., 2020; Paušič et al., 2021). Cover crop mixtures that included crucifer and legume species produced nitrate catch crop services that were on par with or better than those supplied by cover crops that only contained crucifers (Couëdel et al., 2018). Cover crop mixtures containing a legume and a non-legume can provide more efficient green manure and nitrate catch crop services than non-legume cover crops (Tribouillois et al., 2016). Some research has shown that legume-living mulches can reduce nitrogen fertilizer requirements compared to the standard for a crop in the sole (Jeranyama et al., 1998; Radicetti et al., 2018). They observed that intercropped medic cover crop species have a positive yield response of subsequent maize grain and reduced maize fertilizer needs in the following year. In Spain’s semi-arid cold Mediterranean climate, Alonso-Ayuso et al. (2020) found that intercropped cover crop treatments had lower soil inorganic nitrogen in the spring before termination than autumn cover crop treatments (Figure 3). They concluded that intercropping enabled earlier cover crop planting, resulting in more significant biomass accumulation and nitrogen scavenging in autumn.

Figure 3.

The box and whisker of soil inorganic N content (kg N ha⁻¹) that were present at 0 to 40 cm depth in late March before cover crop termination (Source; Alonso-Ayuso et al., 2020).