Cover Crop Monocultures Research Articles

Remote sensing of cover crop legacies on main crop N‐uptake dynamics

AbstractGrowing cover crops promotes soil health as they retain nutrients during autumn/winter and provide organic matter to the soil biota, which in turn supplies nutrients to the main crop upon mineralisation in spring. Different cover crops have varying impacts on soil biology and nutrient dynamics due to the quantity and quality of plant material returned to the soil. To understand these effects, high‐resolution data on crop responses is required. In this study, remote sensing was used to provide such data. The temporal dynamics of soil nitrogen (N) availability and N uptake in barley were studied in response to different cover crop monocultures and mixtures. This was achieved using high‐resolution multispectral images of the main crop acquired from an unmanned aerial vehicle. Alongside this, in‐situ collected plant and soil parameters were used in this 5‐year cover crop field experiment. The results showed that cover crop legacies significantly affected barley N uptake, biomass, and canopy N content. In early June, at peak canopy N, the highest values were observed in barley grown after vetch‐radish or oat‐radish mixtures (84 kg N/ha) and the lowest in barley grown after fallow (63 kg N) or oat (53 kg N/ha on 23rd of June). At the start of the barley growing season, soil microbial biomass was not significantly affected by the cover crop legacies. However, differential N mineralisation between cover crop legacies can be attributed to differences in microbial activity associated with cover crop quantity and quality. This research demonstrates the potential of remote sensing to monitor and understand temporal and spatial variation of crop canopy N in response to cover crop N mineralisation by the soil biota which is an important component of soil health. This approach can contribute to more efficient N use by enabling fine‐tuning of the type, quantity, timing, and location of fertilisation.

Cover crop mixtures do not assemble markedly distinct soil microbiotas as compared to monocultures in a multilocation field experiment

Cover crops are used in cropping systems to enhance ecosystem services, such as soil resilience to erosion or microbial activity. Different cover crops are selected to steer specific processes, but whether cover crop mixtures have an added value over monocultures remains debated. Here, we investigated if cover crop mixtures accumulate soil microbiotas distinct from those of monocultures, potentially leading to more varied microbially-driven soil functions. We performed a field experiment at three locations in the Netherlands, each including nine cover crop monocultures, five- and eight-species mixtures, and a fallow control. After three months, we measured cover crop biomass and profiled soil bacterial, fungal, and arbuscular mycorrhizal fungal communities via amplicon sequencing. The different crop monocultures produced similar biomass across all three locations, and mixtures had average productivity compared to monocultures. The diversity and community structure of soil microbial communities was primarily determined by the geographical location, and then by cover crop treatment at each location. Although the cover crop species affected the soil microbiome differently, cover crop mixtures did neither increase microbial diversity nor the overall community differentiation compared to monocultures. Our results suggest that mixing cover crop species does not significantly influence microbially-driven soil functions, at least in short-term crop rotations.

Role of cover crop roots in soil organic carbon accrual—A review

AbstractAppropriate cover crop (CC) management is an important tool for the improvement of soil carbon stock; however, the relationships between carbon accumulation and CC root traits remain unclear. A literature review was performed to identify the extent and focus of recent research and to answer questions about the role of root traits of CCs in soil C accumulation with regard to species selection, mixture composition and agronomic management. The findings based on the analysis of 69 publications show that a range of root traits such as root biomass, architecture, depth of rooting, root chemical composition, as well as quantity and quality of rhizodeposition, can contribute to soil structure formation and C accumulation. These traits are usually species specific, and it seems that appropriate species combinations in the mixtures can offer the highest potential for optimization of C stock across various environments. However, there has been twice as much recent research on roots of CC monocultures than on mixtures, with little attention paid to agronomic aspects such as plant spatial arrangement or soil tillage in relation to CC root development. Considerations of real management under field conditions could be beneficial in providing greater accuracy of estimation of the contribution of CCs in increasing the SOC stock in croplands.

Long-term cover crop monocultures and mixtures improve soil multifunctionality and shift microbial communities and functions

Long-term cover crops are an emerging approach for achieving sustainable agriculture by reducing fertilizer use and enhancing soil quality. However, soil microbial communities, multifunctionality (i.e., the ability of soil ecosystems to simultaneously provide and maintain multiple ecological functions), and their relationships in cover crops remain poorly understood, especially in cover crop mixtures. To address these issues, here, we conducted a 15-year field experiment under different cover crop deployments (one legume and one brassica grown in monoculture or in mixture) in Carya cathayensis plantations. We found that cover crops significantly increased soil microbial diversity and multifunctionality, especially in mixtures. The bacterial and fungal compositions was significantly affected by cover crops. Interestingly, some taxa may serve as potential indicator species for nutrient status (i.e., bacterial Subgroup_2, Subgroup_6 and Subgroup_17). Cover crops significantly increased beneficial microbial functions (e.g., nitrogen fixation) but decreased pathogen-related functions (e.g., pathotroph). Bacterial and fungal community assemblages under cover crops were dominated by stochastic processes. Cover crops, especially mixtures, had a higher habitat niche breadth (Bcom). In addition, cover crops have more complex and stable bacterial and fungal networks. Partial least squares path modeling further confirmed that the variations in soil multifunctionality were largely related to nutrient resources. Importantly, the relative abundance of rare microbial taxa was also highly correlated with soil multifunctionality. Overall, our study illustrates that the use of cover crops in the long term can foster soil microbial specific taxa and contribute to improved soil multifunctionality. Our study provides support for the benefits of a legume-brassica mixture over monocultures of either and a theoretical basis for exploring cover crops as an ecological management model.

Cover crop monocultures and mixtures enhance bacterial abundance and functionality in the maize root zone.

Cover cropping is an effective method to protect agricultural soils from erosion, promote nutrient and moisture retention, encourage beneficial microbial activity, and maintain soil structure. Re-utilization of winter cover crop root channels by maize roots during summer allows the cash crop to extract resources from distal regions in the soil horizon. In this study, we investigated how cover cropping during winter followed by maize (Zea mays L.) during summer affects the spatiotemporal composition and function of the bacterial communities in the maize rhizosphere and surrounding soil samples using quantitative polymerase chain reaction (PCR), 16S ribosomal ribonucleic acid (rRNA) gene amplicon sequencing, and metaproteomics. We found that the bacterial community differed significantly among cover crop species, soil depths, and maize growth stages. Bacterial abundance increased in reused root channels, and it continued to increase as cover crop diversity changed from monocultures to mixtures. Mixing Fabaceae with Brassicaceae or Poaceae enhanced the overall contributions of several steps of the bacterial carbon and nitrogen cycles, especially glycolysis and the pentose phosphate pathway. The deeper root channels of Fabaceae and Brassicaceae as compared to Poaceae corresponded to higher bacterial 16S rRNA gene copy numbers and improved community presence in the subsoil regimes, likely due to the increased availability of root exudates secreted by maize roots. In conclusion, root channel reuse improved the expression of metabolic pathways of the carbon and nitrogen cycles and the bacterial communities, which is beneficial to the soil and to the growing crops.

Three years of cover crops management increased soil organic matter and labile carbon pools in a subtropical vegetable agroecosystem

AbstractCover crops have been widely adopted to improve soil functions in agroecosystems, including providing carbon (C) inputs that can contribute to soil C sequestration. However, soil C changes may be slow after introducing cover crops in unfavorable environments for soil organic matter (SOM) accumulation, like the Southeast United States subtropical region characterized by a warm humid climate, and coarse‐textured soils. We examined labile C pools as potential early indicators of SOM changes after cover crop introduction in a sandy subtropical vegetable production system. We compared the effects of four cover crop monocultures namely two grasses [sorghum sudangrass, Sorghum bicolor × S bicolor var. Sudanese and pearl millet, Pennisetum glaucum (L.) R. Br.], two legumes (sunn hemp, Crotalaria juncea L., and cowpea, Vigna unguiculata Walp.), and one four‐species mixture on soil organic carbon pools for 3 years. Soil samples were collected at a 15‐cm depth before cover crop planting and post cover crop incorporation to assess changes in SOM, permanganate‐oxidizable carbon (POX‐C), mineralizable carbon (Cmin), and water extractable organic carbon (WEOC). The incorporation of cover crops increased concentrations of SOM, POX‐C, and Cmin in year 3 relative to their baseline values in year 1. Concentration of SOM increased by 0.24 ± 0.05% (mean ± standard error) after 3 years of cover crop management. However, concentrations of WEOC significantly decreased in years 2 and 3 relative to the baseline. Monocultures and the mixture had similar effects on measured C pools, likely due to comparable aboveground biomass production. Our findings highlight the potential of POX‐C and Cmin as early indicators of SOM accumulation driven by cover crops use, as well as the capacity of cover crops to build SOM in similar subtropical systems and coarser textured soils.

On the legacy of cover crop-specific microbial footprints

Apart from improving the physical and chemical condition of arable soils, cover crops have the potential to boost and activate selected soil microbiota that could contribute to improved nutrient cycling and strengthened disease suppressiveness. However, a main crop can only benefit from cover crop-induced microbial shifts if these persist until the onset of the main growing season. Here, we map the persistence of microbiome changes by cover crops over time. We performed a field experiment on a sandy soil with ten different cover crop monocultures belonging to five plant families, one cover crop mixture and a fallow control. Cover crops were grown for 4.5 months under field conditions in 70-L bottomless containers in a random block design with eight replications. We studied the total (DNA-based) and the potentially active (RNA-based) microbial fractions at the onset of the main growing season, and just after the harvest of the main crop, potato (respectively 3.5 and 10 months after cover crop termination), through MiSeq sequencing. All cover crops tested induced shifts in the soil microbiome that lasted at least until the onset of the main growing season. Cover crop treatments gave rise to species and even cultivar-specific microbial footprints, and - although roughly the same trends were observed - DNA-based microbial shifts were not necessarily paralleled by similar changes at RNA level. We conclude that cover crops have the potential to act as handles to steer the soil microbiome in a way that is supportive of sustainable crop production.

Cover crop monocultures and mixtures affect soil health indicators and crop yield in the southeast United States

AbstractThe southeastern United States has a long history of soil degradation due to intensive agriculture and climatic factors. Conservation management practices such as cover cropping aim to improve soil productivity and restore soil health. Our objectives were to evaluate the performance of cover crops and their short‐term effects on soil health indicators for two southeastern soil types. Cover crop experiments were conducted at the Wiregrass (WREC) and Tennessee Valley (TVREC) Research and Extension Centers in Alabama from 2017 to 2021. Cover crop treatments included winter fallow, monocultures, and two‐ and three‐way mixtures of cereal rye (Secale cereale L.), crimson clover (Trifolium incarnatum L.), and forage radish (Raphanus sativus L.) preceding a cotton (Gossypium hirsutum L.)–legume {peanut [Arachis hypogaea L.] or soybean [Glycine max (L.) Merr.]} cash crop rotation. At TVREC, aboveground biomass for both rye and clover were greater than radish. All cover crops, except the radish monoculture, increased soil organic carbon (SOC) 19–30% in the top 5 cm at TVREC compared with winter fallow. Similarly, some cover crop treatments increased permanganate oxidizable carbon (POXC) compared with the fallow control. However, this effect was not observed at WREC. Cover crops did not produce many meaningful aggregate stability differences at either location. Treatments containing rye or clover decreased soil strength. by 14–22% at TVREC after 4 yr of cover crop utilization. Short‐term cover crop use improved selected soil health indicators and reversed soil degradation at one location evaluated, but these benefits were dependent on soil type and cover crop selection.

Post-termination Effects of Cover Crop Monocultures and Mixtures on Soil Inorganic Nitrogen and Microbial Communities on Two Organic Farms in Illinois

Cover crops can continue to affect agricultural systems even after they have been terminated by influencing nitrogen dynamics and by altering soil microbial communities. These post-termination effects can influence soil fertility, weed pressure, and the dynamics of potential plant pathogens in the narrow window of time between cover crop termination and cash crop emergence. We evaluated the post-termination effects of 12 different spring-sown cover crop mixtures and monocultures on soil nitrogen and microbial communities on two different organic farms in Central Illinois (on Lawson silt loam soil) and Northern Illinois (on Virgil silt loam soil). In comparison to control plots with no cover crops, all cover crop treatments significantly reduced soil nitrate levels but increased the potentially mineralizable nitrogen pool following termination. Nitrate levels of cover crop plots approached those of controls after 2 and 4 weeks, respectively, but potentially mineralizable nitrogen levels in cover plots remained elevated for at least 4 weeks following termination. Monocultures of Brassica cover crops showed the greatest decrease in soil nitrate, while Brassicas and unplanted control plots containing high biomass of weeds showed the greatest increase in potentially mineralizable nitrogen in comparison to plant-free control plots. In contrast to their effect on soil nitrogen, cover crops had very limited impact on the composition of soil microbial communities. Overall microbial community composition varied across sites and years, and only soil fungi significantly responded to cover cropping treatments. Nevertheless, we found that some highly correlated groups of soil microbes showed significant responses to soil nitrate and to high plant biomass. Key members of these correlated groups included ammonia-oxidizing organisms and saprotrophic fungi. Our results suggest that cover crops may reduce the potential for springtime nitrogen leaching losses by retaining nitrogen in the soil organic pool, and they may also have impacts on the soil microbial community that are particularly relevant for nitrogen cycling and decomposition of plant residues.

Comparison of cover crop monocultures and mixtures for suppressing nitrogen leaching losses

The 2025 goal of the Chesapeake Bay Program is to reduce agriculture’s nitrogen (N) loading by 20% from 2014 values. Cover cropping is an important best management practice for recycling and conserving N in cropping systems that might otherwise be lost to groundwater via leaching and runoff. The reduction of N leaching by winter cover crops depends largely on precipitation, timing of planting, and the selection of the appropriate crop species. We compared nitrate-nitrogen (NO3-–N) leaching losses among forage radish (Raphanus sativus L.), cereal rye (Secale cereal L.), a forage radish+cereal rye mixture, and no-cover control. Replicated field trials were conducted at the University of Maryland Central Maryland Research and Education Center over 2016–2018. We collected porewater from 60 cm below the ground surface using porous cup lysimeters following rainfall events and used NO3-–N concentrations paired with the HYDRUS 1-D soil moisture model to compare N leaching losses (in kg N ha−1) among cover crop treatments. We show that mean soil porewater NO3-–N concentrations were higher (by 5x) in the no-cover control compared to rye and radish+rye treatments (P < 0.0001). Overall, N leaching losses (kg N ha−1) were highest in the no-cover control plots (P < 0.0001), but the majority of N leaching losses in rye plots occurred during the fall while the majority of radish N leaching losses occurred during the winter and spring (after they winter-killed). The rye and radish+rye mixtures reduced N leaching losses by 80% in both years. Understanding how different cover crop species affect N leaching losses, can help us design cropping systems to minimize N losses to the Chesapeake Bay especially as climate change alters rainfall patterns across the region.

Finding the right mix: a framework for selecting seeding rates for cover crop mixtures.

Cover crop mixtures have the potential to provide more ecosystem services than cover crop monocultures. However, seeding rates that are typically recommended (i.e. seeding rate of monoculture divided by the number of species in the mixture) are non‐optimized and often result in the competitive species dominating the mixture, and therefore limiting the amount of ecosystem services that are provided. We created an analytical framework for selecting seeding rates for cover crop mixtures that maximize multifunctionality while minimizing seed costs. The framework was developed using data from a field experiment, which included six response surface designs of two‐species mixtures, as well as a factorial replacement design of three‐species and four‐species mixtures. We quantified intraspecific and interspecific competition among two grasses and two legume cover crop species with grass and legume representing two functional groups: pearl millet [Pennisetum glaucum (L.) R.Br.], sorghum sudangrass [Sorghum bicolor (L.) Moench × Sorghum sudanense (Piper) Stapf], sunn hemp (Crotalaria juncea L.), and cowpea [Vigna unguiculata (L.) Walp]. Yield–density models were fit to estimate intraspecific and interspecific competition coefficients for each species in biculture. The hierarchy from most to least competitive was sorghum sudangrass > sunn hemp > pearl millet > cowpea. Intraspecific competition of a less competitive species was the greatest when the biculture was composed of two species in the same functional group. Competition coefficients were used to build models that estimated the biomass of each cover crop species in three‐species and four‐species mixtures. The competition coefficients and models were validated with an additional nine site‐years testing the same cover crop mixtures. The biomass of a species in a site‐year was accurately predicted 69% of the time (low root mean square error, correlation > 0.5, not biased, r 2 > 0.5). Applying the framework, we designed three‐species and four‐species mixtures by identifying relative seeding rates that produced high biomass with high species evenness (i.e. high multifunctionality) at low seed costs based on a Pareto front analysis of 10,418 mixtures. Accounting for competition when constructing cover crop mixtures can improve the ecosystem services provided, and such an advancement is likely to lead to greater farmer adoption.

Impact of Cover Crop Monocultures and Mixtures on Organic Carbon Contents of Soil Aggregates

Cover crops are considered an integral component of agroecosystems because of their positive impacts on biotic and abiotic indicators of soil health. At present, we know little about the impact of cover crop types and diversity on the organic carbon (OC) contents of different soil aggregate-size classes. In this study, we investigated the effect of cover plant diversity on OC contents of different soil aggregates, such as macro- (&lt;2000–500 μm), meso- (&lt;500–250 μm), and micro-aggregates (&lt;250 μm). Our experiment included a total of 12 experimental treatments in triplicate; six different monoculture treatments such as chickling vetch (Vicia villosa), crimson clover (Trifolium incarnatum), hairy vetch (Vicia villosa), field peas (Pisum sativum), oilseed radish (Raphanus sativus), and mighty mustard (Brassica juncea), and their three- and six-species mixture treatments, including one unplanted control treatment. We performed this experiment usingdeep pots that contained soil collected from a corn-soybean rotation field. At vegetative maturity of cover plants (about 70 days), we took soil samples, and the soil aggregate-size classes were separated by the dry sieving. We hypothesized that cover crop type and diversity will improve OC contents of different soil aggregate-size classes. We found that cover plant species richness weakly positively increased OC contents of soil macro-aggregates (p = 0.056), whereas other aggregate-size classes did not respond to cover crop diversity gradient. Similarly, the OC contents of macroaggregates varied significantly (p = 0.013) under cover crop treatments, though neither monoculture nor mixture treatments showed significantly higher OC contents than the control treatment in this short-term experiment. Interestingly, the inclusion of hairy vetch and oilseed radish increased and decreased the OC contents of macro- and micro-aggregates, respectively. Moreover, we found a positive correlation between shoot biomass and OC contents of macroaggregates. Overall, our results suggest that species-rich rather than -poor communities may improve OC contents of soil macroaggregates, which constitute a major portion of soil systems, and are also considered as important indicators of soil functions.

Late-seeded cover crops in a semiarid environment: overyielding, dominance and subsequent crop yield

AbstractInterest in cover crops is increasing but information is limited on integrating them into crop rotations especially in the relatively short growing season on the northern Great Plains. A 3-yr research project, initiated in 2009 near Mandan, North Dakota, USA, evaluated (1) what impact cover crops may have on subsequent cash crops yields and (2) whether cover crop mixtures are more productive and provide additional benefits compared to cover crop monocultures. The study evaluated 18 different cover crop monocultures and mixtures that were seeded in August following dry pea (Pisum sativum L.). The following year, spring wheat (Triticum aestivum L.), corn (Zea mays L.), soybean (Glycine max L.) and field pea were seeded into the different cover crop treatments and a non-treated control. A lack of timely precipitation in 2009 resulted in a low cover crop yield of 17 g m2 compared to 100 and 77 g m2 in 2008 and 2010, respectively. Subsequent cash crop yield was not affected by late-seeded cover crops. Cool-season cover crop monocultures were more productive than warm-season monocultures and some mixtures in 2008 and 2010. Relative yield total did not differ from one in any cover crop mixture suggesting that overyielding did not occur. Species selection rather than species diversity was the most important contributor to cover crop yield. Cover crops can be grown following short-season cash crops in the northern Great Plains, but precipitation timing and species selection are critical.

Cover crop residue components and their effect on summer annual weed suppression in corn and soybean

AbstractCover crop residue can act as a mulch that will suppress weeds, but as the residue degrades, weed suppression diminishes. Biomass of cover crop residue is positively correlated to weed suppression, but little research is available regarding the composition of cover crop residue and its effect on weed suppression. Field experiments were conducted to determine the impact of cover crop residue properties (i.e., total carbon, total nitrogen, lignin, cellulose, and hemicellulose) on summer annual weed suppression and cash crop yield. Cover crop monocultures and mixtures were planted in the fall and designed to provide a range of biomass and residue properties. Cover crops were followed by corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. At termination, cover crop biomass and residue components were determined. Biomass ranged from 3,640 to 8,750 kg ha−1, and the carbon-to-nitrogen (C:N) ratio ranged from 12:1 to 36:1. As both cover crop biomass and C:N ratio increased, weed suppression and duration of suppression increased. For example, a C:N ratio of 9:1 is needed to suppress redroot pigweed (Amaranthus retroflexus L.) 50% at 4 wk after termination (WAT), and that increases to 16:1 and 20:1 to have 50% suppression at 6 and 8 WAT, respectively. Similarly, with biomass, 2,800 kg ha−1 is needed for 50% A. retroflexus suppression at 4 WAT, which increases to 5,280 kg ha−1 and 6,610 kg ha−1 needed for 50% suppression at 6 and 8 WAT, respectively. In general, similar trends were observed for pitted morningglory (Ipomoea lacunosa L.) and large crabgrass [Digitaria sanguinalis (L.) Scop.]. Corn and soybean yield increased as both cover crop biomass and C:N ratio increased where no weed control measures were implemented beyond cover crop. The same trend was observed with cash crop yield in the weed-free subblocks, with one exception. This research indicates that cover crop residue composition is important for weed control in addition to biomass.

Are cover crop mixtures better at suppressing weeds than cover crop monocultures?

AbstractCover crops are increasingly being used for weed management, and planting them as diverse mixtures has become an increasingly popular strategy for their implementation. While ecological theory suggests that cover crop mixtures should be more weed suppressive than cover crop monocultures, few experiments have explicitly tested this for more than a single temporal niche. We assessed the effects of cover crop mixtures (5- or 6-species and 14-species mixtures) and monocultures on weed abundance (weed biomass) and weed suppression at the time of cover crop termination. Separate experiments were conducted in Madbury, NH, from 2014 to 2017 for each of three temporal cover-cropping niches: summer (spring planting–summer termination), fall (summer planting–fall termination), and spring (fall planting–subsequent spring termination). Regardless of temporal niche, mixtures were never more weed suppressive than the most weed-suppressive cover crop grown as a monoculture, and the more diverse mixture (14 species) never outperformed the less diverse mixture. Mean weed-suppression levels of the best-performing monocultures in each temporal niche ranged from 97% to 98% for buckwheat (Fagopyrum esculentum Moench) in the summer niche and forage radish (Raphanus sativus L. var. niger J. Kern.) in the fall niche, and 83% to 100% for triticale (×Triticosecale Wittm. ex A. Camus [Secale × Triticum]) in the winter–spring niche. In comparison, weed-suppression levels for the mixtures ranged from 66% to 97%, 70% to 90%, and 67% to 99% in the summer, fall, and spring niches, respectively. Stability of weed suppression, measured as the coefficient of variation, was two to six times greater in the best-performing monoculture compared with the most stable mixture, depending on the temporal niche. Results of this study suggest that when weed suppression is the sole objective, farmers are more likely to achieve better results planting the most weed-suppressive cover crop as a monoculture than a mixture.

Decomposition of mixtures of cover crop residues increases microbial functional diversity

To improve sustainability in agricultural systems, winter cover crops are increasingly replacing fallow to stimulate soil functions that reduce nutrient losses and greenhouse gas production, reduce pests for the next cash crops, increase soil organic matter pools and reduce erosion. Several of these functions are highly dependent on soil microbes decomposing cover crop residues. Since cover crop species differ in their traits it is hypothesized that plant species residue mixtures with complementary characteristics perform better by stimulating soil microbial functional diversity. To test this, residues of cover crop monocultures and mixtures were mixed with agricultural soil in a microcosm experiment, and fungal and bacterial biomass, microbial metabolic potential, greenhouse gas emissions and soil nutrients were measured during 50 days. Fungal biomass increased for all treatments compared to the control (no additions). However, there were no significant differences between cover crop mixtures and monocultures. Biolog ECO plates were used as a proxy for the metabolic potential of the microbial community. The number of substrates used was significantly higher in soil amended with residues of cover crop mixtures indicating an increased number of substrate niches for microbes. C:N ratio of cover crop residues was shown to be an important variable in explaining dynamics of CO2 and N2O emissions. Mixtures of cover crops showed reduced N2O and CO2 emissions compared to monocultures at the start of the experiment, but did not reduce greenhouse gas emissions over the whole incubation period. Adding nitrogen to the cover crop treatment with the highest C:N ratio (oat) did increase N2O emissions, but not CO2 emissions suggesting that decomposition rate of oat residues is not limited by nitrogen availability. Overall, mixtures of cover crops stimulated microbial functional diversity in soil incubations. Although this may have positive implications for soil quality and functioning in agricultural fields, further studies are needed to verify if these results hold under field conditions.

Managing nitrogen through cover crop species selection in the U.S. mid-Atlantic.

Cover crops have the potential to be agricultural nitrogen (N) regulators that reduce leaching through soils and then deliver N to subsequent cash crops. Yet, regulating N in this way has proven difficult because the few cover crop species that are well-studied excel at either reducing N leaching or increasing N supply to cash crops, but they fail to excel at both simultaneously. We hypothesized that mixed species cover crop stands might balance the N fixing and N scavenging capabilities of individual species. We tested six cover crop monocultures and four mixtures for their effects on N cycling in an organically managed maize-soybean-wheat feed grain rotation in Pennsylvania, USA. For three years, we used a suite of integrated approaches to quantify N dynamics, including extractable soil inorganic N, buried anion exchange resins, bucket lysimeters, and plant N uptake. All cover crop species, including legume monocultures, reduced N leaching compared to fallow plots. Cereal rye monocultures reduced N leaching to buried resins by 90% relative to fallow; notably, mixtures with just a low seeding rate of rye did almost as well. Austrian winter pea monocultures increased N uptake in maize silage by 40 kg N ha-1 relative to fallow, and conversely rye monocultures decreased N uptake into maize silage by 40 kg N ha-1 relative to fallow. Importantly, cover crop mixtures had larger impacts on leaching reduction than on maize N uptake, when compared to fallow plots. For example, a three-species mixture of pea, red clover, and rye had similar maize N uptake to fallow plots, but leaching rates were 80% lower in this mixture than fallow plots. Our results show clearly that cover crop species selection and mixture design can substantially mitigate tradeoffs between N retention and N supply to cash crops, providing a powerful tool for managing N in temperate cropping systems.

Plant identity, but not diversity, and agroecosystem characteristics affect the occurrence of M. robertsii in an organic cropping system

Organic farmers rely largely on cultural practices, such as crop rotation and cover cropping, and on biological control to manage pests. Winter cover crops add diversity to agroecosystems and can benefit soil conservation and health, retention and supply of soil nutrients, regulation of arthropods, and crop yields. Within the framework of a three-year experiment to investigate ecosystem services and disservices associated with 12 winter cover crop treatments during the transition of a small grain – corn – soybean production system to organic management, we examined the effects of cash crop and crop management, cover crop species and diversity, arthropods, and soil properties on Metarhizium spp. in a single field site in central Pennsylvania, USA. We used sentinel insect assays with Galleria mellonella L. to determine the relative occurrence of Metarhizium spp. in soil samples from treatment plots. M. robertsii was the only species detected, and therefore Metarhizium species diversity was not related to cover crop diversity. Detection of M. robertsii was lower in cover crop monocultures and mixtures containing brassicas compared to those with legumes. M. robertsii was detected more frequently in the standing cover crops in the corn phase of the rotation than in the soybean phase, but there was no difference in soil samples in the corn and soybeans after cover crop termination. In samples from cover crops, 33% of the variation of percentage mortality of sentinel G. mellonella by M. robertsii was explained by positive correlations with cover crop biomass in the previous fall, the biomass of weeds in the current season (spring), soil moisture, Ca, and timing of sampling in days after planting, and by negative correlations with number of days since disturbance prior to soil sampling and percent sand. In samples from corn and soybeans, 23% of the variation of percentage mortality of sentinel G. mellonella by M. robertsii was explained by positive correlations with soil labile C, electrical conductivity, and activity-density of ground beetles (Carabidae), and by negative correlations with increasing number of days after planting relative to time of sampling, percent sand, and activity-density of mites. The complex interactions of multiple biotic and abiotic factors that shape the soil community requires further research to develop an understanding of how to manage production systems and practices to promote and conserve biological control in the soil.

Suppression of Glyphosate-resistant Canada Fleabane (Conyza canadensis) in Corn with Cover Crops Seeded after Wheat Harvest the Previous Year

AbstractGlyphosate-resistant (GR) and multiple herbicide–resistant (groups 2 and 9) Canada fleabane have been confirmed in 30 and 23 counties in Ontario, respectively. The widespread incidence of herbicide-resistant Canada fleabane highlights the importance of developing integrated weed management strategies. One strategy is to suppress Canada fleabane using cover crops. Seventeen different cover crop monocultures or polycultures were seeded after winter wheat harvest in late summer to determine GR Canada fleabane suppression in corn grown the following growing season. All cover crop treatments seeded after wheat harvest suppressed GR Canada fleabane in corn the following year. At 4 wk after cover crop emergence (WAE), estimated cover crop ground cover ranged from 31% to 68%, a density of 124 to 638 plants m–2, and a range of biomass from 29 to 109 g m–2, depending on cover crop species. All of the cover crop treatments suppressed GR Canada fleabane in corn grown the following growing season from May to September compared to the no cover crop control. Among treatments evaluated, annual ryegrass (ARG), crimson clover (CC)/ARG, oilseed radish (OSR)/CC/ARG, and OSR/CC/cereal rye (CR) were the best treatments for the suppression of GR Canada fleabane in corn. ARG alone or in combination with CC provided the most consistent GR Canada fleabane suppression, density reduction, and biomass reduction in corn. Grain corn yields were not affected by the use of the cover crops evaluated for Canada fleabane suppression.

Weed Suppression in Cover Crop Monocultures and Mixtures

Interest in planting mixtures of cover crop species has grown in recent years as farmers seek to increase the breadth of ecosystem services cover crops provide. As part of a multidisciplinary project, we quantified the degree to which monocultures and mixtures of cover crops suppress weeds during the fall-to-spring cover crop growing period. Weed-suppressive cover crop stands can limit weed seed rain from summer- and winter-annual species, reducing weed population growth and ultimately weed pressure in future cash crop stands. We established monocultures and mixtures of two legumes (medium red clover and Austrian winter pea), two grasses (cereal rye and oats), and two brassicas (forage radish and canola) in a long fall growing window following winter wheat harvest and in a shorter window following silage corn harvest. In fall of the long window, grass cover crops and mixtures were the most weed suppressive, whereas legume cover crops were the least weed suppressive. All mixtures also effectively suppressed weeds. This was likely primarily due to the presence of fast-growing grass species, which were effective even when they were seeded at only 20% of their monoculture rate. In spring, weed biomass was low in all treatments due to winter kill of summer-annual weeds and low germination of winter annuals. In the short window following silage corn, biomass accumulation by cover crops and weeds in the fall was more than an order of magnitude lower than in the longer window. However, there was substantial weed seed production in the spring in all treatments not containing cereal rye (monoculture or mixture). Our results suggest that cover crop mixtures require only low seeding rates of aggressive grass species to provide weed suppression. This creates an opportunity for other species to deliver additional ecosystem services, though careful species selection may be required to maintain mixture diversity and avoid dominance of winter-hardy cover crop grasses in the spring.