26The ability of cover crop mixturesto provide both nitrogen (N) retention and N supply 27 services has been extensively studied in research station experiments, especially with grass-28 legume bicultures. Mixtures are often as effective as grass monocultures at N retention, but the 29 N supply service can be compromised when non-legumes dilute the presence of legumes in a 30 cover crop stand. To study the tradeoffs between N retention and supply when using cover crop 31 mixtures, we measured N retention andsupply in distributedon-farm experiments,developed 32 multiple linear regression models to predict N retention and supply based on cover crop 33 functional characteristics and environmental variables, and synthesized the regression models 34 intoa graphical analysis tool. The experiments took place on three organic farms and a research 35 station in Pennsylvania, USA and tested3-species and 4-species cover crop mixtures in 36 comparison to commonly used grass and legume monocultures.Cover crop treatments were 37 planted between a small grain crop harvested in mid-summer and a maize (Zea mays L.) crop 38 planted the following spring. Potential nitrate (NO 3 -) leaching below 30 cm, an indicator of the 39 N retention service, declined asthe presence of non-legume species in a cover crop 40 increased(r 2 =0.72). Potential NO 3 leaching increased as the August soil NO 3 --N concentration 41 increased and as the fall biomass N content of winter-killed species or canola (Brassica napus L. 42 'Wichita') increased. Relative maize yield, an indicator of the N supply service, decreased as 43fall and spring cover crop biomass carbon-to-nitrogen (C:N) ratios increased and increased as 44 total spring biomass N contentandsoil carbon (C) concentration increased (r 2 =0.56). 45Synthesizing the regression models in a graphical analysis tool revealeda tradeoff between N 46 supply and retention services for cover crop mixtures, where increasing the fractional non- 47Thetradeoff could be minimized by managing environmental conditions and cover crop 49 composition so that potential NO 3 leaching remains low even when the fractional non-legume 50 seeding rate is low.The regression models suggest this could be achieved by maintaining low soil 51 NO 3 --N concentrations prior to cover crop planting in August, not including winter-killed 52 legumes in the mixture, and using non-legume species that are the most efficient at N retention. 53 Thus, with thoughtful management of cover crops and soils, farmers may be able to realize the 54 potential of cover crop mixtures to provide high levels of both N retention and supply services. 55 56 Key Words: cover crop mixture; nitrate leaching; nitrogen mineralization; adaptive management 57 58 Replacing synthetic fertilizer inputs with biologically suppliednitrogen (N) sources and 60 minimizing N losses to the environment are both important goals for farming systems that rely 61 on ecological nutrient management, including organic cropping systems(Drinkwater et al., 2011; 62 Drinkwater and S...
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.
Agroecosystems are increasingly expected to provide multiple ecosystem services. We tested whether and how cover crop selection (identity and number of species) affects provisioning of multiple services (multifunctionality). In a 3-yr study of 10 cover crop treatments and eight ecosystem services, certain services consistently co-occurred. One such service "bundle" included cover crop biomass production, weed suppression, and nitrogen retention. Another set of bundled services included cash crop production, nitrogen supply, and profitability. We also identified tradeoffs: as some services increased, other disservices arose, limiting multifunctionality. However, functionally diverse mixtures ameliorated disservices associated with certain monocultures, thereby increasing cover crop multifunctionality. Core Ideas• Cover crop monocultures and mixtures support multiple ecosystem services.• Service interactions can lead to bundling, or co-occurrence, of certain services.• Service interactions also create trade-offs among services and disservices.• Cover crop mixtures can mitigate disservices to increase multifunctionality.
Legume cover crops can often meet much of the N demand of a crop. There may be, however, an asynchrony between N mineralization from the cover crop residues and crop N uptake, resulting in potentially substantial N loss. We hypothesized that manipulation of hairy vetch (Vicia villosa Roth.) termination and corn (Zea mays L.) planting dates would regulate the quantity of N available from the vetch, the mineralization rate from the vetch residues, and the relative rate of N uptake in the corn. Field experiments were implemented in 2007 and 2008 to study the integrative effects of delaying vetch termination/corn planting through the establishment of three termination/planting dates within the month of May (an early, middle, and late date). Greater vetch biomass was found as the termination date was delayed, with a 360 and 35% biomass gain in 2007 and 2008, respectively, over 4 wk. The soil N content, for all termination dates, followed a similar availability trend across the season in both years, but the quantity of inorganic N in the soil varied depending on termination date. The average corn grain yield in 2007 was 8.0 Mg ha−1 under vetch fertilization, with no difference among vetch biomass levels, and in 2008, ranged between 4.4 and 7.6 Mg ha−1, with significant differences depending on vetch biomass level. Our study concluded that although vetch N availability can be manipulated through termination date, the dependence on climate for vetch biomass levels and N release will complicate year‐to‐year predictability.
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