Managing the trade off between nitrogen supply and retention with cover crop mixtures

Abstract

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 Reduce Nitrate Leaching in Agroecosystems:A Global Meta‐Analysis

“…One such factor is soil N availability (residual soil N left after previous crop uptake in the fall and N mineralized from previous crop residues or soil organic matter) during the fall and spring growth period. In general, NO 3 − leaching increases as soil N availability increases (White et al, 2017). Under such conditions, cover crops could be an effective tool to retain soil N and reduce NO 3 − leaching.…”

“…Since cover crops are typically terminated 2 to 6 wk prior to cash crop planting, there are long periods of time where there are no living plants removing reactive N. Therefore, strategies to reduce NO 3 − leaching should not only focus on growing cover crops to efficiently scavenge N during its growth, but also on efficient use of the N captured by cover crops after termination. To tailor N release from leguminous and broadleaf cover crops with N demand of the succeeding cash crop, we suggest planting leguminous and broadleaf cover crops in mixture with grasses (White et al, 2017).…”

Functional traits in cover crop mixtures: Biological nitrogen fixation and multifunctionality

“…Previous studies have explored legume‐grass intercrops for simultaneously supplying and retaining N within agroecosystems (e.g., Ranells & Wagger, ; White et al., ). Mixtures of legumes and non‐legumes commonly result in facilitation.…”

Crop rotations for increased soil carbon: perenniality as a guiding principle

“…, White et al. ). Cover crops and perennial crops are better‐suited to growing in species mixtures than are grain crops, leaving open the possibility for leveraging functional diversity in mixed‐species cover crops or perennial forages to provide SOC storage and other environmental services (Wortman et al.…”

“…First, we found relatively few cropping system sites testing the typologies of crop rotations-cover cropped and perennial cropped-able to provide some of the greatest environmental benefits in terms of SOC storage. Due to this low testing frequency, we aggregated crop functional types (legume, grass) within perennial and cover cropped rotations, however, it may be that plant functional diversity in mixed species cover crops and perennial crops can influence their provisioning of ecosystem services (Smith et al 2014, Storkey et al 2015, White et al 2017. Cover crops and perennial crops are better-suited to growing in species mixtures than are grain crops, leaving open the possibility for leveraging functional diversity in mixedspecies cover crops or perennial forages to provide SOC storage and other environmental services (Wortman et al 2012, Finney and Kaye 2016, Blesh 2017).…”