Urban agriculture, though often difficult to define, is an emerging sector of local food economies in the United States. Although urban and agricultural landscapes are often integrated in countries around the world, the establishment of mid- to large-scale food production in the U.S. urban ecosystem is a relatively new development. Many of the urban agricultural projects in the United States have emerged from social movements and nonprofit organizations focused on urban renewal, education, job training, community development, and sustainability initiatives. Although these social initiatives have traction, critical knowledge gaps exist regarding the science of food production in urban ecosystems. Developing a science-based approach to urban agriculture is essential to the economic and environmental sustainability of the movement. This paper reviews abiotic environmental factors influencing urban cropping systems, including soil contamination and remediation; atmospheric pollutants and altered climatic conditions; and water management, sources, and safety. This review paper seeks to characterize the limited state of the science on urban agricultural systems and identify future research questions most relevant to urban farmers, land-use planners, and environmental consultants.

Achieving agronomic and environmental benefits associated with cover crops often depends on reliable establishment of a highly productive cover crop community. The objective of this study was to determine if cover crop mixtures can increase productivity and stability compared to single species cover crops, and to identify those components most active in contributing to or detracting from mixture productivity. A rainfed field experiment was conducted near Mead, NE, in 2010 and 2011. Eight individual cover crop species (in either the Brassicaceae [mustard] or Fabaceae [legume] family) and four mixtures of these species (two, four, six, and eight species combinations) were broadcast planted and incorporated in late March and sampled in late May. Shoot dry weights were recorded for sole crops and individual species within all mixtures. Sole crops in the mustard family were twice as productive (2428 kg ha−1) as sole crops in the legume family (1216 kg ha−1), averaged across 2 yr. The land equivalent ratios (LERs) for all mixtures in 2011 were >1.0, indicating mixtures were more productive than the individual components grown as sole crops. Improved performance in mixture may be related to the ecological resilience of mixed species communities in response to extreme weather events, such as hail. Partial LERs of species in the mustard family were consistently greater than those in the legume family, indicating that mustards dominated the mixtures. Results provide the basis for yield‐stability rankings of spring‐sown cover crop species and mixtures for the western Corn Belt.

Previous studies have demonstrated benefi ts of individual cover crop species, but the value of diverse cover crop mixtures has received less attention. Th e objectives of this research were to determine the eff ects of spring-sown cover crop mixture diversity and mechanical cover crop termination method on cover crop and/or cash crop productivity, soil moisture and N, and profi tability in an organic cropping system. An experiment was conducted between 2009 and 2011 near Mead, NE, where mixtures of two (2CC), four (4CC), six (6CC), and eight (8CC) cover crop species, or a summer annual weed mixture were included in a sunfl owersoybean-corn rotation. Cover crops were terminated in late May using a fi eld disk or sweep plow undercutter. Undercutting cover crops increased soil NO 3 -N (0-20 cm) by 1.0 and 1.8 mg NO 3 -N kg -1 relative to disk incorporation in 2010 and 2011, respectively. Cover crop mixtures oft en reduced soil moisture (0-8 cm) before main crop planting, though cover crop termination with the undercutter increased soil moisture content by as much as 0.024 cm 3 cm -3 compared to termination with the disk during early main crop growth. Crop yields were not infl uenced by cover crop mixture, but termination with the undercutter increased corn and soybean yield by as much as 1.40 and 0.88 Mg ha -1 , respectively. Despite diff erences in productivity between spring cover crop mixtures and weed communities, crop yield was not diff erent among these treatments; thus, profi tability of the weed mixture-undercutter treatment combination was greatest due to reduced input costs. ).Abbreviations: CC, cover crop mixture; DAT, days aft er termination; DOY, day of year; NC, weed-free and cover crop-free control; WD, weedy mixture and cover crop-free.

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Organic agriculture aims to build soil quality and provide long-term benefits to people and the environment; however, organic practices may reduce crop yields. This long-term study near Mead, NE was conducted to determine differences in soil fertility and crop yields among conventional and organic cropping systems between 1996 and 2007. The conventional system (CR) consisted of corn (Zea mays L.) or sorghum (Sorghum bicolor (L.) Moench)-soybean (Glycine max (L.) Merr.)-sorghum or corn-soybean, whereas the diversified conventional system (DIR) consisted of corn or sorghumsorghum or corn-soybean-winter wheat (wheat, Triticum aestivum L.). The animal manure-based organic system (OAM) consisted of soybean-corn or sorghum-soybean-wheat, while the forage-based organic system (OFG) consisted of alfalfa (Medicago sativa L.)-alfalfa-corn or sorghum-wheat. Averaged across sampling years, soil organic matter content (OMC), P, pH, Ca, K, Mg and Zn in the top 15 cm of soil were greatest in the OAM system. However, by 2008 OMC was not different between the two organic systems despite almost two times greater carbon inputs in the OAM system. Corn, sorghum and soybean average annual yields were greatest in either of the two conventional systems (7.65, 6.36 and 2.60 Mg ha -1 , respectively), whereas wheat yields were greatest in the OAM system (3.07 Mg ha -1 ). Relative to the mean of the conventional systems, corn yields were reduced by 13 and 33% in the OAM and OFG systems, respectively. Similarly, sorghum yields in the OAM and OFG systems were reduced by 16 and 27%, respectively. Soybean yields were 20% greater in the conventional systems compared with the OAM system. However, wheat yields were 10% greater in the OAM system compared with the conventional DIR system and 23% greater than yield in the OFG system. Alfalfa in the OFG system yielded an average of 7.41 Mg ha -1 annually. Competitive yields of organic wheat and alfalfa along with the soil fertility benefits associated with animal manure and perennial forage suggest that aspects of the two organic systems be combined to maximize the productivity and sustainability of organic cropping systems.