An increasing interest has emerged with respect to the importance of microbial diversity in soil habitats The extent of the diversity of microorganisms in soil is seen to be critical to the maintenance of soil health and quality, as a wide range of microorganisms is involved in important soil functions This review focuses on recent data relating how plant type, soil type, and soil management regime affect the microbial diversity of soil and the implication for the soil's disease suppressiveness The two main drivers of soil microbial community structure, ie, plant type and soil type, are thought to exert their function in a complex manner We propose that the fact that in some situations the soil and in others the plant type is the key factor determining soil microbial diversity is related to the complexity of the microbial interactions in soil, including interactions between microorganisms and soil and microorganisms and plants A conceptual framework, based on the relative strengths of the shaping forces exerted by plant and soil versus the ecological behavior of microorganisms, is proposed

Microbial diversity in soil: Selection of microbial populations by plant and soil type and implications for disease suppressiveness

Various organic technologies have been utilized for about 6000 years to make agriculture sustainable while conserving soil, water, energy, and biological resources. Among the benefits of organic technologies are higher soil organic matter and nitrogen, lower fossil energy inputs, yields similar to those of conventional systems, and conservation of soil moisture and water resources (especially advantageous under drought conditions). Conventional agriculture can be made more sustainable and ecologically sound by adopting some traditional organic farming technologies.

https://www.jstor.org/stable/10.1641/0006-3568(2005)055%5B0573%3AEEAECO%5D2.0.CO%3B2

Environmental, Energetic, and Economic Comparisons of Organic and Conventional Farming Systems

Nitrogen is the most limiting nutrient for crop production in many of the world's agricultural areas and its efficient use is important for the economic sustainability of cropping systems Furthermore, the dynamic nature of N and its propensity for loss from soil‐plant systems creates a unique and challenging environment for its efficient management Crop response to applied N and use efficiency are important criteria for evaluating crop N requirements for maximum economic yield Recovery of N in crop plants is usually less than 50% worldwide Low recovery of N in annual crop is associated with its loss by volatilization, leaching, surface runoff, denitrification, and plant canopy Low recovery of N is not only responsible for higher cost of crop production, but also for environmental pollution Hence, improving N use efficiency (NUE) is desirable to improve crop yields, reducing cost of production, and maintaining environmental quality To improve N efficiency in agriculture, integrated N management strategies that take into consideration improved fertilizer along with soil and crop management practices are necessary Including livestock production with cropping offers one of the best opportunities to improve NUE Synchrony of N supply with crop demand is essential in order to ensure adequate quantity of uptake and utilization and optimum yield This paper discusses N dynamics in soil‐plant systems, and outlines management options for enhancing N use by annual crops

Enhancing nitrogen use efficiency in crop plants

The integration of cover crops into cropping systems brings costs and benefits, both internal and external to the farm. Benefits include promoting pest-suppression, soil and water quality, nutrient cycling efficiency, and cash crop productivity. Costs of adopting cover crops include increased direct costs, potentially reduced income if cover crops interfere with other attractive crops, slow soil warming, difficulties in predicting N mineralization, and production expenses. Cover crop benefits tend to be higher in irrigated systems. The literature is reviewed here along with Michigan farmer experience to evaluate promising cover crop species for four niches: Northern winter (USDA Hardiness Zones 5-6), Northern summer (Zones 5-6), Southern winter (Zones 7-8), and Southern summer (Zones 7-8). Warm season C 4 grasses are outstanding performers for summer niches (6-9 Mg ha -1 ), and rye (Secale cereale L.) is the most promising for winter niches (0.8-6 Mg ha -1 ) across all hardiness zones reviewed. Legume-cereal mixtures such as sudangrass (Sorghum sudanese L.)-cowpea (Vigna unguiculata L.) and wheat (Triticum aestivum L.)-red clover (Trifolium pretense L.) are the most effective means to produce substantial amounts (28 Mg ha -1 ) of mixed quality residues. Legume covers are slow growers and expensive to establish. At the same time, legumes fix N, produce high quality but limited amounts (0.5-4 Mg ha -1 ) of residues, and enhance beneficial insect habitat. Brassica species produce glucosinolate-containing residues (2-6 Mg ha -1 ) and suppress plant-parasitic nematodes and soil-borne disease. Legume cover crops are the most reliable means to enhance cash crop yields compared with fallows or other cover crop species. However, farmer goals and circumstances must be considered. If soil pests are a major yield limiting factor in cash crop production, then use of brassica cover crops should be considered. Cereal cover crops produce the largest amount of biomass and should be considered when the goal is to rapidly build soil organic matter. Legume-cereal or brassica-cereal mixtures show promise over a wide range of niches.

Evaluating Cover Crops for Benefits, Costs and Performance within Cropping System Niches

A fundamental shift has taken place in agricultural research and world food production. In the past, the principal driving force was to increase the yield potential of food crops and to maximize productivity. Today, the drive for productivity is increasingly combined with a desire for sustainability. For farming systems to remain productive, and to be sustainable in the long-term, it will be necessary to replenish the reserves of nutrients which are removed or lost from the soil. In the case of nitrogen (N), inputs into agricultural systems may be in the form of N-fertilizer, or be derived from atmospheric N2 via biological N2 fixation (BNF).

Biological nitrogen fixation: An efficient source of nitrogen for sustainable agricultural production?

Relying more on biological N 2 fixation has been suggested as a way to meet one of the major challenges of agricultural sustainability. A 15 N study was conducted to compare the fate of applied legume and fertilizer N in a long-term cropping systems experiment. Nitrogen-15-labeled red clover (Trifolium pratense L.) and (NH 4 ) 2 SO 4 were applied to microplots within the low-input and conventional cropping systems of the Farming Systems Trial at the Rodale Institute Research Center in Penn-sylvania. The 15 N was applied to soil and traced into corn (Zea mays L.) in 1987 and 1988. Residual 15 N was also traced into second-year spring barley (Hordeum vulgare L.). Legume and fertilizer 15 N remaining in soil was measured and loss of N was calculated by difference

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Fate of Legume and Fertilizer Nitrogen-15 in a Long-Term Cropping Systems Experiment

Cover crops often reduce density and biomass of annual weeds in no-till cropping systems. However, cover crops that over-winter also have the potential to reduce crop yield. Currently, there is an interest in annual medics (Medicago spp.) and other annual legumes that winter-kill for use as cover crops in midwestern grain cropping systems. A 2-yr study was conducted at East Lansing and the Kellogg Biological Station, Michigan, to investigate the influence of annual legume cover crops on weed populations. Two annual medic species [burr medic (M. polymorpha cv. Santiago) and barrel medic (M. truncatula Gaertn. cv. Mogul)], berseem clover (Trifolium alexandrinum L. cv. Bigbee), and medium red clover (Trifolium pratense L.) were no-till seeded as cover crops into winter wheat (Triticum aestivum L.) stubble in a winter wheat/corn (Zea nays L.) rotation system. Density of winter annual weeds were between 41 and 78% lower following most cover crops when compared with no cover control in 2 out of 4 site years, while dry weight was between 26 and 80% lower in all 4 site years. Impact of cover crops on the density of summer annual weeds was infrequent; however, weed dry weights were reduced by 70% in 1995 following burr medic and barrel medic. Dry weight of perennial weeds before corn planting were 35 to 75% lower following annual legumes compared with the control, while weed density was not affected. This study indicated a potential for annual legumes to reduce weed density and growth in no-till corn grain systems.

Weed suppression by annual legume cover crops in no-tillage corn

The objective of this study was to quantify the N contribution from different alfalfa (Medicago sativa L.) plant parts to a subsequent corn (Zea mays L.) crop, various soil fractions, and a 2nd yr spring barley (Hordeum vulgare L.) crop. The study was conducted at two field locations in Michigan, on a Capac loam (fineloamy, mixed, mesic, Aeric Ochraqualf) in East Lansing (EL) and on an Oshtemo sandy loam (coarse-loamy, mixed, mesic, Typic (Hapludalf) at the Kellogg Biological Station (KBS) in Hickory Corners

Quantifying the Nitrogen Contribution from Alfalfa to Soil and Two Succeeding Crops Using Nitrogen-15

The rising real prices of purchased inputs is driving smallholder maize (Zea mays L.) production towards lower levels of inorganic fertilizer. Legume intercrops are a source of plant N that can be produced locally and offer a practical complement to inorganic fertilizers. Field experiments conducted on a loamy sand (Typic Kandiustalf) soil evaluated the impact of relay-intercropping two legume crops, cowpea (Vigna unguiculata L.) and sunnhemp (Crotolaria juncea L.) into smallholder maize in Zimbabwe. The objectives were to quantify: (i) biomass and N yield of intercropped legumes, (ii) the impact of the legumes on companion maize yield and N uptake, and (iii) the response of a subsequent maize crop to legumes. Dry matter yield ranged from 0.6 to 4.6 Mg ha -1 for cowpea and 0.9 to 2.9 Mg ha -1 for sunnhemp, over two years. At the most, cowpea and sunnhemp produced 154 and 82 kg N ha -1 , respectively. Companion maize grain yields were not reduced when the legumes were relay-intercropped into maize fertilized at 0 to 60 kg N ha -1 . However, maize yields were reduced 18 to 31% when maize + legume intercrops were fertilized at 120 kg N ha -1 . In the subsequent year, maize grain yields were increased by 8 to 27% following maize + legume when no fertilizer N was applied, compared with maize following maize. Legumes reduced fertilizer needs of a subsequent maize crop by 36 kg N ha -1 . Intercropped annual legumes and small amounts of inorganic fertilizer offers a strategy to meet the N needs on smallholder farms.

Relay-intercropping of sunnhemp and cowpea into a smallholder maize system in Zimbabwe

Frost-seeding a legume into an established stand of winter wheat (Triticum aestivum L.) or interseeding a legume into a small grain at planting has potential to provide the benefits of a legume green manure while still allowing for the harvest of a revenue-producing crop. Field studies were conducted at three Michigan locations to quantify N accumulation by alfalfa (medicago sativa L.) and red clover (Trifolium pratense L.) frost-seeded into winter wheat or interseeded with oat (Avena sativa L.), and to evaluate the response of a subsequent corn (Zea mays L.) crop to legume and fertilizer N (...)

O. B. Hesterman

Papers

Fate of Legume and Fertilizer Nitrogen-15 in a Long-Term Cropping Systems Experiment

Weed suppression by annual legume cover crops in no-tillage corn

Quantifying the Nitrogen Contribution from Alfalfa to Soil and Two Succeeding Crops Using Nitrogen-15

Relay-intercropping of sunnhemp and cowpea into a smallholder maize system in Zimbabwe

Forage Legume-Small Grain Intercrops: Nitrogen Production and Response of Subsequent Corn