| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a Dep. of Agronomy, Univ. of Florida, Inst. of Food and Agricultural Sciences, Gainesville, FL 32611
b Dep. of Entomology and Nematology, Univ. of Florida, Inst. of Food and Agricultural Sciences, Gainesville, FL 32611
* Corresponding author (jmscholberg{at}ifas.ufl.edu)
Received for publication February 1, 2005.
 |
ABSTRACT |
|---|
 TOPNOTES ABSTRACT INTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
Abbreviations: GM, green manure
|
INTRODUCTION |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
A GM, a crop used primarily as a soil amendment and a nutrient source for subsequent crops, may provide such an alternative. Unlike synthetic N fertilizers, legumes utilized as GM represent a potentially renewable source of on-farm, biologically fixed N and may also fix and add large amounts of C to cropping systems (Hargrove, 1986; Sharma and Mittra, 1988). Providing adequate soil N fertility with application of animal manure products may result in soil P loading (because manure N/P ratios are often much lower than those maintained by plants; Royer et al., 2003; Hao et al., 2004) or soil salinization (due to high ion concentrations in animal manure; Hao and Chang, 2003). Such excess P application and soil salinization may be avoided by use of leguminous GMs (Eigenberg et al., 2002). Green manures grown on site do not incur the often inhibitive handling and transportation costs of other organic inputs. The slow release of N from decomposing GM residues may be better synchronized with plant uptake than sources of inorganic N, possibly increasing N-uptake efficiency and crop yield while reducing N leaching losses (Abdul-Baki et al., 1996; Agustin et al., 1999; Aulakh et al., 2000; Cline and Silvernail, 2002). Green manure approaches may also drive long-term increases of soil organic matter and microbial biomass (Goyal et al., 1992, 1999; Chander et al., 1997; Biederbeck et al., 1998), further improving nutrient retention and N-uptake efficiency. When used in place of fallow, well-chosen GM may reduce erosion (Dapaah and Vyn, 1998), reduce nutrient or pesticide losses (Delgado et al., 2001; Gaston et al., 2003), and suppress weeds (Phatak et al., 1987; Dyck and Liebman, 1995; Burgos and Talbert, 1996) and specific crop pests (Bugg et al., 1990; Caswell et al., 1991). Green manures may also offer habitat or resources for beneficial organisms (Bugg et al., 1991; Nicholls and Altieri, 2001).
Historically the primary approach for maintaining soil fertility in intensive cropping systems around the world, GM use in modern agricultural systems has been nearly replaced by synthetic fertilizer, weed, and pest control inputs after the post-World War II development of the agrochemical industry (Smil, 2001; Dinnes et al., 2002). Combined with technological advancements in mechanization and (in the USA) the effects of costly government support programs created after the Great Depression, use of such agrochemical inputs increased yields while reducing farm expenses and crop-rotation requirements necessitated by many GM techniques. Economic gains from these changes, however, have not been experienced uniformly by all farmers. Large costs may also be deferred to the future through environmental degradation, farm consolidation and overspecialization, and government spending associated with reliance on current technologies (see also Schaeffer, 1997).
While organic farm surveys indicate widespread use of cover crops and GM (Organic Farming Research Foundation, 1999, 2004), it remains unclear which GM–cover crop species are used, how they are used, and the type and degree of production benefit that GM–cover crop use provides. Although ideologically favorable to GM approaches, the expansion of the organic agricultural sector has involved use of animal-based products as well as botanical extracts and a limited number of allowable synthetics. Arguably, these materials find use as simple substitutes for conventional inputs rather than employment in farm-based, diversified, whole-systems approaches to agriculture as originally envisioned by the organic movement.
Despite limited but significant successes in research and on-farm settings, GM-based cropping systems have regained little parity with current conventional and organic approaches to crop production. Conventional inputs—and many organic inputs—deliver readily known and adjustable levels of nutrients or active ingredients. Such materials often have well documented, consistent patterns of availability or action. Green manures, however, are biological organisms affected by the cropping environment, regularly confounding direct control by farm managers. Economic competitiveness of GM may thus require delivery of multiple services rather than the "one-for-one" approach more effective with chemical inputs, animal-based products, and botanical extracts. (Multiple services could include, for example, provision of biologically fixed N, pest and weed control, increase of soil organic matter, and reduction of soil erosion or agrochemical loss.) Even within such multifaceted approaches, however, procurement of particular "keystone" services may be required as an initial condition for system acceptability. Satisfying crop demand for N, often the most limiting nutrient for plant growth, frequently appears as such a keystone obstacle to GM reintroduction.
The breadth of useful species, growing environments, and management strategies summarized in Tables 1
to 4 highlight the complexity of options for GM approaches to crop production. Yet proper assessment of GM techniques requires an even greater understanding of the site-specific relationships between the life cycles of the plants used (both GM and subsequent economic crops), the production environment (climate, weather, soil, and pests), and management options (for example: type, patterns, and timing of tillage, planting, irrigation, and fertility and pest control inputs, as well as production goals). The challenge of properly reintegrating GM techniques back into our menu of agronomic options will require a cooperative effort to more systematically examine these interrelationships and develop more deliberate, knowledge-intensive support systems.
|
|
|
|
|
|
GREEN MANURE RELATIONSHIPS WITH ENVIRONMENT AND MANAGEMENT |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
Genetic differences (species and variety) may dictate that some legumes grow larger and accumulate more N than others. Environment (temperature, soil type, and nutrient and water availability) and management (e.g., planting density and timing, mowing, and pest control) may further alter performance of individual GM species (Kouyate et al., 2000; Ross et al., 2001; Steinmaier and Ngoliya, 2001). Because they do not derive direct sales profit, GM species are often chosen that require acceptably low levels of nutrient, irrigation, and pest control inputs and often fit into otherwise unplanted fallow periods. Legume GM species are often preferable to nonlegumes because they supply their own N, but in production scenarios where N is less limiting, where a specific GM service other than high N supply (such as allelopathy) is sought, or where legumes do not perform well, nonlegumes or mixtures of legumes and nonlegumes may be more advantageous. Desirability of GM may also include or exclude ability to reseed, growth habit (e.g., upright, prostrate, or viney), aggressiveness, and presence of toxic or allelopathic chemicals affecting livestock, crops, or plant pests; these characteristics are often controlled at the species or variety level.
Biological N fixation (for legumes) and overall N accumulation during growth are primary factors governing the adequacy of a GM as an N source (see Tables 1–4 for reports of GM N contents). Estimates of N accumulation for leguminous GMs and the relative contribution of biological N fixation in this process ranges broadly depending on soil fertility, water availability, and GM species. Generally speaking, most legumes accumulate N from biological fixation when demand cannot be met by uptake of N from the soil (Gardner, 1985). For example, sunn hemp (Crotalaria juncea L.) has been estimated to fix 27 to 39% (Ramos et al., 2001), 72 to 81% (Ladha et al., 1996), and 91% (Seneratne and Ratnasinghe, 1995) of its total N in different study locations and conditions. Reduction of soil N through competition generally increases rates of biological N fixation by legumes. Karpenstein-Machan and Stuelpnagel (2000) found that the relative contribution of N fixation to hairy vetch (Vicia villosa Roth) and crimson clover (Trifolium incarnatum L.) N increased when intercropped with an increasingly larger proportion of cereal rye. The same study showed greater N accumulation for optimal mixtures of legumes with cereal rye than legumes alone. However, water stress and deficiency of nutrients other than N may significantly reduce fixation (Gardner, 1985).
Climate probably limits GM species selection more than any other single factor. In very cold climates, temperate GM species survive during the spring, summer, and fall. As one moves to warmer climates, increasing winter temperatures permit temperate GM species to persist during winter months while tropical GM species become better suited during warmer months. Where the lowest temperatures remain above freezing, tropical GM species may survive all year, and high temperatures may begin to exclude the use of temperate GM species altogether. However, light levels, precipitation, soil type, and pest pressures also interact with temperature to determine how specific GM species will perform in a given location. For example, temperate legumes at higher latitudes often attain most production during the long, cool days of early fall and late spring (Cline and Silvernail, 2001). At lower latitudes, however, daylight hours with consistently cool temperatures are far more limited.
In many environments, seasonality may relate as much or more to dry or wet periods. Transpiration of soil water by GM poses a major concern for rain-fed systems in semiarid environments (Vigil and Nielson, 1998; Brandt, 1999). Especially in regions with annual precipitation around 500 mm or less, management of well-adapted GM such as wheat, pea or lentil (Lens culinaris Medik.) must prevent reduction of subsequent crop stand or yield due to soil water depletion (Bullied and Entz, 1999; Baumhardt and Lascano, 1999). Early termination of GM or timing of termination before periods of natural soil water recharge may or may not mitigate such problems (Schlegel and Havlin, 1997; Thiessen Martens et al., 2001). In low-rainfall climates, or in higher rainfall climates on sandy soils, establishment of small-seeded GMs such as clovers and medics may also be particularly dependent on timely rainfall when irrigation is not available. Large-seeded GMs planted at deeper depths may have better establishment potential in such environments, but are still vulnerable to periodic failure (Keeling et al., 1996). Where convenient, use of self-reseeding GM may better ensure successful establishment (Walsh et al., 2001).
Green manure performance and patterns of subsequent effects often differ based on gross differences in soil textural type. Green manure growth and N accumulation are usually greatest on loamy soils due to their relatively high inherent fertility, nutrient and water retention capacity, and microbial biomass (see Tables 1–4 for notes on soil types within studies). With their high potential to retain released N, such soils also help mediate short-term benefits from GM to subsequent crops, even when considerable lag times exist between peak GM N release (from decomposition) and subsequent crop N uptake. On sandy soils and in warm, humid climates, even short lag times between peak GM N release and subsequent crop N demand can result in significant N leaching losses (Nelson and King, 1996; Wyland et al., 1996; Weinert et al., 2002). To derive acceptable benefits, GM-based systems on such soils may need to make greater use of intercropping GM with economic crops or manipulation of GM decomposition (see discussion below). Compared with inorganic N approaches, GM approaches on sandy soils may provide benefit by temporarily increasing N immobilization potential during lag times to prevent leaching losses (Green and Blackmer, 1995), effectively requiring more massive and recalcitrant GM as well as sources of significant N (whether from GM or from supplementary sources).
Sandy soils may also present different biotic problems than related in much of the literature. Many temperate legumes that perform well on fine-textured soils are poorly adapted to low nutrient levels and pests (particularly nematodes) common in sandy soils (see below). Also, successful rhizobial inoculation of temperate leguminous GM in sandy soils is sometimes problematic. In such soils, pest, inoculation, and nutrition problems can interact in a confounding manner, creating variable GM performance within different areas of the same plot or field. Extreme clay content, on the other hand, may limit the effectiveness of GM species sensitive to restricted soil aeration (in wet climates or seasons) or low soil water potential (in dry climates or seasons). In general terms, plants used as GM may have different adaptations to soil pH and fertility that vary widely across soil textural types or climatic regions (see Tables 1–4 for notes on GM study environments).
The growing season of a GM must fit the demands of a particular crop rotation, which often means GM is planted during fallow periods with weather unfavorable for optimal production of economic crops. In temperate environments on fine-textured soils, winter-hardy legumes such as vetch, clover, and medics are capable of accumulating large amounts of biomass (7–10 t ha–1) and N (150–250 kg N ha–1; Table 3) and delivering substantial N benefit to subsequent spring-planted crops. A number of investigators in such environments have shown that well-managed temperate leguminous GMs can fully satisfy N requirements of subsequent crops such as sweet corn (Zea mays L. cv. Rugosa; Griffin et al., 2000; Cline and Silvernail, 2002). Studying fertigated, mulched tomato (Lycopersicon esculentum Mill.) in Maryland, Abdul-Baki et al. (1996) found that plastic mulch with 112 kg ha–1 inorganic N (recommended rate) produced lower yields than hairy vetch, crimson clover, and hairy vetch plus rye live mulches combined with 56 kg ha–1 inorganic N. In tropical environments, sunn hemp, cowpea [Vigna unguiculata (L.) Walp.], and mungbean [V. radiata (L.) R. Wilczek] may also accumulate large amounts of biomass and N (Table 1). Because no freezes occur in tropical environments, these legumes may be followed immediately by frost-sensitive crops. For example, studies in Asia have shown such GM to be capable of supplying the N requirements of rice (Oryza sativa L.), again on fine-textured soils (Agustin et al., 1999; Aulakh et al., 2000; Ladha et al., 2000).
However, GM–crop considerations are frequently more complex. Nutrient-demanding crops pose particular challenges to GM-based approaches, especially in soils with poor exchange capacity and when peak crop nutrient demand occurs well after release by the GM. For example, N recommendations for sweet corn on sandy soils in Florida fall between 180 and 200 kg N ha–1 (Hochmuth and Cordasco, 2000). This crop is generally planted in the spring to avoid pest pressures and low-light conditions that occur in the rainy summer season. Modern sweet corn varieties are bred for adaptation to high plant population and high inputs; even small reductions in N availability may significantly reduce grade for human consumption (which depends heavily on ear size and uniformity of fill, USDA, 1997) as well as ear yields (Cherr, 2004). However, providing adequate GM N supply during early spring in Florida is particularly difficult because well-adapted tropical GM species die and decompose during winter freezes (leading to potentially large N leaching losses on sandy soils) and temperate leguminous GM species appear poorly adapted to the region's variable weather, soils, and nematode pests. Options may exist to improve the effectiveness of GM for spring-grown sweet corn in Florida (e.g., intercropping or manipulation of GM decomposition). However, use of fall- or winter-grown crops, crops with lower N demand or less sensitivity to N reductions, crops better adapted to low plant populations, food crops without price premiums for large fruits, and crops grown for animal forage or feed may more readily afford integration of GM-based approaches into existing systems.
|
PEST CONTROL WITH GREEN MANURES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
Suppression of parasitic nematodes by GM also exemplifies the importance of highly specific GM–environment–management interactions. Different crop species, and even different varieties of the same crop species, vary in their resistance to different nematode species. Plants may also show different levels of susceptibility to regional races and local isolates of nematode species. Overuse of resistant crop varieties can select for "resistance-breaking" nematodes (McSorley, 2001). Crop rotation with a nonhost or nematode-suppressant GM may help reduce such selection pressures by providing an opportunity to disrupt nematode life cycles. For example, a number of GM species act as nonhosts or suppressors of one or more Meloidogyne species (root-knot nematodes): castor (Ricinus communis L.), iron-clay cowpea (Vigna unguicalata cv. Iron Clay), showy crotalaria (Crotalaria spectabilis Roth), joint-vetch (Aeschynomene Americana L.), marigolds (Tagetes minuta L. and T. erecta L.), sesame (Sesamum indicum L. cv. Paloma), sunn hemp, barley (Hordeum vulgare L.), green panic grass [Megathyrsus maximus (Jacq.) B. K. Simon & S. W. L. Jacobs], glycine [Neonotonia wightii (Wight & Arn.) J.A. Lackey], horsebean [Canavalia ensiformis (L.) DC], velvetbean (Mucuna spp.), and Sudex [Sorghum bicolor (L.) Moench x S. x drummondii (Steud.) Millsp. & Chase; Sipes and Arakaki, 1997; Al-Rehiayani and Hafez, 1998; McSorley, 1999].
On the other hand, some GM species may exacerbate infestations of plant-parasitic nematodes by acting as hosts. In Hawaii, Sipes and Arakaki (1997) found populations of Meloidogyne spp. on taro [Colocasia esculenta (L.) Schott] increased significantly following alfalfa (Medicago sativa L.), cowpea (variety unreported), lablab [Lablab purpureus (L.) Sweet], hairy vetch, mustard (Brassica napus L.), oat (Avena sativa L. cv. Coker), Rhodes grass (Chloris gayana Kunth), cereal rye (cv. Danka), annual ryegrass (Lolium multiflorum Lam. cv. Alamo), siratro [Macroptilium atropurpureum (DC.) Urb. cv. Siratro] and wheat (Triticum aestivum L., multiple cultivars). In Florida, McSorley (1999) found high root-knot nematode populations on roots of potential GM species of pearl millet [Pennisetum typhoides (Burm. f.) Stapf & C.E. Hubb syn P. glaucum (L.) R. Br.] and Japanese millet (Echinochloa frumentacea Link). Sunn hemp has been found to be a poor host of reniform nematodes (Rotylenchulus reniformis), yet may support a slow population increase with time (Caswell et al., 1991; Wang et al., 2003b). Some GM species known as nonhosts or direct suppressors of Meloidogyne spp. may have other undesirable characteristics, or may vary in their adaptability to a particular environment and management system. As discussed above, GM species well suited for control of one type of nematode may show susceptibility to others. Al-Rehiayani and Hafez (1998), working in Idaho, found varieties of buckwheat and mustard to be non- or poor hosts for a Meloidogyne chitwoodi race, while Sipes and Arakaki (1997) found opposite results with Meloidogyne javanica in Hawaii. Nematode management with GM thus requires specific information on plant host status. Generally, GM species closely related to subsequent cash crops often host similar nematodes.
Green manures may control pests indirectly by providing habitat for organisms that feed on or parasitize weeds, insects, and nematodes. Greenhouse studies in Florida using sandy soil have shown that sunn hemp can increase omnivorous and predatory nematodes on soils with low organic matter (<2%), though perhaps not enough to control parasitic nematodes such as Meloidogyne spp. (Wang et al., 2003a). Wang et al. (2001) found application of sunn hemp residues to a silty clay at a rate of 10 g dry residue kg–1 dry soil enhanced nematode-trapping fungi. Studying cucurbit crops (Cucurbita spp.) with buckwheat refuges, Platt et al. (1999) found numbers of insect predators and parasitoids caught on sticky traps increased by 2 to 19 times as one moved toward buckwheat refuges from 20 to 35 m away. However, the method of biological control differs among pests, with some controlled by general increases of biological activity while others require development of proper habitat for specific antagonists (Bugg et al., 1991; Davis et al., 1996; Wang et al., 2003a, 2003b). Efficacy of biological control may also depend on space and time. For example, Platt et al. (1999) found striped cucumber beetle (Acalymma vittatum F.) populations increased beyond the economic threshold at distances >10 m from buckwheat refuges and when buckwheat stopped flowering. Such spatially and temporally explicit information regarding the effects of GM use on both pest and beneficial arthropods remain rare, especially for field-scale studies that also quantify economic crop yields and have control treatments with standard chemical pest suppression.
Allelopathic chemicals released by specific GM species may directly inhibit weed growth, although allelopathy is highly specific to GM species, environment, residue management, and target organism (Blackshaw et al., 2001; Caamal-Maldonado et al., 2001; Inderjit, 2001). Small-seeded weeds may be particularly susceptible to growth-reducing stresses (Davis and Liebman, 2001). Allelopathic chemicals and delayed release of N from decomposing GM may thus reduce small-seeded weed growth more than that of large-seeded crops, providing such crops with a critical early season advantage (Dyck et al., 1995; Petersen et al., 2001).
|
BUILDING SOIL ORGANIC MATTER WITH GREEN MANURES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
Generally, organic matter associated with the smaller size soil fractions—silt and clay—may experience more physical and chemical protection from decomposition than that associated with larger (sand) size fractions. For example, on a loam soil having about 35% sand and 1.6% soil organic C, Kandeler et al. (1999) found roughly 50 to 75% of soil organic C and microbial biomass N associated with the clay-size fraction (<2 µm) and roughly 90 to 95% of soil organic C and microbial biomass N existing within silt and clay size fractions together (<63 µm). Organic matter and microbial biomass in fine-textured soils may, therefore, show greater and more rapid response potential to GM approaches. In two similar experiments after roughly 10 yr of a pearl millet and wheat rotation on a low-organic-matter (
0.40–0.50% organic C) sandy loam (65–69% sand) in India, Goyal et al. (1992, 1999) found combinations of inorganic fertilizer and organic amendments (wheat straw, animal manure, or sesbania GM) generally increased soil organic C, total N, microbial biomass C, and enzyme activity more than inorganic fertilizer alone in the top 15 cm of soil (plow layer). Still, with manure–residue additions in these studies varying between 8 and 15 t ha–1 annually, relative increases in soil organic C amounted to only about 5 to 15% during the entire study periods. Realization of greater SOM increases, especially in hot, humid, sandy environments, may require greater or more consistent additions of recalcitrant residue, especially under conventional tillage (see Tables 1
–4 for summary of GM biomass accumulations in literature; see also Yadav et al., 2000). In some cold environments, soils may possess large amounts of organic matter with slow turnover rates; changes in residue management may affect SOM only slowly under such conditions (Franzluebbers and Arshad, 1996; Pikul et al., 1997). Finally, while long-term and large-scale studies may demonstrate increased economic crop yield in response to higher SOM (Kanchikerimath and Singh, 2001; Majchrzak et al., 2001), the timing and amount of N mineralization from soils and residues following GM generally have major effects on subsequent economic crop yields in the short term (Vyn et al., 2000; Soon et al., 2001; see below).
|
MANIPULATING NITROGEN SUPPLY WITH GREEN MANURES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
Green Manure Management
Nitrogen release from plant residues depends on a large number of interactive factors including chemical composition and N concentration, temperature, and water availability (Andren et al., 1992; Schomberg et al., 1994). These factors in turn depend on many of the subjects in Fig. 1. Additionally, GM residues may alter root growth and N uptake patterns of subsequent crops (see below).
Some researchers have found N-substitution values for GM in excess of actual GM N accumulation, suggesting that GM N is sometimes taken up more efficiently than chemical fertilizer N or that GM modifies the soil environment, crop growth, or both such that greater crop N uptake is possible (e.g., Agustin et al., 1999; Yadav et al., 2000; Prasad et al., 2002). On the other hand, GM N release can occur before (generally in warmer environments; Sainju and Singh, 2001) or after (generally in cold environments; Griffin and Hesterman, 1991; Shrestha et al., 1999) peak N demand from subsequent crops. In these cases, manipulation of residue quality through proper selection of GM or GM mixtures, tillage, and planting densities and timing may better synchronize leguminous GM-N release with subsequent crop demand.
Residue Management
Decomposition and N release generally occur faster for residues with lower C/N ratios and lignin and polyphenol contents (Seneviratne, 2000). Optimum temperature and water availability for soil-based decomposition are usually around 35°C and field capacity, respectively (Vigil and Kissel, 1995; Katterer et al., 1998; Lomander et al., 1998). Mathematically, investigators often characterize decomposition (biomass or C loss) and N loss as negative exponential declines with time, with rate directly affected by one or more decay-rate constants depending on temperature, water and N availability, and chemical quality of the residue (see also Douglas and Magdoff, 1991; Dou et al., 1996; Quemada et al., 1997). For example, Somda et al. (1991) used a litterbag study of a number of legumes and nonlegumes; C/N and lignin/N ratios were generally lower for legumes (8:1–27:1, and 2:1–9:1, respectively) than for nonlegumes (27:1–186:1, and 4:1–44:1, respectively), and decay-rate constants of both fast and slow pools were greater for legumes. Kuo and Sainju (1997) showed that mixing hairy vetch residue with increasingly large proportions of cereal rye and annual ryegrass residues slowed the relative rate of N release. Working in Georgia, USA, Ranells and Wagger (1996) also found faster decomposition and N release for hairy vetch and crimson clover grown alone than when grown with cereal rye, but still found no net N immobilization in any treatment (including rye alone).
Leaf C/N ratio and lignin content is generally much lower than for stems or roots of the same plant. In most studies, leaf decomposition and N release occurs significantly faster than for other tissues. Prolonged periods of N immobilization are often recorded for recalcitrant stems and roots (Collins et al., 1990; Cobo et al., 2002). Investigating nutrient release of about a dozen legumes, Cobo et al. (2002) found that leaves decomposed five times faster than stems, decomposition was closely related to cell wall content, and N release was most dependent on the lignin/N ratio. Both Cobo et al. (2002) and Collins et al. (1990) showed that the decomposition rates of leaf and stem mixtures were intermediate to leaves and stems decomposing alone, but faster than predicted by summing individual leaf and stem decomposition rates. These studies suggest that fungal decomposers may redistribute N from leaves to more recalcitrant tissues during decomposition.
Nitrogen contributions from belowground tissues of GM (roots and root nodules) are difficult to determine due to the rapid turnover of these tissues and possible root exudation of N. For example, Ramos et al. (2001) determined that 39 to 49% of all N accumulated by swordbean [Canavalia ensiformis (L.) DC.] and velvet-bean [Mucuna aterrima (Piper & Tracy) Holland] was below ground, and 10 to 12% of all accumulated N was transferred to the soil by root and nodule turnover and root exudation. In a 3-yr study, Griffin et al. (2000) reported 56, 46, and 38% of total biomass and 32, 28, and 19% of total N in roots at final sampling for alfalfa, cereal rye, and hairy vetch plus rye intercrop, respectively; however, both of these studies occurred on soils that were fine textured, high fertility, or both. On a sandy soil, Cherr (2004) found roots accounted for as little as 10% of the total biomass and 3% of the total N content at final sampling for sunn hemp, and 13 to 22% of total biomass and 7 to 10% of total N content at final sampling for temperate legumes such as blue lupine (Lupinus angustifolius L.) and cahaba white vetch (Vicia sativa L.). Although root biomass can be relatively small, Puget and Drinkwater (2001) showed that C from lignified roots may persist in the soil longer than that of highly labile shoots, increasing the long-term importance of root contributions to SOM and possibly also soil organic N.
Soil incorporation of plant residues may speed decomposition and N release by buffering temperature and water regimes relative to the surface (Mansoer et al., 1997; Thonnissen et al., 2000a). Schomberg et al. (1994) found greater N immobilization potential for sorghum [Sorghum bicolor (L.) Moench) and wheat residue on the soil surface, although initial N immobilization was more rapid when the residues were buried. At peak immobilization (5 mo–1 yr or more), sorghum and wheat residues tied up 150 to 170% of their initial N content. For these low-N residues, net N immobilization lasted >1 yr on the soil surface (study ended after 1 yr) and only 1/3 yr for buried residues. Near-complete decomposition and N release of soil-incorporated legumes has been found to require 15 to 20 wk in temperate environments (Bowen et al., 1993), but may take place in only 2 to 6 wk in tropical environments (Thonnissen et al., 2000a). Although it may slow the process somewhat, surface decomposition in warm, high-precipitation environments on sandy soils may still result in rapid N loss despite the presence of recalcitrant residue (Mansoer et al., 1997; Cherr, 2004). In such systems, it may be critical that economic crops are either planted as soon as possible after termination of GM or direct planted into adequately suppressed GM. In cool environments, however, slow surface decomposition under very dry conditions or soil cooling underneath residue mulch in wet conditions may necessitate soil incorporation of GM residue, selection of a lower biomass GM, or both (Fortin and Pierce, 1990; Holderbaum et al., 1990; Pikul et al., 1997).
Growth Management
Growth management may also exert effects on GM residue quantity and quality. Generally, leaf tissue fractions dominate shoots during early season growth, while stems become increasingly important as time goes on (Gallaher, 1991; Mansoer et al., 1997; Cherr, 2004). Low plant populations may increase the proportion of GM as stem or lignin, while higher plant population may favor greater leaf and nonstructural carbohydrate production (Gallaher, 1991; Marshall, 2002) especially in GMs with upright growth. High plant populations may facilitate early season production with earlier canopy closure, while lower plant populations may improve later production as compensatory growth takes effect; however, extremely low or high plant populations may reduce production by overcoming GM's ability for compensatory growth or exacerbating competition for nutrients, water, and light as plants become larger. Compensatory behavior may be aided by cutting plants to induce branching if and when they possess indeterminate growth habit, and competition may be reduced by mowing or grazing plants if and when they possess growing points below mowing or grazing height (Stopes et al., 1996; Shrestha et al., 1999; Ross et al., 2001; Marshall, 2002). Termination date may also affect the fraction of plant biomass and N as leaf, stem, reproductive, or senesced tissue if the GM responds to late-season changes in temperature and photoperiod; lengthening the GM growing season may or may not, therefore, increase GM biomass and N content (Cline and Silvernail, 2001; Sainju and Singh, 2001).
In addition to providing greater weed control, intercropping and live mulching with GM may help better synchronize GM N release with subsequent crop N demand, especially in warm, humid climates or on coarse soils (Dapaah and Vyn, 1998; Zemenchik et al., 2000; Jeranyama et al., 2000; Blackshaw et al., 2001). These practices, however, must be carefully managed to prevent crop–GM competition (Guldan et al., 1996; Ghaffarzadeh, 1997; Jeranyama et al., 1998; Rao and Mathuva, 2000). For example, in Georgia, USA, Phatak et al. (1999) developed a successful relay cropping system in which a cool-season GM of crimson clover is permitted to reseed itself annually and cotton is no-till planted as crimson clover declines at the onset of warm weather (see also Hartwig and Ammon, 2002).
Subsequent Crop Management
Due to different nutrient release characteristics and effects on soil water, temperature, and biota, crop root growth patterns may be markedly different following GM than chemical fertilizer. Because its N release is driven by decomposition, GM may represent a source of slow-release N. Spatial distribution of GM residue may be heterogeneous, creating localized areas of N release and other GM-mediated impacts (Mahmoudjafari et al., 1997). Green manures may have effects on soil moisture transfers, temperature, and populations of root-parasitizing organisms such as nematodes (Hartwig and Ammon, 2002).
Under conventional tillage, the use of GM or other crop residues or organic amendments appears to change crop root length density (RLD) within the plow layer. Pallant et al. (1997) and Nickel et al. (1995) reported greater RLD at depths below 12 to 15 cm for conventionally tilled corn with GM or crop residues, while chemically fertilized corn may maintain greater RLD in the upper soil depths (Nickel et al., 1995; Goldstein, 2000) and sustain greater root damage from pests (Goldstein, 2000). For potato, Opena and Porter (1999) reported that organic amendments (mixture of waste potato compost plus cattle manure and sawdust) also increased RLD in the 0- to 30-cm plow layer; however, surface-applied residues may encourage crop root proliferation near the soil surface and near the plant, especially when residues show potential for significant N release on decomposition (Thorup-Kristensen and van der Boogaard, 1999; Cherr, 2004). Altered patterns of root growth may benefit crop performance only if nutrient and water availability in root-explored areas remain adequate. Few studies, if any, have documented how changes in the amount and distribution of irrigation and supplementary fertilizer N affect uptake patterns, growth, and reproductive yield of economic crops following GM.
|
ECONOMIC ANALYSES OF GREEN MANURES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
Internally, a particular approach to crop production will affect economic profit and risk. Farmers may also have to consider input, transition, and opportunity costs associated with GM (Ali and Narciso, 1996; Ali, 1999). External factors aside, GM approaches are more often found to be economically superior to conventional approaches when capable of providing multiple services (Young et al., 1994; Ghaffarzadeh, 1997; Ali, 1999), when GM replaces costly conventional inputs such as fallow management or plastic mulches (Wyland et al., 1996; Ellis et al., 2000), when one or more species from multispecies GM mixtures serves as an economic crop (Painter et al., 1995; Ghaffarzadeh, 1997), and when strict GM crops are replaced with crops that provide food or feed while residue is left in the field (Ali, 1999). Variable weather or other environmental patterns may also alter year-to-year profitability of GM approaches (Vigil and Nielson, 1998). Use of GM or GM plus reduced tillage approaches may also economically justify and ecologically mitigate the use of other inputs (Young et al., 1994; Painter et al., 1995).
|
FUTURE GREEN MANURE RESEARCH NEEDS |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
Practical information about the composition and N concentration of GM as it changes during a growing season is often lacking. Most studies report only end-of-season values for GM biomass and N content and concentration. This poses an obstacle to GM adoption because growing time in an on-farm production system differs from that studied in research, creating yet another way GM biomass and composition in an on-farm setting may differ from reported findings. Repeated sampling of GM with time provides much more meaningful information for development of appropriate GM selection and management. Additionally, repeated measurements of GM leaf area index and light interception coupled with reliable weather data (temperature and solar radiation) are needed to build simple predictive models for GM growth.
A critical need also exists for growth analysis of GM-amended crops. Final yields and crop biomasses may not reflect patterns of crop growth and N accumulation throughout a growing season. For example, in Florida, Cherr (2004) found GM had significant benefit for spring-planted sweet corn during early- to midseason; however, GM-amended corn fell behind conventionally fertilized treatments in terms of late-season ear fill. These differences led investigators to seek information about patterns of late-season corn root growth and water and N availability as affected by GM residue. In such a case, growth analysis revealed critical information necessary for understanding GM-based system behavior. To develop appropriate management techniques, information is also needed regarding how changes in the amount and distribution of irrigation and supplementary N fertilizer affect the availability and uptake of water and N by economic crops following GM.
Most existing information on GM performance comes from studies conducted in temperate or tropical environments on fine-textured soils, the results of which may not apply to regions with intermediate climates (warm but with winter freezes) or sandy soils. Studies on sandier soils in Florida, Zimbabwe, Zambia, and India have shown reduced performance for temperate GM species and low benefits from tropical GM species (Gallaher, 1993; Jeranyama et al., 2000; Steinmaier and Ngoliya, 2001; Cherr, 2004). Development of GM techniques appropriate for such regions will require approaches that better manipulate GM N accumulation and timing of N release.
If GM does not supply adequate N to meet the requirements of subsequent crops, then supplementary inorganic N may be required to prevent yield reductions. Many studies have compared the use of GM alone against synthetic fertilizers (Carsky et al., 1999), and others have also investigated GM used in combination with synthetics (Ladha et al., 2000). These studies, however, usually do not establish optimums for chemical N rate, whether used alone or with GM. This makes it difficult to assess how much (if any) chemical N is required in addition to GM for optimal production. Some studies establish optimal chemical N rates for cut-and-carry systems where GM is not grown in place (Prasad et al., 2002), but this does not reflect common agricultural practice in developed countries.
Although some studies have documented the ability of GM to host beneficial organisms or suppress insect pests (Bugg et al., 1991; Nicholls and Altieri, 2001), little field-scale data exists on insect population dynamics across space and time and the effects of pest attack on economic crop yield when GM is substituted for insecticide. Without this data, and without comparisons to control treatments with conventional or organic insecticides, it is impossible to determine when and where GM use can substitute for such insecticides in field-scale production.
Needs for Whole-Systems and Participatory Approaches to Green Manure Research
The traditional style of agricultural research and technology transfer may poorly suit the development of GM-based approaches to crop production. Such a traditional style involves separated, stepwise phases of initial planning, small-plot trials, larger scale studies under more realistic conditions, and finally the dissemination of finished technology to farmers (Wuest et al., 1999). Green manure-based alternatives to crop production are often not viable without the provision of multiple benefits. Because of the complex interrelationships between GM, management, and environment, this may demand adjustments of the entire farm system including the choice of economic crops, crop rotation, cultural practices, and marketing. The complexity of issues arising on the whole-farm level far exceeds the information gained in traditional, station-based research (Delate, 2002; Langeveld et al., 2005). Because planning and station-based research of GM usually occurs before (or without) dissemination and adoption–adaptation phases, there is little opportunity for planned feedback (van de Fliert and Braun, 2002). Application of GM in on-farm trials as a substitute technology, rather than use of appropriate GM-based systems, often fails as a result.
Our traditional research and development methods must be complemented with suitable farmer participation as well as whole-system evaluations. Effective involvement of farmers can help determine appropriate criteria for cropping-system evaluation, farmer needs and preferences, improved methods of dissemination and extension, and feedback. Such participatory elements can provide improved linkage and overlap between the planning, research, dissemination, and adoption–adaptation phases (van de Fliert and Braun, 2002). For example, this allows GM studies to be designed with complete farming systems in mind, GM research itself to become a more visible form of dissemination, and feedback from the dissemination and adoption–adaptation phases to help guide ongoing planning and research (Delate, 2002; Mueller et al., 2002).
In station-based settings, stakeholder groups have cooperatively planned and managed research and dissemination of results from fields or experimental farms (Delate, 2002; Mueller et al., 2002). Alternatively, Stoorvogel et al. (2004) describe selection of a limited number of representative, reliable, and visible farms for collaboration with academic researchers. Langeveld et al. (2005) integrated these concepts with the "nucleus and pilot" approach. Here, the performance of an entire system is evaluated on a researcher-controlled experimental farm under realistic conditions. The experimental farm (the nucleus) serves as an example for pilot farmers, who then test and evaluate the system in on-farm settings and communicate their experiences to both the researchers and other farmers.
Resource limitations restrict the number of whole systems that can be compared with each other, and management of such systems often changes continuously (Langeveld et al., 2005). Factorial studies, sometimes conducted within whole-systems studies, are important to provide an understanding of specific processes governing GM benefits (Drinkwater, 2002). Farmers make decisions based on multiple criteria that generally change with time and differ between individuals (Pannell, 1995; Kroma and Flora, 2001). Understanding how these criteria affect GM use may not be possible without on-farm, participatory, and whole-systems research approaches. On the other hand, maintenance of consistent data sets and GM evaluation may require that such approaches be complemented with on-station, researcher-controlled studies. Participatory studies often suffer from high farmer dropout rates and lack of effective communication between farmers and researchers. Bentley (1994) argued strongly that participatory research may be best suited to commercial farmers and farm sectors that have: (i) preexisting relationships with researchers and research institutions; (ii) highly organized ownership–management; and (iii) similar socioeconomic status with researchers. It may also be critical to identify early-stage farmer collaborators whose market orientation provides a benefit for the use of GM, such as those engaged in organic or direct market farming, and researchers must effectively screen GM approaches before on-farm study to reduce the risk of economic loss and discouragement for farmer collaborators. Finally, GM researchers should better involve organizations already networked with potential farmer collaborators. For example, the nearly 60 accredited organic certification agencies in the USA (USDA, 2005) have high levels of contact with farmers and other organizations with inherent interest in GM. Such agencies could provide crucial partners in the search for farmer collaborators and for subsequent dissemination, market development, and monitoring of GM approaches with other farmers.
|
CONCLUSIONS |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
|
ACKNOWLEDGMENTS |
|---|
|
NOTES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
|
REFERENCES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONGREEN MANURE RELATIONSHIPS WITH...PEST CONTROL WITH GREEN...BUILDING SOIL ORGANIC MATTER...MANIPULATING NITROGEN SUPPLY...ECONOMIC ANALYSES OF GREEN...FUTURE GREEN MANURE RESEARCH...CONCLUSIONSREFERENCES |
|---|
