Plants for Biological N Fixation

3.1. Grain legumes. The main grain legumes in industrialized countries include soybean (Glycine max L.), pea (Pisum sativa L.), lentil (Lens culinaris Medikus), lupin and bean (Phaseolus spp.). The area dedicated to grain legume production has increased dramatically in the past 20 years. Some of the more dramatic examples include lupin production in western Australia (117,939 ha in 1980; 2,042,000 ha in 2000) and dry pea production in western Canada (49,300 ha in 1980; 1,219,000 ha in 2000) (FAOSTAT 2001). On the other hand, only modest gains in soybean and dry pea area were recorded for EU countries during this period, and soybean production in the USA has remained stable at approximately 29 million ha during the past 20 years (FAOSTAT 2001).

With the exception of Phaseolus spp., most of the aforementioned grain legume species can supply much of the N required for their own growth through BNF. Grain legumes also contribute N to following crops. In western Canada, it is estimated that 10 to 15 kg ha"1 of N are contributed for each 1000 kg ha"1 pea seed harvested (Wright, 1990). BNF by grain legumes is influenced by soil and edaphic factors including soil pH, moisture and indigenous soil N status. Higher BNF by field pea (31% increase) and lentil (10% increase) under no-till compared with tilled systems in western Canada was attributed to a soil environment more conducive to Rhizobia bacteria function (Matus et al 1997). A positive effect of no-till on BNF of soybean was reported in Brazil by Andrade and Hungria (this volume).

3.2. Forage and pasture legumes. Major forage legumes in temperate regions include alfalfa (Medicago sativa L.) and clovers (e.g. Red clover, Trifolium repens L.). Most of these crops are harvested for hay, though some direct grazing by livestock also occurs (Entz et al. 2001a). Area under forage production is decreasing in some industrialized countries. For example, fodder maize (Zea mays L.) has displaced significant areas of alfalfa hay production in EU countries. In Argentina, a significant perennial pasture hectarage has been replaced with a grain legume (mainly soybean)/maize system (Panigatti 1992). In the North America Northern Great Plains, 7.8 and 3.8 million hectares are currently dedicated to perennial forage legumes for hay production and grazing, respectively (Entz et al 2001a).

BNF by alfalfa and clover crops is well documented. In dry subhumid Manitoba, Canada, Kelner et al (1997) established that net N additions of an alfalfa hay crop were 84, 148 and 137 kg ha"1 in the first, second and third years of the stand, respectively. Most of the N benefits of alfalfa to following grain crops are captured in the first two or three grain crops (Campbell et al 1990), though significant additions have still been detected 13 years after alfalfa crop termination (Hoyt 1990).

Major forage legume species in humid areas of Uruguay and Argentina include red clover and Lotus spp. These species, often grown in combination with perennial forage grasses (e.g. Festuca spp. and Lolium spp.), are rotated with cereal and oilseed grain crops, often in an eight year rotation (half forage; half grain). In a long-term study at La Estanzuela, Uruguay, Rossello (1992) observed that four years of a legume grass mixture (grazed) increased total N in the 0 to 10 cm soil depth by 500 kg ha"1. This organic N was found to be quite unstable after the forage stand was terminated. Rossello strongly suggests that a better understanding of N loss mechanisms (i.e. denitrification, leaching, selective soil erosion) is necessary. Agronomic practices that have been shown to increase grain crop utilization of the biologically-fixed N elsewhere [no-till forage termination systems (Mohr et al 1999); elimination of post forage fallow periods (Campbell et al 1994)] are now being implemented in Uruguay.

A unique cropping system, which has been used by farmers in southern Australia for over 40 years, involves annual, self-seeding Medicago ("medic") and Trifolium spp. grown in sequence with grain and oilseed crops (Grace et al 1995). The single year medic and clover pasture plants have a high BNF potential and are capable of supplying most or all of the N for one or two following cereal grain crops (Grace et al. 1995). BNF of these medic and clover plants is sometimes limited by residual herbicides used in the grain phase of the rotation (B. Bellotti, personal communication).

Forage-based cropping systems provide benefits other than BNF, benefits that increase the overall sustainability of crop production. For example, deep roots of alfalfa can extract nitrate-N which may have leached below the rooting zone of annual grain crops (Campbell et al 1994). Scientists at the University of Minnesota (J. Lamb; M. Russelle and co-workers) are currently developing and testing non-BNF alfalfa cultivars for deep nitrate extraction, though conventional (i.e. N-fixing) alfalfa cultivars are capable of significant subsoil nitrate extraction (Campbell et al. 1994). Perennial forage legumes are also used in salinization management (Entz et al 2001a).

3.3. Green fallow and cover crop legumes. Fallow periods are often included in grain production systems in semi-arid zones in an effort to replenish soil water reserves and control weeds (Campbell et al 1990). Green fallow refers to a practice where short-duration legume crops are grown during this fallow period. Suitable legume species and their soil water use characteristics have been described by Biederbeck and Bouman (1994). BNF by legumes during the green fallow phase can contribute a fertilizer N replacement value of up to 150 kg ha"1 (Badaruddin, Meyer 1989). A cultivar of Chickling vetch (Lathyrus sativus L.), 'AC Greenfix', was recently developed specifically for green fallow (Biederbeck, personal communication). AC Greenfix has high tolerance to indigenous inorganic soil N and has a high water use efficiency.

In wetter zones, where continuous grain cropping is feasible, cover crops offer an opportunity to add N to the soil system. These cover crops can be relay cropped with grain or vegetable crops, and are often used in orchards. Heat and water resources for relay cropping legume cover crops with winter wheat have recently been documented for western Canada (Thiessen Martens, Entz 2001). The fertilizer N replacement value of relay cropped legume cover crops in Manitoba range from 0 to 70 kg ha"1 (Hoeppner 2001). Trials are currently underway at the University of Manitoba to adapt self-seeding medics for use as cover crops in western Canadian cropping systems.

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