Grain Legumes in Northern Great Plains : Impacts on Selected Biological Soil Processes

Cropping systems in the Northern Great Plains have shifted from fallow-based to legume-based systems. The introduction of grain legumes has impacted soil organisms, including both symbiotic and nonsymbiotic N-fixing bacteria, pathogens, mycorrhizae and fauna, and the processes they perform. These changes occur through effects of legume seed exudates, rhizosphere exudates, and decomposing crop residues. The legume–Rhizobium symbiosis results in dinitrogen (N2) fixation that adds plant available N into the soil system. It is estimated that about 171 million kg N2 was fixed by field pea (Pisum sativum L.), lentil (Lens culinaris Medik.), dry bean (Phaseolus vulgaris L.), and chickpea (Cicer arietinum L.) crops in the Canadian Prairies in 2004, representing 7% of the total fertilizer-N (2580 million kg) used by Canadian prairie farmers in that year. Similarly, an estimated 40 million kg N2 was fixed by field pea, lentil, and dry bean (including chickpea) crops in U.S. agroecosystems in 2004. Some of the fixed N2 is recycled for the benefit of nonlegume crops grown after grain legumes. Many other associations benefit from the legume in a cropping system, including mycorrhizal associations that improve plant nutrient and water uptake, changes in the pathogen load and disease development, and overall changes in the soil community. Legumes contribute to greenhouse gas (N2O and CO2) emissions during nitrification and denitrification of fixed N. However, because less fertilizer-N is used in legume-based cropping systems, overall greenhouse gas emissions are usually less than those in fertilized monoculture cereals. Therefore, grain legumes in Northern Great Plains have positive effects on agriculture by adding and recycling biologically fixed N2, enhancing nutrient uptake, reducing greenhouse gas emissions by reducing N fertilizer use, and breaking nonlegume crop pest cycles. C ROTATIONS have historically been the predominant cropping system in the semiarid Canadian Prairies and Northern Great Plains of the United States, mainly to reduce the risk of crop failure resulting from soil moisture deficits (Spratt et al., 1975; Grant et al., 2002). It has long been recognized that fallow-based rotations reduce soil organic matter, primarily because little plant C is returned to the soil, but also due to increased soil erosion during the fallow phase (see Janzen, 2001, for a historical perspective of concerns about organic matter loss). Soils in southern Saskatchewan and the Palouse region of the U.S. Pacific Northwest, lost about 50% of the original organic matter after more than 80 yr of wheat-fallow cropping (Campbell and Souster, 1982; Papendick and Parr, 1997; Schnitzer et al., 2006). With the advent of conservation tillage systems (minimumor zero-tillage) in the 1970s, more soil moisture was conserved than what was possible under conventional-tillage. Continuous cropping became a viable option for producers because soil moisture was conserved in such tillage systems; consequently, fallow acreages in Canada and the United States began to decline (Fig. 1). Since monoculture cereal cropping often results in pest buildup (Pedersen and Hughes, 1992; Bailey et al., 2001), other crops were sought to include in rotations, as crop diversification was required (see Tanaka et al., 2002, for a historical perspective of cropping systems). Pulse crops became attractive rotation crops, and their acreages increased (Fig. 2). In the Canadian Prairies, field pea is grown on the greatest acreage, while lentil, chickpea, and dry bean are also grown but to a lesser extent (Fig. 2a). In the U.S. Northern Great Plains, dry bean, including chickpea, are the most common, but field pea and lentil are also grown (Fig. 2b). The introduction of these pulse crops to the previously fallow-based agricultural systems of the Northern Great Plains has many implications, including changes in soil biology and the resultant biological processes. In this paper we discuss the effects of pulse crops on soil microbiological and faunal relationships and some of the processes that occur during, or as a result of, these interactions. The interactions occur mostly in the zone around legume seeds (spermosphere) at planting, around roots (rhizosphere), and inside roots (in nodules or endophytically) during crop growth, and within and near decomposing crop residues (detritusphere) after harvest. In this paper, these zones of activity are described first, followed by a general discussion of the importance of soil biota in sustainable agriculture and the effects that legumes have. The impact of grain legumes on N2 fixation, N uptake, N cycling, greenhouse gas emissions, and biological pest control are then discussed. Examples from the Northern Great Plains are cited when available, otherwise examples from outside the region are given to illustrate some points. Similarly, examples on grain legumes will be cited where available, otherwise points will be illustrated using other legumes as examples. MAJOR ZONES OF INTERACTION BETWEEN LEGUMES AND SOIL ORGANISMS The spermosphere, rhizosphere, root nodules, and detritusphere are the major zones of interaction between legumes and soil microorganisms. The spermosphere is the soil zone surrounding, and influenced by, seeds. The size of the spermosphere is thought to extend anywhere from 5 to 10 mm from the seed (Short and Lacy, 1976; Stanghellini and Hancock, 1971). The term pathozone was coined to indicate the zone of influence of the spermosphere in terms of pathogen chemotaxis N.Z. Lupwayi, Agriculture&Agri-FoodCanada, Box 29, Beaverlodge, AB, Canada T0H 0C0; and A.C. Kennedy, USDA-ARS, 217 Johnson Hall, Washington State Univ., Pullman, WA 99164-6421. Received 8 Nov. 2006. *Corresponding author (LupwayiN@agr.gc.ca). Published in Agron. J. 99:1700–1709 (2007). Symposium Papers doi:10.2134/agronj2006.0313s a American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA R e p ro d u c e d fr o m A g ro n o m y J o u rn a l. P u b lis h e d b y A m e ri c a n S o c ie ty o f A g ro n o m y . A ll c o p y ri g h ts re s e rv e d .