Abstracts |
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
(N-2) fixation that adds plant available N into the soil system. It is estimated that about 171
million kg N-2 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 N-2 was fixed by field pea, lentil,
and dry bean (including chickpea) crops in U.S. Agroecosysterns in 2004. Some of the fixed N-2 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 N-2, enhancing
nutrient uptake, reducing greenhouse gas emissions by reducing N fertilizer use, and breaking
nonlegume crop pest cycles. |