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a International Plant Nutrition Institute, 102-411 Downey Rd., Saskatoon, SK S7N4L8 Canada
b Agriculture and Agri-Food Canada, 5403 1st Ave. South, Lethbridge, AB, Canada T1J 4B1
c Dep. Land Resources and Environmental Sci., Montana State Univ., P.O. Box 173120, Bozeman, MT 59717-3120
* Corresponding author (ajohnston{at}ipni.net)
a International Plant Nutrition Institute, 102-411 Downey Rd., Saskatoon, SK S7N4L8 Canada
b Agriculture and Agri-Food Canada, 5403 1st Ave. South, Lethbridge, AB, Canada T1J 4B1
c Dep. Land Resources and Environmental Sci., Montana State Univ., P.O. Box 173120, Bozeman, MT 59717-3120
* Corresponding author (ajohnston{at}ipni.net)
Received for publication April 24, 2007.
THE FOLLOWING PAPERS were presented at the symposium "Pulse Crop Ecology in North America: Impacts on Environment, Nitrogen Cycle, Soil Biology, Pulse Adaptation, and Human Nutrition," held during the 2005 ASA–CSSA–SSSA annual meetings in Salt Lake City, UT.
Pulse crops, also known as grain legumes, have become a major component of the North American diet. While many of us think of pulse crops as something we grow for export and consumption abroad, their use in North American household and restaurant food products continues to grow. And for good reason. Pulse crops have been shown to be an excellent source of protein, dietary fiber, and phytochemicals essential for the human diet. Pulse crops are finding a fit in the crop rotations on many farms in the Northern Great Plains region. Breaking pest cycles common to monoculture, reducing the use of fertilizer N, and increasing the marketing opportunities available to the grower have increased interest and area seeded to pulse crops. The genesis of this symposium was the realization that pulse crops have increased in land occupancy from 400,000 ha in 1991 to 3,000,000 ha in North America in 2006, primarily in the northern Great Plains, and are impacting importantly the agroecology of this region (Statistics Canada, 2005; USDA-National Agricultural Statistics Service, 2005). A North American conference committee was formed to decide on topics for timely review.
Pulse crops provide amino acids, minerals, carbohydrates, lipids, and vitamins. They also have a number of secondary compounds that have been linked to good health, including nonnutritive phytochemicals such as isoflavones, saponins, catechins, and anthocyanidins. Together these pulse grain constituents can play an important role directly in the human diet, and also indirectly via animal production. Future research will characterize the genetic diversity of both nutrient and phytochemical composition in pulse crops, and help to develop breeding and selection strategies to enhance these plant components. There is large variability in the level and absorption of pulse crop nutrients and compounds by animals. This variability extends to the actual plant species, the production environment, and the processing. In addition, soil microbial and nutrient supply and plant exudates also impact the production environment of the pulse crop. Even with this uncertainty, pulse grains have found their way into human and livestock diets in most parts of the world.
From an environmental viewpoint, greenhouse gas emissions are a major issue. The inclusion of pulse crops in rotation with cereal and oilseed crops is considered to have a significant positive impact on greenhouse gas emissions, given the absence of fertilizer N in pulse crop production. Nitrous oxide emissions are water-mediated events, with emissions being very low in dry areas and higher in wet areas. In addition, N2O emissions require a soil supply of nitrate-N, often residual nitrate-N left by the previous crop or the product of recently applied fertilizer or livestock manure. Field research in semiarid regions indicates that N2O emissions were lower from field pea crops when compared with a fertilized cereal, and no difference was observed when cereals were grown on pulse crop versus cereal crop stubble. As a result, there would be a net decline in N2O emissions with pulse crop production, given the reduced N fertilizer used with these crops. Managing N in the soil appears to be the governing factor related to N2O emissions from cropped fields, regardless of crop type.
Pulse crops have a significant impact on soil biology, increasing soil microbial activity from seeding to long after harvest. Along with N fixation, the rhizophere activity of the crop plays a major role in plant nutrient uptake. Mycorrhizal activity enhances plant phosphorus and zinc uptake. Endophytic rhizobia enter the cells of nonlegume crops in rotation and influence nutrient uptake in nonlegumes. While pulse crop residues do have low nutrient content, microbes can decompose them more easily than cereals. Pulse crops influence nutrient cycling in soils by increasing microbial activity and nutrient content. Future research will focus on the role pulses play in influencing nonpulse crop growth and development, as well as their impact on plant health and soil biology.
Pulse crops are always credited for N fixation and subsequent benefits to crops in rotation. However, grain legumes are high-protein crops, removing the vast majority of the N fixed in the harvested seed. As a result, it is the root and aboveground biomass, as well as the reduced N immobilization of the pulse residues, that contribute to the subsequent crops in rotation. Results from the semiarid regions of North America suggest that the actual N contribution from pulses is not that high, often <20 kg N ha–1. While the yield response of growing a cereal on pulse residue would imply a greater N benefit relative to oilseed stubble, only a portion of this can be attributed to added N. Estimating the N credit from pulse crop residue is difficult, and the range of estimated N supply from pulse crop seed yields indicates that N benefits cannot be related to shoot or grain yields. Fixation is the key to not only increasing pulse crop yields, but also influencing the residual N left after a pulse crop is harvested. Continued efforts to improve N fixation should be the focus of researchers interested in the benefits of grain legumes to cropping systems.
When we hear "presentation on changing climate" we are often overwhelmed by the uncertainty that exists in the output of model predictions. However, there is one thing that most people can agree on, that being we have an increasing amount of carbon dioxide in the atmosphere with time. Whether this is causing global warming or cooling is often debated, and whether these changes are going to impact on crop production is continually debated (Schneider, 1994; Skinner and Majorowicz, 1999; Smith and Almaraz, 2004). While changing atmospheric carbon dioxide levels may impact on crop growth, these differences are often minor relative to the impact of changing crop management practices (e.g., seeding date, fertilizer rate, variety selection). Add to this the impact of changes in available soil water, shifting weed populations, and soil fertility changes, it seems that pulse crops will need to adapt to changing environments. Ultimately, the genetic variability available to plant breeders to influence crop improvement will play a major role in pulse crop adaptation to climate change.
Pulse crops have had a major impact on the agroecology and economy of Great Plains agriculture. Their contribution to increasing crop diversity has had a positive impact on farming systems of the region. Continued research evaluating the nutritional benefits, production impact, and adaptability of pulse crops will ensure a secure future for this crop in Great Plains agriculture.
This symposium has brought together the knowledge that impacts the environment, the crop itself, and the ecological reserves of the soil surrounding the growing pulse crop. The symposium also linked the agricultural capacity of pulse crops with the human wellness agenda. The papers presented identify important questions regarding pulse crops, and provide an excellent summary on the state of knowledge at this point in time. Further study will confirm, refute, and add to this knowledge of the increasingly important role that pulse crops play in North American agroecosystems.
ACKNOWLEDGMENTS
Appreciation is extended to program coordinators Dr. George Clayton, Agriculture and Agri-Food Canada, Lethbridge, AB, Canada, and Dr. Perry Miller, Montana State University, Bozeman, MT, for their guidance and assistance during the planning of the symposium. Program committee members included Pat Carr, Neil Harker, Guy Lafond, Reynald Lemke, Kent McKay, Kevin McPhee, Fred Muehlbauer, John O'Donovan, Fran Walley, and Bob Zentner.
REFERENCES
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