|INGRAM, L - University Of Sydney|
|SCHUMAN, GERALD - Retired ARS Employee|
Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/9/2014
Publication Date: 9/18/2014
Publication URL: http://handle.nal.usda.gov/10113/60376
Citation: Ingram, L.J., Schuman, G., Parkin, T.B., Mortenson, M.C. 2014. Trace gas fluxes from a northern mixed-grass prairie interseeded with alfalfa. Plant and Soil. 386:285-301.
Interpretive Summary: Grazing cattle is the predominant agricultural use of the grasslands that make up much of the Northern Great Plains. While they are crucial grazing areas, due to low rainfall, low soil fertility and severe climatic conditions, vegetation production and associated quality is often low, limiting livestock production. While we can’t change the weather, we do have the ability to improve soil fertility by planting legumes. Legumes, by combining with microbes called rhizobium, have the ability to remove nitrogen gas from the air and convert it to a form of nitrogen that legumes can use. Over time as legumes die and decompose, this nitrogen improves the overall soil fertility. However while improving the soil fertility is good for ranchers in that they are able to increase the productivity and forage quality of their rangelands there is concern that this increase in soil nitrogen may result in greater amounts of the gas of nitrous oxide being produced. This gas, which, along with carbon dioxide and methane, are the three most important greenhouse gases that are believed to be responsible for causing climate change. The aim of this study was to determine if the increase in soil nitrogen that occurs in these grasslands over time after a legume was seeded into them, results in an increase in nitrous oxide and methane being produced. In addition we measured these same greenhouse gases in native grasslands and also native grasslands that had a nitrogen fertilizer added to them. We found that there were generally no differences in the amount of nitrous oxide released between the three different treatments (native, legume and fertilized rangeland). We also found that prairies which had a legume interseed into them generally took up methane to a much greater extent than either native grasslands or fertilized grasslands. Overall it would appear that seeding a legume into a prairie grassland is a win-win situation. Not only does it improve soil fertility and increase the overall production and forage quality and thus livestock production but also there is no increase in the amount of greenhouse gases released from these grasslands.
Technical Abstract: The role of legumes in improving soil fertility, forage quantity and quality is well established, however what is less clear is the extent that the nitrogen fixed by legumes may drive increased trace gas emissions. A chronosequence study in native prairie that had been interseed with the legume alfalfa for 38, 16, and 5 years was undertaken in northwestern South Dakota in order to investigate the impact on the greenhouse gases, nitrous oxide and methane. In addition, comparisons were made to native grasslands that had not been interseeded with alfalfa or a one-time addition of ammonium nitrate. Across all three interseeding years there were few significant differences between the three treatments (native, interseeded or fertilized). Despite greater amounts of soil nitrogen, the interseeded treatment released as much nitrous oxide as the native grasslands, with the exception of the oldest interseeded site where it was higher (but not significantly so). This suggests that the nitrogen being fixed was most likely taken up by native vegetation and reducing the nitrogen available for denitrification. Methane uptake was generally greatest on grasslands interseeded with alfalfa and least on grasslands that had had nitrogenous fertilizers added to them, though again these differences were not significant. The results of this study suggest that while soil nitrogen was greatly increased (up to 40% in the oldest interseeded site) and improved overall productivity, it did not result in overall increase in trace gas production relative to native grasslands.