Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2000
Publication Date: 3/1/2002
Citation: GLASENER, K.M., WAGGER, M.G., MACKOWN, C.T., VOLK, R.J. 2002. CONTRIBUTIONS OF SHOOT AND ROOT NITROGEN-15 LABELED LEGUME NITROGEN SOURCES TO A SEQUENCE OF THREE CEREAL CROPS. SOIL SCIENCE SOCIETY OF AMERICA JOURNAL. 66(2):523-530. Interpretive Summary: Legume cover crops can be grown as a green manure to provide a source of nitrogen (N), the primary limiting nutrient for cereal crops. For resource poor small farms, this may be the only source of N that the farmer can provide. Tropical small farms may benefit greatly from the use of legume green manure. Using N stable isotope techniques, we determined the contributions of above ground (shoots and leaf litter) and below ground sources of N supplied by two different tropical legumes that had different chemical properties that potentially could affect their decomposition. A no-till cereal crop sequence of corn-upland rice-corn was grown after the legumes. The primary source of N for the cereal crops was derived from above ground legume tissue, primarily because more N was present in these tissues than the roots of legumes. Recoveries were the same for the two legumes, even though the potential decomposition rate of the legumes differed. Above ground legume N contributed 15% of the total N accumulation of the cereal crops, while the legume roots provided only 3%. A substantial contribution (28%, root + shoot sources) to the N needs of cereals is realized for the crop immediately following the legume cover crop and extends even to the third cereal crop (11%) grown in a sequence over a 15-month period. This research demonstrated that the decomposition of legume residues, particularly legume shoots, make a meaningful contribution to the N economy of cereal crops grown in the tropics. These results will be useful to agronomists, crop consultants, and producers seeking to understand effective use of legume cover crops as a source of N for cereal crops.
Technical Abstract: A field experiment quantified the amount of N in cereal crops that was derived from preceding tropical legume cover crops having different chemical composition and mineralization potential. Desmodium ovalifolium Guillemin and Perrottet and Pueraria phaseoloides (Roxb.) Benth., grown in 6.0 m2 microplots, were labeled with foliar-applied urea containing 99 atom m% 15N. A cereal rotation of corn (Zea mays L.)-rice (Oryza sativa L.)-cor followed the legumes. Cereal accumulation of legume N from either the shoot (shoot + leaf litter) or the root-soil sources was evaluated by spatially separating the legume N sources. Initially the Desmodium shoot N source contained 316 kg N/ha and roots contained 12.5 kg N/ha. Pueraria shoots and root N sources initially contained 262 and 14.8 kg N/ha, respectively. About 9% of the initial N of each legume shoot was recovered in the total aboveground tissues from the three cereal crops, while 49% of Desmodium and 28% of Pueraria root-soil N sources were recovered. After harvest of the last cereal crop of the rotation, the soil (0 to 75 cm) contained an average of 1.3% of the initial legume N sources as mineral N. Soil organic N contained an average of 18% of the N initially present in the shoot and root-soil N sources of the legumes. Decomposition of legume residues, particularly legume shoots, make a meaningful contribution to the N economy of cereal crops grown in the tropics. A substantial contribution (28%, root + shoot sources) to the N needs of cereals is realized for the crop immediately following the legume cover crop and extends even to the third cereal crop (11%) grown in a sequence over a 15-mo period.