Testing the Origin of Agricultural Nitrogen Sources in Soil, Plant, and Water.

Nitrogen (N) is one of the major nutrients for agriculture. In agriculture, several N sources, such as soil organic matter, chemical N, and manure N are used for crop production. When these N sources are mineralized, they can be converted into nitrate-N. Nitrate in the soil can be used by plant, stayed in soil, and lost through soil profile into ground water. It is not easy to differentiate where the nitrate-Ns come from because the nitrate-Ns from these N sources have similar chemical structures. One way to look at it is to use natural stable isotope, ¹⁵N. Natural N stable isotopes consist of 99.636% of ¹⁴N and 0.37% of ¹⁵N. Therefore, the objectives of this study are to test the feasibility whether natural stable isotope method (¹⁵N) can be used as an indicator of N sources in soil, plant, and drained water and to investigate whether mineralized N (nitrate-N) through soil profile in the drained water is from N fertilizer or from soil N.Corn was planted on a sandy soil, Becker, MN in 2012. Experimental design was randomized complete block design with 4 replications. 224 kg N ha^-1 of 8 nitrogen sources was used for N leaching trial. N sources were control (no N fertilizer), urea, ESN, Super U, UAN, dairy, turkey, and chicken. Additional water was supplied with an irrigation system. Soil, plant and drained water samples during the growing season were collected and prepared for total N and ¹⁵N analysis. Water samples were collected every week using lysimeter, which was installed 0.9 meter deep. Initial soil pH, nitrate-N concentration, Bray-P, and K concentration were 5.2, 7.4, 12, and 51, respectively. Average corn grain yield was 3.7, 7.1, 10.5, 8.9, 4.3, 7.4, 5.6, and 7.8 Mg ha^-1 for control, urea, ESN, super U, UAN, chicken, dairy, and turkey manures, respectively. Nitrate-N leaching in drained water was observed during the growing season.