The Amount and Nitrogen-15 Content of Nitrate in Soil Profiles from two Central Illinois Fields in a Corn-Soybean Rotation

This paper reports the concentration and the δ¹⁵N (per mill ¹⁵N excess) of nitrate present in soil cores as a function of depth and time. The cores were taken at approximately monthly intervals from two central Illinois fields which were in a corn and soybean rotation. During the period of October 1971 through April 1972, the mean δ¹⁵N value of nitrate present at all depths of both fields was + 1.2 ± 1.5. The most striking variation from this mean occurred after the application, on 4 May 1972, of anhydrous ammonia to the field being sown to corn, when a dramatic decrease was observed in the δ¹⁵N values of the nitrate in the 0–30 layer of that field. The difference in the δ¹⁵N values of the nitrate found in the two fields disappeared after about 2 months. The δ¹⁵N values of the ammonium ion found in the soil after application of fertilizer increased with time as the concentration of ammonium decreased and the concentration of nitrate increased. These observations indicate that the observed changes following fertilization in δ¹⁵N of nitrate were due primarily to isotopic discrimination associated with incomplete conversion of ammonium to nitrate. The δ¹⁵N values of the nitrate in both fertilzed and unfertilized fields were higher in late summer than in the period October to May. The nitrate content of the 150-cm profile did not show significant variation with time in either field from October 1971 until 4 May 1972, when anhydrous ammonia was applied to the field being sown to corn. The nitrate concentration in the soil profiles of both fields decreased during the period of crop growth. Fertilizer application and crop uptake appeared to exert a larger influence on the total amount of nitrate in the soil profile than did movement of nitrate out of the 150-cm profile by leaching, since rainfall in amounts sufficient to redistribute the nitrate within the profile did not lower the total amount of nitrate in the profile.