Recovery of starter nitrogen-15 fertilizer with supplementarily applied ammonium nitrate on irrigated potato

Nitrogen contamination in ground water of potato (Solanum tuberosum L.) producing areas has indicated a need for improved management of N and water, particularly on sandy soil. Therefore, a field experiment was conducted with the objective of following the recovery and partitioning of starter¹⁵NH₄ and¹⁵NO₃ into potato plant tops and tubers in conjunction with additional supplementarily applied NH₄NO₄. Potato plants treated with starter¹⁵NH₄ or¹⁵NO₃ tended to increase the percent recovery of starter¹⁵N in tubers sampled from one growth time to the next to reach nearly 40% recovery toward the end of the season. Whole plants reached peak recovery of around 50% of the starter¹⁵N near mid-season. From then on, there was a trend for loss of starter¹⁵N by senescence, defoliation or translocation to the roots. The percent recovery of starter¹⁵N was significantly higher at final tuber harvest (not whole plants) for the treatment with starter¹⁵NH₄ at 112 kg ha^-1 combined with 112 kg ha^-1 of supplemental N as compared to the treatment with 112 kg ha^-1 of starter¹⁵NH₄ plus 224 kg ha^-1 of supplemental N. This difference may have been a result of isotope dilution. Early in June the accumulation of starter¹⁵NO₃ in whole plants was about five times as high as that from starter¹⁵NH₄. Later there was no difference in percent recovery of these two forms of N. The temporary delay in starter¹⁵NH₄ uptake was probably related to slow nitrification early in the season instead of preferential uptake of starter¹⁵NO₃.