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The percentage of applied fertilizer N taken into plants is often estimated by measuring the difference in plant N uptake between treated and check plots. This method has often overestimated plant N uptake. The objectives of this study were to (i) compare the recovery of fertilizer N in corn (Zea mays L.) as calculated by the difference and isotopic methods and (ii) track fertilizer N in the plant–soil system using isotopic enrichment of N. Sixteen N plots (3 m × 3 m) were established on a Hecla fine‐sandy loam (sandy, mixed, frigid Oxyaquic Hapludolls) and replicated four times in a completely random design. Corn received sidedressed band applications (15 cm from row and 5 cm deep) of 15N‐enriched and nonlabeled urea N at 135 kg N ha−1 in 1993 and 1994. Plant uptake of fertilizer N as estimated by the isotopic and difference methods was 45% and 39% in 1993 and 40% and 22% for 1994, respectively. Nearly 42% and 36% of the applied labeled N was accounted for in the soil at the end of 1993 and 1994, respectively. The difference method did not overestimate plant N uptake because of high soil N availability. Lower corn yield potential in 1993 was the consequence of a cooler, shorter growing season. This climatic difference had less effect on the results generated by the isotopic method. Reasons for the N deficit in this investigation are speculative since no attempts were made to measure gaseous emissions; however, denitrification and/or leaching are thought to be the primary mechanisms.

While numerous studies have evaluated the efficacy of outdoor rainfall simulations to predict P concentrations in surface runoff, few studies have linked indoor rainfall simulations to P concentrations in surface runoff from agricultural fields. The objective of this study was to evaluate the capacity of indoor rainfall simulation to predict total dissolved P concentrations [TP(<0.45)] in field runoff for four dominant agricultural soils in South Dakota. Surface runoff from 10 residue-free field plots (2 m wide by 2 m long, 2-3% slope) and packed soil boxes (1 m long by 20 cm wide by 7.5 cm high, 2-3% slope) was compared. Surface runoff was generated via rainfall simulation at an intensity of 65 mm h(-1) and was collected for 30 min. Packed boxes produced approximately 24% more runoff (range = 2.8-3.4 cm) than field plots (range = 2.3-2.7 cm) among all soils. No statistical differences in either TP(<0.45) concentration or TP(<0.45) loss was observed in runoff from packed boxes and field plots among soil series (0.17 < P < 0.83). Three of four soils showed significantly more total P lost from packed boxes than field plots. The TP(<0.45) concentration in surface runoff from field plots can be predicted from TP(<0.45) concentration in surface runoff from the packed boxes (0.68 < r(2) < 0.94). A single relationship was derived to predict field TP(<0.45) concentration in surface runoff using surface runoff TP(<0.45) concentration from packed boxes. Evidence is provided that indoor runoff can adequately predict TP(<0.45) concentration in field surface runoff for select soils.

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