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The experiments reported in this paper were designed (i) to determine whether it is possible to detect the presence of fertilizer nitrogen in nitrate produced by four central Illinois soils during laboratory incubation by using variations in the natural abundance of 15N, (ii) to use such variations to estimate the fractional contribution of fertilizer nitrogen to the nitrate produced, and (iii) to compare these estimates with those based on the use of fertilizer nitrogen which was artificially enriched with 15N. The mean δ 15N per mill excess) value of nitrate produced during incubation of the soils which had received unlabeled (NH4)2SO4 was significantly lower in 11 out of 12 experiments compared with that produced by unfertilized soils. As expected, estimates of percent fertilizer derived nitrogen based on variations in natural abundance of 15N were less precise than those based on the use of (NH4)2SO4 artificially enriched with 15N. The use of natural variations in 15N abundance to compute the percent fertilizer derived nitrate resulted in a lower estimate than the estimate based on 15N enriched material in seven experiments. This underestimation appeared to be related to the so called “priming effect” and/or the exchange of fertilizer and soil nitrogen.

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Synopsis This paper describes two phytotrons, wherein simplicity and low cost of construction were achieved, along with close control of the environmental factors of day‐temperature, night‐temperature, photoperiod, light intensity and quality, and humidity.