A comparison of the isotope recovery and difference methods for determining nitrogen fertilizer efficiency

In an experiment with sorghum on a medium deep red soil (Udic Rhodustalf) at Patancheru, India, where¹⁵N-labeled urea was applied at different rates during the 1981 rainy season, the apparent (ARF) and isotope recovery fractions (¹⁵NRF) were appreciably different, particularly at lower rates of fertilizer application. The fertilizer rates were corrected for losses of fertilizer nitrogen, that were estimated from the differences in the amounts of¹⁵N recovered in the soil and the crop, and the known amounts of¹⁵N applied. Introducing these ‘effective’ fertilizer rates, the apparent discrepancy between ARF and¹⁵NRF could be explained if it were assumed that the¹⁵N immobilized in the organic soil fraction was not remineralized during the course of the growing season. In the difference method, the equivalent amount of nitrogen at natural abundance released in exchange for fertilizer nitrogen (5 atom % xs¹⁵N) immobilized in the organic nitrogen fraction is treated as ‘fertilizer nitrogen’, since no distinction is made between¹⁴N and¹⁵N. In the isotope-dilution method, the nitrogen at natural abundance mineralized during biological interchange is not considered fertilizer nitrogen, and therefore the assumed effective amount of fertilizer nitrogen available to the crop is less than in the difference method.