A comparison of the isotope-dilution and the difference method for estimating fertilizer nitrogen recovery fractions in crops. III. Experimental

AbstractFertilizer nitrogen (N) recovery fractions may be calculated by the difference or the isotope-dilution method. With the first method an apparent recovery fraction (ARF) and with the latter method a 15N recovery fraction (15NRF) is calculated. The two methods may give different results, ARF generally being higher than 15NRF, particularly in the higher range of ARF-values. Fertilizer N recovery fractions in crops calculated by the two methods were compared using experimental data derived from the literature. The experimental results could be largely explained on the basis of theoretical relationships between 15NRF and ARF, based on the assumptions of (1) complete and instantaneous mixing of initial soil mineral N and fertilizer N, and (2) zero order kinetics for plant uptake, mineralization and immobilization reactions. In the lower range of ARF-values, values of 15NRF exceeded those of ARF, presumably because the plant derives its N from fertilizer as well as soil N, even if there is no crop response to applied N. In the higher range of ARF-values, 15NRF tended to be lower than ARF, presumably because of the occurrence of mineralization-immobilization turnover, which causes the 15N content of the mineral N pool to decrease. This phenomenon, referred to as ‘pool substitution’, may be enhanced by increased uptake efficiency of soil N by fertilized crops or by increased mineralization in fertilized treatments. From an agronomic point of view, the use of ARF would be preferred, as this quantity accurately reflects the overall effect of fertilizer application on crop N uptake. 15NRF is a meaningful quantity in rebalance studies. It accurately estimates the fertilizer N recovery in the crop, provided 4N released in mineralization-immobilization turnover in soil is not considered fertilizer N. The theory developed in the present series of papers may contribute to the understanding of the dynamics and transformations of fertilizer N in soil-crop systems and thus help to develop criteria for efficient and effective fertilizer N management