A comparison of the isotope-dilution and the difference method for estimating fertilizer nitrogen recovery fractions in crops. I. Plant uptake and loss of nitrogen

Abstract

The apparent recovery fraction (ARF) of applied nitrogen (N) by a crop is calculated as the difference between the total N uptake by crops from fertilized and unfertilized treatments per unit N applied. The &sup15;N recovery fraction (&sup15;NRF) is calculated as the amount of 15N-labeled N recovered in fertilized crops per unit 15N-labeled N applied. The relationship between ARF and 15NRF is discussed on the basis of a complete-mixing model for the distribution of 15N-labeled N over different N pools in the soil-crop system. Mineralization-immobilization turnover in soil is not considered in the model. It is shown that in the lower range of ARF values, i.e., on soils high in available N, values of 15NRF are likely to exceed those of ARF. This is because the fertilizer N mixes with the soil mineral N pool and thus the plant derives its N from applied as well as soil N, even if there is little or no crop response to applied N. In the higher range of ARF values, i.e., in N-deficient soils, values of 15NRF may be lower than those of ARF due to an increased uptake efficiency of soil N in fertilized treatments. Loss of N either from the fertilizer or from the mixed soil mineral N pool, reduces the range of values of 15NRF and ARF. From an agronomic point of view, ARF is a meaningful quantity as it accurately reflects the overall effect of fertilizer application on crop N uptake, whereas 15NRF is a meaningful quantity in 15N tracer studies on N fertilizer use efficiency and N balances in soil-crop systems. In the absence of mineralization-immobilization turnover in soil, the fertilizer N recovery in the crop is accurately estimated by 15NRF.