A comparison of the isotope-dilution and the difference method for estimating fertilizer nitrogen recovery fractions in crops. II. Mineralization and immobilization of nitrogen uri icon

abstract

  • The recovery fractions of fertilizer nitrogen (N) by crops can be calculated by the difference or the isotope-dilution method. In the difference method, an apparent recovery fraction (ARF) is calculated from the N uptake by fertilized and unfertilized crops, whereas in the isotope-dilution method a N-15 recovery fraction ((NRF)-N-15) is calculated from isotope-ratio analysis and N uptake by fertilized crops. The recovery fractions calculated by the two methods are compared on the basis of simplified models for the distribution of N-15-labeled N over different N pools in the soil-crop system, considering plant uptake and mineralization-immobilization in soil. In N-deficient soils, ARF-values are likely to be higher than those of (NRF)-N-15, due to pool substitution. Plant uptake and immobilization do not affect the relationship between ARF and (NRF)-N-15, as these processes do not discriminate between N-15 and N-14. However, immobilization reduces the range of values of ARF and (NRF)-N-15. Mineralization is the main factor causing the discrepancy between ARF and (NRF)-N-15. it results in dilution of the soil mineral N pool with soil-derived N and thus affects the N-15/N-14 ratio in the soil mineral N pool. The combined action of mineralization-immobilization and plant uptake increases the effect of dilution of the soil mineral N pool by (re)mineralized soil N. The effects of plant uptake and mineralization-immobilization on the relationship between (NRF)-N-15 and ARF are discussed and analytical expressions are derived for the relevant quantities considered in the model. The difference and the isotope-dilution method measure essentially different quantities and complementary information can be derived from recovery fractions calculated by the two methods, thus contributing to the understanding of fertilizer N-dynamics and transformations in soil.

publication date

  • 2003
  • 2003