Influence of pool substitution on the interpretation of fertilizer experiments with ¹⁵N

Abstract

The uptake of labelled and unlabelled N by wheat was measured in pot and field experiments with "N-labelled fertilizer. Soils from two sites on the same series were used in the pot experiment; one had been bare-fallowed for 22 years and contained 1.6% organic C, the other had been under grass for many years and contained 3.8% organic C.Fertilizer N increased the uptake of unlabelled soil N in both soils, i.e. there was a positive 'added nitrogen interaction' (ANI). There was no AN1 in the field experiment. A simulation model is used to show how positive ANIs can arise as a result of 'pool substitution'-labelled inorganic fertilizer N standing proxy for unlabelled inorganic soil N that would otherwise have been immobilized. In the low-organic fallow soil, pool substitution accounted for the whole of the observed AN1 and fertilizer N did not enhance either gross or net mineralization of soil N. Pool substitution also operated in the high organic grassland soil, but here net mineralization of soil N increased with increasing additions of fertilizer, giving rise to a 'real' AN1 in addition to the larger 'apparent' AN1 caused by pool substitution. This increase in net mineralization is probably caused by a decrease in immobilization of N as fertilizer N additions increase, not by an increase in gross mineralization of soil N. For pool substitution to operate, fertilizer N and soil inorganic N must occupy the same pool. This occurred in the pot experiment but not in the field experiment, where fertilizer and soil inorganic N remained separate and there was no ANI. When pool substitution occurs, fertilizer use efficiency is predictably lower as measured by the isotopic method than as measured by the conventional non-isotopic procedure.

Nitrogen use efficiency and fertiliser fate in a long-term experiment with winter cover crops

“…The distribution of 15 N profiles followed the pattern described by other authors, with the majority of the N R-soil in the upper 0.40 m and small amounts below 0.80 m (Jenkinson et al, 1985;Hart et al, 1986;Rao et al 1991;Reddy and Reddy, 1993;Ottman et al, 2000;Gabriel and Quemada, 2011). This strong retention in the upper layer is explained primarily by pool substitution between 14 N and 15 N in the microbial biomass and the stable organic matter fraction (Jenkinson et al, 1985;Hart et al, 1986;Rao et al 1991;Timmons and Cruse, 1991) and to a lesser extent in the maize root system (Gabriel and Quemada, 2011). When compared to the results of 2007/2009 in the same experiment field, the 15 N substitution seems to be favoured by larger fertiliser application or lower fertiliser use efficiency.…”

Eficiência nutricional e aproveitamento do nitrogênio pelo capim-marandu de pastagem em estágio moderado de degradação sob doses e fontes de nitrogênio

“…Segundo Hart et al (1986), o nitrogênio do fertilizante estimula a atividade biológica do solo e o maior crescimento das raízes das plantas adubadas, fazendo com que o N de um maior volume de solo seja absorvido por essas plantas. Figura 2 -Eficiência de utilização do N (A) e recuperação do N aplicado (B), em função de doses e anos de recuperação da pastagens do capim-marandu (média de três cortes por ano).…”

Recovery of 15N-labeled fertilizer by spring bread wheat at different N rates and application times

“…Moreover the early-season application of N fertilizer could also have allowed more time for pool substitution and resulted in greater isotopic dilution with soil N, which would decrease the Ndff values (Jansson and Persson 1982). The microbial needs could already be satisfied with native mineral soil N and fertilizer N applied at seeding, so that fertilizer N applied at booting could remain more available to plant (Hart et al 1986;Mary et al 1988).…”