The residual value of fertiliser N in crop sequences: An appraisal of 60 years of research using 15N tracer

Abstract

Over the past 60 years, more than 100 experiments have been conducted using manufactured and organic fertilisers labelled with the stable isotope 15N. Organic N forms included crop residues, green manures, composts, animal manures and slurries in either crop sequences or successive cuttings of pasture grasses. As stated in the pioneering paper of Low and Piper (1957), the use of 15N-labelled materials is essential to follow the dynamics of fertiliser N in the soil-plant system. The majority of studies used 15N-labelled manufactured fertilisers.In general there was no difference of N fertiliser form on the 15N recovery in the 1st residual crop (R1) and in subsequent crops (R2, R3, Rn…). The 15N recovery was <10% of added N in the R1 crop and less in subsequent crops (R2, R3), with the exception of studies where severe water stress limited crop growth in the year of N application (Yr0). High mineral N remained in the root zone which was assimilated by the subsequent crops, giving a higher residual value in the R1 crop than was normally observed.The similarity of the results from more than 100 studies, and a boot strapping analysis, suggested that dilution of the 15N by the large unlabelled mineralisable soil organic N pool compromises the use of the techniques to explore residual uptake of immobilised 15N. The isotope technique does, however, enable researchers to monitor the rate of incorporation of the N into microbial biomass and more recalcitrant organic N forms within the soil organic matter.