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

Over the past 60 years, more than 100 experiments have been conducted using manufactured and organic fertilisers labelled with the stable isotope 15 N. 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 15 N-labelled materials is essential to follow the dynamics of fertiliser N in the soil-plant system. The majority of studies used 15 N-labelled manufactured fertilisers. In general there was no difference of N fertiliser form on the 15 N recovery in the 1st residual crop (R 1 ) and in subsequent crops (R 2 , R 3 , R n …). The 15 N recovery was <10% of added N in the R 1 crop and less in subsequent crops (R 2 , R 3 ), with the exception of studies where severe water stress limited crop growth in the year of N application (Yr 0 ). High mineral N remained in the root zone which was assimilated by the subsequent crops, giving a higher residual value in the R 1 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 15 N by the large unlabelled mineralisable soil organic N pool compromises the use of the techniques to explore residual uptake of immobilised 15 N. 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. [ABSTRACT FROM AUTHOR]