
%0 Journal Article
%A Rochester, IJ 
%A Constable, GA 
%A Macleod, DA 
%T Cycling of fertilizer and cotton crop residue nitrogen
%J Soil Research
%D 1993/10/01
%V 31
%N 5
%P 597-609
%K Nitrogen;N Cycling;Mineralization;Immobilization;Cotton;
%R https://doi.org/10.1071/SR9930597
%U https://www.publish.csiro.au/paper/SR9930597 
%X Mineral N (nitrate and ammonium) contents were monitored in N-fertilized soils supporting cotton crops to provide information on the nitrification, mineralization and immobilization processes operating in the soil. The relative contributions of fertilizer N, previous cotton crop residue N and indigenous soil N to the mineral N pools and to the current crop's N uptake were calculated. After N fertilizer (urea) application, the soil's mineral N content rose rapidly and subsequently declined at a slower rate. The recovery of 15N-labelled urea as mineral N declined exponentially with time. Biological immobilization (and possibly denitrification to some extent) were believed to be the major processes reducing post-application soil mineral N content; the decline could not be accounted for by crop N uptake alone. Progressively less N was mineralized upon incubation of soil sampled through the growing season. Little soil N (either from urea or crop residue) was mineralized at crop maturity. Cycling of N was evident between the soil mineral and organic N pools throughout the cotton growing season. Considerable quantities of fertilizer N were immobilized by the soil microbiomass; immobilized N was remineralized and subsequently taken up by the cotton crop. A large proportion of the crop N was taken up in the latter part of the season when the soil mineral N content was low. We suggest that much of the N taken up by cotton was derived from microbial sources, rather than crop residues. The application of cotton crop residue (stubble) slightly reduced the mineral N content in the soil by encouraging biological immobilization. 15N was mineralized very slowly from the labelled crop residue and did not contribute significantly to the supply of N to the current crop. Recovery of labelled fertilizer N and labelled crop residue N by the cotton crop was 28 and 1%, respectively. In comparison, the apparent recovery of fertilizer N was 48%. Indigenous soil N contributed 68% of the N taken up by the cotton crop. 
