Do organic amendments improve the synchronism between soil N supply and wheat demand?

Microbial immobilization-remineralization and ammonium fixation-release govern N conservation and N availability, but the magnitude may differ according to organic amendments types and fertilization stages. Besides, few studies have explored the N supply under different organic amendments and the synchronism between N supply and crop demand. Thus, a pot experiment with different N amendments (equivalent N) was conducted using 15N cross-labeling. All wheat straw or pig manure was basal applied. 25% of the ammonium sulfate (AS) was basal dressed and 75% was top dressed. The promoting effect of straw addition on microbial N immobilization weakened with straw decomposition. Net immobilization still occurred at 29 days after straw addition. Straw addition hindered the fixation of top dressed AS-N, whereas manure addition encouraged it. In all cases, the absorption of AS-N relied more on the release of fixed ammonium (35.78%-51.53%) than the mineralization of soil microbial biomass N (SMBN) (21.41%-42.07%), whereas the opposite result was obtained for straw- or manure-N (2.62%-12.57% and 41.44%-84.27%, respectively). A greater proportion of manure-N was absorbed by wheat directly independent of the mineralization of SMBN or release of fixed ammonium compared with that of straw-N. The negative effect of straw addition on fertilizer N uptake was primarily due to the poor synchronism between N supply and wheat demand, although the total amount of fertilizer N supply by the mineralization of SMBN and the release of fixed ammonium were not significantly different. In comparison, the effect of manure addition was inferior to that of straw addition from the standpoint of both the strength of microbial N immobilization and retarded N uptake. The results provide beneficial information for optimizing fertilization measures to synchronize N supply with crop demand.