Effects of Hydroquinone, Dicyandiamide and Straw Additions on Wheat Yield and the Pathways in Fertilizer N Retention and Supply.

Fertilizer N can be conserved through microbial immobilization and fixation by soil clay minerals, and then subsequently remineralized and released to meet crop demand. These processes were significantly influenced by N management practices, such as addition of urease inhibitor and nitrification inhibitor, and crop residuals returning. In the present study, a pot experiment was conducted to clarify the effects of addition of urease inhibitor (hydroquinone, HQ), nitrification inhibitor (dicyandiamide, DCD), and wheat straw on microbial immobilization and minerals fixation of fertilizer N, and wheat yield. In the treatments without straw, both DCD and HQ additions increased soil microbial biomass N (SMBN), and the largest increment was obtained in the treatment with combined application of DCD and HQ. In the treatments receiving straw, SMBN averagely increased by a factor of 2.2. Combined application of straw and DCD further increased SMBN, whereas the reverse was true in the treatment with the addition of HQ and straw. Both inhibitors were favorable for raising the fixed NH₄⁺ fraction and retarding the fixed NH₄⁺ release. In the treatments without straw, the release of fixed NH₄⁺ increased by 60.9%, 39.8% and 96.3% due to the addition of DCD, HQ and DCD + HQ from the heading stage to the maturity stage, synchronizing soil N supply with crop demand. Straw addition decreased fixed NH₄⁺ on average, and diminished regulative effect of inhibitors on fixed NH₄⁺ release. However, contents of fixed NH₄⁺ in treatments receiving straw were all greater than the treatments without straw at the end of the experiment. Both inhibitors increased crop yield and N use efficiency in the treatments without straw returning, whereas significant reduction in yield was observed in the treatments receiving straw, regardless of the presence of inhibitors. It was primarily attributed to the intensified competition for N between crop and soil microorganisms because of the addition of organic carbon. The combination of straw and DCD further decreased crop yield, whereas HQ can help alleviate the yield reduction caused by the addition of straw.