Fertilization with different forms of nitrogen is an important measure to regulate nitrogen metabolism of wheat (Triticum aestivum L.). Soil microorganisms and enzymes play significant roles in nutrient transformation and decomposition. Their activities may affect how different forms of nitrogen fertilizer influence nitrogen metabolism in wheat. To understand the effects of nitrogen form on soil nitrogen bacteria group and soil enzyme activity, pot experiments using the wheat cultivar ‘Yumai 50’ were carried out in a sandy loam at the Experimental Farm of Henan Agricultural University in 2009 and 2010. The soil contained 14.29 g/kg organic matter, 1.50 g/kg total N, 16.9 mg/kg Olsen-P and 195.95 mg/kg NH₄OAc-K. Each pot (30 cm diameter, 38 cm tall) was filled with 20 kg of sieved dry soil. Nitrogen forms were NH^-₂-N as CO(NH₂)₂-N, NH₄⁺-N as NH₄HCO₃, and NO₃^--N as NaNO₃. The nitrification inhibitor dicyandiamide was applied to each pot. Prior to sowing, each pot received 3.06 g N, 2.9 g P₂O₅, and 3.3 g K₂O; an additional 2.04 g N was also applied to each pot during the wheat elongation stage. Seven plants from each pot were selected when the plants had five leaves. The experiment was arranged in a completely randomized design with 15 replications, and all pots were managed in the same way. Rhizosphere soil samples from a depth of 5-20 cm for each treatment were taken.
at elongation stage (March 25), anthesis (April 30), 14 days after anthesis (May 14), and 28 days after anthesis (May 28). Soil samples were put into sterile bags and transported to the lab as quickly as possible. Part of each soil sample was sieved through a 1 mm screen for analysis of (1) the activities of amination, nitrobacteria, nitrite bacteria, and denitrifying bacteria and (2) inorganic nitrogen (NH₄⁺-N and NO₃^--N) contents, and part was air-dried for (3) determination of soil enzyme (urease and protease) activities. This study showed that the activity of nitrogen bacteria group and enzymes in rhizosphere soils planted with wheat responded differently to nitrogen form. Overall, amination, nitrobacteria, nitrite bacteria, denitrifying bacteria, and protease activities graphed as inverted "V"-shapes that peaked 14 days after anthesis. Urease activity peaked at the jointing stage and was much higher than the activities of amination, nitrobacteria, nitrite bacteria, denitrifying bacteria, and proteases in all studied stages. The different nitrogen forms had different effects on soil nitrogen bacteria group, soil enzyme activity, and inorganic nitrogen. When CO(NH₂)₂-N was applied to Yumai 50, the activities of urease, protease, and amination in the rhizosphere soil increased substantially. The content of NH₄⁺-N in the soil was highest when NH₄HCO₃ was applied, and the content of NO₃^--N in the soil was highest when CO(NH₂)₂-N was provided. Therefore, CO(NH₂)₂-N could promote decomposition and the use of organic nitrogen, and the conversion of ammonia in the soil was promoted under NaNO₃. Nitrogen form affected the content of organic nitrogen in the soil by affecting nitrogen bacteria and the activities of enzymes in the rhizosphere.