Nitrification is an important biochemical process in the soil, and a cause of soil nitrogen loss. In nature, some plant species can exude from their roots some substances that may inhibit the activity of soil ammonia oxidizing bacteria. This kind of nitrification inhibitory substances is generally referred to as biological nitrification inhibitor (BNI). Sorghum is such a plant species that exudes from its roots methyl hydroxy phenyl propionate (MHPP), which has great inhibitory effect on soil nitrification. To explore underlying mechanism of sorghum roots exuding MHPP, sorghum seedlings were cultivated in hydroponic systems using ammonium and nitrate, separately. Root exudates were collected after a given period of time for analysis of MHPP content with a High Performance Liquid Chromatograph (HPLC) and hydrion content with the micro-titration method. After the collection of root exudates, Sorghum roots were also collected for isolation of plasma membrane of root cells with the two phase method for determination of H+ ATPase activity. Results show that in the hydroponic system with ammonium sorghum roots exuded MHPP at a rate of about 3.52 μmol g^-1 root FW d^-1 and proton at a rate of 16.49 μmol g^-1 d^-1, while in the other, they did almost zero. And in the former, sorghum roots exuded, while in the latter they did almost neither. Moreover, with increasing concentration of ammonium in the hydroponic medium, roots exudation of MHPP and proton increased in rate, too. So, it is obvious that ammonium nitrogen promotes release of MHPP from sorghum roots. However, it was found in further study that although apparently the release of MHPP from sorghum roots was affected by form of nitrogen, the exudation of MHPP was always correlated to that of hydrions in the process, and root cells exuding proton depended on activity of the plasma membrane H^＋ pump. It was also found in the study that MHPP release from sorghum roots was actually regulated by plasma membrane H^＋ ATPase activity. In the absence of nitrogen sources, it is feasible to use fusicoccin or vanadate to affect proton pump activity and hence release of MHPP from sorghum roots. Statistic analysis of all the data acquired in the experiment shows that the release of MHPP and proton was nearly at a ratio of 1:4 and their correlation coefficient r was 0.98. Therefore, it can be held that in the hydroponic system using ammonium nitrogen the secretion of a large volume of MHPP from sorghum roots is to a certain extent induced by enhanced plasma membrane H^＋ ATPase activity resulting from absorption of ammonium ion, which can be deemed as an important physiological mechanism that can be used to prevent ammonium oxidation in rhizosphere and improve nitrogen utilization efficiency.