Nitrogen loading levels affect abundance and composition of soil ammonia oxidizing prokaryotes in semiarid temperate grassland

Global nitrogen deposition has profound impact on the terrestrial ecosystem including the semiarid temperate grassland, causing vegetation community shifts and soil acidification. Little is known regarding the effect of nitrogen (N) deposition on the belowground microbial communities. This study aimed to examine the response of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to added N in semiarid temperate grassland. We studied the changes of AOB and AOA by using molecular techniques targeting amoA genes along a urea fertilization gradient, i.e., 0, 1, 2, 4, 8, 16, 32, 64 g N m(-2) year(-1), in a 6-year field experiment of semiarid temperate grassland, Inner Mongolia of China. AOB community responded to urea-N substrate clearly, and N addition rates 2-4 g N m(-2) year(-1) induced an increase in its abundances and the shift of its composition. However, AOA community remained unchanged and the highest N loading at 64 g N m(-2) year(-1) even decreased its abundance. Moreover, higher N loading rates (more than 16 g N m(-2) year(-1)) significantly decreased the diversity of AOB but not AOA, as indicated by the decrease of its Shannon and Evenness indices. The relative long-term nitrogen loading of more than 2-4 g N m(-2) year(-1) resulted in diversity loss of AOB in this semiarid temperate grassland. Increasing N loading altered AOB abundance and composition, but AOA showed nonsignificant changes.