“…However, these widely-observed phenomena were entirely attributed to some purely chemical agents, before the discovery of bacterial ammonia oxidizers in actively nitrifying soils (Winogradsky, 1890). Although the responsible microorganisms and relevant mechanisms still remained elusive, the positive correlations between aerobic bacterial numbers and nitrate concentrations began to be noticed (Fred and Graul, 1916;Noyes and Conner, 1919). These early discoveries paved the way for more convincing studies confirming that nitrification was a universal phenomenon in a wide range of acidic soils, and stimulated scientific interests to identify the responsible nitrifiers in soils (De Boer and Kowalchuk, 2001).…”