Effects of pH and mineralisation on nitrification in a subtropical acid forest soil

Effects of pH on nitrification have been explored widely; however, few researchers have investigated the compound effects of pH and mineralisation on nitrification and responsible nitrifiers in subtropical forest soils. An acid subtropical forest soil was subjected to pH treatments by liming for 40 days at 25°C and 60% water holding capacity. After 40 days, gross N transformation rates in the samples were evaluated using the 15N tracing technique and model. Ammonia-oxidising bacteria (AOB) and ammonia-oxidising archaea (AOA) were quantified by quantitative polymerase chain reaction. The results showed that gross rates of mineralisation (P < 0.01), NH4+ oxidation to NO3− (P < 0.01), and dissimilatory NO3− reduction to ammonium (DNRA, P < 0.05) significantly increased with elevated soil pH. The NH4+ oxidation to NO3− rates and N mineralisation rates were significantly correlated (P < 0.05). The abundance of AOB (P < 0.05) but not AOA amoA genes dramatically increased in the elevated pH treatments after 40 days. We suggest that the N mineralisation was sensitive to elevated pH in the subtropical acid forest soil, which increased the supply rate of nitrification substrate and the favourable pH microsites for nitrifiers. Interactions between elevated pH and mineralisation have a synergistic and stimulatory effect on development of AOB and their associated nitrifying activity.