pH-induced changes in fungal abundance and composition affects soil heterotrophic nitrification after 30 days of artificial pH manipulation

Heterotrophic nitrification occurs extensively and is important for NO3- production in acidic soils. However, whether low soil pH stimulates heterotrophic nitrification is unknown and the potential microbial driver is unclear. Thus, a pH gradient (3.5, 4.5, 5.5, 6.5, 7.5) was manipulated in forest (SF with initial pH 4.5) and cropland (SC with initial pH 5.5) soils in subtropical China to illustrate the effect of soil pH on heterotrophic nitrification. After 30 days of pH regulation, 1% C2H2 was used to inhibit autotrophic nitrification and reveal heterotrophic nitrification via N-15-labelling experiments. During 30 days of pH regulation, soil microbial properties (e.g. gene abundance and composition of fungi and bacteria) were also determined to study the potential microbial driver of heterotrophic nitrification. The results showed that the gross heterotrophic nitrification rates increased from < 0.3 mg N kg(-1) day(-1) in the pH 7.5 treatments to > 1 mg N kg(-1 )day(-1) in the pH 3.5 treatments, and the contribution of heterotrophic nitrification to the total nitrification was enhanced to more than 60% in the low pH treatments in both SF and SC. With soil acidification, more organic than inorganic N substrate was used in heterotrophic nitrification. Fungi showed a positive correlation with the gross heterotrophic nitrification rate (P < 0.01) and with the contribution of heterotrophic N-15-NO3- production to total N-15-NO3- production (P < 0.01), suggesting that fungi were the dominant heterotrophic nitrifiers in acidic soils. In addition, Phialocephala, Chloridium, and Tararomyces may have the potential for heterotrophic nitrification in our studied acidic soils. The present study suggested the decreasing soil pH could affect fungal abundance and composition, in turn, stimulate heterotrophic nitrification after a short term of pH regulation.