Differential effects of ammonium and nitrate addition on soil microbial biomass, enzymatic activities, and organic carbon in a temperate forest in North China

Purpose Ammonium and nitrate are the main components of anthropogenic nitrogen (N) from atmosphere, while their ratio varies worldwide. However, it remains unclear whether forest soil ecosystem changes differ when various ratios of mixed N are added. Methods Ammonium and nitrate were mixed in different ratios (3:7, 4:6, 5:5, 6:4, and 7:3) and forest soils were fertilized for four years. Then, the soil pH, microbial biomass, enzymatic activities, and organic carbon (SOC) were determined. The potential mechanism was analyzed using structural equation modeling. Results Ammonium addition induced a greater fungal biomass decrease than nitrate addition (-0.618 vs. -0.329). The fungal biomass decrease further led to a decline in degradation enzymes, which resulted in SOC accumulation. Phosphatase activity increased and correlated with C-degrading enzymatic activities after N addition, indicating that phosphorous may become the limiting factor that controls degradation. Both ammonium and nitrate addition caused soil acidification ( P < 0.05), but the acidification did not affect the enzymatic activities ( P > 0.05). Moreover, when the added mixed N component ratios were closer to the ambient N component ratios, fewer changes in soil microbial biomass, enzymatic activities, and SOC were observed. This can be explained by the home-field advantage, because soil microbes may have adapted more readily to the ambient N components of the sample site and display fewer responses when the added N is nearly similar to the ambient N types. Conclusion When designing N addition field experiments, the type of N compounds should be considered, besides the amount and duration.