ObjectivesAmmoxidation, driven by the ammonia-oxidizing microorganisms, is a rate-limiting step of nitrogen transformation processes. The aim of this study was to explore the main factors which influenced abundance of ammonia-oxidizing microorganisms under a corn-soybean rotation system and a long-term fertilization for 37 years in a brown soil.

MethodsSoil samples (0–20 cm) were collected from the nine treatments of the long-term fertilization trial: no fertilization (CK), low chemical N input (N₁), high chemical N input (N₂), chemical N and P input (N₁P), chemical N, P and K input (N₁PK), pig manure (M₂), pig manure and chemical N (M₂N₁), pig manure, chemical N and P (M₂N₁P) and pig manure, chemical N, P and K (M₂N₁PK). The abundance values of ammonia-oxidizing microorganisms were analyzed using qPCR methods. Main factors influencing the abundance of ammonia-oxidizing microorganisms were found out by the redundancy analysis (RDA) of soil chemical properties and the abundance of ammonia-oxidizing microorganisms.

ResultsThe pH, the soil contents of organic matter, total N, alkali-hydrolysable N, available K, available P, ammonium and nitrate in the organic fertilizer treatments were significantly higher than those in the CK and chemical fertilizer treatments. The soil organic matter, alkali-hydrolysable N, available K, available P and total N was in order of manure treatments > chemical fertilizer treatments > CK. Compared with the CK, the pure chemical fertilization decreased soil pH, while the manure application increased soil pH. The pH values of soil were the lowest in one with the N₂ treatment and the highest in the M₂ treatment. The abundances of ammonia oxidizing bacteria (AOB) were 0.94 × 10⁶ – 5.77 × 10⁷ copies/g dry soil, and the abundances of ammonia oxidizing archaea (AOA) were 3.56 × 10⁶ – 1.22 × 10⁷ copies/g dry soil in different fertilization treated soils, respectively. The abundances of AOB and AOA in manure treatment soils were significantly higher than those in the CK and chemical fertilizer treatment soils. The abundances of AOB and AOA treated with the manure were the highest, while the abundances of AOB and AOA only treated with the chemical nitrogen were the lowest. The redundancy analysis showed that the main factors which affected the abundances of AOB and AOA were soil pH, organic matter, alkali-hydrolysable N, available P and available K, and these factors had positive correlations with the abundances of AOB and AOA.

ConclusionsThe long-term fertilization under a rotation influenced the abundances of ammonia-oxidizing microorganisms most likely by significantly changing the chemical properties of brown soil. The long-term application of organic fertilizer improved soil nutrient contents and played an important role in maintaining the abundances of ammonia-oxidizing microorganisms in brown soil. Simultaneously, the results provide a basis for regulating abundances of AOA and AOB by changing the pH values, the contents of organic matter, total nitrogen, alkali-hydrolysable nitrogen, available phosphorus and available potassium in the future.