Ammonia oxidizer abundances
AOA and AOB amoA copy numbers ranged from
9.3×108~2.69×109 and
8.08×106~3.97×108copies per g soil during the N addition period (Fig. 1a, b). Nutrient
addition did not significantly affect AOA amoA copy numbers but
increased those of AOB. AOA and AOB displayed a similar pattern during
the N addition cessation period (Fig. 1c, d). AOA amoA copy numbers
remained constant and ranged from
4.05×108~3.95×109copies g-1soil,
while AOB amoA copy numbers increased significantly in previously
fertilized plots. AOB amoA gene copy numbers were ten times higher in
R90 than in the unfertilized plots (R0). Since their discovery as
ammonia oxidizers in 2015, comammox strains have only been identified
after terminated nutrient addition. Comammox Nitrospira clade A
and B ranged from 4.32×104 to
5.30×105 copies g-1 soil, lower than
AOA and AOB. Comammox Nitrospira clade B decreased in R60 but
remained unchanged in other treatments (Fig. 1e, f).
AOB amoA copy numbers positively correlate with
NH4+-N (r= 0.74, p < 0.001)
and NO3−-N (r =0.67, p <
0.001) in the N addition plots, whereas AOB amoA copy numbers were
significantly negative correlated with soil pH (r = -0.47, p <
0.05) (Table S3). In contrast, AOB amoA copy numbers positively
correlated with microbial biomass N (r= 0.49, p < 0.05) and
negative correlated with total N (r= -0.46, p < 0.001), total
C (r= -0.52, p < 0.001), and
NH4+-N (r=- 0.40, p < 0.05)
after nutrient addition ceased (Table S3).