Response of comammox Nitrospira to nitrogen fertilization in acidic paddy soil
In order to reveal the differences in response and sensitivity of comamox Nitrospira and incomplete nitrifiers to nitrogen addition in acidic rice fields,a pot experiment was conducted to investigate the impact of urea fertilization,in conjunction with growth stage,on complete nitrifiers,and to compare their responsiveness to nitrogen(N)fertilization with that of incomplete nitrifiers in acidic paddy soils.The research results revealed that irrespective of fertilization conditions,comammox Nitrospira(Clade A:3.24×108-7.24×108 copies∙g-1,Clade B:2.14×108-1.48×109 copies∙g-1)rather than ammonia-oxidizing archaea(AOA:2.00×107-4.37×107 copies∙g-1)and nitrite-oxidizing bacteria(AOB:1.35×107-3.31×107 copies∙g-1),were the dominant ammonia oxidizers across tillering,heading,and maturity stages.Neither urea fertilization(Clade A:P=0.762,Clade B:P=0.398)nor growth stage(Clade A:P=0.264,Clade B:P=0.237),nor their interaction(Clade A:P=0.316,Clade B:P=0.294),significantly influenced comammox Nitrospira clades A and B,as their populations remained consistent across the fertilizer treatments and seasons.It is plausible that the environmental factors governing comammox bacterial abundance in urea-fertilized and unfertilized soils may vary.In the absence of urea,the abundance of comammox Nitrospira clades A(R=-0.73,P=0.027)and B(R=-0.75,P=0.019)exhibited a negative correlation with soil N,suggesting that increased ammonia,ammonification,and organic N may not be conducive to comammox bacterial growth and activity in unfertilized soils.Conversely,in the presence of urea,comammox bacteria,especially clade A,were notably influenced by soil pH and redox potential(Eh).Notably,urea application had the potential to alter the interplay between complete and incomplete nitrifiers,as evidenced by the absence of a positive mode of population co-variation of clade B with AOA and AOB following urea application.
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