Effects of agricultural land use on the differentiation of nitrifier communities and functional patterns from natural terrestrial ecosystems

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Human activities severely affect the global nitrogen (N) cycle. Croplands receive intensive N fertilization; consequently, cropland and natural ecosystem differentiation often results in community and functional variation in N-transforming microbes, including nitrifiers, which perform nitrification central to N cycle. However, evidence of such variation is mostly limited to ammonia oxidizers (AO) in local fields, excluding soil heterogeneity and nitrite-oxidizing bacteria (NOB); the variation under diverse climatic and soil conditions is not comprehensively understood. We conducted a large-scale survey of 131 cropland and natural sites in China. The community patterns of ammonia-oxidizing bacteria (AOB) and NOB differed significantly between croplands and some natural ecosystems, whereas ammonia-oxidizing archaea (AOA) were not affected by ecosystem type. The AOB population and nitrification potential (NP) were significantly higher in agroecosystems than in natural systems except wetlands. Fewer co-occurrence interactions involving nitrifiers were observed in croplands than in natural ecosystems except forests, systematically indicating the ecological diversification of nitrifiers in potential microbial associations among these habitats. Ecosystem type, pH, organic matter (OM), total phosphorus (TP), mean annual temperature (MAT) and mean annual precipitation (MAP) were primary drivers of nitrifier community and functional shifts. This study provides the first large-scale evidence of overall nitrifier community (i.e., AOA, AOB and NOB) and potential functional shifts between agroecosystems and natural environments, enabling predictions of terrestrial N cycle under foreseeable natural land use conversions and global climate change.