Responses of complete and incomplete ammonia oxidizers to farmland reversion to forest in Ferralsols

Competition for ammonia is a key factor in the evolution of ammonia-oxidizing microorganisms. Recent studies have shown that complete ammonia oxidizing bacteria (comammox) have a higher affinity for ammonia compared to ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB), potentially driving nitrification in soils with low nitrogen levels. This study investigated the effects of different land uses (Old-Growth Forest, OGF, over 50 years; Returning Farmland to Forest, RFF, lasting 20 years; Crop Land, CL, over 30 years) on soil nitrification and ammonia oxidizers in Ferralsols. Our results showed that both the nitrification potential and nitrite oxidation potential in Crop Land soil were significantly higher than those in Old-Growth Forest soil and Returning Farmland to Forest soil. The abundance of AOA, Nitrobacter-like nitrite-oxidizing bacteria (NOB) nxrB genes and comammox clade B was higher in Old-Growth Forest soil than in Returning Farmland to Forest and Crop Land soils. Conversely, the abundance of AOB, Nitrospira-like nitrite-oxidizing bacteria (NOB) nxrA genes and comammox clade A amoA genes in Crop Land soil was substantially higher compared to those in Old-Growth Forest soil and Returning Farmland to Forest soil. Our results suggested that distinct niche differentiation among AOB, AOA, NOB, and comammox exists in this Ferralsols, with N availability being a key factor driving this differentiation.