Nitrification and Nitrifying Communities in Tropical Soils of Varying Edaphic Properties.

Abstract Nitrification is a key process in soil N dynamics, but relatively little is known about it in tropical soils. In this study, we examined soils from Trinidad with the overall goals of determining the relation between abundance and community structure of nitrifiers with nitrification levels and edaphic characteristics. Nine soils were examined that ranged in texture from sands to clays and spanned pH 4 to 8. Enumeration of ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) by qPCR amplification of bacterial v. archaeal amoA genes, showed that in all soils, AOA dominated over AOB, with the former ranging from ca. 105 to 106/g soil and the latter ca. 103 to 104/g soil. AOA abundance was negatively correlated (p < 0.001) with levels of soil organic carbon, clay and ammonium, but showed no correlation to soil pH. The abundance of AOB was not correlated to any soil property examined. Nitrification potential rates (NPR) had a positive correlation with clay and pH (p < 0.001), but no correlation to abundance of either AOA or AOB. Structures of AOA and AOB communities, as determined by amoA terminal restriction fragment (TRF) analysis, differed significantly between soils (analysis of similarity p < 0.001). Environmental variables best explaining archaeal amoA TRF clustetring were NH4-N, total N and total Kjeldahl N. In contrast, bacterial amoA TRF clustering was best explained by phosphorus, bulk density and iron. Collectively, the results indicated that while AOA or AOB could not be unequivocally determined as dominant drivers of nitrification, edpahic factors potentially shaping these microbial communities could be identified, and consisted of differing sets of soil properties. The latter finding could provide new insights into the question of AOA v. AOB niche differentiation, and suggest that these communties are not necesarily shaped by a common set of soil properties.