The limited effects of carbonaceous material amendments on nitrite-oxidizing bacteria in an Alfisol

Carbonaceous materials are soil conditioners that affect nitrogen cycles. However, how carbonaceous materials influence nitrite-oxidizing bacteria (NOB) is yet unclear. In this study, we investigated the NOB community and its potential activities under different treatments (control, biochar, straw, limestone, biochar + limestone, and straw + limestone) in an Alfisol, a type of arable soil depleted in calcium carbonate but enriched in aluminum- and iron-bearing minerals. Treatments with limestone increased soil pH, and straw inputs caused an increment of available potassium (AK). Ammonia (NH₄⁺) was inversely changed under the straw and biochar + limestone amendments. None of the treatments significantly impacted the abundance of Nitrobacter (nxrA) or the potential nitrite oxidation activity (PNO). The abundance of Nitrospira (nxrB) increased in the biochar + limestone-treated samples and was significantly correlated with PNO, pH, and AK. High-throughput sequencing results showed that the α-diversity of NOB did not change in response to the treatments. The dominant Nitrobacter OTUs were affiliated within the Clusters 3, 4, 8, and 9 (a new cluster named in this study), while those of Nitrospira were in the lineage II and Namibian soil cluster 2. The limited compositional variation for Nitrobacter was explained by pH, and that for Nitrospira by pH, TN, and NH₄⁺. Among all available data in this study, the richness of Nitrospira was the most important predictor (73%) for PNO. Therefore, we assumed that the community of nitrite oxidizers (Nitrospira) could be relatively redundant in function, supported by the observation that the carbonaceous inputs did not impact either the potential activity or the α-diversity but did affect the abundance and community composition.