Changes in alpine grassland type drive niche differentiation of nitrifying communities on the Qinghai‒Tibetan Plateau

Abstract

Nitrification is a critical process of nitrogen (N) cycling performed by ammonia-oxidizing archaea (AOA) and bacteria (AOB), nitrite-oxidizing bacteria (NOB), and complete ammonia oxidation (comammox). However, the performance of these nitrifying groups in alpine grasslands has rarely been documented. This study investigated their abundance and community structure in four alpine grasslands on the Qinghai‒Tibetan Plateau with quantitative PCR and high-throughput sequencing. Potential ammonia oxidation (PAO) was measured and partitioned by inhibitor method. PAO was mainly dominated by AOA. PAO and potential nitrite oxidation (PNO) significantly decreased, while the apparent half-saturation constant (Km) had no significant change along a grassland degradation gradient. Phylogenetic analyses revealed that the predominant AOA, AOB, and NOB were Nitrososphaera, Nitrosospira, and Nitrospira lineages, respectively. Network analyses revealed that the alpine nitrifying co-occurrence network had a modular structure that was mainly shaped by taxonomic relatedness and environmental relationships, indicating different nitrifying species occupy disparate niches. Soil pH, ammonium, and moisture were the main drivers for variation in AOA, AOB, and NOB communities, respectively. Here, we provide evidence of niche differentiation in nitrifying communities with changes in grassland type. Moreover, these findings support a theoretical foundation for predicting the repercussions of grassland degradation on soil nitrifying communities in alpine ecosystems.