Niche differentiation and functional complementarity between rare and abundant taxa sustain grassland soil bacterial diversity

Soil bacterial communities are central to grassland ecosystem functioning, yet how abundant and rare bacteria taxa differ in ecological strategies, community formation, and environmental responses remains unclear. Here we show, using 278 soil samples from grasslands across China, that these groups have contrasting but complementary ecological roles. Abundant taxa are mainly adapted to nutrient-poor conditions, tolerate a wider range of environments, and are shaped largely by random ecological processes. Rare taxa are more often associated with nutrient-rich conditions, occupy narrower environmental niches, and are governed more strongly by environmental selection. Machine-learning analyses identify pH and aridity index as the main drivers of diversity and community assembly in both groups. Predicted functions also differ: abundant taxa contribute more to carbon fixation, whereas rare taxa play greater roles in nitrogen cycling. Abundant bacterial functions depend mainly on geography and community composition, whereas rare bacterial functions rely more on diversity and respond more strongly to environmental change, highlighting their complementary contributions. Abundant and rare soil bacteria in Chinese grasslands show contrasting trophic strategies, assembly processes, and functions that underpin bacterial biodiversity, with rare taxa more environmentally sensitive and soil pH regulating both groups, based on analyses of 278 samples and machine learning.