The application of biochar and organic fertilizer substitution regulates the diversities of habitat specialist bacterial communities within soil aggregates in proso millet farmland.

Biochar and organic fertilizer substitution have been utilized to improve crop yields and promote sustainable agricultural development. However, few studies have focused on how biochar and organic fertilizer substitution regulate the diversities of bacterial meta-communities and sub-communities (i.e., habitat specialists and generalists) within soil aggregates on the Loess Plateau. A field experiment, containing six treatments, namely no fertilizer (CK), no fertilizer plus 10⁴ kg ha⁻¹ biochar (CK + B), traditional N fertilizer (TF, 150 kg N ha⁻¹), traditional N fertilizer plus 10⁴ kg ha⁻¹ biochar (TF + B), organic fertilizer replacing (OF), and organic fertilizer replacing plus 10⁴ kg ha⁻¹ biochar (OF + B), was adopted to explore the influences of biochar and organic fertilizer substitution on the size distribution, nutrient levels, extracellular enzymatic stoichiometry, bacterial meta-community and sub-community diversities, structures, ecological networks, and assembly processes within bulk soil, large macro-aggregates (> 2 mm), small macro-aggregates (0.25–2 mm), and micro-aggregates (< 0.25 mm) in proso millet farmland. Results showed that compared to CK + B treatment, OF + B treatment enhanced the distribution of large macro-aggregates while decreasing the distribution of micro-aggregates. OF + B treatment also markedly elevated the contents of total nitrogen (TN), soil organic carbon (SOC), nitrate nitrogen (NO₃⁻-N), ammonium nitrogen (NH₄⁺-N), as well as activities of C-, N-, and P-acquiring enzymes, enhanced the carbon limitation, and reduced the nitrogen limitation in the bulk soil and macro-aggregates. Compared to CK treatment, OF and OF + B treatments significantly increased the observed OTUs of specialist bacterial communities in large and small macroaggregates, while having no effect on generalist bacterial communities. The impacts of biochar and fertilizer additions, as well as soil aggregates, on the community structure and composition of specialists were greater than those of generalists. RDA analysis showed that NH₄⁺-N content had a significant influence on the meta-community and sub-community structure within three aggregates. Ecological network analysis indicated that OF + B treatment increased the whole bacterial network complexity and stability compared to CK + B treatment. Meanwhile, the bacterial network of specialists with biochar amendment was more complex and tighter than that without biochar. Null-model analysis showed that biochar and fertilizer regimes rarely affected the assembly processes of the bacterial meta-community and sub-community within bulk soil and three aggregates, mainly determined by stochastic process. These observations offer a theoretical basis for understanding the intrinsic relationships between bacterial microbial communities and soil aggregates regulated by biochar and fertilization regimes in the semi-arid region. Highlights: Biochar and fertilizer regimes had a profound influence on the richness and structure of bacterial communities in specialized habitats. Organic fertilizer substitution markedly enhanced the bacterial richness of habitat specialists in macro-aggregates. NH₄⁺-N was the important variable regulating the bacterial community and sub-community structure within soil aggregates. Bacterial network of habitat specialists with biochar addition was more complex and tighter than that without biochar [ABSTRACT FROM AUTHOR]