Soil aggregate size modifies the impacts of fertilization on microbial communities

Soil microorganisms play an essential role in the redistribution of soil aggregate sizes due to the micro environmental changes, but little is known about the changes in microbial communities (especially fungi) and their activities within different microenvironments due to agronomic managements. Here, we examined the influence of fertilization (inorganic and inorganic plus organic amendments) on soil aggregate distribution and the microbial communities within aggregates at the site of a long-term fertilization experiment. Significant impacts on soil aggregate distribution and distinct changes in fungal and bacterial communities in soil aggregates due to fertilization were observed, with the fungal community exhibiting more marked changes compared with the bacterial community. Soil carbon and nitrogen contents in aggregates were the most closely related to changes in microbial communities associated with soil aggregates. The changes of microbial taxa abundance in aggregates were generally accompanied by the redistributions of these taxa among all size aggregates, showing the preference for specific ecological niches for these taxa under fertilizations. Both applications of nitrogen alone (N) and inorganic plus organic amendment (NPKM) contributed to mega-aggregate formation. N treatment caused the low total fungal counts in mega-aggregates and carbon depletion, whereas it increased the abundance of N2O-producing fungi in both mega-and macro-aggregates, denitrifying bacteria in micro-aggregates and nitrifying bacteria in macro-aggregates. In contrast, NPKM treatment mitigated/prevented the sharp shifts of the abundance of these microbes mentioned above with nitrogen fertilization, and increased the ectomycorrhizal fungi abundance in all soil fractions as well as carbon accumulation in soil. Our results suggest that the responses of soil bacteria and fungi to fertilizations are modified by their ecological niches, which responses are responsible for the variation of ecosystem services.