Fertility thresholds in the black soils of Northeast China: a shift from soil organic matter molecular composition to bacterial richness dominance

Soil organic matter (SOM) and bacterial communities are key regulators of soil fertility, yet the synergistic regulatory effect remains poorly elucidated. In this study, 180 black soil samples spanning a wide and continuous fertility gradient were collected through large-scale field sampling in Northeast China. Metabonomics and high-throughput sequencing were employed to characterize the SOM molecular composition and bacterial community structure. Multidimensional analysis identified a response threshold (integrated fertility index, IFI of around 0.8) for both bacterial communities and SOM molecular variables to soil IFI, where the key driver of soil fertility shifted from SOM molecules to bacterial communities. Based on the response threshold, all the samples were divided into two groups: the low-fertility group with IFI<0.8 and the high-fertility group with IFI>0.8. Specifically, in the low-fertility group (IFI<0.8), IFI was only positively correlated with SOM molecular composition, which explained 52.81% of the total variation in IFI. While in the high-fertility group (IFI>0.8), IFI was only positively correlated with bacterial richness, which explained 70.77% of the total variation in IFI. These findings highlight the importance of targeted soil management strategies: prioritizing SOM monitoring in low-fertility soils and bacterial community assessment in high-fertility soils. This study provides a basis for precision agricultural practices aimed at optimizing and sustaining soil fertility in the black soil region of Northeast China.