Bacteria and fungi are core microorganisms in diverse ecosystems, and their cross-kingdom interactions are considered key determinants of microbiome structure and ecosystem functioning. However, how bacterial-fungal interactions mediate soil organic carbon (SOC) dynamics remains largely unexplored in the context of artificial forest ecosystems. Here, we characterized soil bacterial and fungal communities in four successive planting of Eucalyptus and compared them to a neighboring evergreen broadleaf forest. Carbon (C) mineralization combined with five C-degrading enzymatic activities was investigated to determine the effects of successive planting of Eucalyptus on SOC dynamics. Our results indicated that successive planting of Eucalyptus significantly altered the diversity and structure of soil bacterial and fungal communities and increased the negative bacterial-fungal associations. The bacterial diversity significantly decreased in all Eucalyptus plantations compared to the evergreen forest, while the fungal diversity showed the opposite trend. The ratio of negative bacterial-fungal associations increased with successive planting of Eucalyptus due to the decrease in SOC, ammonia nitrogen (NH4+−N), nitrate nitrogen (NO3−−N), and available phosphorus (AP). Structural equation modeling indicated that the potential cross-kingdom competition, based on the ratio of negative bacterial-fungal correlations, was significantly negatively associated with the diversity of total bacteria and keystone bacteria, thereby increasing C-degrading enzymatic activities and C mineralization.

Synthesis and applications: Our results highlight the regulatory role of the negative bacterial-fungal association in enhancing the correlation between bacterial diversity and C mineralization. This suggests that promoting short-term successive planting in the management of Eucalyptus plantations can mitigate the impact of this association on SOC decomposition. Taken together, our study advances the understanding of bacterial-fungal negative associations to mediate carbon mineralization in Eucalyptus plantations, giving us a new insight into SOC cycling dynamics in artificial forests.

The experimental site is located in the state-owned Daguishan Forest Farm in Hezhou City, Guangxi Zhuang Autonomous Region, China (111°20’5’’E, 23°58’33’’N). The mean annual temperature in this area is 19.3℃, with mean annual precipitation and evaporation of 2,056 mm and 1,200 mm, respectively. The soil type is classified as red soil (i.e., ferralsols). A total of 12 plots (20 m wide × 30 m long) were established to collect soil samples in triplicate representing four generations of Eucalyptus plantation. In each treatment, the Eucalyptus trees were at the same stage of development (i.e., 4 years after planting). The treatments included the first generation (PrG) of Eucalyptus reforestation, the second generation (SeG) regenerating after the PrG was cut, the third generation (ThG) regenerating after the SeG, and the fourth generation (FoG) regenerating after the ThG. An evergreen broadleaf forest with three adjacent plots was selected as the control (CK), which was a precursor to the Eucalyptus plantation. All the plots were located within a 5 km² area. The Eucalyptus species planted in these plots was a hybrid of Eucalyptus urophylla S.T. Blake × Eucalyptus grandis Hill ex Maiden (Eucalyptus urograndis).