Fertilizer application practices are one of the major challenges facing agroecology. The agrobenefits of combined application of green manure and chemical fertilizers, and the potential of green manure to replace chemical fertilizers are now well documented. However, little is known about the impact of fertilization practices on microbial communities and tice yield. In this study, the diversity of bacterial and fungal communities, symbiotic networks and their relationship with soil function were analyzed in five fertilization treatments (N: 100% nitrogen fertilizer alone; M: green manure alone; MN₆₀: green manure couple with 60% nitrogen fertilizer, MN₈₀: green manure couple with 80% nitrogen fertilizer; and MN₁₀₀: green manure couple with 100% nitrogen fertilizer). First, early rice yield was significantly higher by 12.6% in MN₁₀₀treatment in 2021 compared with N. Secondly, soil bacterial diversity showed an increasing trend with increasing N fertilizer application after green manure input, however, the opposite was true for fungal diversity. Microbial interaction analysis showed that different fertilizer applications changed soil microbial network complexity and fertilizer-induced changes in soil microbial interactions were closely related to soil environmental changes. Random forest models further predicted the importance of soil environment, microorganisms and rice yield. Overall, nitrogen fertilizer green manure altered rice yield due to its effects on soil environment and microbial communities. In the case of combined green manure and N fertilizer application, bacteria and fungi showed different responses to fertilization method, and the full amount of N fertilizer in combination with green manure reduced the complexity of soil microbial network. In contrast, for more ecologically sensitive karst areas, we recommend fertilization practices with reduced N by 20–40% for rice production. Graphical Abstract

To investigate the effects of biochar on soil carbon composition and transformation, the effects of 1%, 2%, and 5% mass ratios of banana and cassava straw biochar on carbon dioxide release, total organic carbon (TOC), soluble organic carbon (SOC), and enzyme activity in soil were studied in incubation experiments at a constant temperature in the laboratory. The results showed that the cumulative CO₂ emissions from cassava straw were 15.82 (1% addition ratio) and 28.14 μg·kg⁻¹ (2%), which were lower than those from banana straw, i.e., 46.77 (1%) and 59.26 μg·kg⁻¹ (2%). After culture, the total organic carbon contents of cassava straw were 8.55 (5%), 5.27 (2%), and 3.98 μg·kg⁻¹ (1%), which were higher than those of banana straw, i.e., 6.31 (5%), 4.23 (2%), and 3.16 μg·kg⁻¹ (1%). The organic carbon mineralization rate in each treatment showed a trend of increasing first, then decreasing, and finally stabilizing. There was a very significant positive correlation between catalase and urease activity in soil with cassava straw biochar and between catalase activity and SOC mineralization with banana straw biochar. It plays an important role in the transformation and decomposition of organic carbon. These results show that the application of biomass carbon can significantly improve the organic carbon content and enzyme activity of farmland soil, increase the cumulative mineralization amount and mineralization rate of SOC, and thus increase the carbon sequestration capacity of soil.

Introduction: As a soil amendment, Bamboo charcoal helps to contributes to the improvement of soil carbon sequestration, but its effect on the accumulation and transformation of different soil organic carbon in soil of karst forests is not clear.Methods: The research focused on three distinct forest land succession stages: virgin forest, secondary forest, and planted forest. A 60-day indoor constant temperature culture experiment was conducted, applying bamboo charcoal to the soil of the three forest lands at four different addition ratios: 0%, 1.0%, 2.0%, and 4.0%. The analysis aimed to study the characteristics of SOC mineralization, different carbon fractions of organic carbon, and soil enzyme activity.Results: The findings revealed that bamboo charcoal application led to an increase in the organic carbon (SOC) content within the three forest soils. Moreover, the organic carbon content showed an increase corresponding to the increased proportion of bamboo charcoal, with the highest SOC content observed in the planted forest land with 4.0% bamboo charcoal. The overall performance of the C₀/SOC value in the three forest soils was ranked as follows: planted forest < secondary forest < virgin forest (C₀: the mineralization potential of soil organic carbon). In both planted and secondary forest soils, the C₀/SOC value increased after the application of bamboo charcoal. However, in the virgin forest soil, the application of 1.0% and 4.0% bamboo charcoal reduced the C₀/SOC value, while the application of 2.0% bamboo charcoal increased the C₀/SOC value. Particularly the C₀/SOC value of the planted forest soil without bamboo charcoal was the smallest at 0.047, whereas that in the virgin forest soil with 2.0% bamboo charcoal had the largest value at 0.161.Discussion: Herein, appropriate human intervention can enhance the carbon sequestration potential of forest soil, in different succession stages within the karst area. However, the external application of bamboo charcoal does not significantly improve the carbon sequestration potential in the planted and secondary forest. Notably, applying a higher proportion (4.0%) of bamboo charcoal can enhance the organic carbon sequestration potential, particularly in the virgin forest soil, representing the climax community of forest succession.