Plantation conversion of Eucalyptus promotes soil microbial necromass C accumulation

Context

Stand conversion in subtropical regions has altered soil physicochemical properties and microbial communities, leading to changes in microbially mediated processes, such as microbial necromass C (MNC) formation and accumulation. However, previous studies on the effects of stand conversion on MNC are lacking, leading to gaps in our understanding regarding the influence of long-term stand conversion on MNC accumulation in different soil layers and the relative importance of soil properties for regulating MNC.

Aims

In this study, we used field surveys and soil analysis to assess the effects of converting a Eucalyptus forest into other planted forest (broadleaf mixed forest [BM] and Acacia mangium × Acacia auriculiformis forest [AM]) on soil properties, enzyme activity, microbial community composition, and MNC after conversion 20 years in Guangdong, South China.

Results

We found that the content of soil organic C (SOC) in the surface soil (0–10 cm after litter removal) increased by 64.9 % when Eucalyptus was converted to AM, whereas there was no significant difference in the subsurface soil (10–20 cm). β-1,4-glucosidase (BG) and β-1,4-N-acetaminophen glucosidase (NAG) activity increased significantly, while leucine aminopeptidase (LA) activity decreased significantly in the surface soil. In the subsurface soil, BG activity did not change significantly; nonetheless, acid phosphomonoesterase (AP) activity decreased. The fungal, bacterial, and gram-negative bacterial biomass did not significantly differ among the different forests in the surface soil, but the fungal, bacterial, gram-positive, and gram-negative bacterial biomass decreased significantly in the subsurface soil. The ratio of fungi to bacteria was highest in the BM, whereas the ratio of gram-positive to gram-negative bacteria was highest in the AM. Soil fungal and microbial necromass C and the ratio of fungal to bacterial necromass C increased significantly in the surface soil when Eucalyptus was converted to AM. The contribution of MNC and fungal necromass C to SOC content significantly increased by 22.20 % and 26.23 %, respectively, when Eucalyptus was converted to AM. The main controlling factors of MNC accumulation in the surface soil were pH and total N, whereas soil enzyme activity (BG related to C-acquisition) was the dominant determinant of MNC accumulation in the subsurface soil.

Conclusion

Our study provides evidence that converting Eucalyptus to AM may promote MNC accumulation in the surface soil by changing soil pH and TN content to affect soil enzyme activity and microbial community structure, and ultimately changed MNC accumulation. Therefore, developing effective forest management practices, such as reasonable stand conversion may help to enhance forest SOC accumulation by increasing MNC accumulation.