Under the guidance of Prof. LIU Zhanfeng, YUAN Ye, a postgraduate student, and LI Yue, a Postdoctoral researcher from South China Botanical Garden of Chinese Academy of Sciences conducted a 10-year field nitrogen and phosphorus addition in a tropical coastal forest to investigate the effects of nutrient fertilization, soil aggregate fractions and their interactions on the concentrations of soil microbial residues and their contribution to SOC accumulation.

Nitrogen (N) decomposition could intensify soil phosphorus (P) availability limitation, thus influence the below-ground carbon (C) cycling in tropical forests. Soil microbial residues, always indicated by amino sugars, are important sources of stable soil organic carbon (SOC) and contribute more than 50% of SOC. However, their concentrations and contribution to SOC altered by N and P fertilization with the physical protection of soil aggregates are not well understood. Thus, a ten-year field fertilization experiment was conducted to investigate the effects of nutrient fertilization, soil aggregate fractions and their interactions on the concentrations of soil microbial residues and their contribution to SOC accumulation in a tropical coastal forest.

The results of this study showed that continuous P addition greatly decreased the concentrations of microbial residues and their contributions to SOC in different aggregate fractions. The accumulation of microbial residues depended on the balance between the production and degradation of microbial products. The suppressing effect of P on microbial residues and their contribution to SOC might be caused by increased microbial metabolism due to alleviated microbial P limitation, and the accelerated SOM decomposition by increased residue-decomposing enzyme activity. Contrasting to the suppressing effect of P, N addition had an overall no significant effect on the concentrations of microbial residue and their contributions to SOC, this could be explained by the N-induced opposing microbial controls over the degradation and production of microbial residues.

Soil aggregation plays an important role in mediating SOC stabilization and turnover and it can also shape the microhabitats of microorganisms. In this study, soil aggregation had a substantial impact on the distribution of soil microbial residues. The interactive effects between soil aggregate fraction and nutrient addition on the concentrations of microbial residues and their contribution to SOC were not significant, suggesting a weak role of physical protection by soil aggregates in mediating microbial responses to the altered soil nutrient availability.

The findings in this study suggested that the P-induced decrease in the contribution of microbial residues might be unfavorable for the stability of SOC in such N-rich and P-poor tropical forests, and this information is critical for us to better understand the role of tropical forests in the global C cycling.

For further reading, please refer to: https://doi.org/10.1111/gcb.15407.