Objective To study the effect of biological carbon on soil iron reduction in Chinese fir plantations under the condition of abundant rainfall and long-term soil water saturation, and analyze the community structure changes of iron-reducing bacteria and phosphorus solving bacteria in soil, for clarifying the influence of biochar on soil iron reduction and its relationship with phosphorus form transformation.

Method Based on the red soil of Chinese fir plantations, Chinese fir leaves under the forest were collected and fired to produce 300℃ and 500℃ biochar. Biochar was added at 0, 1% and 3% for 40 days of indoor culture.The basic chemical nutrients of soil were analyzed. The content of different phosphorus forms in soil was determined by the modified Hedley method, and the community structure of soil phosphate solubilizing bacteria and iron reducing bacteria was analyzed by high-throughput sequencing technology.

Result The content of soil active phosphorus increased with the increase of the amount of biochar, and most of them were the H₂O-Po and NaHCO₃-Pi. The content of Residual-P decreased with the increase of biological carbon. The copy number of genes of Fe (III)-reducing bacteria in flooded treatment group was higher than that in non-flooded treatment group, and increased with the increase of biochar firing temperature under the same water condition. The content of ferrous ions in flooded treatment group was much higher than that in non-flooded treatment group, and decreased with the increase of biochar addition. The soil chemical properties such as pH, TC and TP contents in the flooded group were higher than those in the non-flooded treatment group, and increased with the increase of biochar addition amount. The richness of soil phosphorus solubilizing bacteria community increased with increasing firing temperature, and the community structure and diversity of phosphorus solubilizing bacteria also increased with increasing firing temperature.

Conclusion The anaerobic conditions promote the reduction of Fe (III). The addition of biochar changes the chemical properties of soil, affects the community structure and diversity of phosphorus solving bacteria and the growth microenvironment of iron reducing bacteria. Under the double action of microorganism and Fe (III) reduction, the conversion of residual phosphorus and sodium hydroxide phosphorus into aqueous phosphorus and sodium bicarbonate phosphorus are promoted, and the availability of soil phosphorus is enhanced. Therefore, in the southern regions with abundant rainfall, the application of biological carbon in Chinese fir plantation can improve the soil nutrient status and provide enough phosphorus for the growth of Chinese fir.