Effect of biochar addition on soil microbial biomass and metabolic activities of carbon sources in Lou soil

Objectives The residual effect of different biochar after 4 to 5 years’ application was investigated, in order to provide data and theory references for suitable use of biochar in Lou soil amendment.

Methods A field experiment was carried out on Lou soil of Guanzhong Plain, Shaanxi Province. The tested biochar was made from trunk and branch of fruit trees, and was once added into soil in 5 levels (0, 20, 40, 60 and 80 t/hm²). Five years later, the soil microbial biomass C, N, P contents and the C/N ratio were analyzed; the microbial carbon metabolic activities were studied by the chloroform fumigation–extraction and biolog-ECO plates.

Results When the biochar addition was 40–60 t/hm², the soil microbial biomass C increased significantly. When the biochar addition was above or equal to 40 t/hm², the soil microbial biomass C/N ratio increased significantly. The biochar addition had no significant effect on soil microbial biomass N and P contents. When the biochar addition was 20 t/hm², the seasonal fluctuation of soil microbial biomass C increased significantly. When the biochar addition was 40–60 t/hm², the seasonal fluctuation of soil microbial biomass C/N increased significantly. When the biochar addition was 20–60 t/hm², the seasonal fluctuation of soil microbial biomass P decreased significantly. The biochar addition did not influence the seasonal fluctuation of soil microbial biomass N significantly. The biochar addition had no significant effects on carbon source metabolic activities. However, the addition of high amount of biochar had a tendency to reduce the overall carbon source metabolic activities of soil microorganisms. When the biochar addition was 60 t/hm², the richness index decreased significantly, while the evenness index increased significantly. When the biochar addition was above or equal to 60 t/hm², Shannon-Wiener index, Simpson index decreased significantly. The biochar addition had no significant effects on the utilization rates of soil microorganisms to use C sources such as carbohydrates, amino acids, polymers, aromatic chemicals and amines. When the biochar addition was 60 t/hm², the utilization rate of soil microorganisms using carboxylic acids decreased significantly. Carbohydrates, carboxylic acids and amino acids were the preferable carbon sources with higher carbon utilization efficiency for soil microorganisms.

Conclusions The biochar addition rate significantly affects the soil microbial biomass and microbial functional diversity even after 4 to 5 years’ application. When the biochar addition was 40 t/hm², the soil microbial biomass C and C/N increased significantly, and there is no significant seasonal fluctuation in soil microbial biomass P. When the biochar addition was above 40 t/hm², the overall carbon source metabolic activities of soil microorganisms, richness index, Shannon-Wiener index and Simpson index indicating soil microbial functional diversity, and the utilization rates of soil microorganisms to use C sources such as carbohydrates, amino acids, and amines shows a decreasing trend. Therefore, biochar application must be controlled within a reasonable range to avoid adverse effects on soil.