Microbial mechanism of biochar addition on nitrogen leaching and retention in tea soils from different plantation ages.

The effect of biochar additions on N leaching and retention in tea soils and its microbial mechanism are still unclear. In this study, effects of biochar additions at rates of 0, 3% and 6% on N leaching, N retention and microbial responses in two tea soils with 20- and 60-year plantation ages were investigated under application with 15N-labeled urea. The results showed that cumulative mass of leached NH4+-N, NO3--N and TN was reduced by 20.9%-91.9%, 35.1%-66.9% and 40.0%-72.8% under biochar additions, respectively. The retention of TN in soil was increased by 1.2%-5.8% under biochar amendment. Fertilizer-N in the leachate was reduced by 28.8%-62.1%, while fertilizer-N retention in the soils was enhanced by 3.2%-23.9% with biochar application. Biochar addition of 6% showed the highest mitigation of N leaching and enhancement of TN retention across the two soils. Biochar additions increased soil microbial biomass and enzyme activities and changed the bacterial community composition, indicating that biochar addition increased the microbial N requirement, stimulated soil N cycling, including nitrification and denitrification processes, and enhanced microbial N immobilization in the tea soils. Those microbial responses to biochar addition were higher in 60-year-old soil relative to 20-year-old soil, leading to a higher enhancement of N retention and mitigation of N leaching. Soil pH was the prime factor that influenced soil microbes, and it strongly correlated with microbial biomass, enzyme activity, the relative abundance of dominant phyla and α-diversity indices. Therefore, the enhancement of microbial biomass, activity and shifts of bacterial community composition related to N cycling in response to biochar additions that increased the soil pH could be an important mechanism to better understand the biochar-induced N leaching mitigation and N retention enhancement in tea soils under different plantation ages.