Estimation of the fine root biomass in a Japanese cedar (Cryptomeria japonica) plantation using minirhizotrons

We estimated fine root biomass in a Japanese cedar (Cryptomeria japonica) plantation using a min-irhizotron technique. Since data obtained from minirhizo-trons are limited to the length and diameter of fine roots observed on minirhizotron tubes, data conversion is necessary to determine the fine root biomass per unit soil volume or unit stand area. We first examined the regression between diameter squared and weight per unit length of fine roots in soil core samples, and calculated the fine root biomass on minirhizotron tubes from their length and diameter. Then we determined conversion factors based on the ratio of the fine root biomass in soil core samples to that on minirhizotron tubes. We examined calculation methods, using a single conversion factor for total fine root biomass in the soil for depths of 0–40cm (Cal1), or using four conversion factors for fine roots in the soil at 10-cm intervals (Cal2). Cal1 overestimated fine root biomass in the lower soil or underestimated that in the upper soil, while fine root biomass calculated using Cal2 better matched that in soil core samples. These results suggest that minirhizotron data should be converted separately for different soil depths to better estimate fine root biomass.