Fine root turnover in a northern hardwood forest: a direct comparison of the radiocarbon and minirhizotron methods

We examined fine root turnover using both the minirhizotron and radiocarbon methods within the organic horizon of a northern hardwood forest to better understand discrepancies in turnover estimates obtained using these methods. The recently developed radiocarbon method estimates the mean age of organic matter by comparing its radiocarbon content to recorded atmospheric radiocarbon levels, which peaked in the 1960s as a result of thermonuclear weapons testing. The radiocarbon content of fine roots harvested from minirhizotron tubes did not differ from that of roots collected from the soil, suggesting these two methods sampled the same population of fine roots. However, long-term observation of fine root survivorship using minirhizotrons showed that root age distribution is positively skewed, causing systematic overestimation of fine root turnover by the minirhizotron method and underestimation by the radiocarbon method. We developed a parametric regression model of fine root survivorship. Our estimate of fine root turnover (about 30% per year) using this variation of the minirhizotron method was supported by radiocarbon data considered in conjunction with fine root age distribution.