Reconciling 14C and minirhizotron-based estimates of fine-root turnover with survival functions.

The turnover of fine-roots is a crucial component for the input of C to the soil. The amount of root litter input is depending on estimates of turnover times from different techniques. Turnover times from fine-root cameras (minirhizotrons) often yield 75% higher root litter input estimates than turnover times estimated with the bomb-radiocarbon signature of fine roots. We introduce a generic framework for the analysis of fine-root 14C with different survival functions. So far, mostly an exponential function has been used to estimate the turnover time and mean age of fine roots. In the context of the introduced survival function framework we clarify the terms turnover time, mean residence time, mean longevity, and mean age, which are commonly used in studies of root turnover. Using a unique time series of fine-root 14C ( Fröberg 2012), we test if survival functions other than the exponential function are better in accordance with turnover-time estimates commonly found with other methods. A survival function that corresponds to a two-pool model was best in agreement with minirhizotron-based estimates (mean residence time of 1.9 y). We argue that using fine-root 14C and minirhizotron time-to-death data together would give the best constraints on fine-root turnover. At the same time this could allow quantifying systematic biases inherent to both techniques. [ABSTRACT FROM AUTHOR]