Discrepancy in fine root turnover estimates between diameter-based and branch-order-based approaches: a case study in two temperate tree species

Fine root turnover plays a key role in carbon(C) budgets and nutrients cycles in forest ecosystems.However,the difference between branch-order-based and diameter-based approaches in estimating fine root turnover is still unclear.We studied root biomass turnover based on multiplying root standing biomass by turnover rate(inverse of median root longevity) in two Chinese temperate tree species,Fraxinus mandshurica Rupr.and Larix gmelinii Rupr.The minirhizotron(MR) technique was used to estimate longevities for first and second order roots,and total roots(R total) apparent on the MR tube surface.The corresponding biomass for each root group was estimated by soil monolith.The difference in biomass turnover between R total and the sum of the first and second order roots was used to represent the discrepancy between diameter-and order-based approaches.First order roots had shorter life spans and higher biomass turnover rates than the second order roots in both species.Biomass turnover estimated by the order-based method for F.mandshurica and L.gmelinii were 155.4 g m-2 a-1 and 158.9 g m-2 a-1,respectively,in comparison with 99.5 g m-2 a-1 and 117.7 g m-2 a-1 estimated by the diameter-based method,indicating that the diameter-based approach underestimated biomass turnover.The most probable reason was that the order-based method enhanced separation of the heterogeneous root population into relatively homogenous root groups with varying turnover rates.We conclude that separating fine root pool into different branch orders can improve the accuracy of estimates for fine root turnover,as well as the understanding of the belowground C allocation and nutrient cycling at ecosystem level.