Effects of root order on fine root decomposition

Background/Question/Methods Roots differ widely in their turnover rates, which among other factors, depends on order of branching and on species differences in root construction.  This study evaluates decomposition of fine roots by root order of a range of temperate tree species that varied widely in rates of root turnover.  We assessed how functional and structural differences among roots affected decomposition rate. Using the root litterbag method, we separated the first four root orders of four tree species into two distinct functional and structural root categories, 1^st & 2^nd order finer roots and 3^rd and 4^th order coarser roots. In addition, a third category was established using new white root growth when present. Oven-dried root samples were buried under their own species canopy in a 33-year-old common garden planting near Siemianice, Poland.

Results/Conclusions In contrast to hypothesized rates of decomposition our results show that the finest root orders decomposed slower than coarser 3^rd & 4^th order roots as well as new white root growth. Coarser 3^rd & 4^th order roots of Tilia cordata, Larix decidua and Pinus sylvestries lost ~ 35 % mass after 36 months. The 3^rd & 4^th order roots of Acer pseudoplatanus lost slightly less than 30 % mass within 19 months. Mass loss of 1^st & 2^nd order roots only ranged from 10 – 20 %, except for Larix decidua, which lost 31 % of its original mass during 36 months. Only new white root growth followed the predicted trend of fastest decomposition with rates of ~ 30 % after 14 months. The C:N ratio of the coarser order roots was in general greater than that in the finer root orders, except for Pinus sylvestries where the C:N ratio of 3^rd & 4^th root order was similar to that of the 1^st & 2^nd order roots. In all three root orders the phosphorus and magnesium content decreased significantly during the first three months, while nitrogen and calcium immobilized in most root orders. Carbon turnover in the infertile sandy forest soil is slower than what might be predicted because the majority of root mortality is observed in 1^st & 2^nd order roots, which are often missed in traditional root decomposition approaches.