Decomposition of the finest root branching orders: linking belowground dynamics to fine-root function and structure

Root turnover is fastest in the finest roots of the root system (first root order). Additionally, tissue chemistry varies among even the finest root orders and between white roots and older, pigmented roots. Yet the effects of pigmentation and order on root decomposition have rarely been examined. We separated the first four root orders (all,1 mm) of four temperate tree species into three classes: white firstand second-order roots; pigmented firstand second-order roots; and pigmented thirdand fourth-order roots. Roots were enclosed in litterbags and buried under their own and under a common species canopy in a 34year-old common garden in Poland. When comparing decomposition of different root orders over 36 months, pigmented thirdand fourth-order roots with a higher C:N ratio decomposed more rapidly, losing 20–40% of their mass, than pigmented firstand second-order roots, which lost no more than 20%. When comparing decomposition of roots of different levels of pigmentation within the same root order over 14 months, pigmented (older) firstand secondorder roots lost ;10% of their mass, while white (younger) firstand second-order roots lost ;30%. In contrast to root mass loss, root N content declined more rapidly in the firstand second-order roots than in thirdand fourth-order roots. In higher-order roots, N increased in the first 10 months from ;110% to nearly 150% of initial N content, depending on species; by the end of the study N content had returned to initial levels. These findings suggest that, in plant communities where root mortality is primarily of pigmented firstand second-order roots, microbial decomposition may be slower than estimates derived from bulk fine-root litterbag experiments, which typically contain at least four root orders. Thus, a more mechanistic understanding of root decomposition and its contribution to ecosystem carbon and nutrient dynamics requires a fundamental shift in experimental methods that stratifies root samples for decomposition along more functionally based criteria such as root order and pigmentation, which parallel the markedly different longevities of these different root classes.