Root decomposition in forest ecosystems: Chemical or morphological control?

Background/Question/Methods

Roots are the major type of plant tissue that contributes to soil organic carbon. The rate at which roots decompose and the fraction of their carbon that is captured by soils varies between tree species. Previous studies have shown that coexisting species in forests can vary greatly in root chemical and morphological traits. Our study was designed to test the extent to which variation these traits affect decomposition and soil carbon sequestration rates. Compared to tulip roots (Liriodendron tulipifera), elm roots (Ulmus americana) have higher lignin:nitrogen ratio, but finer diameter and greater root tip abundance. If morphological traits explain more variation in decomposition rates than chemical properties, then elm roots are expected to decompose faster than tulip roots due to their higher surface area and finer morphology, which causes them to easily break apart into soil particulate organic matter. However, if chemical properties are more significant than morphological traits in determining decomposition rates, then it is expected that tulip roots will decompose faster than elm roots, since they are more chemically labile. In addition to comparing decomposition rates between these two species, we tested if their decomposition rates vary based on the identity of neighboring tree species. Since microbial communities can adapt to the quality of locally available nutrients, it is expected that decay rates will exhibit a ‘home field advantage,’ being accelerated for roots that are more similar to those of neighboring tree species.

Results/Conclusions

Soil and roots of each species were collected from Jennings Woods, Ohio. Entire root systems and roots that were dissected and pooled as 1^st and 2nd order roots or 3rd and 4th order roots were added to 238 litterbags containing soil and placed under replicate trees of each species. Bags were left to decompose in Jennings Woods and periodically harvested over 42 weeks. Our results show that the effects of chemistry and morphology on decomposition depends on the section of the root system considered. No support was found for ‘home field advantage.’ The decay patterns in 1^st and 2^nd order roots suggest that morphology is more important in determining decomposition rates. In contrast, decomposition in 3^rd and 4^th order roots seems to be controlled by chemical composition. Based in these contrasting results, future studies should focus on the role that functional traits have on decay rates belowground and the ways that contrasting root morphologies and tissue quality may affect the dynamics of soil carbon pools.