Saheed Olaide Jimoh (@sahjim05), University of Wyoming, discusses his article: Traits associated with the conservation gradient are the strongest predictors of early-stage fine root decomposition rates
Background
Fine root decomposition is a process that converts dead plant materials into their component elements, supporting nutrient cycling, regulating carbon storage, and sustaining organic matter pools in ecosystems. This process can be impacted by morphological and chemical fine root traits, as well as phylogeny, resulting in variation in the rate of decomposition at different stages. Understanding how fine root traits correlate with decomposition within a phylogenetic framework can enhance our knowledge of nutrient cycling and carbon storage in terrestrial ecosystems.
The root economics space (RES) is defined by two orthogonal belowground tradeoffs – the conservation and collaboration axes. The conservation axis (root nitrogen – root tissue density) is depicted by fast to slow resource gradients, while the collaboration axis (specific root length – root diameter) is represented by a tradeoff between the do-it-yourself and outsourcing of resource acquisition, likely shaped by relationships with mycorrhizal fungi. On the conservation axis, acquisitive species with high root nitrogen provide higher substrate quality that enhances decomposition, while conservative species have dense root tissues that are more resistant to decomposition. On the collaboration axis, thin-rooted species with high surface area-to-volume may decompose faster because of higher exposure to microbes. Low-diameter roots may decompose faster but thick roots may also exhibit fast decomposition when the cortical tissues preferred by microbes are of high quality.
Our question
Which fine root traits best predict early-stage fine root decomposition rates across species while accounting for phylogeny?
Hypothesis
We hypothesized that early-stage decomposition (pml) would be more strongly associated with the root conservation axis than with the collaboration axis, and chemical root traits (root nitrogen, phosphorus, cellulose, phenol, tannin, lignin, lignin-to-nitrogen ratio, and lignin-to-phosphorus ratio) would be better predictors of early-stage decomposition (pml) than the morphological RES traits (root tissue density, root diameter, and specific root length).
The study
We selected 63 of the most common and abundant tree species in Aotearoa New Zealand, grown under standardized conditions in a potting medium. We collected fine roots (i.e., thin absorptive roots) from nine individuals per species and scanned them to measure root traits. To assess early-stage decomposition, pre-weighed fine root samples were placed in litter bags on the forest floor, with mesh bags preventing material loss, and left to decompose for six months. Litter bags were retrieved from the field, and roots were carefully sorted from litter, cleaned of soil, and dried in an oven at 60oC for 72 hours. Proportion mass loss over six months was estimated as pml = 1 – (final dry mass / initial dry mass).
Key findings
The proportion mass loss rate showed a strong phylogenetic signal, and closely related species decomposed at similar rates. Fine root decomposition is more closely related to the conservation than the collaboration axis.
The percentage mass loss was negatively related to root tissue density, positively related to root diameter, and not correlated with specific root length. Root lignin and root cellulose are the strongest predictors of decomposition rates.
Early-stage fine root decomposition was most strongly driven by tissue quality traits, such as high root nitrogen and low lignin-to-nitrogen ratio, which all align with the conservation axis of the root economics space. While root diameter had a weaker influence, it played an undeniable role in early-stage fine root decomposition. Some thick-rooted species decomposed faster, likely because of the higher-quality inner (cortical) tissue, while thin-rooted species decomposed more slowly, likely because of their higher cellulose concentration that maintains the structural integrity of small-diameter roots.
These results advance our understanding of the relationships between fine root decomposition and functional traits, indicating that decomposition is more closely related to traits on the conservation axis, while root chemical composition are key regulators of decomposition.
