Decomposition of plant roots and associated fungal mutualists is a dominant process in ecosystem carbon cycles, yet is woefully understudied compared to decomposition of leaf litter, particularly for the finest order roots that have the highest turnover. Much less is known about how roots decompose within the soil matrix, and whether the litter traits that influence leaf litter similarly influence root decomposition, or how well coordinated these influential traits are across leaves and roots.

Dr. Sun Tao, a researcher from Institute of Applied Ecology, Chinese Academy of Sciences, compared long-term (6 years) in situ decomposition dynamics of leaf litter and first-order roots (as opposed to a fixed diameter cutoff) across 35 co-occurring woody species of a temperate forest ecosystem.

They specifically accounted for mycorrhizal type and its impact on leaf and first-order root decomposition by including nearly equal numbers of EM and AM plant species.

By measuring a large number of leaf and first-order root traits (31 morphological and chemical traits), they found that First-order roots decomposed more slowly (k = 0.11 ± 0.01 yr-1) than did leaf litter (0.35 ± 0.02 yr-1), losing only 35% of initial mass on average after six years of exposure in the field. In contrast to leaf litter, non-lignin root C chemistry (non-structural carbohydrates, polyphenols) accounted for 82% of the large interspecific variation in first-order root decomposition.

They also found that leaf litter from ectomycorrhizal (EM) species decomposed more slowly than that from arbuscular mycorrhizal (AM) species, whereas first-order roots of EM species switched, after two years, from having slower to faster decomposition compared to those from AM species.

The fundamentally different dynamics and control mechanisms of first-order root decomposition compared to those of leaf litter challenge current ecosystem C models, the recently suggested dichotomy between EM and AM plants, and the idea that common traits can predict decomposition across roots and leaves. Aspects of C chemistry unrelated to lignin or nitrogen, and not presently considered in decomposition models, controlled first-order root decomposition; thus, current paradigms of ecosystem C dynamics and model parameterization require revision.

The study entitled "Contrasting dynamics and trait controls in first-order root compared to leaf litter decomposition" has been accepted in PNAS. 

This research is financially supported by the State Key Program of China, the State Key Program of China, Natural Science Foundation of China and Key Research Program of Frontier Sciences of Chinese Academy of Sciences.