Decomposition of fine root organic matter among temperate trees species
Open Access
- Author:
- Goebel, Marc
- Graduate Program:
- Ecology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 22, 2013
- Committee Members:
- David Eissenstat, Dissertation Advisor/Co-Advisor
Jason Philip Kaye, Committee Member
Margot Wilkinson Kaye, Committee Member
Eric Zenner, Committee Member
Dr Sarah Hobbie, Special Member - Keywords:
- decomposition
fine roots
root pigmentation
root order
litter bags
nitrogen
root persistence
soil depth
root neighbors - Abstract:
- The finest roots of plants contribute considerably to the biogeochemical cycle, especially during their death and decomposition. The pool of dead roots can be of similar or higher magnitude as the living fine root biomass pool. While root turnover is fastest among root order branches with a diameter less than 1 mm, 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 utilized root litter-bags and pre-installed minirhizotron tubes (MR) to examine rates of decomposition and rates of persistence between 1st & 2nd and 3rd & 4th order roots among temperate tree species at a common garden site in southwestern Poland. In the root litter-bag study, roots were enclosed in litterbags and buried under their own and under a common species canopy in a 34-year-old common garden in Poland. When comparing decomposition of different root orders over 36 months, pigmented 3rd and 4th order roots with a higher C:N ratio decomposed more rapidly, losing 20 to 40% of their mass, than pigmented 1st and 2nd order roots, that 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) 1st & 2nd order roots lost approximately 10% of their mass, while white (younger) 1st and 2nd order roots lost about 30%. In contrast to root mass loss, root N content declined more rapidly in the 1st and 2nd order roots than in 3rd and 4th order roots. In higher order roots, N increased in the first 10 months from about 110% to nearly 150% of initial N content depending on species; by the end of the study N content had returned to initial levels. The findings of our root litter bag study suggests that in plant communities where root mortality is primarily of pigmented 1st and 2nd order roots, microbial decomposition may be slower than estimates derived from bulk fine root litter bag experiments, which typically contain at least four root orders. For the MR study, we took image sessions on MR tubes two months prior to a trenching application. Persistence of dead fine roots was assessed by tracking severed roots on trenched minirhizotron tubes, from August 2004 until August 2007. We observed 902 roots, during the two months before trenching around MR tubes, of which 97 % were less than 1 mm in diameter. New roots growing next to severed roots are a major accelerator of severed root decomposition (< 1 mm) in all temperate tree species. Persistence of severed roots increased with soil depth, while increasing soil microbial biomass carbon, labile soil carbon content and earthworms decreased severed-root persistence, as did soil porosity and percent clay content. Severed older roots had higher root persistence than severed younger roots of the two maple tree species. Persistence of dead roots was relatively long in this sandy, infertile soil, ranging from 320 to greater than 974 days, depending on species. In an additional study, using remaining sample material from the root litter-bag study, we addressed the recalcitrance of lignin in fine root orders. Due to small root specimen availability we utilized the FTIR technique to observe changes in peak area around the FTIR spectra wave number of 1510 cm-1, which is linked to oscillations of aromatic ring structure, the basic molecular structure of monolignol units. Contrary to our expectations, estimates of initial lignin concentration did not show significant differences between 1st & 2nd and 3rd & 4th order roots in the three species. In contrast to Acer. platanoides and Tilia cordata, Pinus sylvestris was the only species that showed variations in lignin concentration over time in the two root-order categories. Our results confirm the recalcitrant nature of lignin. However, our results warrant further detailed verification, since oscillations of aromatic ring structure at a wave number of 1510 cm-1 could also represent other chemical compounds, e.g. phenolic or suberin. In conclusion, 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 parallels the markedly different longevities of these different root classes. In addition, the hidden nature of belowground process needs to be approached with different methods at the same time in order to overcome limitations of individual techniques.