Fine root morphology as a driver of root and soil organic carbon decay rates

Background/Question/Methods

Roots regulate soil carbon (C) input, but root decomposition rates and root impacts on soil organic C turnover (SOC) are uncertain. This uncertainty is, partly, caused by the heterogeneity of root systems, which vary in diameter distributions and tissue chemistry. We evaluated how root diameter distributions affect root and SOC decomposition. Roots from eight Panicum virgatum (switchgrass) cultivars were analyzed for root diameter size-class distribution and C:N ratio. Roots from each cultivar were mixed with C₃ soil according to five root diameter size-class treatments: (1) 0-0.5 mm, (2) 0.5-1 mm, (3) 1-2.5 mm, (4) a 1:1:1 mixture of roots from each diameter size class, and (5) a mixture combining diameter classes in proportions representing measured size distributions for each cultivar. All treatments were incubated for 90 days under laboratory conditions. Respired CO₂ was measured throughout and the microbial community structure was measured at termination of the experiment. Carbon-13 isotope techniques were used to partition respiration into root- versus native SOC-derived C.

Results/Conclusions

Results indicated: (1) specific root length differed among root systems of the cultivars, (2) root decomposition rates within the three size classes varied by cultivar, but were not correlated with cultivar differences in root C:N ratios or the soil microbial community, (3) root diameter size class affected root and SOC decomposition, and (4) mixing roots of different diameters did not lead to synergistic increases in decomposition. We conclude that intraspecific variation in root architecture is significant and that root properties are an important trait for shaping decomposition processes.