Convergence of soil organic matter chemical composition under contrasting vegetation types

Background/Question/Methods

Aboveground and belowground litter properties influence litter decay on annual-decadal timescales but their impact on long-term SOM storage is unclear.  The objective of this study was to examine the role of litter from vegetation types representing extremes of plant lifespan and tissue chemical composition in determining soil organic matter chemistry and long-term C storage.  We collected litter and soils from an old-growth coast-redwood forest and an adjacent tree-less prairie, at one site with identical climate, topography, and parent material. Solid-state CP MAS ¹³C NMR, soil density fractionation and ¹⁴C modeling were applied in order to: 1) compare the chemical structures of vegetation tissues (aboveground and belowground) to that of particulate soil fractions free in the soil matrix (free light fraction, or fLF) or located inside soil aggregates (occluded light fraction, or oLF) at two soil depths; 2) relate the soil density fractions chemical composition to the stability of their carbon.

Results/Conclusions

Based on CP MAS ¹³C NMR, we found strong differences between coast-redwood and prairie tissues, and between plant tissues and surface (0-10 cm) and deep (50-70 cm) soil organic matter pools with varying degree of physical protection from decomposers activity.  On average, coast-redwood tissues (both aboveground and belowground) contained more aromatic C (C and O substituted aryl C), more lipids (alkyl C) and fewer carbohydrates (O-alkyl C) than prairie tissues. However, despite clear differences in the relative abundance of major C functional groups between the tissues of the two vegetation types, the chemical composition of surface soil fractions differing in degree of physical protection and C turnover time (fLF and oLF) changed consistently from the initial litter input, with a similar increasing accumulation of alkyl C and aromatic functional groups, and loss of O-alkyl C. The degree of decomposition increased from fLF to oLF in a similar manner under both vegetation types and the C ages and mean turnover times of the SOM fractions were similar at both ecosystems. Preliminary data on deep soil fractions have confirmed the same trend.  In summary, our findings suggest that litter properties may have a small role in controlling soil organic matter C storage and cycling.