Root Litter Decomposition Slows with Soil Depth

Novel use of 13C to quantify how deeper root inputs affect soil carbon storage.

The Science

Clever use of 13C isotopes revealed that plant tissues decompose more slowly the deeper they are in the soil profile. The restriction to decay was breaking down the coarse root particulates into finer particles that bacteria can transform.

The Impact

These results help bolster strategies for enhancing soil carbon sequestration and sustainable bioenergy production based on promoting deeper rooting by plants. Model results suggested that the lack of root exudates in deep soil limits microbial processes.

Summary

Although over half of the world’s soil organic carbon (SOC) is stored in subsoils (>20 cm deep), there are few studies examining in situ decomposition in deep soils. Researchers at Lawrence Berkeley National Laboratory added 13C-labeled fine roots to three depths (15 cm, 55 cm, and 95 cm) in the soil of a Ponderosa pine forest in California. They measured the amount of root-derived carbon remaining over 6, 12, and 30 months, in different soil fractions and in microbial phospholipid fatty acids (PLFAs). Root decomposition in the first 6 months was similar among all depths but diverged significantly by 30 months because decomposition at 95 cm nearly stopped. Mineral associations were not the cause of slower decomposition at depth because similar amounts of applied root carbon were recovered in the dense fraction at all depths. The largest difference among depths was in the amount of root carbon recovered in the coarse particulate fraction, which was much greater at 95 cm (50%) than at 15 cm (15%). There was more fungal and gram-negative bacteria biomass in the surface soil, and these groups may have facilitated rapid breakdown of particulates; they preferentially incorporated the added root carbon relative to native SOC. Simulations of these soils using the CORPSE model, which incorporates microbial priming effects and mineral stabilization of SOC, reproduced patterns of particulate and mineral-associated SOC over both time and depth and suggested that a lack of priming by root exudates at depth could account for the slower breakdown of particulate root material.

Principal Investigator

Margaret Torn
Lawrence Berkeley National Laboratory
mstorn@lbl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This material is based on work supported by the Terrestrial Ecosystem Science program of the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy Office of Science, under Contract No. DE AC02-05CH11231.

References

Hicks Pries, C. E., B. N. Sulman, C. West, and C. O'Neill, et al. "Root litter decomposition slows with soil depth". Soil Biology and Biochemistry 125 103–114  (2019). https://doi.org/10.1016/j.soilbio.2018.07.002.