Long Term Decomposition: The Influence of Litter Type and Soil Horizon on Retention of Plant Carbon and Nitrogen in Soils

Litter type affects initial decomposition rates, but soil horizon affects mechanisms of long-term soil carbon stabilization.

The Science

In one of the few studies examining litter decay over a decade, Lawrence Berkeley National Laboratory (LBNL) scientists used stable isotope labels to trace plant litter–derived carbon and nitrogen as they decomposed and formed soil organic matter (SOM). They found that the litter type (needles or roots) and the soil environment (organic or mineral horizon) both affected decomposition, but at different timescales.

The Impact

This research helps bridge the gap between studies of litter decomposition and SOM by tracing how litter becomes SOM over a decade. The results back the recent paradigm shift in the understanding of soil carbon research be demonstrating that the long-term retention of litter-derived carbon and nitrogen soil is an ecosystem property dependent on the soil horizon in which the litter was placed.

Summary

The scientists found that the legacy of the type of plant inputs (root or needle litter) affected total carbon and nitrogen retention over 10 years, but that the soil horizon affected how the litter-derived SOM is stabilized in the long term. In the organic (O) horizon, litter was retained in the coarse particulate size fraction (>2 mm) over 10 years, likely due to conditions that limited its physical breakdown. In the mineral (A) horizon, litter-derived carbon and nitrogen were retained in a finer size fraction (<2 mm), likely due to association with minerals that prevent microbes from accessing the carbon and nitrogen. Litter type had no effect on the stabilization of litter-derived carbon and nitrogen in mineral-associated pools. After 10 years, 5% of initial carbon and 15% of initial nitrogen were retained in organo-mineral associations, which form the most persistent organic matter in soils. Very little litter-derived carbon moved vertically in the soil profile over the decade, but nitrogen was significantly more mobile.

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, within the U.S. Department of Energy Office of Science under contract number DE-AC02-05CH11231.

References

Hicks Pries C., J. A. Bird, C. Castanha, and P. J. Hatton, et al. "Long term decomposition: The influence of litter type and soil horizon on retention of plant carbon and nitrogen in soils". Biogeochemistry 134 5–16  (2017). https://doi.org/10.1007/s10533-017-0345-6.