PNNL

Abstract

The pathways carbon (C) takes from plant litter to soil organic matter (SOM) underlies our understanding of how soil C stocks and microbial decomposition will respond to climate change, and determining whether natural soil C sinks can be enhanced to offset potential losses. Long-term ecosystem-scale litter manipulations and innovations in the molecular characterization of SOM provide a unique opportunity to explore these dynamics. We incubated soils from a 20-year litter-input experiment for 525 days and asked how litter quantity and source (i.e., roots versus aboveground litter) affected soil C cycling, including microbial function and the size and molecular composition of C pools defined by degree of mineral association. Input exclusion led to a 30% loss of soil C, attributable largely to the non-mineral-associated C fraction, and was accompanied by declines in soil C decomposition. The absence of roots led to a shift in the microbial catabolic profile, though there was no evidence that root litter was preferentially stabilized. Carbon did not accumulate under litter addition. However, direct examination of the finest mineral fraction with Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) revealed dramatic changes to the chemical composition of carbon. In particular, lipid content increased proportionally with input addition, and was subsequently mineralized during incubation, indicating that this apparently stable fraction is metabolically active. Moreover, non-metric dimensional scaling (NMS) showed that both litter treatments and incubation caused the molecular composition of SOM to diverge, and explained 79% of the variance in soil respiration. We conclude that the path from litter to soil is more complex than previously thought and mineral-associated SOM thought to be stable may be microbially available.