The Path From Litter to Soil: Insights Into Soil C Cycling From Long‐Term Input Manipulation and High‐Resolution Mass Spectrometry

Abstract

The path of carbon (C) from plant litter to soil organic matter (SOM) is key to understanding how soil C stocks and microbial decomposition will respond to climate change and whether soil C sinks can be enhanced. Long‐term ecosystem‐scale litter manipulations and molecular characterization of SOM provide a unique opportunity to explore these issues. 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 C cycling, microbial function, and the size and molecular composition of C pools. Input exclusion led to a 30% loss of soil C, attributable largely to the nonmineral‐associated C fraction, and to declines in soil C decomposition. The absence of roots caused a shift in the microbial catabolic profile, though there was little evidence that root litter was preferentially stabilized. Although C pool size did not change with litter additions, Fourier transform ion cyclotron resonance mass spectrometry analysis of the finest mineral fraction revealed dramatic changes to the chemical composition of carbon. Lipid content increased proportionally with input addition and was subsequently mineralized during incubation, indicating that this fraction was metabolically active. Moreover, nonmetric dimensional scaling showed that both litter treatments and incubation caused the molecular composition of SOM to change. We conclude that the path of C from litter to soil may involve labile pools and root‐driven microbial activity associated directly with SOM in the soil mineral matrix otherwise previously hypothesized to be stable.

Spatial gradients in soil-carbon character of a coastal forested floodplain are associated with abiotic features, but not microbial communities

“…In contrast, the deeper soil depths were more similar to older organo-mineral complexed C in terrestrial soils across various ecosystems and land uses (Conant et al, 2011;Dungait et al, 2012;Jobbágy and Jackson, 2000;Gleixner, 2006, 2008). The lack of any systematic gradients in the mineral-associated soil C provides further evidence in support of these interpretations, in addition to previous studies showing mineral-associated soil C to be less responsive to environmental forcings, relative to water soluble C (Reynolds et al, 2018).…”

“…The systematic shifts observed in the molecular signatures compared to non-significant changes in bulk C chemistry shows that molecular-level investigations are particularly relevant to process-based resolution of C biogeochemistry. The absence of bulk C signals mimicking molecular C signals parallel studies indicating rapid change in molecular constituents of the soil C pool with no change in gross C content (Graham et al, 2018;Reynolds et al, 2018). A faster turnover time of C has been observed in microbial biomass as compared to bulk soil organic matter (Kramer and Gleixner, 2008), which is likely to impact microbe-mediated biochemical C transformations and lead to chemically complex heterogeneous C signatures likely to be missed in bulk analysis (Tfaily et al, 2015).…”

Spatial gradients in the characteristics of soil-carbon fractions are associated with abiotic features but not microbial communities

“…In contrast, the deeper soil depths were more simi- lar to older organo-mineral complexed C in terrestrial soils across various ecosystems and land uses (Conant et al, 2011;Dungait et al, 2012;Jobbágy and Jackson, 2000;Gleixner, 2006, 2008). The lack of any systematic gradients in the mineral-associated soil C provides further evidence in support of these interpretations, in addition to previous studies showing mineral-associated soil C to be less responsive to environmental forcings, relative to water-soluble C (Reynolds et al, 2018).…”

“…The systematic shifts observed in the molecular signatures compared to non-significant changes in bulk C chemistry show that molecular-level investigations are particularly relevant to process-based resolution of C biogeochemistry. The absence of bulk C signals mimicking molecular C signals parallel studies indicating rapid change in molecular constituents of the soil-C pool with no change in gross C content (Graham et al, 2018;Reynolds et al, 2018). A faster turnover time of C has been observed in microbial biomass as compared to bulk soil organic matter (Kramer and Gleixner, 2008), which is likely to impact microbe-mediated biochemical C transformations and lead to chemically complex heterogeneous C signatures likely to be missed in bulk analysis (Tfaily et al, 2015).…”

Self-Similarity, Leaf Litter Traits, and Neighborhood Predicting Fine Root Dynamics in a Common-Garden Forest

“…Based on the above logic, we address the central hypothesis that the genetic and phenotypic neighborhood has a significant impact on fine root dynamics through leaf-litter input to soils. Mechanistically, if belowground responses are more sensitive to neighbor effects than a given focal tree, sensitivity could vary depending on neighbor foliar mass inputs, neighbor foliar quality, neighbor genetic identity, and neighbor dissimilarity to the focal tree (e.g., Binkley and Giardina, 1998;Reynolds et al, 2018;Figure 1). Variable foliar mass inputs could alter the soil environment with implications for fine root dynamics even with no differences in leaf litter chemistry.…”