Lignin, Cutin, and Suberin Decomposition and Stabilization in C Saturated Soils.

Soils exhibiting C saturation provide the opportunity to study the controls on C stabilization with increasingly limited C sequestration potential.  By examining the changes in C biochemistry with increased soil C saturation at a manure trial in Lethbridge, Alberta, in which SOC responded asymptotically to increased manure C inputs, the effect of C biochemistry on C stabilization and C saturation was assessed.  Lignin (an aromatic plant-derived SOC component) and cutin and suberin (aliphatic plant-derived SOC components) were quantified using CuO oxidation and coupled gas chromatography-mass spectroscopy in whole soils and soil fractions (i.e. macroaggregated coarse particulate organic matter (M-cPOM) and silt+clay (M-SC), intra-micro-within-macroaggregate particulate organic matter (M-iPOM), and easily dispersed silt+clay (SC)) across the manure treatments.  Carbon-normalized lignin signatures increased significantly with increased manure input only in the bulk soil and the chemically protected (SC) fractions; while, cutin and suberin signatures did not significantly differ across manure treatments for any fractions.  The partitioning of lignin to each soil fraction, as determined by the change in C-normalized lignin signatures from the bulk soil to each fraction, differed significantly across treatments for the M-cPOM and M-iPOM fractions, but not for the SC and M-SC fractions.  The partitioning of cutin and suberin, on the other hand, was not significantly different across treatments.  These results indicate that with C saturation, and decreasing stabilization potential, there is no preferential preservation of lignin, cutin, and suberin in chemically protected (SC) and physically + chemically protected (M-SC) fractions.  Furthermore, despite theories of innate lignin recalcitrance, depleted lignin C signatures with saturation in the non-protected (M-cPOM) and physically protected (M-iPOM) fractions indicate that lignin is actually preferentially decomposed when not protected by organo-mineral associations.  This determines that the role of innate lignin, suberin, and cutin biochemistry does not control soil C stabilization and is, therefore, of minimal importance in soil C saturation.