Root chemistry effects on soil organic matter fractions persist in topsoil and subsoil of a well-drained Alfisol

Subsoils have been identified as a potential target for building soil C stocks, given the older age and slower cycling of organic matter at depth. Plant roots deliver C deep into the soil profile, but there is limited experimental evidence as to which root traits are most effective at increasing subsoil C. To that end, we conducted a 13C natural abundance study to trace C from roots of differing quality into organic matter fractions in a shallow and deep soil layer of an Alfisol in Lexington, KY. The roots of four species were packed into nylon-wrapped tubes with soil and incubated in the field at depths of 0–10 cm and 40–50 cm for one year. Roots with high guaiacyl lignin (G-lignin) and suberin had more of their C retained in light particulate organic matter (lPOM), while those with high p-hydroxyphenyl lignin and N compounds had more C transferred into mineral-associated organic matter (MAOM). The effects of litter type and chemical composition on lPOM and MAOM formation persisted at both depths. More litter-derived C was recovered in lPOM than MAOM at both depths, but this effect was more pronounced in the subsoil than topsoil (30.2% as lPOM and 8.6% as MAOM in the subsoil; 21.3% as lPOM and 10.4% as MAOM in the topsoil). Overall, our results suggest that lPOM is an important reservoir for recent inputs of root litter in the subsoil, and this fraction can be enhanced by adding roots with high G-lignin and suberin.