Substrate quality effects on stabilized soil carbon reverse with depth

High-quality plant inputs to the soil with low lignin to nitrogen ratios have been conceptualized to foster the formation of soil organic matter (SOM) and carbon (C) in stabilized soil compartments, such as aggregates and organo-mineral associations (MAOM), in dependence of the soil's capacity to store additional C (i.e., C saturation deficit). Yet, evidence for these conceptualizations from field experiments is scarce and it is unclear whether factors commonly not included in laboratory experiments (such as soil depth) may modulate the influence of substrate quality on SOM formation. We, thus, tested the effect of substrates with different qualities (from European alder and Scots pine) on C stocks in bulk soil, free particulate and aggregate-occluded organic matter, and MAOM in soils at various ages after reclamation and at different soil depths in a common-garden field experiment. Our results revealed that substrate quality generally acted in the predicted way in the upper investigated soil layer (0-5 cm depth), i.e., high-quality substrates increased C stocks in MAOM. Surprisingly, the effect of substrate quality reversed with depth (5-15 cm; i.e., low-quality litters were associated with higher C stocks in MAOM), potentially due to a higher sensitivity (or priming) of native SOM in response to high-quality substrates at depth or a lower probability of such substrates reaching deeper soil layers. These patterns appeared to be largely independent of the soils' theoretical capacity to stabilize additional C, which was exceeded in all investigated soils. Based on the results of our field experiment, we highlight the need to refine recent conceptualizations by focusing on environmental factors that are commonly not accounted for in laboratory-scale experiments and that may alter the direction in which substrate quality acts to influence the formation of SOM.

Automated summary

High-quality plant inputs to the soil generally increase carbon stocks in MAOM in the upper soil layer, but the effect of substrate quality may reverse with depth. Further research is needed to consider environmental factors that can alter the direction in which substrate quality influences SOM formation.