The Fate of Vascular Plant Biomolecules with Depth and Cultivation in a Canadian Grassland Soil.

Examination of soil management and depth effects on the mechanisms of soil organic matter (SOM) protection will deepen our understanding of soil responses to changing environmental variables.   We examined the interactions between SOM biochemical composition across depth and functionally separated pools of aggregate-protected, mineral-protected, and non-protected SOM in two different management treatments.  Clay loam (Typic Boroll) soil cores were collected from a no-till treatment with continuous fallow-flax-wheat rotations in place since 1978 and a native shortgrass prairie that had never been cultivated at the Agriculture Canada Experimental Farm in Scott, Saskatchewan.  We measured the total soil organic carbon (SOC) and the cupric oxide oxidation products of seemingly recalcitrant plant-derived compounds, aromatic lignin phenols and aliphatic cutin + suberin substituted fatty acids.  These measurements were taken across a shallow depth gradient of 0-5 cm and 5-20 cm in the bulk soil, two particulate organic matter fractions (POM) (intra-microaggregate protected POM and non-protected coarse POM) and three mineral fractions (microaggregate protected silt, non-aggregated silt, and non-aggregated clay).

As expected, SOC depth gradients were greater in the native grassland than the no-till soil, a feature consistent across all fractions, but attenuated in the mineral and aggregate protected SOC fractions.  Depth gradients for lignin/OC or cutin+suberin/OC differed across fractions or treatments.  Cutin + suberin/SOC concentrations remained consistent with depth in all fractions from the no-till soil, while decreasing significantly with depth in all grassland fractions.  Lignin/OC concentrations exhibited opposing depth effects, remaining consistent with depth in the mineral and aggregated grassland fractions and decreasing with depth in the no-till mineral and aggregate fractions.  When not protected, coarse POM lignin/OC was depleted with depth in the grassland, but did not change in the no-till soil. These results point to distinct differences in biochemical-specific protection or degradation of SOM components with no-till cultivation.  The strong cutin+suberin/SOC gradient reflects the root architecture of grassland soils, with increased root density and surface area in the upper 5 cm leading to increased cutin+suberin SOC concentrations.  Relict effects of tillage prior to no-till adoption in 1978 as well as lesser root density gradients explain the less pronounced SOC and cutin + suberin/SOC gradients in the no-till soil, but fail to clarify the contrasting treatment effects of lignin/SOC protection with depth in these two systems.