Rooting Abundance Governs How Soil Organic C Promotes Soil Aggregation Across a Steep Precipitation Gradient

Abstract:

Roots play important roles in the soil as regulators of water distribution and as direct and indirect sources of carbon-rich, soil aggregating compounds throughout the profile. However, rooting depth distributions are changing on a global scale in the Anthropocene. We assessed how varied rooting abundances with depth can influence the distribution and mechanisms of protection for soil organic C (SOC) throughout soil profiles across a steep precipitation gradient in the US Great Plains. Predictably, soils supporting annual agricultural crops exhibited lower root abundances than native or post-agricultural systems, and lower SOC concentrations near the surface, consistent with radiocarbon signatures suggesting greater amounts of contemporary SOC at the surface of native sites in comparison to agricultural fields. Below ~50 cm, radiocarbon signatures suggest greater mobility of fresh, surface-derived SOC under agriculture. There, SOC exhibits a greater propensity to help form macroaggregates at depth in more mesic locations, and microaggregates where rainfall is more limited. Increased precipitation was linked to a greater abundance of coarser roots and macroaggregates particularly under native prairie, suggesting that the gain in organic matter (OM) and enhanced soil microbial activity in these soils promote its binding with microaggregates throughout the profile. However, in cultivated soils, greater fine root abundance in drier conditions appears to induce the breakdown of aggregates at depth as the plants mine the soil volume for water and nutrients. These aggregates may undergo stabilization given the role roots play in water uptake and hydraulic redistribution through the profile, promoting wetting and drying cycles and altering infiltration rates, especially considering the shrink-swell capacity of these smectite-rich soils. Thus, as changes in rooting abundance with depth due to human activity affect the vertical carbon distribution in soils, there is an adaptation in the mechanisms of aggregate stabilization that likely is mediated by water availability across the precipitation gradient as it governs OM decomposition. Consequentially, alterations in the soil structure will affect the soil’s capacity to store and move water, solute, and gases, and the CO₂ feedbacks to the atmosphere.