Buried Surface Horizons in Saskatchewan Croplands Provide Insight into Microbial Soil Carbon Cycling and Development of Persistent Organic Matter

The accumulation of soil from hillslope erosion in depositional landscape positions can bury the original A horizon soil, leading to a disconnect of C-rich surface soils from surface environments due to burial. The buried C then serves as the primary C for the buried native microbial community. The goal of this thesis was to quantify and characterize the soil organic C (SOC) and examine microbial decomposition of this C in buried surface horizons, decades after burial. Surface (Ah) soils were sampled at 0-10 cm, buried surface (Ahb) soils were identified around 40-50 cm deep, and buried subsoil (Bmb) soils were sampled directly below Ahb soils. Buried surface (Ahb) soils had a SOC content (25.6 mg OC g-1 soil) similar to the corresponding surface (Ah) soil (30.9 mg OC g-1 soil), but greater than Bmb subsoil horizons (8.7 mg OC g-1 soil). Microbial abundance was reduced in Ahb vs. Ah horizons (Total PLFA: Ah = 77.2 nmol g-1 soil, Ahb = 26.8 nmol g-1 soil), however enough viable biomass remained to suggest microbial C cycling was still occurring in Ahb horizons. Over the course of a 100-day controlled incubation experiment, the susceptibility of SOC in Ahb horizons to mineralization to CO2 was 4x lower than corresponding Ah horizons. Carbon mineralization was strongly correlated with mineral-associated organic matter (Rho = -0.67, p < 0.01) in Ah horizons but not in Ahb horizons. Microbial community structure in Ahb horizons was distinct from surface soils, suggesting co-development of the community with increasingly persistent C as decomposition proceeded in the decades after burial. Similar quantities and concentrations of microbial necromass (amino sugars) remained in the Ahb vs. Ah horizon but were strongly correlated with clay only in the Ahb horizons (Rho = 0.77, p < 0.001). Further, microbial communities in Ahb horizons were more thermodynamically efficient at utilizing added glucose than Ah horizons (72 vs. 64 % energy retained) but had a lower calorespirometric ratio under basal metabolism (841 vs 3106 kJ mol-1 CO2-C), suggesting communities are adapted to intense C recycling due to energy limitations of the buried C. This research highlights the co-development of distinct microbial communities with persistent C in buried soils with limited fresh C inputs and examines C persistence in soils after repeated microbial decomposition.

Name thesis:degree_name

Master of Science (M.Sc.)

Level thesis:degree_level

Masters

Discipline thesis:degree_discipline

Soil Science

Grantor

University of Saskatchewan

Year dc:date.issued

2024

Author dc:creator

• Mitchell, Amanda Dawn

Advisor dc:contributor.advisor

• Helgason, Bobbi

Committee members dc:contributor.committeemember

• Arcand, Melissa
• Schoenau, Jeff
• Comeau, Louis-Pierre

• Carbon Cycling
• Persistent C
• Microbial Communities
• Thermodynamic Efficiency
• Mineral-Associated Organic Matter

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/10388/16370

OAI identifier oai:identifier

oai:harvest.usask.ca:10388/16370