Date of Award

8-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Plant, Soil and Environmental Sciences

Major Professor

Sean M. Schaeffer

Committee Members

Jennifer M. DeBruyn, Sindhu Jagadamma, Robert T. Ladd, Jie Zhuang

Abstract

A better understanding of the mechanisms of soil organic matter (SOM) stabilization is necessary for improving soil quality, especially in agroecosystems. This doctoral dissertation research studied the effects of long-term conservation agricultural management practices on the accumulation of newly added labile carbon (C) and microbially derived SOM. To study their accumulation in soil, newly added labile C was represented by carbon-13 (13C) labelled glucose and the microbially derived SOM was represented by amino sugars.Short-term drying-rewetting cycles are common in surface soils, especially in agroecosystems, which may have different effects on different C pool. Understanding the accumulation and mineralization of newly added labile C in soil during drying-rewetting cycles is important for predicting soil organic C (SOC) storage in long-term. A 24-day incubation in microcosms was conducted with an agricultural soil under 36 years of conservation management. I added 13C-labelled glucose and applied different frequencies of drying-rewetting cycles to the microcosms. At the end of the 24-day incubation, 0.08%-1% of the added glucose C was incorporated into the extractable organic C (EOC) pool, 4%-27% of the added glucose C was incorporated into the microbial biomass C (MBC) pool, and 0.7%-5% of the added glucose C was incorporated into the hydrogen peroxide (H2O2)-resistant C pool. The drying treatment induced higher recovery of the added glucose C in each C pool. The vetch cover crops are more favorable for the stabilization of newly added labile C under repeated drying-rewetting cycles. Structural equation model shows that chemical association and biochemical recalcitrance rather than physical protection are major controls of labile C sequestration in soil under drying-rewetting cycles.Understanding the physical, chemical, and microbial processes controlling the retention of microbial residues in soil is essential for predicting the accumulation of microbially derived SOM. I measured amino sugar concentration, C and nitrogen (N) concentrations microbial respiration rate, extracellular enzyme activity, and soil aggregate composition in an agricultural soil under 31-years of conservation management. Structural equation models show that physical protection plays a critical role in muramic acid stabilization, while microbial activity and substrate availability are more critical for glucosamine.

Orcid ID

http://orcid.org/0000-0001-5420-260X

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