The accumulation of new carbon input and microbial residues in soil under long-term conservation agricultural management practices

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.