Roles of residue management, microbes and aggregation in soil carbon stabilization under semiarid, irrigated corn
Oleszak, Hanna, author
Cotrufo, M. Francesca, advisor
Stewart, Catherine, advisor
Trivedi, Pankaj, committee member
With atmospheric carbon dioxide levels continuously on the rise, it is critical that we focus our efforts on sequestering carbon (C) to slow global warming. To maximize these efforts, it is furthermore important to understand the pathways by which plant C inputs form soil organic carbon (SOC), as these pathways may inform the efficiency and duration of C stabilization. No-tillage is often recommended as a universal tool to draw C into the soil, yet literature reports mixed effects of tillage practices on C accrual. To maximize our efforts and best recommend agricultural practices for C sequestration, it is important to understand how the incorporation of residue within the mineral soil and disturbance associated with tillage impact plant residue C dynamics, as mediated by changes in microbial community and soil structure. While microbes play the active role in decomposing organic matter, soil structure can act as a gatekeeper to microbial accessibility to organic matter; thus, the effects of disturbance and residue incorporation upon the interplay of these two variables is highly important to consider. We used 13C labeled plant residue to track the movement of residue C in incorporated vs. surface-applied residue treatments in irrigated, semiarid corn for a period of 30 months. Both carbon dioxide (CO2) fluxes and soil cores were tested for total C and 13C enrichment to quantify residue-derived C contribution to CO2 efflux and to C accrual in the mineral soil over time, respectively. Furthermore, aggregate size fractionation and microbial community (via phospholipid fatty acids, PLFAs) were analyzed to assess how residue placement location and disturbance affect the mechanisms behind residue decomposition, and ultimately soil C stabilization. The incorporation of residue in the mineral soil resulted in significantly greater SOC formation efficiencies and greater SOC accrual in the first year, compared to the surface-application of residue. However, differences in SOC accrual subsided after 30 months, even though higher CO2 losses were measured in the surface applied residue treatments after 30 months. Residue-derived microbial biomass was greater in INC than SA or SA-NR at all timepoints, although this was only significant at 6 and 12 months. Residue-derived microbial community composition differed between early and later stages of decomposition, as well as between disturbed and undisturbed treatments. Mean weight diameters of aggregates featured a seasonal trend, with greater mean weight diameters in the fall. Furthermore, INC had significantly higher MWD at 5-10 cm at 6 months compared to surface-applied treatments, while disturbed treatments (INC and SA-NR) had significantly higher MWD's at depth at 12 months than SA. MWD was strongly correlated to residue-derived SOC in incorporated treatments, but not in surface-applied treatments. Finally, SOC formation efficiencies were more strongly correlated to residue-derived F:B in the incorporated treatment, compared to surface-applied treatments. Residue C recovery, SOC formation efficiencies, and residue-derived microbial biomass indicate that the incorporation of residue stimulates SOC formation through the DOC-microbial pathway and the physical transfer path concurrently, while the surface-application of residue follows a shift in SOC formation pathways from DOM-microbial SOC formation to physical transfer of residue. Additionally, correlations between residue-derived SOC, residue-derived F:B, and MWD indicate that the protection of residue C largely relies on aggregation when residue is incorporated.
Includes bibliographical references.
Includes bibliographical references.