Dissolved Organic Matter Transformations in Wet Tropical Soils: The Effects of Redox Fluctuation
EMSL Project ID
48643
Abstract
In tropical systems, changes in climate will likely affect both the amplitude and periodicity of redox oscillations due to predicted increases in warming and precipitation intensity. Our research will measure how shifts in soil oxygen/redox patterns affect the fate of complex soil C substrates. Redox conditions are a major driver of soil trace gas fluxes, microbial community dynamics and carbon stabilization, but are poorly constrained in most soil biogeochemical process models and underestimated in representations of upland soils. In humid and wet tropical soils, low redox events are particularly common, occurring on daily to weekly timescales and driven by high biological oxygen demand, high moisture (limiting diffusion), warm temperatures and abundant labile carbon. These oscillations prime tropical soils for rapid C, Fe, P and N cycling, and regulate mechanisms of both carbon stabilization (mineral sorption) and loss (dissolved organic matter (DOM) leaching). Although the importance of tropical soils in the global C cycle is clear, we have a surprisingly poor understanding of how soil C cycling in these systems with inherently low climatic variability will respond to climate change; this makes predicting future climate impacts extremely difficult. Better forecasting of soil C cycling in wet tropical soils depends on a mechanistic understanding of organic matter-mineral interactions, and more detailed knowledge of the chemical nature of DOM. Previous research has shown that while a significant amount of carbon is stabilized via association with Fe hydro(oxide) minerals common to tropical soils, Fe also may be responsible for up to 50% of C oxidation in periodically anoxic soils. Fe-OM complexes are highly vulnerable to variable redox effects, and can rapidly solubilize and re-precipitate in response to local Eh conditions. Losses of dissolved organic carbon (DOC) are thought to be larger in tropical soils than temperate soils because of high rainfall, substantial substrate supplies, and rapid decomposition rates. However the role of DOM in tropical soil C cycling and its contribution to SOM formation are very poorly understood. This is in part because its very chemical nature is so complex and poorly constrained, although microbial metabolites, EPS and necromass likely comprise a significant portion.
In our proposed EMSL research, we will use isotope tracing and molecular characterization of both mineral retained and dissolved soil C following manipulations of soil redox. DOM pools will be characterized with high mass accuracy using high field FT-ICR mass spectrometry, while mineral sorbed organic matter will be measured with a multi-modal imaging approach, including C60-FTICR and NanoSIMS. We hypothesize that shifts in soil O2 availability and Fe (hydr)oxide mineral crystallinity will have a significant effect on microbial C processing, leading to altered degradation of complex C compounds and mineral stabilization. Our results will directly benefit attempts to reduce uncertainties in model predictions of tropical soil carbon balance.
Project Details
Project type
Special Science
Start Date
2014-12-23
End Date
2016-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members