Does microbial consumption of plant litter produce diverse pools of dissolved organic matter?
EMSL Project ID
60180
Abstract
Microbial consumption and transformation of plant litter is becoming a widely recognized control on soil organic matter (SOM) persistence. Although the reasons for the increased resiliency of microbially transformed SOM are largely unknown, we hypothesize that the molecular compounds produced through microbial decomposition increase the overall molecular diversity of the resulting dissolved organic matter (DOM) pool. Molecular diversity, the evenness and richness of distinctly different molecular compounds, is emerging as a mechanism governing DOM retention time in soil. Increased DOM molecular diversity is expected to impede its usage and uptake by microbes due to the thermodynamic and physical limitations of each individual molecule occurring at relatively low concentrations. Whether DOM molecular diversity is dependent on microbial necromass (dead microbial cells) or is the result of extracellular microbial oxidative transformations of plant litter also remains in question. The aims of our research are: (1) to determine the relative contribution of microbial inputs (e.g., necromass, oxidatively transformed plant litter) versus non-microbially transformed plant litter inputs to DOM, (2) to identify how microbial consumption of plant litter influences the DOM molecular diversity and persistence, (3) to determine the contribution of microbial necromass to the DOM pool over time, and (4) to use a novel 18O-H2O pulse-chase experimental approach with high resolution mass spectrometry and isotopic ratio mass spectrometry to track microbial transformation of plant litter. We hypothesize that DOM persistence relies on microbial extracellular transformations of plant litter rather than contributions from necromass, due to the increased molecular diversity of the resulting DOM pool.We will incubate a soil microbial community with plant litter (blue grama grass) in a mineral matrix and at three decomposition stages (early and late) pulse the microcosm with 18O enriched H2O and chase with H2O with natural abundance 18O-H2O. After each pulse, the soil samples will be extracted with both water and a choloroform:methanol (2:1, v:v) solution. Water extractable organic matter (WEOM) is targets DOM pools of biologically-active C, while the chloroform:methanol extraction targets metabolites, proteins, and lipid extractable (MPLEx) pools of DOM. Both extractions will be analyzed using Liquid Chromatograph Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (LC-FTICR-MS) to track microbial transformations of DOM and to quantify DOM molecular diversity over time. DNA extractions will be analyzed with Isotope Ratio Mass Spectrometry (IRMS) to determine relative inputs of microbially-derived OM to the DOM pool over time. Upon measurement of DOM molecular diversity as a function of microbial decomposition during ‘early’ and ‘late’ stages of decomposition, we will use this harvested DOM as a substrate for a microbial community and measure mineralization rates from the two DOM substrates with differing molecular diversities. These mineralization trials will inform us if DOM molecular diversity influences DOC persistence. Together we expect that these strategies will allow us to identify the contributions to the overall DOM pool from active microbial cells, dead cell material (e.g., necromass), recently oxidized plant-derived organic compounds and plant-derived DOM that has yet to be oxidatively degraded by microorganisms. Our approach will contribute to our understanding of how microbial transformations increase DOM resilience and test a newly developing method of tracking the microbially driven global carbon cycle.
Project Details
Project type
Exploratory Research
Start Date
2021-12-01
End Date
2022-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members