Molecular-level studies of soil organic matter responses to environmental change
Abstract
Soil organic matter (SOM) is critical to ecosystem function and sustainability. Variation in the earth’s climate has raised concerns about the stability of SOM stocks in various regions around the world.
Part of this uncertainty stems from the lack of molecular-level information about SOM and its response to environmental change. The chemical complexity and heterogeneity of SOM requires the use of advanced, molecular-level methods to determine SOM responses to ecosystem changes. The combination of SOM biomarkers (as measured by gas-chromatography/mass spectrometry: GC/MS) and analysis by nuclear magnetic resonance (NMR) facilitates a detailed analysis of SOM components. These methods were applied to three different field studies: 1) an in situ 14-month soil warming in a mixed forest in southern Ontario, Canada, 2) enriched CO2 and N addition in the Duke Forest, USA, and 3) permafrost disturbance the Canadian Arctic. The objective was to measure and quantify molecular-level changes to SOM composition in varying environments using SOM biomarkers and NMR.
Enhanced lignin oxidation and cuticular carbon sequestration was observed with soil warming. In addition to this, labile constituents such as carbohydrates decreased significantly. Fungal activity, as measured by phospholipid fatty acids (PLFAs), increased. This study demonstrated that lignin, which is sometimes believed to be stable, was indeed susceptible to enhanced degradation by soil fungi. In addition, increased cuticle preservation and a reduction of usable carbon suggest that SOM composition was shifting to a more recalcitrant form over time.
We also employed molecular-level methods to investigate the composition and degradation of SOM from the Duke Forest Free Air CO2 Enrichment (FACE) experiment. Increased fresh carbon inputs into the forest floor under elevated CO2 were observed. The ratios of fungal to bacterial PLFAs and Gram-negative to Gram-positive bacterial PLFAs decreased in the mineral soil with N fertilization, indicating an altered soil microbial community composition. Moreover, the acid to aldehyde ratios of lignin-derived phenols increased with N fertilization, suggesting enhanced lignin degradation in the mineral soil. This suggested that microbial decomposition of SOM constituents, such as lignin and cuticle-derived compounds, was promoted under both elevated CO2 and N fertilization.
High temperatures and substantial rainfall in July 2007 in the Canadian High Arctic resulted in permafrost active layer detachments (ALDs) that redistributed soils throughout a small watershed in Nunavut, Canada. Increased concentrations of extracted bacterial PLFAs and large contributions from bacterial protein/peptides in the NMR spectra at recent ALDs suggested increased microbial activity. PLFAs were appreciably depleted in a soil sample where ALDs occurred prior to 2003. However an enrichment of bacterial derived peptidoglycan was observed by solution-state NMR and enhanced SOM degradation was observed by 13C solid-state NMR. These data suggest that a previous rise in microbial activity, as is currently underway at the recent ALD site, led to degradation and depletion of labile SOM components.
Collectively, these studies highlight the detailed insight into ecosystem function that can be obtained using a combination of molecular-level techniques. The techniques and application to study SOM dynamics in changing ecosystems will be presented and discussed in detail.