Molecular Understanding of Soil C Processes through Advanced Fticr-MS Characterization.

The focus on ecosystem stress and climate change is currently relevant as researchers and policymakers strive to understand the feedbacks between soil C dynamics and climate change. Successful development of chemical/molecular profiles that link soil microbiology with soil carbon (C) to ascertain soil vulnerability and resilience to climate change would have great impact on assessments of soil ecosystems in response to global climate change. Additionally, better understanding of the dynamics of soil organic matter (SOM) plays a central role to climate modeling, fate and transport of carbon, and mineral SOM interactions. Current methods used to characterize organic matter in the solid and aqueous phases lack molecular and elemental detail that is necessary to develop predictive, mechanistic models of the key processes that operate in the belowground ecosystem. Recent advances in the area of ultrahigh resolution mass spectrometry (UHR MS) have improved this situation. The use of UHR MS, in particular Fourier transform ion cyclotron resonance MS (FT-ICR MS), usually coupled with electrospray ionization (ESI), has enabled the examination of molecules, directly from mixtures, with ultrahigh mass resolution and sub-ppm mass accuracy. The use of FT-ICR MS has, however, been limited to dissolved organic matter (DOM) mixtures collected from rivers, lakes and estuaries. Here we demonstrate how UHR MS can be used to gain information of the molecular composition of soil organic matter. EMSL’s extensive expertise and capabilities in UHR MS proteomics were leveraged to develop extraction protocols for the characterization of carbon compounds in SOM, thereby providing the chemical and structural detail needed to develop mechanistic descriptions of soil carbon flow processes. Solvent extractions are the most commonly used procedures to prepare extracts from soil due to their ease of use, efficiency and wide applicability. Our experiments have allowed us to identify thousands of individual compounds in complex soil mixtures with a wide range of C content representing diverse ecosystems within the USA. This UHR technique generated large databases of chemical formulas for individual samples which could be searched for specific compounds and compound classes or integrated across the entire dataset to summarize the molecular characteristics of SOM (e.g. aromaticity, elemental ratios, and degree of unsaturation). Moreover, our experiments have shown that the yield of the chemical extraction was dependent on (1) the type of solvent used and its polarity, (2) sample-to-solvent ratios and (3) the chemical and physical nature of the samples including their origins. Hexane, a non-polar organic solvent, was efficient in extracting lipid and lipid-like compounds regardless of soil origin or organic carbon %. For samples with high organic carbon %, acetonitrile extracted a wide range of compounds characterized with high O/C ratios, characterized as polyphenolic compounds that were not observed with methanol extraction. Soils extracted with pyridine showed a similar molecular distribution to those extracted by methanol. The results from these experiments show that solvent extraction followed by UHR MS is a promising tool to understand the dynamics of soil organic matter. The information gained from this study has been used on different user proposals assessing the sensitivity of soil carbon decomposition and feedbacks to climate change. We present examples of several of these studies and demonstrate its potential in several studies involving the simulated change or ecosystem gradients.