Project Funding: 2014 - 2017

NRA: 2013 NASA: Carbon Cycle Science   

Funded by USDA

Abstract:
Whether soils become a source or sink of C under future climate and land use change is of great societal importance because soils represent the largest pool of terrestrial organic carbon (C). The response of terrestrial vegetation composition and productivity to these changes has been studied and well documented over time, but the interactions between carbon inputs, soil microbiota, and soil mineralogy that ultimately sequester carbon into stabile pools has received much less attention. The uncertainty in future projections of soil C is mirrored by the uncertainty in our understanding of the mechanisms controlling C stabilization and destabilization in soils. Molecular weight and biocomplexity of C are no longer considered indicators of its recalcitrance. Instead, the current conceptual framework highlights C flow through a ‘filter’ of soil microbes, and describes C as ‘agglomerated’ into submicron pores and/or associated with reactive mineral surfaces. This conceptual framework of C stabilization in soils lacks key information about how flow and fate of C are affected by variation in 1) carbon inputs 2) microbial populations and 3) mineral assemblages. It is essential to correctly understand the molecular level mechanisms of C stabilization before estimating large temporal and spatial estimations of changes in terrestrial C storage. Our objective is to examine how different forms of carbon entering the soil environment interact with soil microbial populations and mineral assemblages to stabilize carbon, and to use these data to inform models that can estimate changes in soil C cycling over a large range of temporal scales and climate regimes. This approach supports DOE Carbon Cycle Science objectives to develop, test, and simulate process-level understanding of terrestrial ecosystems and USDA’s objective to improve our understanding of the C cycle such that we may more fully understand the C consequences of land use change. Our methodology is first to take advantage of a natural ‘climosequence of chronosequences’ along the Pacific from the Channel Islands in southern California to Santa Cruz in central California and soils near the Mattole River in Northern California. Each location contains soils ranging in age from 6000 to 250,000 years. Climate and soil age are well understood drivers of C storage in soils, but few studies have addressed the interaction of these two drivers simultaneously. Secondly, we will utilize state of the art spectroscopic techniques that will inform a novel terrestrial C model. Methodologies include stable isotope probing (through model compound additions and plant labeling) coupled to confocal micro-Raman spectroscopy (SIP-RAMAN) to quantify the flow of specific forms of labeled C through the microbial community and to evaluate the types of C stabilized into different soil fractions and/or mineral surfaces. Detailed 2-D micro-Raman maps of soil C speciation, 13C incorporation, and mineralogy at the micron scale can also be coupled to fluorescence in-situ hybridization (FISH) probing of bacterial, archeal, and fungal groups. These techniques will be evaluated in comparison with synchrotron-based micro X-ray microscopy and micro-X-ray fluorescence mapping and 13C-PLFA techniques. These data, and other data including radiocarbon content of different soil fractions, will then be used to inform a reactive transport based soil C model (i.e., Crunchflow). Crunchflow is a multiphase reactive transport model that allows for prediction of the transport, reactivity and isotopic fractionation of organic C including organic-mineral interactions. The incorporation of organic C processes into Crunchflow represents a new approach to simulating soil C over long timescales and provides an ideal tool to integrate the molecular, mineralogical, and isotopic aspects of this study.