Abstract
Direct evidence-based approaches are vital to evaluating newly proposed theories on the persistence of soil organic carbon and establishing the contributions of abiotic and biotic controls. Our primary goal was to directly identify the mechanisms of organic carbon stabilization in native-state, free soil microaggregates without disrupting the aggregate microstructure using scanning transmission x-ray microscopy coupled with near edge x-ray absorption fine structure spectroscopy (STXM-NEXAFS). The influence of soil management practices on microaggregate associated-carbon was also assessed. Free, stable soil microaggregates were collected from a tropical agro-ecosystem in Cruz Alta, Brazil. The long-term experimental plots (>25 years) comparing two tillage systems: no-till and till with a complex crop rotation. Based on simultaneously collected multi-elemental associations and speciation, STXM-NEXAFS successfully provided submicron level information on organo-mineral associations. Simple organic carbon sources were found preserved within microaggregates; some still possessing original morphology, suggesting that their stabilization was not entirely governed by the substrate chemistry. Bulk analysis showed higher and younger organic carbon in microaggregates from no-till systems than tilled systems. These results provide direct submicron level evidence that the surrounding environment is involved in stabilizing organic carbon, thus favoring newly proposed concepts on the persistence of soil organic carbon.
Highlights
The increasing concentration of carbon dioxide (CO2) in the atmosphere has stimulated a wide array of research exploring mitigation options to counteract climate change
Carbon STXM-NEXAFS spectroscopy showed that soil organic carbon (SOC) distributed heterogeneously (Fig. 1a,f, and Supplementary Fig. S1a), inside what appeared to be protective micro- and nano-casings that are made of various minerals such as aluminosilicates, Fe oxyhydroxides, Ca phosphates etc
In NTR, spectrum d (Fig. 1e; green spectrum) is unique compared to b and c spectra (Fig. 1e; red and yellow spectra), suggesting the area represented by cluster d (Fig. 1d) is chemically distinct from other areas
Summary
The increasing concentration of carbon dioxide (CO2) in the atmosphere has stimulated a wide array of research exploring mitigation options to counteract climate change. Scanning transmission x-ray microscopy coupled with near edge x-ray absorption fine structure spectroscopy (STXM-NEXAFS) is a powerful technique that can image and obtain chemical information in micrometer-sized soil samples at nanometer-scale resolution with minimal disturbance to the specimen[10] This technique generates element-specific component maps for a broad range of biologically important elements (i.e., C, N, O, P, and S), alkaline metals (i.e., Na, Ca, K, Mg), first row transition metals (i.e., Mn, Fe), Al, and Si11 and has been used by many researchers in the recent past to study soil C and mineral associations[8,9,12,13,14]. The primary goal of this study was to search for direct evidence of the mechanisms of SOC stabilization using STXM-NEXAFS whilst incurring minimal disturbance to the original aggregate microstructure These observations were supported by appropriate bulk chemical analyses (total organic C/TOC, 13C nuclear magnetic resonance/NMR, and high-performance liquid chromatography/HPLC) to reveal the effect of long-term management practices on microaggregate-associated carbon
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