Root-driven weathering impacts on mineral-organic associations in deep soils over pedogenic time scales
[4];
[5];
[1]
- Univ. of Massachusetts, Amherst, MA (United States)
- Geosciences and Environmental Change Science Center, Denver, CO (United States)
- U.S. Geological Survey, Menlo Park, CA (United States)
- Univ. of Arizona, Tucson, AZ (United States)
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Stanford Synchrotron Radiation Lightsource (SSRL), Menlo Park, CA (United States)
Plant roots are primary weathering agents in deep soils, yet the impact of resulting mineral transformations on the vast deep soil carbon (C) reservoir are largely unknown. Root-driven weathering of primary minerals causes the formation of reactive secondary minerals, which may protect mineral-organic associations (MOAs) for centuries or millennia. Conversely, root-driven weathering may also transform secondary minerals, potentially enhancing the bioavailability of C previously protected in MOAs. Here we examined the impact of root-driven weathering on MOAs and their capacity to store C over pedogenic time scales. To accomplish this, we compared soil that experienced root-driven weathering, resulting in the formation of discrete rhizosphere zones in deep soil horizons (100-160 cm) of the Santa Cruz Marine Terrace chronosequence (65ka-226ka), with adjacent soil that experienced no root growth. Using a combination of radiocarbon, mass spectrometry, Mössbauer spectroscopy, and X-ray spectromicroscopy approaches, we characterized transformations of mineral-organic associations in relation to changes in C content, turnover and chemistry across four soils ranging in age from 65 to 226 ka. We found that the onset of root-driven weathering (65-90ka) increased the amount of C associated with poorly crystalline iron (Fe) and aluminum (Al) phases, particularly highly-disordered np-goethite. This increase coincided with greater C concentrations, longer C residence times, and a greater abundance of microbially-derived C. Continued root-driven weathering (137-226ka) did not significantly change the amount of C associated with crystalline Fe and Al phases, but resulted in a decline in the amount of C associated with poorly crystalline metal phases. This decline coincided with a decrease in C concentrations and potential turnover rates, and a shift toward plant-derived C. In contrast, soil not affected by root-driven weathering showed comparatively low amounts of C bound in poorly crystalline Fe and Al phases regardless of soil age and, correspondingly, lower C concentrations and estimated residence times. Furthermore, our results demonstrate that root driven-formation and disruption of poorly crystalline Fe and Al phases is a direct control on both C accrual and loss in deep soil. This finding suggests that root impacts on soil C storage are dependent on soil weathering stage, a consideration that is critical for future predictions of the vulnerability of deep soil C to global change.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States); Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- Grant/Contract Number:
- AC02-76SF00515; AC05-76RL01830
- OSTI ID:
- 1574857
- Alternate ID(s):
- OSTI ID: 1558176; OSTI ID: 1573352
- Report Number(s):
- PNNL-SA-137958; TRN: US2001188
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 263, Issue C; ISSN 0016-7037
- Publisher:
- Elsevier; The Geochemical Society; The Meteoritical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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