Root-Driven Weathering Impacts on Deep Soil Carbon.

Plant roots reshape the soil environment by releasing approximately 25-40% of their photosynthetically fixed carbon (C) as root-derived compounds. While root-derived organic compounds are recognized as an important source of soil C, their role in promoting weathering reactions has been overlooked. Root-driven weathering may generate mineral-organic associations, which protect up to 91% of deep soil C for centuries to millennia. In contrast, root-driven weathering may also cause mineral transformations, potentially disrupting mineral-organic associations. Hence root-derived C may not only initiate C accumulation in deep soils, but also diminish C stocks through disruption of mineral-organic associations. Yet, the cumulative impact of root-driven weathering on mineral-organic associations is largely unknown. Therefore, our overarching goal was to examine root-promoted transformations of mineral-organic associations, and related changes in C storage, in deep soil. To accomplish this goal, we examined root impacts on soil C residence time and chemistry, mineralogy and mineral-organic associations across the Santa Cruz Marine Terrace chronosequence (65ka-226ka). As soils aged, and rhizogenic weathering increased, we observed a gradual change from predominantly root-derived to microbially-derived organic matter via mass spectrometry. Mössbauer and sequential extractions showed amorphous Fe (and Al) complexes formed during initial weathering, whereas crystalline (hydr)oxides dominated later weathering stages. X-ray spectro-microscopy revealed strong spatial associations between C and Fe during initial weathering stages, indicative of protective mineral-organic associations. In contrast, later weathering stages showed weaker spatial relationships between C and Fe. We conclude that initial root-induced weathering creates metal-organic complexes, protecting root-derived C from decay. As root-driven weathering proceeds, minerals transform into more crystalline phases that retain lower amounts of microbially-derived C. Our results suggest that root-induced weathering reactions are primary drivers of the formation and disruption of mineral-organic associations, and are thus critical for future predictions of the vulnerability of deep soil carbon to climate change impacts.