Mineral and Metabolic Constraints on Carbon Cycling Within and Export From Mountainous Floodplain Soils

Floodplains contain soils rich in organic matter and are important ecosystems for agricultural production worldwide. Importantly, seasonal flooding results in drastic redox fluctuations, which have profound implications for transformations of soil carbon (C). However, we do not know the underlying biogeochemical controls on and spatiotemporal variation of soil C cycling in redox-dynamic floodplains. Using an active meander of the subalpine East River watershed (Gothic, CO), this dissertation first reviews the current understanding of floodplain C import, storage, and export, and further discusses how hydrology and corresponding redox patterns impact mineral and metabolic controls of floodplain C transformations. Using a combination of sequential extractions, physical fractionation, and high-resolution mass spectrometry, Chapter 2 finds that mineral-associated C constitutes a meaningful fraction of total C, suggesting that mineral-organic associations are quantitatively important for floodplain soil C protection. Chapter 3 investigates mechanisms of anaerobic protection that drive floodplain C cycling. Studying the same meander, metabolite data show evidence for kinetic constraints during flooding, leading to an accumulation of relatively higher molecular weight compounds. Upon drainage, thermodynamic constraints are relieved in the surface soils, but deep soils remain reducing year-round. Chapter 4 aims to resolve the balance between mineral and metabolic constraints on floodplain C loss, across extreme low (2018) and high (2019) river discharge years. Using similar techniques as above, in combination with environmental monitoring, we found that reducing conditions (1) decreased the stability of iron-carbon associations, causing concomitant mobilization of iron and DOC, and (2) resulted in the selective preservation of reduced DOC compounds; both findings were stronger during extreme flooding. Connecting these mechanisms to riverine exports of floodplain C showed shifts in the balance of inorganic versus organic C export during extreme drought or flooding. Climate change is shifting the extent of flooding in mountainous floodplains, with prolonged megadrought expected to drive more low water years. Thus, understanding the mechanisms that control floodplain C cycling will be crucial to maintaining healthy floodplains that can sustain food productivity and clean water