From Soils to Streams: Connecting Terrestrial Carbon Transformation, Chemical Weathering, and Solute Export Across Hydrological Regimes

Soil biota generates carbon that exports vertically to the atmosphere (CO2) and transports laterally to streams and rivers (dissolved organic and inorganic carbon, DOC and DIC). These processes, together with chemical weathering, vary with flow paths across hydrological regimes; yet an integrated understanding of these interactive processes is still lacking. Here we ask: How and to what extent do subsurface carbon transformation, chemical weathering, and solute export differ across hydrological and subsurface structure regimes? We address this question using a hillslope reactive transport model calibrated using soil CO2 and water chemistry data from Fitch, a temperate forest at the ecotone boundary of the Eastern temperate forest and mid-continent grasslands in Kansas, USA. Model results show that droughts (discharge at 0.08 mm/day) promoted deeper flow paths, longer water transit time, carbonate precipitation, and mineralization of organic carbon (OC) into inorganic carbon (IC) (similar to 98% of OC). Of the IC produced, similar to 86% was emitted upward as CO2 gas and similar to 14% was exported laterally as DIC into the stream. Storms (8.0 mm/day) led to carbonate dissolution but reduced OC mineralization (similar to 88% of OC) and promoted DOC production (similar to 12% of OC) and lateral fluxes of IC (similar to 53% of produced IC). Differences in shallow-versus-deep permeability contrasts led to smaller difference (<10%) than discharge-induced differences and were most pronounced under wet conditions. High permeability contrasts (low vertical connectivity) enhanced lateral fluxes. Model results generally delineate hillslopes as active CO2 producers and vertical carbon transporters under dry conditions, and as active DOC producers and lateral carbon transporter under wet conditions.