Soil is the largest terrestrial carbon pool (Batjes, 2014). Vertical fluxes of CO 2 into the atmosphere have been extensively studied (e.g., Bond-Lamberty et al., 2020;Chapin et al., 2006;Jian et al., 2021). Lateral fluxes of dissolved organic and inorganic carbon (DOC and DIC) from terrestrial to aquatic systems have been increasingly recognized for emitting substantial amounts of CO 2 along river corridors (e.g., Barnes et al., 2018;Battin et al., 2009;Regnier et al., 2013). Vertical and lateral carbon fluxes, however, are often studied separately within disciplinary boundaries (Brookfield et al., 2021;Grimm et al., 2003). Their connections, partitioning, and relationship to carbon transformation across gradients of hydroclimatic and subsurface conditions have remained poorly understood. As a result, quantifications of carbon transformation rates and fluxes have remained highly uncertain (Duvert et al., 2018).Organic carbon (OC) transformation and chemical weathering (i.e., carbonate dissolution and precipitation) are key processes that produce dissolved and gaseous carbon and drive terrestrial carbon dynamics. Their rates depend on hydroclimatic conditions (Figure 1) that are bound to change under future climates with intensifying hydrological extremes including droughts and storms (Ault, 2020;Mastrotheodoros et al., 2020). Soil respiration rates often increase with temperature (Lloyd & Taylor, 1994) but peak at 50%-70% water saturation (Yan et al., 2018).

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Determination of soil carbonate is important for numerous chemical and physical soil processes in arid and semi-arid zones. Here, we modify a conventional method to more easily determine active CaCO 3 (ACC) fraction in soils. Unlike the conventional method where the oxalate used up after reaction with carbonate is determined by titration with KMnO 4 , the proposed method uses the difference between total carbonate analyzed before and after reaction with oxalate to determine ACC. The method compared well to the conventional approach; the procedure was faster, did not require KMnO 4 , and appeared to be unaffected by exchangeable Ca. The proposed method is well-suited for samples across a wide range of total carbonate.

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