Storm pulse responses of fluvial organic carbon to seasonal source supply and transport controls in a midwestern agricultural watershed

Storm events are the primary mechanisms of delivering fluvial organic carbon (OC) in both dissolved (DOC) and particulate (POC) forms but their sources and flow pathways can vary with seasonal land use and weather. Within the low relief and poorly drained landscapes of a predominantly agricultural watershed in Eastern Iowa, six storm events were monitored for DOC and POC concentrations over a two hydrological year period in order to investigate the export mechanisms, landscape connectivity, and hydro-climatological controls of fluvial OC under representative events and associated management practices. Event-driven dynamics favored POC over DOC, where POC accounted for 54-94 % of total OC export during events, highlighting a sampling-driven bias against POC in the absence of event monitoring. The disparity between POC and DOC export exhibited a seasonal effect, where the POC:DOC export ratio was low (1.3-1.7) for October events while June/July events yielded a much higher value (up to a value of 14.7). The relationships between event DOC and POC export, Normalized Difference Vegetation Index of landscapes, and antecedent wetness conditions suggest a strong interaction or competing influences between vegetation coverage and runoff-generation threshold. While we recognize the low statistical power of the limited data set (n = 6), the storm events could be binned into two clusters: a "bare soil" period and a crop "rapid growth" period. Specifically, intra-storm variations in OC concentration and concentration-discharge (C-Q) hysteresis patterns demonstrated a seasonally-dependent access to contributing OC sources, which can be viewed as the rapid liberation of DOC during the "bare soil" period, and a progressive leaching of terrestrial DOC during the "rapid growth" period. Although high resolution event monitoring of fluvial carbon is rare this work highlights the importance of such efforts to predict C sourcing and transformation in inland water systems under variable land use and across seasons.