Transition of Carbon-Nitrogen Coupling Along an Anthropogenic Disturbance Gradient in Subtropical Small Mountainous Rivers

Abstract

The commonly observed inverse relationship between dissolved organic carbon (DOC) and nitrate (NO3-) concentrations in aquatic systems can be explained by stoichiometric and thermodynamic principles regulating microbial assimilation and dissimilation processes. However, the interactive effects of human activities and dissolved oxygen (DO) conditions on the DOC and DIN (dissolved inorganic carbon, mainly composed of NO3--N, and NH4+-N) relations are not well identified, particularly in subtropical small mountainous rivers (SMRs). Here, we investigated the export and relations among DOC, NO3--N and NH4+-N in 42 Taiwan SMRs along an anthropogenic disturbance gradient. Results showed the island-wide mean concentrations of the three solutes in streams are generally low, yet the abundant rainfall and persistent supply still lead to disproportional high DOC and DIN yields. Due to the low DOC availability (energetic constraints) in Taiwan SMRs, the inverse DOC－NO3--N relation usually does not exist under well-oxygenated conditions, regardless of low or high human disturbance. However, a significant inverse relationship between DOC－NO3--N would emerge in highly-disturbed watersheds under low-oxygenated conditions (mean annual dissolved oxygen < 6.5 mg L-1), while excess N accumulates as NH4+-N rather than NO3--N. In such conditions, the controlling mechanism of DOC – DIN relations would shift from energetic constraints to redox constraints. Although riverine concentrations of DOC, NO3--N, and NH4+-N could be elevated by human activities, the transition of DOC－DIN relation patterns is directly linked to DO availability. Understanding the mechanism that drives C-N relations is critical for assessing the ecosystem function in the delivery and retention of DOC and DIN in aquatic ecosystems.

Keywords: dissolved organic carbon (DOC), Dissolved inorganic nitrogen (DIN), Small mountainous rivers (SMRs), Ecological stoichiometry, Thermodynamic constraint, Taiwan