Responses of C/N Coupling to Nutrient Additions in Subtropical Mountainous Rivers in Taiwan

Abstract

<p>C-N (carbon-nitrogen) coupling in streams which reflects ecosystems’ demand for energy and nutrients can help understand the ecosystem status and health. However, the DON (dissolved organic nitrogen) response to different nutrient addition is not clear, particularly in the C-limited streams. In this study, incubations with nutrient additions (DIN, dissolved inorganic nitrogen and/or DOC, dissolved organic carbon) were conducted using water samples from agricultural and pristine watersheds in northern Taiwan to examine the responses of C-N coupling to different disturbance levels. Results showed that during the incubation, DON concentration only increased when DIN was added, whereas no significant changes was found in DON with DOC addition. Specifically, DON could increase up to 360% - 460% in the pristine watersheds, while it only increased by 20% - 80% in the agricultural watersheds. Compared to DOC, DON significantly responded to DIN addition, indicating apparent nutrient demand in pristine watersheds. Besides, the DOC:DON ratio in the pristine watersheds dropped from around 10 to 2, while the ratio in the agricultural watersheds only dropped from 0.8 to 0.5. Such decrease might indicate that DIN addition can greatly increase DOM (dissolved organic matters) lability in pristine watersheds. Interestingly, all DOC concentrations dropped to around 0.4 to 0.5 mg/L in all different addition combinations, including DOC addition. Although DIN and/or DOC addition can stimulate relevant biogeochemical processes, a presumable limitation of DOC (0.4 to 0.5 mg/L) might be a tipping point of these processes, which required further investigation.</p> <p><strong>Keywords:</strong> carbon-nitrogen coupling, small mountainous rivers (SMRs), Taiwan</p>