Reactive macronutrient ratios as predictors for nitrate cycling in stream ecosystems

Human activities have significantly altered macronutrient concentrations in surface waters, impacting both ecological functions and water quality. Typically, research assesses this alteration and its effects from a single macronutrient perspective. Alternatively, we propose that macronutrient perspectives need to be integrated via a stoichiometric framework via carbon (C) : nitrogen (N) : phosphorus (P) ratios. These ratios may help to assess and improve natural attenuation at ecosystem and catchment level. From the C:N:P perspective, agricultural practices have resulted in a stoichiometric N surplus in temperate stream ecosystems, an issue of which German streams are a prime example.  In contrast, Florida's streams are characterized by a P surplus relative to N and C due to high geological background P supply.  Our study encompasses five streams in Germany and Florida, covering a wide range of C:N:P ratios, each characterized by distinct catchment characteristics. Here, we ask whether C:N:P ratios are the main driver of microbial nitrate-N uptake, irrespective of other differences between the two regions. Through streamside mesocosm and microcosm laboratory experiments employing an isotope tracer approach, we compared nitrate uptake. Additionally, we manipulated C:N:P ratios to assess the short-term effects on nitrate uptake and measured retention in the streamside mesocosm experiment. Enhancing our understanding of the interconnectedness of biogeochemical cycles enables the development of management recommendations for stoichiometric restoration in highly impacted stream ecosystems. This research contributes valuable insights towards sustainable practices and the preservation of aquatic ecosystems facing nutrient-related challenges and water security.