CNP patterns at catchment scales 


Background

The inputs of macro-nutrients (carbon (C), nitrogen (N) and phosphorous (P)) to freshwater ecosystems have been increasing throughout the last decades1. These ecosystems often connect terrestrial and marine environments and their biogeochemistry can play an important role in mediating macro-nutrient exports. As this mediation is influenced by macro-nutrient stoichiometry2, we see the need to understand what controls C:N:P patterns across freshwater ecosystems. Because those patterns are commonly studied on small scale communal levels, less is known about their behavior on catchment scales.

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1Vitousek et al., „Human Alteration of the Global Nitrogen Cycle“.
2Graeber et al., „Bioavailable DOC“.
Conceptual classification of CNP ratios with a ternary plot (from Graeber et al., 2021)
Conceptual classification of CNP ratios with a ternary plot (from Graeber et al., 2021)


Objective

The main objective of this group is to identify the drivers of C:N:P ratios in freshwater ecosystems.


Research questions / hypotheses

We hypothesize that observed macro-nutrient stoichiometric ratios are the result of different biogeochmical and hydrological processes, such as nutrient mobilization and assimilation by algae, plants and bacteria. We speculate that some catchments have more stable (homeostatic) and some more variable (heteroscatic) stoichiometric ratios. As a first step, we want to classify catchments regarding those patterns and try to link them with catchment specific properties, such as land-use, soil type, vegetation or nutrient input.


Data & Methods

We use concurrently measured low-frequency records of dissolved organic carbon (DOC), Nitrate-N (NO3-N) and soluble reactive phosphorus (SRP) from around 200 German catchments along multiple phyiscal and socio-economical gradients. We use ternary plots (see figure) to visualize and classify the DOC:NO3-N:SRP ratios of those catchments and search for common patterns along/ across the above mentioned gradients. Ternary plots offer the visualization of all three macro-nutrients at a time, and also allow us to compare the ratios of catchments to those ideal for different biological groups, such as bacteria (data from Vrede et al.3, shown in the figure).

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3 Vrede et al., „Elemental Composition (C, N, P) and Cell Volume of Exponentially Growing and Nutrient-Limited Bacterioplankton“.



Graeber, Daniel, Youngdoung Tenzin, Marc Stutter, Gabriele Weigelhofer, Tom Shatwell, Wolf von Tümpling, Jörg Tittel, Alexander Wachholz, und Dietrich Borchardt. „Bioavailable DOC: Reactive Nutrient Ratios Control Heterotrophic Nutrient Assimilation—An Experimental Proof of the Macronutrient-Access Hypothesis“. Biogeochemistry, 8. Juni 2021. https://doi.org/10.1007/s10533-021-00809-4
Vitousek, Peter M., John D. Aber, Robert W. Howarth, Gene E. Likens, Pamela A. Matson, David W. Schindler, William H. Schlesinger, und David G. Tilman. „Human Alteration of the Global Nitrogen Cycle: Sources  and Consequences“. Ecological Applications 7, Nr. 3 (1997): 737–50. https://doi.org/10.1890 /1051-0761(1997)007[0737:HAOTGN]2.0.CO;2.
Vrede, Katarina, Mikal Heldal, Svein Norland, und Gunnar Bratbak. „Elemental Composition (C, N, P) and Cell Volume of Exponentially Growing and Nutrient-Limited Bacterioplankton“. Applied and Environmental Microbiology 68, Nr. 6 (1. Juni 2002): 2965–71. https://doi.org/10.1128/AEM.68.6.2965-2971.2002.