Bioavailable DOC: reactive nutrient ratios control heterotrophic nutrient assimilation—An experimental proof of the macronutrient-access hypothesis

• Published in: Biogeochemistry 2021-08, Vol.155 (1), p.1-20
• Main Authors: Graeber, Daniel, Tenzin, Youngdoung, Stutter, Marc, Weigelhofer, Gabriele, Shatwell, Tom, von Tümpling, Wolf, Tittel, Jörg, Wachholz, Alexander, Borchardt, Dietrich
• Format: Article
• Language: English
• Subjects: Access; Accessibility; Ammonium; Ammonium compounds; Aquatic ecosystems; Assimilation; Bacterial leaching; Bioavailability; Biogeochemistry; Biogeosciences; Biological assimilation; Chemical compounds; Dissolved organic carbon; Dissolved organic matter; Dissolved organic phosphorus; Earth and Environmental Science; Earth Sciences; Ecosystems; Emission analysis; Emission measurements; Environmental Chemistry; Experiments; Fluorescence; Fluorophores; Fresh water; Freshwater; Freshwater ecosystems; Heterotrophic organisms; Heterotrophs; Hypotheses; In situ leaching; Inland water environment; Inoculum; Leachates; Life Sciences; Mathematical analysis; Mineral nutrients; Nitrogen; Nutrient balance; Nutrient uptake; ORIGINAL PAPERS; Phosphorus; Ratios; Rivers; Stoichiometry; Uptake
• ISSN: 0168-2563; 1573-515X
• DOI: 10.1007/s10533-021-00809-4

We investigate the “macronutrient-access hypothesis”, which states that the balance between stoichiometric macronutrient demand and accessible macronutrients controls nutrient assimilation by aquatic heterotrophs. Within this hypothesis, we consider bioavailable dissolved organic carbon (bDOC), reactive nitrogen (N) and reactive phosphorus (P) to be the macronutrients accessible to heterotrophic assimilation. Here, reactive N and P are the sums of dissolved inorganic N (nitrate-N, nitrite-N, ammonium-N), soluble-reactive P (SRP), and bioavailable dissolved organic N (bDON) and P (bDOP). Previous data from various freshwaters suggests this hypothesis, yet clear experimental support is missing. We assessed this hypothesis in a proof-of-concept experiment for waters from four small agricultural streams. We used seven different bDOC: reactive N and bDOC:reactive P ratios, induced by seven levels of alder leaf leachate addition. With these treatments and a stream-water specific bacterial inoculum, we conducted a 3-day experiment with three independent replicates per combination of stream water, treatment, and sampling occasion. Here, we extracted dissolved organic matter (DOM) fluorophores by measuring excitation-emission matrices with subsequent parallel factor decomposition (EEM-PARAFAC). We assessed the true bioavailability of DOC, DON, and the DOM fluorophores as the concentration difference between the beginning and end of each experiment. Subsequently, we calculated the bDOC and bDON concentrations based on the bioavailable EEM-PARAFAC fluorophores, and compared the calculated bDOC and bDON concentrations to their true bioavailability. Due to very low DOP concentrations, the DOP determination uncertainty was high, and we assumed DOP to be a negligible part of the reactive P. For bDOC and bDON, the true bioavailability measurements agreed with the same fractions calculated indirectly from bioavailable EEM-PARAFAC fluorophores (bDOC r² = 0.96, p < 0.001; bDON r² = 0.77, p < 0.001). Hence we could predict bDOC and bDON concentrations based on the EEM-PARAFAC fluorophores. The ratios of bDOC: reactive N (sum of bDON and DIN) and bDOC: reactive P (equal to SRP) exerted a strong, predictable stoichiometric control on reactive N and P uptake (R² = 0.80 and 0.83). To define zones of C:N:P (co-)limitation of heterotrophic assimilation, we used a novel ternary-plot approach combining our data with literature data on C:N:P ranges of bacterial biomass. Here, we found