Column experiments show that cycling of both nitrogen and phosphorus is altered by dissolved organic carbon in river sediments

Since agriculture and wider development have altered simultaneously runoff, pollution and natural structures in catchments (e.g. wetlands, floodplains, soil drainage, riparian trees) aquatic ecosystems deviate from background concentrations of N and P, but also organic C (OC). Hence mechanistic studies coupling OC, N and P are needed and whilst data coupling OC:N is becoming more available and interpreted this is not yet the case for aquatic OC:P.  Column flow experiments (excluding light) allow preliminary controlled study of microbial biogeochemical processes in benthic sediments exposed to factorial nutrients (here +C, +NP, +CNP using simple dissolved substrates glucose, nitrate, and phosphate).

Based on the stoichiometric theory, we tested the hypothesis that bioavailable DOC will stimulate the heterotrophic uptake of soluble reactive P (SRP) and dissolved inorganic nitrogen in stream sediments. Glucose-C additions increased nutrient uptake, but also NP additions enhanced consumption of native and added OC. The effects of C addition were stronger on N than P uptake, presumably because labile C stimulated both assimilation and denitrification, while adsorption (unaffected by the presence or not of OC) formed a part of P uptake. Internal biogeochemical cycling lessened net nutrient uptake due to N and P recycling into dissolved organically-complexed forms (DOP and DON).

Simple column experiments point to mechanisms whereby availability of organic carbon can stimulate N and P sequestration in the bed of nutrient-polluted streams. This should promote further studies coupling OC with N and, especially P, towards better knowledge and ability to incorporate coupled macronutrient cycles into nutrient models and, potentially, ecosystem management.