Abstract

Concurrent with nutrient pollution, agriculture has significantly impacted the quantity, composition, and bioavailability of catchment-derived dissolved organic carbon (DOC) in stream ecosystems. Based on the stoichiometric theory, we tested the hypothesis that bioavailable DOC will stimulate the heterotrophic uptake of soluble reactive P (SRP) and inorganic nitrogen in stream sediments. In a simplified laboratory column flow-through study, we exposed stream sediments to additions of glucose, nitrate, and phosphate alone and in combination (+C, +NP, +CNP), and calculated gross and net changes in DOC and nutrients via a mass balance approach. Our results show that glucose-C increased nutrient uptake, but also that NP additions resulted in the enhanced consumption of both native and added organic C. The effects of C addition were stronger on N than P uptake, presumably because labile C stimulated both assimilation and denitrification, while part of the P uptake was due to adsorption. Internal cycling affected net nutrient uptake due to losses of dissolved organically-complexed P and N (DOP and DON). Overall, our study shows that increases in the stoichiometric availability of organic carbon can stimulate N and P sequestration in nutrient-polluted stream sediments. Future studies are required to assess the effects of complex organic carbon sources on nutrient uptake in stream sediments under different environmental conditions, and whether these stoichiometric relations are relevant for ecosystem management.

Highlights

  • Pollution of surface waters by excess nitrogen and phosphorus has become globally recognised over the last few decades [1,2,3]

  • We used Bayes factor statistics to test whether we find evidence of a significant effect of the dissolved organic carbon (DOC), CNP, or NP treatment on the difference of solute concentrations (DOC, NO3, NH4, soluble reactive P (SRP), dissolved organic nitrogen (DON), dissolved organic P (DOP))

  • Our results show that glucose-C addition increased net DOP release by 2 μmol that partially offset the net 7 μmol glucose-stimulated SRP uptake

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Summary

Introduction

Pollution of surface waters by excess nitrogen and phosphorus has become globally recognised over the last few decades [1,2,3]. There is a widely understood change in aquatic DOC concentration and forms associated with DOC release from sources of humified soil C [10], effluents and urban areas [11,12] This change in DOC sources could be of significant importance for the N and P losses as, based on the stoichiometric theory, the availability of DOC to microbiota is the primary driver controlling assimilative and dissimilative. For microbial biomass C (MBC) there were similar values for the Ctreat and CNPtreat columns, but nearly an order of magnitude lower for NPtreat (Table 2). In contrast increased MBP concentrations for all but Ctreat relative to background sediment suggested P incorporation into microbial biomass in all columns exposed to P inflow solutions

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