The biotic contribution to the benthic stream sediment phosphorus buffer

Abstract

Benthic stream sediments interact strongly with phosphorus (P) and can buffer dissolved reactive P (DRP) concentrations. The sediment P buffer can be measured with the sediment equilibrium phosphate concentration at net zero sorption (EPC₀), which often correlates well with DRP. Yet, it is unclear how much of this P affinity in sediments is attributable to biotic (microbial P demand) or abiotic (sorption) processes. To clarify the role of biotic processes on EPC₀, we used two experiments with benthic sediment from 12 streams. First, sediments sterilized by γ-irradiation increased in EPC₀ compared to fresh sediments by a median of 83%. This increase in EPC₀ was likely a result of cell lysis, where microbial biomass P (2.4 to 22.6 mg P kg¯¹) was re-adsorbed to sediment surfaces. This data also shows that the sediment microbial biomass is a significant, yet under-reported biotic stock of P in streams compared to their photic zone counterpart (i.e., periphyton). In a second experiment, fresh sediment EPC₀ was measured after alleviating potential limitation of carbon (C) and nitrogen (N) for microbial growth. Sediment EPC₀ did not change with C addition and decreased slightly (0.5 µg P L¯¹ or ~ 5% decrease) with N addition, suggesting these sediments strongly buffered DRP towards the EPC₀ in spite of biotic demand. Together, these experiments suggest that sediment EPC₀ was primarily abiotic in nature but that sediments may subsidize biotic P requirements through desorption. Further work is needed on whether this relation holds for streams with different substrate, geology, and nutrient inputs.