Oxygen isotopes of phosphate and soil phosphorus cycling across a 6500 year chronosequence under lowland temperate rainforest

Phosphorus (P) availability declines during ecosystem development due in part to chemical transformations of P in the soil. Here we report changes in soil P pools and the oxygen isotopic signature of inorganic phosphate (δ¹⁸Oₚ) in these pools over a 6500-year soil coastal dune chronosequence in a temperate humid environment. Total P declined from 384 to 129 mg P kg⁻¹ during the first few hundred years of pedogenesis, due mainly to the depletion of primary mineral P in the HCl-extractable pool. The δ¹⁸Oₚ of HCl-extractable inorganic P initially reflected the signature of the parent material, but shifted over time towards (but not reaching) isotopic equilibrium. In contrast, δ¹⁸Oₚ signatures of inorganic P extracted in water and NaHCO 3 (approximately 9 and 39 mg P kg⁻¹, respectively) were variable but consistent with isotopic equilibrium with soil water. In the NaOH-extractable P pool, which doubled from 63 to 128 mg P kg⁻¹ in the early stages of pedogenesis and then gradually declined, the δ¹⁸Oₚ of the extracted inorganic P changed from equilibrium values early in the chronosequence to more depleted signatures in older soils, indicating greater rates of hydrolysis of labile organic P compounds such as DNA and increase involvement in P cycling as overall P availability declines through the sequence. In summary, this application of δ¹⁸Oₚ to a long-term soil chronosequence provides novel insight into P dynamics, indicating the importance of efficient recycling through tight uptake and mineralization in maintaining a stable bioavailable P pool during long-term ecosystem development

Nitrogen cycling of active bacteria within oligotrophic sediment of the Mid-Atlantic ridge flank

Microbial ecology within oligotrophic marine sediment is poorly understood, yet is critical for understanding geochemical cycles. Here, 16S rRNA sequences from RNA and DNA inform the structure of active and total microbial communities in oligotrophic sediment on the western flank of the Mid-Atlantic Ridge. Sequences identified as Bacillariophyta chloroplast were detected within DNA, but undetectable within RNA, suggesting preservation in 5.6-million-year-old sediment. Statistical analysis revealed that RNA-based microbial populations correlated significantly with nitrogen concentrations, whereas DNA-based populations did not correspond to measured geochemical analytes. Bioenergetic calculations determined which metabolisms could yield energy in situ, and found that denitrification, nitrification, and nitrogen fixation were all favorable. A metagenome was produced from one sample, and included genes mediating nitrogen redox processes. Nitrogen respiration by active bacteria is an important metabolic strategy in North Pond sediments, and could be widespread in the oligotrophic sedimentary biosphere