Phosphorus cycling in marine sediments: Advances and challenges

Phosphorus (P) is a critical macronutrient for all living cells, as it provides the phosphate-ester backbone of nucleic acids, plays a crucial role in the transmission of chemical energy by the ATP molecule and is also a structural constituent in many cell components. While P cycling in the marine water column has been the subject of many studies and reviews, many aspects of the sedimentary P cycling remain poorly understood. This is mainly due to low P concentrations in sediments and analytical difficulties associated with isolating distinct P forms from sediment samples. The aim of this review is to highlight the advances made in our understanding of P cycling in marine sediments as a result of improved instrumentation and novel approaches. Some of these techniques range from sediment sequential extractions (SEDEX) to spectroscopic techniques (P-31 NMR) and innovative isotope tracer experiments (stable oxygen ratios in phosphate, radioactive P isotopes). While our understanding of P cycling has considerably improved, numerous aspects of sedimentary P cycling including the fate of organic P in sediments as a function of redox conditions, organic matter content and burial depth, as well as turnover rates of different sedimentary P compounds remain poorly constrained. The techniques presented in this review, along with yet to be utilized innovative approaches used in soil and lacustrine sediment P research, will play an important role in addressing these important questions and in furthering our understanding of this critical component of the P global biogeochemical cycle.