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 (31P 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.