6.1 Future Trends and Opportunities

With increasing fertilizer prices and the negative environmental issues such as eutrophication of aquatic systems and phosphorus contaminations in surface and groundwater that affect water quality, it is now, more than ever, vital to better understand the environmental P cycle. Studies during the last 10 years showed that the δ18OP method is a promising tool to study the environmental P cycle under field conditions and the importance of microbes for available P in soils (Tamburini et al. 2012). The oxygen isotopes in phosphates have helped in identifying P inputs in Lake Erie and Chesapeake Bay, some of the aquatic systems most struck by eutrophication (Elsbury et al. 2009; Joshi et al. 2015; Paytan et al. 2017; Depew et al. 2018). It has the potential to track P derived from glyphosate in the environment (Li et al. 2016), to name just a few highlights. The δ18OP could also provide parameters for environmental models (Jaisi et al. 2017). Modeling P cycling and including P in carbon and nitrogen models is becoming more important and the interest to also include P in life cycle assessments is increasing, especially the mineralization of organic P and its quantification are of interest for modelers (Vereecken et al. 2016; Helfenstein et al. 2018; Thum et al. 2019).

6.2 Need for Inter-laboratory Studies for Quality Control

With an increasing number of researchers using the δ18OP method to investigate P cycling in the environment, it is necessary to conduct an inter-laboratory comparison study for the purification protocol as well as the measurement of silver phosphate with the TC/EA-IRMS like Watzinger et al. (2021) did. While nowadays silver phosphate is the analyte of choice, there are different protocols for purifying extracts and precipitating silver phosphate (see, e.g., Paytan and McLaughlin 2011). Even when using the same protocol, every laboratory is most likely amending the protocol for their needs, using different equipment, consumables, and chemicals. For better comparability of different δ18OP studies, there is a need for an inter-laboratory study similar to other protocols, like for the extraction of microbial P (Bergkemper et al. 2016). Equally important is an international silver phosphate standard which should be easily available to most laboratories (Watzinger et al. 2021).

6.3 From Laboratory to Field Studies in Different Ecotones

For the δ18OP method to progress, further fundamental research, as well as field and laboratory studies, need to be conducted. To the best of our knowledge, the effect of synthesizing enzymes on the δ18OP has not been investigated yet, despite the importance of those enzymes in the P cycle. Furthermore, the effects of some processes like P uptake on the δ18OP were only shown in laboratory studies but are missing proof from field studies. The majority of δ18OP studies so far was conducted in countries in the northern hemisphere. An increase of studies in the southern hemisphere is therefore desirable. This will advance the knowledge about the variables influencing δ18OP values as countries in the global South tend to have different ecosystems, climates, and soil types compared to the North. Two aspects that need to be addressed in further field studies are the importance of soil P concentrations, i.e., P limitation and surplus, and microorganisms (community structure and activity) for the δ18OP of different soil P pools. Is there for example a relationship between P concentrations and δ18OP values and could thus the δ18OP be used as an indicator for P limitation? This will not only help with the interpretation of δ18OP data but will ultimately lead to a better understanding of P cycling in the environment.