Sammi and I visited Rothamsted Research, North Wyke in Devon to help us perfect a technique for extracting inorganic phosphate from soil samples, so we can analyse these for their oxygen isotope composition.
But firstly, why are we interested in isotopes of phosphorus?
Phosphorus is a key nutrient for all life, critical for the development of cells and functioning of DNA and RNA. It is therefore one of a few key elements which are fundamental for the development of all living things along with nitrogen and carbon. When these elements are lacking in the environment, they are often described as limiting nutrients. For this reason, modern farming practices have developed specific fertilisers which help increase the levels of phosphorus and nitrogen in the soil system. This has helped us drastically improve crop yields. However, where these nutrients are lost into streams and rivers, they can promote the growth of algae and damage, often delicate, natural ecosystems. It is therefore important to understand and trace how these nutrients behave when added into the soil system. This is where isotopes can play their part….
For many years, nitrogen cycling in soil systems has been characterised by the analysis of nitrogen (15N) and oxygen (18O) isotopes in nitrate (NO31-). However, phosphorus only has one stable isotope (31P), and until recently, it has only been possible to extract the 18O signature of phosphate (PO43-) in clean waters (e.g. seawater). However, ground-breaking work undertaken in 2010 at ETH Zurich has made it possible for us to extract inorganic phosphate from soils (which also have many organic phosphorus compounds), so we can now start to trace the phosphate cycle far more closely. It is this method for extracting inorganic phosphorus that we have been working on for the last week.
Extracting inorganic phosphate
Whilst quite complex and time consuming (so I don’t go into details here), the extraction technique is based around a few relatively simple principles.
At the first stage, soil samples are treated with acid to release the inorganic phosphate into solution. After this, there are several stages where phosphate compounds are precipitated out of solution and washed to remove any unwanted contaminants containing oxygen, which would interfere with the analysis. The final stage is to add a silver solution which precipitates with the phosphate to form silver phosphate crystals- it is these crystals we analyse for their oxygen isotope composition
|From L-R: First the soil must be filtered to break up any large particles, a messy job; Filtering out the bright yellow APM|
crystals, this is where all the phosphate has been trapped.
Now that we have a working method to use at the BGS’s Stable Isotope Facility, we hope to be able to work on a whole range of new projects for which this isotope technique is critical. We believe this technique will be invaluable to further our understanding of the interactions of fertilisers and soils systems (this is the focus of Sammi’s PhD) but that we could also apply this technique to studies of phosphate pollution, phosphate source tracing and potentially palaeoclimate reconstructions. Watch this space….
Sammi and I would both like to thank Dr Verena Pfahler and Dr Steve Granger for hosting us at Rothamsted Research and to The University of Nottingham and Scotland’s Rural College for sponsoring our visit. Sammi would also like to thank UoN and SRUC for co-funding her PhD project. We hope to have some great data for you soon!