Dear Rena, If you used Ammonium sulfate fertilizer and saw increased crude protein you can be pretty sure from past researchers that it resulted from your N application.

Less than one half of applied fertilizer however is very used by the plant most being lost into the environment with potential untoward environmental issues on water quality.

There are two research ways used to approach how N is partitioned. One can tag the Nitrogen by using a N15 source of ammonium sulfate and determine its movement using radioisotopic assay. Second is the developing a nutrient budget analysis Calculate the total Nitrogen before during and after in the plant and in your water you grow your Ulva in.

Hope this gets you started.

Is your Ulva edible or what is the purpose of your work?

Best Regards and Good Luck. P

Dear Rena,

My suggestion is same as that of  Paul.

here is a paper for your reference.

Michael W. Taylor, Neill G. Barr, Coral M. Grant and T. Alwyn V. Rees,Changes in amino acid composition of Ulva intestinalis (Chlorophyceae) following addition of ammonium or nitrate,Phycologia (2006) Volume 45 (3), 270–276.

Good luck!

 Ms.Rena,Dr.Hepperly has given pertinent reply.What is the medium used to grow the Ulva?If it is water/seawater, the ammonium N content in water can be estimated  at the initiation and end of the feeding study.The difference will give unutilized Ammonium N.If some part of added  ammonium N is converted to nitrate,then NO3-N also needs to be estimated .It is also possible to extract unutilized Ammonium/nitrate  in Ulva  body using some extractant and estimate the mineral N.The total ammonium N added minus the  balance/difference in water and mineral N in Ulva give the proportion of added N converted into protein. The same can be done with 15N more appropriately.

Thank you Dr. Paul, Dr. Jackson and Dr. Annangi for your answers and suggestions. I appreciate it very much and it would be a big help in my on-going study. 

To Dr. Paul,

Yes, the species of Ulva I am studying is edible and I am conducting an experiment if it could be a potential source of protein in abalone diet. 

To Dr. Annangi,

The medium I am using for Ulva is seawater. You're suggestion is very much appreciated, noted and I will consider it. 

To Dr.Jackson,

Thank you very much for the reference journal. 

Regards,

Rena

If you have access to emissionspectroscopic N15 analysis somewhere, this would be the method of choice to track the fate and origin of N compounds in your Ulva system.

Conduct a preplanting analysis before N-fertilizer application, at a particular growth period collect samples of study specie and analyse for N content/compositon.and a post planting N- analysis. Results should tell you more, you can threreafter make inferences.