I have been monitoring the δ15N of two species of floating plants over 5 concentrations of NO3 and NH3.
The measurements have been done over 16 days, with solution changes every 7 days to avoid nutrient limitation.
The idea was to look at the time integration period of these plants and the rate of change in δ15N ratios level out around day 12 – suggesting plants are then at approx. isotopic equilibration with the solutions.
All plants were grown in a green house, with pH between 6-7.5, a 12:12 light dark regime, temp 20-22 deg C, with adequate phosphorus and iron.
The problem:
In both species, for both NO3 and NH3, if I calculate fractionation between day 12 plant δ15N and the source δ15N at each concentration I see a pattern of increasing fractionation with lower N availability.
So effectively there seems to be an inverse relationship between fractionation and concentration.
This was the exact opposite of what I expected, as I thought that when there was limited N, plants took it up without much differentiation, but at excess N, plants could be choosier about taking 14N over 15N = more fractionation).
I feel like I am missing something obvious and wondered if anyone had some “off the top of their head” ideas as to why I might be seeing this?
It might help if we knew whether the NO3 and NH3 treatments were separate or combined, and how you prevented oxidation of ammonia. You say the P and Fe were adequate. How do you know? What about other nutrients?
Hi Paul,
The treatments were separate, Hoagland’s solution (but made up without N, so as not to mess with our concentrations) was added to ensure sufficient micronutrients, P was measured each week to confirm that it remained above 6.0 mg/L (Luond 1980, 1983, Rejmánková 1979, 1982, Goopy & Murray 2003) and we had water samples analysed for Fe each week as well – and confirmed they were
Hi Jacklin,
possibly other details could be of interest. Which part of the total N in water was taken up by plants by day 12? Did this differ strongly betweem low-N and high-N treatments? Could you see any change in d15N of dissolved N during 7 days?
Can be collected, analyzed for d15N, and used to estimate the extent of denitrification, initial composition of the nitrateThe d15N of total soil N is affected by many factors including soil depth, vegetation, climate, particle size, etc.; however, two factors, drainage and influence of litter, have a consistent and major influence on the 15N values (Shearer and Kohl, 1988). Gormly and Spalding (1979) attributed the inverse correlation of nitrate- d15N and nitrate concentration beneath agricultural fields to increasing denitrification with depth. Denitrification causes the d 15N of the residual (unreacted) nitrate to increase exponentially as nitrate concentrations decrease.
Could you see any change in d15N of dissolved N during 7 days?
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