15n enrichment during nitrification in soils

[lfhall]

First, let me preface that I'm a new PHREEQC user.
I'd like to set up an inverse model to calculate the initial 15N value for the soil 15N contributions (as NH4+) to the percolatolation I've collected in my soil percolation sampler.  the knowns in my sampler include 15N total (15Nnitrate + 15Ntotal, mass fraction total nitrate + mass fraction NH4+, along with major ion chem) I also have the major ion chem of the irrigation water added.

My solution species are: ammonium sulfate fertilizer added- (NH4)2SO4 →2NH4 + SO42-  , NH4 + 2O2→ NO3- + H2O +2H+, NH2 + H+ → NH4+

I do have soil pH, C:N, the 18O/D for the water. again the major ion chem, and isotopic values. Lots of pieces here, How would I break this down in PHREEQC. are there examples I can follow?

[dlparkhurst]

For starters, I would write the conceptual mole-balance equations for nitrogen 15. You may be able to get results without even using inverse modeling in PHREEQC.

As for major ion mole balance modeling, you need to have the right information. You need to know the composition of one solution that evolved from another known solution composition. Perhaps you can use a sample or rain/pure water as the initial member and a more evolved soil/ground water sample for the other. You also need potential reactants with well defined stoichiometry that can be used as unknowns to account for changes in major ion composition. 

[lfhall]

I'm continuing to work through my object of modeling nitrification in soil water.

How have others eventually solved the problem of representing nitrification and ammonia volatilization with PHREEQC?
Conceptually it seems there are two separate issues; the biological transform of the NH4 to nitrate, and the kinetic transforms; volatilization of a portion of the NH4 from the initial inorganic source, and the isotopic fractionation. 


the most applicable examples I've found related to the biological transforms are the related posts:

my initial solution can be represented by:
SOLUTION 1 Trail Creek - Irrigation
    temp      5.47
    pH        7.35
    pe        4
    redox     pe
    units     ppm
    density   1
    Alkalinity 149
    Ca        44
    Cl        0.73
    K         0.38
    Mg        11
    Na        0.74
    S(6)      8.8
    -water    1 # kg

and the final solution would be
SOLUTION 2 DSP sampler
    temp      7.8
    pH        6.7
    pe        4
    redox     pe
    units     ppm
    density   1
    Alkalinity 379
    Ca        130
    Cl        1.2
    K         0.65
    Mg        2
    Na        27
    S(6)      21.8
    -water    1 # kg

solution species would be:
NH4+ + 3H2O  = NO3- + 10H+ + 8e-
NH4+ = NH3 + H+

the hope is to plot the resulting mole concentrations similar to the Drew_2006 example.

Reference to examples would be appreciated

[dlparkhurst]

Depends on how much work you want to do.

The following is an example of kinetic (REACTION) conversion of NH4+ to NO3-. Formation of N2(aq), is repressed.

Code: [Select]

SOLUTION_SPECIES
# eliminate N2(aq)
2 NO3- + 12 H+ + 10 e- = N2 + 6 H2O
#-log_k 207.08
-log_k 0
RATES
Nitrification
10 o2 = TOT("O(0)")
20 amm = TOT("N(-3)")
30 k = 1/86400
40 rate = k*o2/(1e-5 + o2)*Amm/(1e-5 + amm)*amm
50 SAVE rate*TIME
SOLUTION
-units mmol/kgw
pH  7 charge
N(-3) 1
C   1
END
USE solution 1
REACTION
O2 1
2.5e-3 in 10
USER_GRAPH 1
    -headings               time O(0) Amm N(5)
    -axis_titles            "Moles of O2 reacted" "Moles per kilogram water" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X RXN
20 GRAPH_Y TOT("O(0)"), TOT("N(-3)"), TOT("N(5)")
  -end
    -active                 true
END
For isotopes, Phreeqc uses a non-typical approach (see https://pubs.usgs.gov/wri/wrir02-4172/#pdf and
https://pubs.er.usgs.gov/publication/70026610). The idea is to define another "element" [N15], analogous to N, but perhaps with slightly different log Ks for aqueous species because of the mass difference. Most important, the rates of reaction would be slightly different, resulting in fractionation. The database iso.dat provides some of this information.

For the full nine yards, it would be necessary to get more complex by merging the approach of Amm.dat, which splits ammonia (Amm) from N, so that disequilibrium of N redox states can be modeled. That would require generating Amm, [Amm15], N, and [N15]. Denitrification could require more definitions. So, depends on how complicated you want to get, or if you want to use the more typical approach using fractionation factors.