Update event handling to allocate to litter N pool for harvest

[Alomir]

We don't track biomass N explicitly, but we DO track litter N. In handling harvest events, we are losing N from aboveground transfer to the litter pool.

[Thomas-Sedhom]

Hi @Alomir
Can I work on this issue ?

[Alomir]

Sure, go for it - thanks! Let me know if you have any questions.

[Alomir]

Most of the action here will be in src/sipnet/events.c:processEvents()

[Thomas-Sedhom]

Hi @Alomir
After carefully exploring the SIPNET model and the harvest event issue, I have a few questions to make sure I understand the correct approach.
So When a harvest event happens, some fractions are removed and others are transferred (aboveground and belowground). This affects carbon and nitrogen in plant leaves, wood, fine roots, and coarse roots also Because of the transfer part, litter carbon and nitrogen in the soil should increase.
For carbon, this is clear to me. We already track plant carbon explicitly (leaf, wood, fine root, coarse root) in the Environment struct, and we use these values to update litter carbon through the fluxes.

typedef struct Environment {
  // (g C * m^-2 ground area)
  double plantWoodC;
  // carbon in leaves (g C * m^-2 ground area)
  double plantLeafC;
  // carbon in coarse roots (g C m^-2 ground area)
  double coarseRootC;
  // carbon in fine roots (g C m^-2 ground area)
  double fineRootC;
}

However, for nitrogen, we do not track N for each plant component. So my question is about how we should correctly update litter nitrogen during harvest events.
So should we derive litter nitrogen from litter carbon using a fixed C:N ratio? For example:

const double litterAdd = fracTA * (envi.plantLeafC + envi.plantWoodC) +
                                 fracTB * (envi.fineRootC + envi.coarseRootC);
const double litterNAdd = litterAdd / CN_percentage_constant; 
fluxes.eventOrgN += litterNAdd / climLen

Or should we explicitly track nitrogen for each plant component (leaf, wood, roots), similar to carbon, which would require extending the Environment struct and updating other parts of the model?

If my understanding is not correct, I would really appreciate some clarification on the intended design.
Thank you for your time

[avinxshKD]

@Alomir can i try this out?? thanks

[Alomir]

@Thomas-Sedhom Sorry for the delayed response!

You are correct that we are using fixed C:N ratios, and that is what is used to update the litter pool - nice investigation work. The only tweak is that we have separate fixed C:Ns for the different biomass pools:

• envi.plantWoodC: params.woodCN
• envi.plantLeafC: params.leafCN
• envi.fineRootC: params.fineRootCN
• envi.coarseRootC: params.woodCN

Yes, we use the same CN for both wood and coarse roots.

[Alomir]

@avinxshKD Sorry to say no, but @Thomas-Sedhom already started on this.