Changes between Version 157 and Version 158 of DevelopmentActivities/ORCHIDEE-DOFOCO


Ignore:
Timestamp:
2018-02-17T18:48:45+01:00 (6 years ago)
Author:
luyssaert
Comment:

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  • DevelopmentActivities/ORCHIDEE-DOFOCO

    v157 v158  
    193193 
    194194=== Litter decomposition === 
    195 EXPLAIN the share to fungivore parameter (externalize first)and why it was introduced 
    196  
    197 EXPLAIN 
     195After large-scale dieback events (with a closed n-cycle, i.e., impose_cn = n), so much soil mineral N becomes immobilized to decompose litter that too little N is left for plant regrowth. To address this, we implicitly represent the action of fungivores, which eat the decomposing fungi and release N for the plants and increase N turnover rates. We set aside a fraction of qd (stomate_litter.f90) which becomes available for plant uptake in nitrogen_dynamics. This fraction is prescribed by '''fungivores'''  and is allocated to '''n_fungivores''' which represents the nitrogen that is released by the fungivores and that is moved into the variable plant_uptake.  This parameter is not very well constrained and has been calibrated to find a good compromise between plant regrowth and litter decomposition. The share of the n contained in the decomposing fungi that is released as an excrement from the fungivores should be between 0 and 1 and is set in the run.def: 
     196{{{ 
     197FUNGIVORES=0.3 
     198}}} 
     199 
     200After a die-back (with a closed n-cycle, i.e., impose_cn = n) too little N is available to reach the target C:N ratio of the receiving soil pool, meaning that carbon litter can not decompose. If the C:N ratio is not controled it spins out of control reaching values of more than 6000. It target C:N ratio, which is a variable in stomate_litter.f90 can now vary between 1 and '''max_cn''' where the latter can be set in the run.def. Good results were obtained by setting max_cn to 250. max_cn is bound between 1 and 400 (the C:N ratio of wood which is the pool with the highest C:N ration (and biomass) that contributes to the litter). During the simulation the target C:N ratio varies and a couple of decades after the dieback reaches the expected value between 5 and 20. The ecological justification behind a max_cn above the C:N ratio of the fungi is that mycorrhizae receive their C from the plant photosynthesis products. Thus, litter can still decompose even though C is not incorporated into the plant biomass. max_cn can be set in the run.def:               
     201{{{ 
     202MAX_CN = 250 
     203}}} 
     204 
     205ORCHIDEE-CN allows negative n_mineralisation values to be passed to stomate_soilcarbon where they were treated as immobilization. If we have persistent negative n_mineralisation, however, we can reach a situation where we completely deplete our soil_n_min pools to satisfy the demand for immobilization. The model cannot recover from such a condition. This tends to happen when there is no plant recruitment, in which case a generation of trees will all reach their maximum diameter and die at the same time. This creates an enormous amount of litter, meaning our som_input values become relatively large and when we subtract som_input from n_mineralisation, the values become negative. To remedy this, we'll truncate som_input based on the current mineralisation pool, but we'll also limit the occurrence of negative values. We'll set a minimum '''min_n''' for n_mineralisation so we can keep some N in the soil (and so the simulation can recover). This problem was discovered and fixed before adding the fungivores and max_cn. It should be tested whether this fix is still needed. min_n can be set in the run.def:   
    198206{{{ 
    199207MIN_N = .0001 
    200208}}} 
    201  
    202 EXPLAIN 
    203 {{{ 
    204 MAX_CN = 250 
    205 }}} 
    206  
    207209 
    208210=== Litter raking ===