ORCHIDEE-CN-P (former branch ORCHIDEE-CNP)

This page describes the phosphorus cycle in ORCHIDEE-CN-P. It is based on ORCHIDEE-CN, which was extended and corrected as described here: ​https://forge.ipsl.jussieu.fr/orchidee/wiki/Branches/MergeOCN/Goll as well as additional non-documented bugfixes to avoid negative pools due to machine precisions as well as bugs.

The phosphorus cycle is an adaptation of the the model described by ​http://www.biogeosciences.net/9/3547/2012/bg-9-3547-2012.html . Nonetheless, the complexity was substantially increased due to the more detailed representation of the C and N cycle in ORCHIDEE compared to JSBACH.

Schematic representation of the key processes represented in ORCHIDEE-CNP

2. Technical modification to the nitrogen code

I modified the code a bit to avoid problems and redundant code. More information here: ​https://forge.ipsl.jussieu.fr/orchidee/wiki/DevelopmentActivities/ORCHIDEE-CNP/TechnicalMods

3. Conceptual modifications to the nitrogen cycle

3.1 soil mineral N concentration in soil solution

Following Smith et al (2014), I introduced the use of the maximum water holding capacity of soils (max_var_eau) to approximate pore space which to derive the average soil mineral N concentration in solution. The use of the actual water volume can not be recommended as we this would lead to high N concentration in soil water when soil water is very low. As we do not account for the inhibition of replenishment of mineral N in the soil solution around roots when soil water is scarce.

3.2 soil mineral N dynamics

Several modification of the original OCN (Zaehle & Friend, 2010) in ORCHIDEE-CN I kicked out and instead use the original formulation are used again. See code for details. Still, the mineral N stocks are unrealistic high. One approach to solve this is to use the modification to OCN described in the SI of Zaehle et al. (2011) (​http://://www.nature.com/ngeo/journal/v4/n9/extref/ngeo1207-s1.pdf).

The general problem with ORCHIDE-CN is that several details of soil mineral N were changed, which are all not tested. The easiest way would be to use the OCN formulation and parameters as OCN is well tested.

2016/08/08: I implemented the possibility to use the original OCN formulations for (1) nitrification, (2) denitrification, (3) rain drainage losses, and (4) anaerobic microsites. In general, OCN uses simplified version of the ORCHIDEE-CN formulations to make sure that model runs stable on a global scale. When I suspect that OCN is buggy (frac_nh3), I keep the ORCHIDEE-CN formulation (I asked SZ to confirm the bugs). The code compiles but is not tested in debug mode and for mass conservation.

2016/08/24: When rain rain drainage is activated N losses are so high, that mineral N pools are close to zero. The parametrisation of rain drainage is anyway not needed for the 11 layer hydrology so I will not take over the rain drainage losses from OCN.

2016/08/24: The OCN anaerobic microsites reduce soil mineral N levels for 4 test sites to levels which are reasonable. The approach of OCN is a simplification of the one in ORCHIDEE-CN (DNDC) avoiding instability related to the diffusion term. In respect of global application of the model, without stability in calculations a superior process representation is useless. We thus take over anaerobic micro sites following OCN.

2016/08/24: The OCN denitrification is a simplification of the one in ORCHIDEE-CN (DNDC) avoiding instability related to bacterial growth. We need to prescribe maximal and minimal bacterial population sizes in the ORCHIDEE-CN formulation to avoid the extinction of bacteria and thus to zero denitrification. The more elegant simplification in OCN is in general preferable than prescribing ranges. Soil mineral NO3 levels show a drastically reduced daily variability with the OCN formulation while NO3 stocks are comparable to the stocks in ORCHIDEE-CN. We thus take over the OCN formulation.

2016/08/24: The OCN nitrification compared to ORCHIDEE-CN (DNDC) uses a simplified temperature response function and a higher linear scaling factor (1.2 (not sure from where SZ took this value) compared to 0.2 (Schmidt et al. 2000)). As the NO3 levels are commonly much lower than NH4 in ORCHIDEE a higher baseline nitrification rate as in OCN makes sense. We thus take of the OCN formulation.

Final mineral N dynamics: The mineral N dynamics in ORCHIDEE-CN-P are now exactly as described in Zaehle et al. (2011), except that I did not take over the rain drainage formulation as we use an improved hydrological scheme compared to Zaehle et al. (2011) which is able to simulate the rain drainage as a result of more soil layers (11 layers).

The mineral N stocks of the test sites are all in realistic ranges. Global pattern correlation test with N15 derived fraction of denitrification to total losses needs still to be done.

3.3 Biological N2 fixation (BNF)

3.3.1 Modified Cleveland et al. (1999) scheme

We introduce a module which computes BNF as a function of NPP, tissue C:N, and tissue & N:P. This approach is based on Cleveland et al (1999), Thornton et al. (2007), and Goll et al. (2012). When using the analytical spinup the BNF must be read in from a file to speed up the spin up. The file should contain the reference BNF rates for the respective climatic conditions. Such files are currently missing, but should be generated as soon as the model is ready.

3.3.2 Biological N2 fixation (BNF) from Yingping

The Cleveland scheme is heavily criticized due to good reason. Yingping Wang will provide maps with annual rates of BNF computed based more or less on Wang et al. (2007) for the historical period. To do so, Yingping needs TRENDY style simulations with a calibrated model.After publication of his model, he will share the code with us & we can compute BNF dynamically in ORCHIDEE.

4. New input files

You can find information on the new input files needed for the P cycle here: ​https://forge.ipsl.jussieu.fr/orchidee/wiki/DevelopmentActivities/ORCHIDEE-CNP/newInput

5. unresolved Issues

There are still unresolved issue with the model. They are listed here: ​https://forge.ipsl.jussieu.fr/orchidee/wiki/DevelopmentActivities/ORCHIDEE-CNP/issues

6. Howto install, compile & run the model

You can find information on how to setup the simulations here: ​https://forge.ipsl.jussieu.fr/orchidee/wiki/DevelopmentActivities/ORCHIDEE-CNP/howtoUse

7. Some results

You can find some basic plots here: ​https://forge.ipsl.jussieu.fr/orchidee/wiki/DevelopmentActivities/ORCHIDEE-CNP/results