CLM4.5-CN: Near-zero soil N₂O (nitrification/denitrification) over SE China in summer despite comparable N deposition

[xzeng]

Hi all,
I’m running a regional RegCM-CLM4.5 (CN + nitrification/denitrification enabled) simulation over Asia. After a long spin-up, the 2020 warm-season soil N₂O emissions from both nitrification and denitrification become near zero over Southeast China (105–125E, 18–32N), while North China shows reasonable/large emissions. This seems inconsistent with the fact that N deposition input and early-season mineral N pools are not particularly low.


Below I summarize area-weighted regional means from monthly CLM h0 outputs (Feb–Jul 2020, pre-modification). Key findings:

1. N deposition entering soil (NDEP_TO_SMINN) in SE China is of the same order as North China through the period (SE: ~14–33 kgN/ha/yr; NC: ~22–32 kgN/ha/yr).
2. Mineral N pools collapse in SE China in Jun–Jul:
   • SMIN_NH4 drops from ~0.14 gN/m³ (Feb–May mean) to ~7.4e-4 gN/m³ (Jun–Jul mean).
   • SMIN_NO3 drops from ~6.3e-3 gN/m³ to ~1.6e-4 gN/m³, essentially a “NO3 vacuum.”
     Meanwhile, North China SMIN pools increase strongly in summer.
3. Process suppression appears in SE China only in summer:
   • F_NIT/POT_F_NIT declines from ~1 in Feb–May to 0.68 in Jun and 0.15 in Jul.
   • F_DENIT/POT_F_DENIT declines from ~0.9 to 0.47 in Jun and 0.06 in Jul.
     North China stays near 1 for both ratios.
4. Consistent with (2–3), total soil N₂O in SE China drops by ~100× in Jun–Jul, while North China peaks.
5. SE China also shows very large microbial immobilization (ACTUAL_IMMOB) relative to deposition in late spring–summer (IMMOB ~50–59 kgN/ha/yr vs NDEP_TO_SMINN ~14 kgN/ha/yr), suggesting strong competition for mineral N. However, even with relatively high deposition, the summer “mineral-N vacuum + scalar suppression” seems to shut down nitrification/denitrification.

Questions to the community:

• Has anyone seen a similar summer collapse of SMIN pools and N₂O fluxes in humid/forested subtropical regions in CLM4.5-CN/ND?
• Which diagnostics would you recommend to pinpoint the bottleneck?
• Are there known parameter settings that can lead to excessive immobilization or overly strong moisture/anaerobic suppression in warm wet regions?

Any suggestions or similar experiences would be greatly appreciated. I can share more detailed daily diagnostics if helpful.
Thanks!

Input - Substrate - Competing sink Comparison (SE vs North China)

month	SE NDEP→SMINN (kgN/ha/yr)​	NC NDEP→SMINN​	SE IMMOB (kgN/ha/yr)​	NC IMMOB​	SE Plant uptake (kgN/ha/yr)​	NC Plant uptake​	SE SMIN_NH4 (gN/m3)​	NC SMIN_NH4​	SE SMIN_NO3 (gN/m3)​	NC SMIN_NO3​
202002	32.9​	23.9​	26.5​	1.59​	10.5​	0.0157​	0.189​	0.246​	0.00991​	0.00175​
202003	21.0​	22.0​	0.0272​	6.14e-06​	3.79​	0.00426​	0.294​	0.267​	0.000764​	3.3e-05​
202004	11.6​	31.6​	28.2​	19.4​	4.38​	11.7​	0.0634​	0.525​	0.0107​	0.0105​
202005	16.4​	27.6​	46.4​	20.5​	6.76​	14.7​	0.0124​	0.660​	0.00388​	0.0379​
202006	14.3​	23.9​	50.4​	21.0​	7.23​	16.6​	0.00104​	0.713​	0.00031​	0.148​
202007	13.8​	22.6​	58.5​	22.0​	7.68​	15.5​	0.000438​	0.507​	9e-06​	0.369​

Process Intensity versus N₂O (SE vs North China)

month	SE N2O_nit (kgN/ha/yr)​	NC N2O_nit​	SE N2O_denit (kgN/ha/yr)​	NC N2O_denit​	SE total N2O​	NC total N2O​	SE F_NIT/POT​	NC F_NIT/POT​	SE F_DENIT/POT​	NC F_DENIT/POT​	SE TSOI_10CM (K)​	NC TSOI_10CM​
202002	0.00261​	0.000327​	0.144​	0.0103​	0.147​	0.0106​	1.00​	1.00​	0.925​	0.970​	273.8​	262.5​
202003	5.71e-05​	1.26e-06​	7.11e-05​	9.78e-08​	0.000129​	0.000001​	1.00​	1.00​	0.679​	1.00​	264.5​	254.8​
202004	0.000369​	0.00144​	0.0822​	0.0476​	0.0826​	0.0490​	0.998​	1.00​	0.966​	0.958​	273.4​	268.6​
202005	0.000130​	0.00533​	0.0289​	0.1169​	0.0290​	0.122​	0.972​	1.00​	0.935​	0.977​	281.2​	273.2​
202006	1.30e-05​	0.0153​	0.00117​	0.194​	0.00118​	0.209​	0.681​	1.00​	0.468​	0.983​	287.4​	281.9​
202007	7.35e-07​	0.0198​	7.87e-05​	0.153​	8.08e-05​	0.172​	0.150​	1.00​	0.0583​	0.973​	290.2​	290.2​