soil mineral N drop to almost 0 after maize harvest

[Hongzhen Luo]

Dear all,

I would like to run a simulation on monoculture of maize, the soil was set as following:

(defhorizon "Ap F.S.L." FAO3
    (description "0-30 cm")
    (dry_bulk_density 1.32 [g/cm^3])
    (clay 0.087)
    (silt 0.278)
    (sand 0.635)
    (humus 0.032)
    (SOM_fractions 0.30 0.15 0.55)
    (C_per_N 11.0 [g C/g N]))

(defhorizon "B F.S.L." FAO3
     (description "30-60 cm")

    (dry_bulk_density 1.40 [g/cm^3])
    (clay 0.135)
    (silt 0.144)
    (sand 0.721)
    (humus 0.025)
    (SOM_fractions 0.30 0.15 0.55)
    (C_per_N 11.0 [g C/g N]))

(defhorizon "C F.S.L." FAO3
     (description "60-90 cm")

    (dry_bulk_density 1.40 [g/cm^3])
    (clay 0.145)
    (silt 0.156)
    (sand 0.699)
    (humus 0.024)
    (SOM_fractions 0.30 0.15 0.55)
    (C_per_N  11.0 [g C/g N]))

The target silage maize yield is 13 Mg/ha DW, 135 kg N/ha.

However,  when I tried to run it, either with or without fertilizer at 105 kg N/ha, the soil mineral N after harvest was always much lower (around 5-10 kg) than my mearuements (100 kg N/ha). Even the N mineralization reached 200 kg N/ha and the leaching+denitrification was less than 30 kg N/ha at the that moment.

I am assuming that the N allocated to SMB was too high (250 kg N/ha). Does it make sense? How can I reduce the allocation of N to SMB?

I would appreciate any help or suggestions from you.

Thanks in advance!

kind regards,

Hongzhen 

[Per Abrahamsen]

Dear Hongzhen.

You specify a very poor soil organic quality by putting only 15% of the SOM in the fast SOM2 pool, meaning you will get very little nitrogen from the soil.

You target underfertilize, adding 105 kg N/ha, yet removing 135 kg N/ha.

The result is a very nitrogen poor system.  The maize is likely entirely N limited, meaning crop residuals will have the highest C/N ratio allowed by the crop parameterization.  This means a large immobilization of crop residuals after harvest.

If people really get 135 kg N/ha yield from 105 kg N/ha fertilizer, I'd expect the soil to be rich.  The richest soil we see in Denmark is (SOM_fractions 0.30 0.70 0.00), I'd start by trying that value.

Kindly,

Per A.

 

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[Hongzhen Luo]

Dear Per,

Thank you for the suggestion. Indeed the soil is rich.

I will start with the ratio you suggested and try to fit it with the crop growth. 

kind regards,

Hongzhen