6. Crop Production

6.1. Grassland areas

6.2. Harvested products

Yields of harvested products are provided as input. In Miterra, fresh yields are used for most crops except for grass (which uses dry matter yields), and then dry matter (DM) yields are simply calculated using a DM fraction for each type of crop.

N content in harvested products (Nharvest) is calculated using an N:DM ratio.

Equation 6.1

where:

is the dry matter content in the harvested products (kg DM ha–1).

is the fresh weight yield of the harvested crop product (kg fresh weight ha–1).

is the fraction of dry matter content in the harvested product (kg DM kg–1 fresh weight).

is the N content in harvested products (kg N ha–1).

is the fraction of total N in harvested dry matter (kg N kg–1 DM).

For other nutrient elements (P, Ca, Mg, K, Na, Cl & S), the content in harvested products is calculated in the same way as N:

Equation 6.2

where:

is the content of element X in the harvested products (kg X ha–1).

is the dry matter content in the harvested products (kg DM ha–1).

is the fraction of element x in harvested dry matter (kg X kg–1 DM).

For heavy metal elements (Cd, Cu, Pb & Zn), their content are calculated based on bioconcentration factors (BCF):

Equation 6.3

where:

is the content of element X in the harvested products (kg X ha–1).

is the content of element X in the soil (kg X ha–1).

is the BCF of element X (kg X in crop kg–1 X in soil).

6.3. Cover crops

Cover crop area

The areas of crops growing in the winter are determined for each crop type:

  • For winter wheat & winter barley, they are explicitly known as winter crops and their areas are available from input.

  • For soft wheat, durum wheat, barley, and rye, they can be grown either in spring or winter, and their areas of winter growth is determined with a “winter share” fraction (fw).

In more recent input data, wheat and barley are separated into spring and winter sown varieties. However, in a few cases, areas of soft wheat, durum wheat, and barley are still reported. Their spring and winter sown areas are therefore calculated using the “winter share” fraction.

Based on that we can estimate the areas of non-winter (spring) growth for each crop:

Equation 6.4

For each crop i in the region:

where:

is the area of crop i growing in the spring (ha).

is the total area of the crop i (ha).

is the fraction of crop i that is growing in the winter (unitless). Data are available from SoilCare (Eurostat/Corine).

The area of cover crop growing after each crop is then estimated as:

Equation 6.5

For each crop i in the region:

where:

is the area of cover crop growing after crop i (ha).

is the area of crop i growing in the spring (ha). See Equation 6.4.

is the total area of cover crops in the region (ha). Data are available from Eurostat.

The fraction of cover crop after each crop is calculated as:

Equation 6.6

For each crop i in the region:

where:

is the fraction of the area of cover crop following crop i (unitless).

is the area of cover crop growing after crop i (ha). See Equation 6.5.

is the area of crop i (ha).

Cover crop C and N production

The N uptake by cover crops is based on data from Schroder et al., who provided estimates per environmental zone on the N yield of cover crops based on temperature sum values. However, especially for Mediterranean climates these values seem high and there might be limitation by water and nitrogen. Therefore, the N uptake by catch crops is maximised at 75% of the soil N surplus, and a minimum value of 5 kg N ha–1 is applied as well. Previously a default value of 42 kg N ha–1 was used based on an average C input of 1500 kg ha–1 and a C:N ratio of 35, which is probably too high for most regions.

Equation 6.7

For each crop i in the region:

where:

is the N content in the cover crop following crop i (kg N ha–1).

is the N uptake by the cover crop following crop i (kg N ha–1 year–1), specific to the environmental zones.

Code

Environmental Zone

N uptake by cover crops
(kg N ha–1 year–1)

ALN

Alpine North

5

BOR

Boreal

5

NEM

Nemoral

5

ATN

Atlantic North

30

ALS

Alpine South

10

CON

Continental

20

ATC

Atlantic Central

55

PAN

Pannonian

55

LUS

Lusitanian

90

MDM

Mediterranean Mountains

95

MDN

Mediterranean North

100

MDS

Mediterranean South

100

is the soil N surplus for crop i (kg N ha–1 year–1). Nsurplus is derived by running the model for initialization assuming no cover crops, and using the output on soil N surplus as input for future runs.

The C content in cover crops is determined according to the C:N ratio of the cover crop.

Equation 6.8

For each crop i in the region:

where:

is the C content in the cover crop following crop i (kg C ha–1).

is the N content in the cover crop following crop i (kg N ha–1). See Equation 6.7.

is the C:N ratio of the cover crop (kg C kg–1 N), with a default value of 25.

6.4. Residue removal & incorporation

For annual crops, part of the unharvested residue biomass may be further removed from the field (e.g., straw), and the rest may be incorporated into soil by tillage.

The biomass of unharvested residues, removed residues, and residues incorporated into soil, are calculated using different approaches for annual crops, straw crops, and perennial crops.

In all calculations in this section, the fraction of C content in crop organic matter (fC) is assumed to be 0.45 for all crop types.

Annual crops & grasslands

Most arable crops, except for straw crops (see below), are considered annual crops. The unharvested residues are calculated based on crop yields and harvest index.

Residue removal and incorporation for grasslands are calculated in the same way as annual crops.

Equation 6.9

where:

is the dry matter/C/N content of the residues that is not harvested, including residues both left in and removed from the field (kg DM/C/N ha–1).

is the dry matter/N content in the harvested products (kg DM/N ha–1). See Equation 6.1.

is the harvest index, which is the ratio of harvested biomass to the annual net primary production (unitless).

is the fraction of C content in crop organic matter (kg C kg–1 DM), with a default value of 0.45.

is the ratio of N in harvested products to N in residues (kg N harvested kg–1 N in residues).

The amount of residues removed from the field is determined by a country- and crop-specific removal factor.

Equation 6.10

where:

is the C/N content in the residues removed from the field, or incorporated into soil (kg C/N ha–1).

is the C/N content of the residues that is not harvested, including residues both left in and removed from the field (kg DM/C/N ha–1). See Equation 6.9.

is the fraction of crop residues removed from field (unitless).

Straw crops

Straw crops include cereal crops, maize, rice, sunflower, and rapeseed. Carbon inputs to soil from straw crops consist of above ground (Cabove) and belowground residues (Cbelow). The above ground residue biomass (Mabove) is calculated according to Scarlat et al. (2010):

Equation 6.11

where:

is the fresh weight biomass of the above ground residues (kg fresh weight ha–1).

is the fresh weight yield of the harvested crop product (kg fresh weight ha–1).

are crop-dependent regression parameters (unitless).

Crop a b

Wheat

-0.3629

1.6057

Rye

-0.3007

1.5142

Oats

-0.1874

1.3002

Barley

-0.2751

1.3796

Maize

-0.1807

1.3373

Rice

-1.2256

3.8450

Sunflower

-1.1097

3.2189

Rapeseed

-0.4520

2.0475

The above ground residue biomass is split into stubble (0.45) and straw (0.55). A fraction of the straw is removed, and the remaining part is incorporated into soil along with the stubble. The total C incorporation from above ground residues is thus calculated as:

Equation 6.12

where:

is the fresh weight biomass of the above ground ( Equation 6.11), stubble, or straw residues (kg fresh weight ha–1).

is the C content of the above ground residues that is removed from the field, or incorporated into soil (kg C ha–1).

is the fraction of straw removed from the field (unitless).

is the fraction of dry matter in the stubble and straw (kg DM kg–1 fresh weight).

Crop DM fraction

Wheat, Rye, Oats, Barley

0.85

Maize

0.70

Rice

0.75

Sunflower

0.60

Rapeseed

0.60

is the fraction of C content in crop organic matter (kg C kg–1 DM), with a default value of 0.45.

The belowground residue C is always considered as incorporated, and is calculated according to Taghizadeh-Toosi et al. (2014):

Equation 6.13

where:

is the C content of the below ground residues that is incorporated into soil (kg C ha–1).

is the fresh weight yield of the harvested crop product (kg fresh weight ha–1).

is the fraction of dry matter content in the harvested product (kg DM kg–1 fresh weight).

is the fresh weight biomass of the above ground residues (kg fresh weight ha–1). See Equation 6.11.

is the fraction of dry matter in the stubble and straw (kg DM kg–1 fresh weight). See table in Equation 6.12.

is the fraction of C content in crop organic matter (kg C kg–1 DM), with a default value of 0.45.

is the ratio of root biomass and exudate C (below-ground C) of total net C assimilation (unitless), with a default value of 0.25.

The amount of N in residues removed and incorporated into soil for straw crops is calculated as:

Equation 6.14

where:

is the C/N content of the residues that is not harvested, including residues both left in and removed from the field (kg C/N ha–1).

is the N content of the residues, including both above and below ground parts, that is removed from the field, or incorporated into soil (kg N ha–1).

is the C content of the above or below ground residues that is removed from the field, or incorporated into soil (kg C ha–1). See Equation 6.12 and Equation 6.13.

is the fraction of C content in crop organic matter (kg C kg–1 DM), with a default value of 0.45.

is the fraction of N content in stubble and straw dry matter (kg N kg–1 DM).

Perennial crops

Perennial crops include fruit trees, olive trees, and grapes. Residue inputs from these crops consist of pruned leaves, dead leaves, fruit losses during growth and harvest, dead roots, root exudates, and input from grass cover.

Pruned residues

Yields for pruned branches and leaves are available for rainfed and irrigated fields. Part of the pruned biomass is removed, and the rest incorporated.

Equation 6.15

where:

is the C or N content in the pruned biomass dry matter (kg C/N kg–1 DM).

is the C or N content in the pruned biomass dry matter that is incorporated into soil (kg C/N kg–1 DM).

is the pruning potential for irrigated (i) or rainfed (r) land (kg DM ha–1 yr–1).

is the fraction of area under irrigation (unitless).

is the fraction of pruned biomass that is removed from the field (unitless).

is the C:N ratio of pruning (kg C kg–1 N), with a default value of 60.
Leaf litters

Include input from dead leaves.

Equation 6.16

where:

is the C or N content in leaf litters (kg C/N kg–1 DM).

is the dry matter content in the harvested products (fruits) (kg DM ha–1). See Equation 6.1.

is the ratio of leaf to fruit biomass (unitless).

is the fraction of C content in crop organic matter (kg C kg–1 DM), with a default value of 0.45.

is the C:N ratio of leaf litters (kg C kg–1 N), with a default value of 30.
Dead fruits

Include input from fruit losses during growth and harvest.

Equation 6.17

where:

is the dry matter, C or N content in dead fruits (kg DM/C/N kg–1 DM).

is the dry matter content in the harvested products (fruits) (kg DM ha–1). See Equation 6.1.

is the fraction of fruit biomass lost during growth (unitless).

is the fraction of fruit biomass lost during harvest (unitless).

is the fraction of C content in crop organic matter (kg C kg–1 DM), with a default value of 0.45.

is the fraction of total N in harvested fruit dry matter (kg N kg–1 DM).
Belowground residues

Include root exudates and dead roots.

Equation 6.18

where:

is the C or N content in below ground biomass, including root exudates and dead roots (kg C/N kg–1 DM).

is the dry matter content in the harvested products (fruits) (kg DM ha–1). See Equation 6.1.

is the ratio of rhizodeposition to fruit biomass (unitless).

is the fraction of C content in crop organic matter (kg C kg–1 DM), with a default value of 0.45.

It is assumed that the fine root biomass is the same as root exudates, therefore the left term is multiplied by 2 to give total belowground C input.

is the C:N ratio of roots and root exudates (kg C kg–1 N), with a default value of 30.
Total C and N input to soil

Total C and N input to soil are estimated by aggregating individual input items from above.

Equation 6.19

where:

is the C/N content of the residues that is not harvested, including residues both left in and removed from the field (kg C/N ha–1).

is the C/N content of the perennial residues that is incorporated into soil (kg C/N ha–1).

See Equation 6.15.

See Equation 6.15.

See Equation 6.16.

See Equation 6.17.

See Equation 6.18.
Additional residues from soil cover

The grounds of some orchards are covered by grass, which contribute additionally to soil C and N input:

Equation 6.20

where:

is the C or N content in soil cover biomass in orchards (kg C/N ha–1).

is the dry matter yields of permanent grassland in the region (kg DM ha–1).

it is assumed that grass cover yield is half of the yield of normal grassland.

is the fraction of C content in crop organic matter (kg C kg–1 DM), with a default value of 0.45.

is the fraction of area covered by grass in the orchard (unitless).

is the C:N ratio in grass residues (kg C kg–1 N), with a default value of 20.

Cover crops

In case cover crops are grown, more crop residues will be available. It is assumed that all cover crops are incorporated into soil. The amounts of cover crop N and C incorporated into soil are calculated by Equation 6.7 and Equation 6.8, respectively.

Other lands

For fallow and set-aside lands, which are not cultivated but may have growth of weeds, it is assumed that 250 and 500 kg C ha–1, respectively, are incorporated into soil annually. The C:N ratio of the incorporated residues is assumed to be 30.

Removal and incorporation of other elements

Removal and incorporation of other elements (P, Ca, Mg, K, Na, Cl, S, Cd, Cu, Pb, and Zn) is only calculated for straw crops. The amount removed/incorporated is based on the N removed/incorporated and X:N ratios.

Equation 6.21

where:

is the content of element X in removed/incorporated residues (kg X ha–1).

is the N content in removed/incorporated residues (kg N ha–1).

is the X:N ratio of the residue (kg X kg–1 N).

6.5. Crop uptake & demand

Crop N uptake is simply calculated as the total N content in both harvested products and unharvested biomass.

Equation 6.22

where:

is the total N content taken up by the crop (kg N ha–1).

is the N content in harvested products (kg N ha–1).

is the N content of the residues that is not harvested, including residues both left in and removed from the field (kg DM/C/N ha–1).

Crop demand refers to the rate of N fertilization required to satisfy crop uptake potential. Since not all N in the soil may become available to crops, a higher rate of N than crop uptake must be applied. This is determined by an uptake efficiency factor.

Equation 6.23

where:

is the total N content required to satisfy crop N uptake (kg N ha–1).

is the total N content taken up by the crop (kg N ha–1).

is the crop uptake efficiency factor (unitless).

Crop Type

Grassland

Cereal Crops

Other Crops

ε

1.0

1.1

1.25

6.6. Biological N fixation

Miterra uses a simple, static approach to estimate N fixation by assigning fixed N amounts to different crops. The table below gives the default values assumed by Miterra.

Crop Type N Fixation

Soybeans

80% of Nuptake.

Pulses

70% of Nuptake.

Arable & perennial crops

2 kg N ha–1 yr–1

Grassland

7.5 kg N ha–1 yr–1, assuming no grass-clover mixture.