6. Crop Production

Grassland areas

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

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

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 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 soil (kg X ha–1).

is the BCF of element x.

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 determiend with a “winter share” fraction (fw).

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. 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.

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

The fraction of cover crop after each crop (fcc) is calculated as:

Equation 6.6

For each crop i in the region:

where:

is the area of crop i.

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 uptake by the cover crop following crop i, 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

Lusitenean

90

MDM

Mediterranean Mountains

95

MDN

Mediterranean North

100

MDS

Mediterranean South

100

is the soil N surplus for crop i. 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 by assuming a C:N ratio of 25.

Equation 6.8

For each crop i in the region:

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

is the ratio of N in harvested products to 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 fraction of crop residues removed from field.

Straw crops

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

Equation 6.11

where:

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

are crop-dependent regression parameters.

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 aboveground 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 aboveground residues is thus calculated as:

Equation 6.12

where:

is the removal fraction of straw.

is the fraction of dry matter in the stubble and straw.

Crop DM fraction

Wheat, Rye, Oats, Barley

0.85

Maize

0.70

Rice

0.75

Sunflower

0.60

Rapeseed

0.60

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

is the fraction of dry matter in the product.

is the biomass of aboveground residues (kg ha–1).

is the fraction of dry matter in the stubble and straw.

is the ratio of root biomass and exudate C (below-ground C) of total net C assimilation, 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 fraction of N in stubble and straw dry matter.

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 pruning potential for irrigated land (kg DM ha–1 yr–1).

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

is the fraction of area under irrigation.

is the fraction of pruned biomass that is removed from field.

is the C:N ratio of prunnings, with a default value of 60.
Leaf litters

Include input from dead leaves.

Equation 6.16

where:

is the dry matter yield of the harvested crop product (kg ha–1. Equation 6.1).

is the ratio of leaf to fruit biomass.

is the C:N ratio of leaf litters, with a default value of 30.
Dead fruits

Include input from fruit losses during growth and harvest.

Equation 6.17

where:

is the fraction of yield biomass lost during growth.

is the fraction of yield biomass lost during harvest.

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

Include dead roots and root exudates.

Equation 6.18

where:

is the ratio of rhizodeposition to fruit biomass.

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, with a default value of 30.
Total C input to soil
Equation 6.19
Total N input to soil
Equation 6.20
Additional residues from soil cover

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

Equation 6.21

where:

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 area covered by grass in the orchard.

is the C:N ratio in grass residues 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 N:X ratios.

Equation 6.22

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 N:X ratio of the residue.

Crop uptake & demand

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

Equation 6.23

Crop demand refers to the rate of N fertilization required to satisfy crop uptake. 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 6.24

where:

is the crop uptake efficiency factor.

Crop Type

Grassland

Cereal Crops

Other Crops

ε

1.0

1.1

1.25

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.