13. Heavy Metal (Cd, Cu & Zn) Dynamics

The concentration of the metals Cu, Zn, and Cd in the topsoil and subsoil is determined by a mass-balance equation that describes the inputs, outputs, and accumulation in each layer.

Crop uptake

As with all other nutrients, Cd, Cu and Zn uptake is calculated by multiplying the crop yield with the metal content in crops. For Cu, the crop Cu content is assumed to be independent of soil conditions. For Cd and Zn, the metal concentration in the plant is derived by a non-linear relationship with the metal concentration in the topsoils (layer 0-30 cm), accounting for the impact of soil properties that control the (bio)availability of metals in soils for Cd and Zn (Adams et al., 2004; Brus et al., 2002; De Vries et al., 2008).

Equation 13.1

where:

is the metal (Cd or Zn) concentration in plant (mg kg^–1).

is the total metal (Cd or Zn) concentration in soil (mg kg^–1).

is a coefficient describing the non-linear relationship between the metal concentration in plant and in soil.

Crop	Cd	Zn
Grass	1.22	0.41
Maize	1.08	0.67
Potatoes	0.81	0.68
Sugar beet	0.62	1.13
SprintBarley	1.08	0.67
Wheat & Other crops	0.62	0.45

is a function of pH, soil organic matter content, and clay according to:

Equation 13.2

where:

is the KCl-extracted pH of the soil.

is the percentage of organic matter content in the soil.

is the percentage of clay content in the soil.

are regression coefficients derived for different metal elements and crops.

Metal	Crop
Cd	Grass	1.45	-0.38	0	0
	Maize	0.9	-0.21	0	-0.32
	Potato	0.97	-0.21	-0.2	-0.41
	Sugar beet	1.33	-0.22	0	-0.13
	Sprint barley	0.9	-0.21	0	-0.32
	Wheat & Other crops	0.22	-0.12	-0.33	-0.04
Zn	Grass	2.06	-0.09	1.09	-1.05
	Maize	3.05	-0.31	0	-0.61
	Potato	0	-0.09	-0.64	-0.41
	Sugar beet	2.69	-0.41	-0.71	-0.37
	Sprint barley	3.05	-0.31	0	-0.61
	Wheat & Other crops	1.32	-0.06	0	-0.24

Equation 13.3

Runoff & leaching

The leaching and runoff rate from the topsoil (0–30cm) and subsoil (30–100 cm) is calculated by multiplying the leaching or runoff rate with the dissolved metal concentration. The dissolved metal concentration is related to the reactive soil metal concentration according to a Freundlich equation (De Vries et al., 2008b).

Equation 13.4

where:

is the metal concentration in the soil solution (mmol L^–1).

is the reactive metal concentration in the soil (mol kg^–1).

is a non-linear relationship coefficient.

0.54

0.47

0.74

To convert from mg kg^–1 to mol kg^–1:

in which is the molar mass of the respective metal element.

The value of is calculated as a function of pH, soil organic matter content, and clay:

Equation 13.5

where:

is the pH determined in water (or soil solution).

is the percentage of organic matter content in the soil.

is the percentage of clay content in the soil.

are regression coefficients derived for different metal elements.

Metal
Cd	-4.85	0.58	0.28	0.27
Cu	-3.55	0.48	0.18	0.16
Zn	-4.51	0.39	0.35	0.45

is derived from total metal concentration by accounting for the variation in organic matter and clay content, according to (Römkens et al., 2004):

Equation 13.6

where:

is the percentage of organic matter content in the soil.

is the percentage of clay content in the soil.

is the total metal concentration in the soil (mg kg^–1).

are regression coefficients derived for different metal elements.

Metal
Cd	-0.089	0.022	-0.062	1.08
Cu	-0.331	0.023	-0.171	1.15
Zn	-0.703	0.183	-0.298	1.24