Answer the following two questions – briefly explain and justify your answers.
1:Phytoextraction of potentially toxic elements from soils has been suggested as a possible means of cleaning up contaminated land. You have calculated the fractions of As, Zn and K contained in ‘grass’ (Fplant) at both grassland and woodland sites. If all the ‘grass’ were cut and completely removed from grassland and woodland once each year this fraction would be removed from the soil. How many years would it take to remove 50% of the original As, Zn and K concentrations (as measured in 2018) from the soils? (25%)
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2:Currently the standing biomass of trees at Bagworth Heath is rather low. If the trees grow vigorously and standing biomass of 40 kg m-2 is achieved by 2100, what will be the average C sequestration rate in the Bagworth Heath woodlands (in kg C m-2 y-1) between now and 2100, assuming present day carbon concentrations in the soil remain the same? Given that the average UK citizen emits approximately 11 tonnes CO2 per year, approximately how many people’s CO2 emissions could be offset by tree growth at Bagworth Heath (area = 13 hectares)? (25%)
275 words
Concentrations of arsenic, zinc and potassium in soil and vegetation samples
Your soil and plant samples have been analysed for arsenic, zinc and potassium. You will find the data obtained on the worksheet ‘Elemental Analysis’ with the following numbering system.
Group / sample identification Sample No. Group / sample identification Sample No.
Group1 Grassland Veg 1.1 Group4 Grassland Veg 4.1
Group1 Woodland Veg 1.2 Group4 Woodland Veg 4.2
Group1 Woodland Tree Tissue 1.3 Group4 Woodland Tree Tissue 4.3
Group1 Grassland Topsoil 1.4 Group4 Grassland Topsoil 4.4
Group1 Grassland Subsoil 1.5 Group4 Grassland Subsoil 4.5
Group1 Woodland Topsoil 1.6 Group4 Woodland Topsoil 4.6
Group1 Woodland Subsoil 1.7 Group4 Woodland Subsoil 4.7
Group2 Grassland Veg 2.1 Group5 Grassland Veg 5.1
Group2 Woodland Veg 2.2 Group5 Woodland Veg 5.2
Group2 Woodland Tree Tissue 2.3 Group5 Woodland Tree Tissue 5.3
Group2 Grassland Topsoil 2.4 Group5 Grassland Topsoil 5.4
Group2 Grassland Subsoil 2.5 Group5 Grassland Subsoil 5.5
Group2 Woodland Topsoil 2.6 Group5 Woodland Topsoil 5.6
Group2 Woodland Subsoil 2.7 Group5 Woodland Subsoil 5.7
Group3 Grassland Veg 3.1 Group6 Grassland Veg 6.1
Group3 Woodland Veg 3.2 Group6 Woodland Veg 6.2
Group3 Woodland Tree Tissue 3.3 Group6 Woodland Tree Tissue 6.3
Group3 Grassland Topsoil 3.4 Group6 Grassland Topsoil 6.4
Group3 Grassland Subsoil 3.5 Group6 Grassland Subsoil 6.5
Group3 Woodland Topsoil 3.6 Group6 Woodland Topsoil 6.6
Group3 Woodland Subsoil 3.7 Group6 Woodland Subsoil 6.7
Elemental concentrations obtained for As, Zn and K in soil and plant samples are reported in mg kg-1 on a dry weight basis.
Using the concentrations of As, Zn and K in soil and plant samples you can calculate the corresponding soil-plant concentration ratios (CRs).
It is reasonable to assume that herbs and trees absorb most of the elements found in their tissues from the upper part of the soil profile. Therefore, when calculating CRs use the topsoil concentrations only.
or
where
CRplant = the soil-plant concentration ratio (dimensionless)
CRtree = the soil-tree concentration ratio (dimensionless)
Cplant = the concentration of an element within the plant tissues (mg kg-1)
Ctree = the concentration of an element within the tree tissues (mg kg-1)
Csoil = the concentration of the same element within the topsoil (mg kg-1)
Organic carbon content of soil and vegetation samples
The organic carbon content of the soil samples (not vegetation) has been estimated based on the mass lost after combustion overnight at 500?C. This ‘Loss on Ignition’ value has been used to calculate the organic carbon content, CORG, in g kg-1.
Conveniently, approximately 50% of the dry mass of wood consists of carbon so the concentration of carbon within a tree’s wood (dry) is assumed to be 500 g kg-1 DW. For the ‘grass’ the concentration of carbon can be assumed to be 450 g kg-1 DW.
Though the carbon content of vegetation is not derived from the soil (actually by photosynthetic fixation of atmospheric C) it is still convenient for our purposes to calculate a soil-plant concentration ratio for C, in the same way as we did for As, Zn and K.
In situ bulk density and depth of topsoil
The in situ bulk densities of topsoil in grassland and woodland areas which have been calculated using your samples are presented in kg m-3 (on a dry weight basis) in the worksheet ‘Soil Data’ (NB. bulk densities only presented for topsoils).
In the same worksheet you will find the depth of topsoil (down to the topsoil-colliery spoil boundary) in m.
Standing Biomass
The dry standing biomass of grassland and woodland vegetation is provided on the ‘Vegetation Data’ worksheet in kg m-2.
The dry standing biomass of trees, which is calculated using the Forestry Commission Mensuration Tables, is also presented on the ‘Vegetation Data’ worksheet, in kg m-2.
Data Presentation
Using the data provided in Bagworth Heath Student Data 15Feb18.xlsx you should calculate and tabulate median values for the following data for presentation in your report.
Concentrations (dry weight basis) of arsenic, zinc, potassium and carbon in soil and vegetation samples.
Concentration ratios for each element for ‘grass’ and tree tissues (calculate individual CRs using each group’s data, then calculate median CRs from these 6 replicate values).
In situ bulk densities of topsoils (subsoil bulk density was not measured).
Standing biomass of ‘grass’ and trees.
Look at your tabulated data critically and ask yourself if the values look reasonable. If any data appear odd (eg. very high or very low values, one element very different to the others), check through the raw data in the spreadsheet and re-check your calculations.
Mass Balance Calculations
When you are satisfied that you have calculated everything correctly use the median values you tabulated previously to calculate the total mass of As, Zn, K and C in the soil-vegetation system at both the grassland and woodland sites, as follows.
where
Mtot = total mass of element in the soil plus vegetation expressed on a spatial
(ground area) basis (mg m-2)
Csoil = the measured concentration of element in the top soil (mg kg-1 DW)
? = the depth of topsoil (m)
?soil = the dry bulk density of the topsoil (kg m-3)
CRtree = the concentration ratio for the element in tree tissue
CRplant = the concentration ratio for the element in ‘grass’ tissue
Mtree = the standing biomass of tree tissue (kg m-2)
Mplant = the standing biomass of ‘grass’ tissue (kg m-2)
As a check on your arithmetic, calculate the concentration of element in the top soil as follows, using the Mtot value you obtained in the previous calculation.
If the Csoil value you calculate is not the same as the Csoil value that was measured then check your calculations.
Tabulate the calculated elemental masses (in mg m-2¬¬, g m-2¬¬ or kg m-2¬¬, as appropriate).
Next, calculate the fraction of the total elemental mass which is present in the soil, ‘grass’ and tree compartments, respectively, using the following three equations.
As a final check on your calculations the sum of Fsoil, Ftree and Fplant should be 1.0, exactly.
present the results of your mass balance calculations graphically so that the reader (me!) can see immediately and clearly the relative distributions of As, Zn, K and C in the three component (soil, ‘grass’, tree) system at Bagworth Heath.
