y = 1.8324x + 457.8

R2 = 0.2994

y = 2.8349x + 316.36

R2 = 0.6919

0

200

400

600

800

1000

1200

1400

1600

1800

0 50 100 150 200 250 300 350 400 450

Dispersible P (mg P / kg bulk soil)

P t

ot

(mg

/kg

)

Meghna Ganges

From: Ali et al. (2003); Dhaka, BUET Symposium

Ganges

Floodplain

Meghna

Floodplain

Potential phosphorus and arsenic release in dispersed

particulate form from Bangladesh rice fields

Maria Martin*, Jakeer Hossain, Simona Sciuto, Luisella Celi, Teresa Borda, Elisabetta Barberis

Di.Va.P.R.A. – Chimica agraria e Pedologia – Università degli Studi di Torino – Via L. da Vinci 44; 10095 Grugliasco – ITALY

*E-mail: maria.martin@unito.it

Soils can loss important amounts of As and P in solution or in particulate

form especially during monsoon season. The potential transfer of P from

Bangladesh soils to waters has received scant attention and the potential

As transfer associated to dispersed particles is generally not considered.

1.43.815.40.8213.77.86.711.40.63st.dev

1.63.07.20.288.413.014.24.20.55st.dev

median

mean

max

min

Ganges

median

mean

max

min

Meghna

3.017.643.41.3069.424.751.811.08.01

3.116.745.01.4367.818.548.532.97.68

6.023.170.74.9690.839.462.550.88.38

0.86.921.30.5931.95.437.48.45.62

3.811.224.40.9540.628.456.37.85.80

4.011.523.40.9940.527.356.89.25.88

7.917.143.31.7455.454.187.521.07.30

1.55.813.30.5123.66.932.83.44.91

g/kg%

Fe ocFe red

bFe totC org<20 µm aSandSiltClaypH

MeghnaMeghna

FloodplainFloodplain

Satkhira

Jessore

Meherpur Brahmambaria

Chandpur

n=8

n=10 n=10n=6

n=6 n=10

n=10

GangesGanges

FloodplainFloodplain

Comilla

Gopalganj

The most As affected

zones belong to the

Ganges Floodplain and

Meghna Floodplain soil

series.

Irrigation causes a continuative input of

As and P in soils, although mineral

fertilizers remain the main P source

In Bangladesh rice cultivation covers nearly 75% of the agricultural surface. In the dry season, paddy fields are irrigated with groundwater, often polluted with arsenic, phosphorus, iron and other elements.

1.20MRPP

0.30AsAs

18.1FeFe

mg L-1

Table 1. Selected physical and chemical characteristics of the soil samples. The sampled soils were

30 for each soil series.

a: clay+fine silt; b: reducible iron (oxalate-ascorbate); c: oxalate extractable iron

0

20

40

60

80

100

120

140

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Dispersible As %

Dis

pers

ible

P %

Meghna Ganges

y = 2.3883x + 1.3133

R2 = 0.7706

y = 4.6896x + 0.8258

R2 = 0.5616

0

5

10

15

20

25

30

35

40

45

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Dispersible As (mg As / kg bulk soil)

As to

t (m

g/k

g)

Meghna Ganges

Table 2. Results of the DESPRAL dispersion test. Selected characteristics of the dispersed soil particles

Table 3. Phosphorus and arsenic forms in the bulk soil and in the dispersed particles

In the present work samples representative of rice

field soils of the Ganges Floodplain (GF) and

Meghna Floodplain (MF) series were chemically and

physically characterized and the potential transfer of

P and As with dispersed particles was evaluated using a simple water dispersion test (DESPRAL).

Dispersible particles

Meghna

0.390.650.2413.83371.5174.97.063.31.19.92.7258st.dev

0.892.151.4915.16701.56117.90.74-8.74.229.74.79138median

0.941.791.2019.97382.49103.93.67-8.91.721.44.92139mean

2.153.452.0067.416176.23303.531.1-2.43.942.610.5258max

0.400.580.555.52540.7736.60.39-17.80.47.61.0560min

Ganges

0.670.840.679.96051.7199.613.75.53.58.62.4042st.dev

1.822.551.499.68741.25110.87.38-13.94.433.24.25129median

1.862.551.5413.310061.79132.514.5-14.35.132.24.40130mean

3.864.702.9439.326816.62413.037.8-320.549.59.40224max

0.700.900.123.01580.1822.60.49-261.313.00.5850min

mg kg-1 (Suspended Solid)mg kg-1 (bulk soil)mg kg-1mV% (clay)%(fs+c)g/kg soil

FeERAsERePERdAs <20µmP <20µmPAscPPbSoluble P

(water)ζa<2µm<20µm<2µm<20µm

1.40

5.03

4.93

7.30

2.00

1.27

3.39

3.39

6.94

1.45

PSI f

(X/logC)

3.41.611.75.313.64.81.080.9715.52.623023.433069.43254st.dev

5.04.741.120.757.83.80.990.4514.14.833497.573749.25529median

7.22.826.814.832.97.51.410.8418.66.8735210.639212.52610mean

15.97.159.825.168.921.04.783.9871.425.1174735.879740.121477max

3.90.813.95.95.81.60.390.212.91.621382.251763.68353min

Ganges

23.92.528.39.736.02.20.450.2932.32.803133.703244.41326st.dev

6.14.957.323.557.83.50.330.3829.82.413882.973803.84675median

19.75.558.323.348.74.10.460.4240.83.374494.404735.55660mean

84.611.110754.3161.711.02.081.15113.111.99153213.8150518.191564max

2.40.38.52.64.41.80.050.073.00.67580.73740.90135min

Meghna

%mg kg-1

AsOlsen/AstotPOlsen/PtotPAs/As totPP/PtotPAsh/AsOlsenPPg/POlsen

As (KH2PO4-)eOlsen AsOlsen PAso

dPocAs red

bP redaAs totP tot

y = 1.090x + 0.245

R2 = 0.724

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

As Olsen (mg / kg)

As H

2P

O4- (

mg

/ k

g)

a Pred and b Asred: extracted with oxalate-ascorbate; c Po and d Aso: oxalate-extractable; e As(H2PO4-): As extracted with P addition of 50 mmol/kg soil; f PSI: phosphorus sorption index; g PP and h PAs: particulate P and As (mg/kg soil)

y = 4.47x + 3.3

R2 = 0.774

0

10

20

30

40

50

60

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Dispersible solids (g/l)

Dis

pers

ible

P (

% P

tot)

Meghna Ganges

y = 9.6385x + 2.0294

R2 = 0.4626

y = 21.47x - 11.182

R2 = 0.5822

0

20

40

60

80

100

120

140

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Dispersible solids (g/l)

Dis

pe

rsib

le A

s (

%A

s to

t)

Meghna Ganges

The Ganges Floodplain soils are calcareous, with silt-loam to silt-clay texture, while the

Meghna Floodplain soils are noncalcareous with mainly silt-loam texture. The organic C

content is low to medium and does not significantly differ between the two series. The Ganges

floodplain soils have a higher total Fe content; however, in the MF soils a greater portion of it

is oxalate extractable and hence in a more reactive form.

Nearly 30% of the < 20µm total granulometric fraction (fine silt+clay) was water-dispersible in the MF soils

and 20% in the GF soils; while the clay fraction was proportionally less dispersible. Although the Zeta

Potential had a large internal variability, the clay particles from the more dispersible MF soils appeared with

a higher negative charge density. This could be due to the lesser amount of Ca in these soils.

Phosphorus in the dispersion water varied widely and did not correlate with Olsen P, while As in water was

under the detection limit. The dispersed particles of both soil series were enriched in P and As compared

with the bulk soil, in particular the MF soils; however, AsER was always nearly 1.5 times higher than PER.

These results suggest particulate dispersion as one of major processes for potential P and As transfer from soil to surface waters,

especially in the Meghna Floodplain soils. This must be carefully evaluated, especially considering the important rainfall events in the

monsoon season,

Dispersible As and P

are positively related

The proportion of dispersible As and P tended to increase with the concentration of suspended solids

Olsen As and As extracted with

50 mmol/kg P are correlated and

the slope of the line is close to 1

Dispersible As and P increase with their total content

The average total soil P was extremely variable even at microscale level, but did not statistically differ between the two

series. The same great variability was observed for Olsen P, that passed from very low levels to overfertilisation. The P

status of soils is probably related to great difference in fertiliser inputs, suggesting the pressing need of a more rational

management of P fertilisation. The large variability of As in soils was also expected, depending on the contamination of

the irrigation water and on field and water management.

Labile and dispersible P forms were higher in the MF compared with GF soils, while the amounts of total and labile As

forms were higher in the GF ones. However, when considering the proportional mobility of As both in suspension and

in solution, compared to total As soil content, it always resulted more pronounced in the MF series, that hence could be

more prone to nutrient and contaminant losses by runoff.

AcknowledgmentsAcknowledgments.. This study has been funded by MIUR, PRIN Project, 2006, and by World Wide Style Program by

CRT Foundation, Turin. Prof S.M.I. Huq, University of Dhaka, is gratefully acknowledged for his collaboration.

a ζ: zeta potential of dispersed particles; b PP= particulate P; c PAs: particulate As; d PER: P enrichment ratio; e AsER: As enrichment ratio; fFeER: Fe enrichment ratio