Phosphorus Losses

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Phosphorus is lost from croplands via erosion or runoff. Quantifying phosphorus losses in eroding agricultural soils is particularly uncertain, as erosion rates vary widely even within a single field. It is also because that few nations have comprehensive, periodic inventories of their soil erosion.

The phosphorus loss from croplands can be roughly estimated based on the amount of topsoil erosion and average phosphorus content. A crop takes up the majority of the nutrients it requires from topsoil. The topsoil is often identified as the 'plough layer', that is, the 20-30 cm depth of soil which is turned over before seedbed preparation. The volume of topsoil in the plough layer is around 2500 m ha- and weighs approximately 20001.

A ton of fertile topsoil can contain 0.6-3.0 kg of phosphorus, based mainly on US and European agricultural practices.

It has been estimated that the annual soil erosion from agricultural systems of the United States, China, and India was 3.0, 5.5, and 6.6Gtyr-1, respectively. Since these three countries hold 23% of global agricultural lands, the global soil erosion could reach 66 Gt yr- . Some of the most serious soil erosion takes place in the agricultural systems of Southeast Asia, Africa, and South America. Hence, the real global soil erosion loss could be even higher, perhaps as much as 75 Gtyr-1.

The erosion intensity from croplands varies a lot among countries, ranging from 0.5 to 4001 ha- yr- . Worldwide, soil erosion is highest in the Asia, Africa, and South America, averaging 30-401 ha-1 yr-1 of soil loss. It was suggested that the global average erosion rate is at least 201 ha-1 yr-1. The lowest erosion rates occur in the United States and Europe where they average about 101 ha- each year. It is evident that soil erosion in the US has been reduced by soil conservation policies; a national survey showed that the total soil erosion between 1982 and 1992 decreased by 32%. The annual sheet and rill erosion rate in the US fell from an average of 10tha-1 in 1982 to 7.7 t ha-1 in 1992, and the wind erosion rate fell from an average of 8.11 ha- yr- to 5.91 ha- yr- over the same period. Assuming global erosion rate averaging 25tha-1 gives the soil loss of 38.5 Gtyr-1 from cropland (cf. Table 1). Furthermore, most of the loss is permanent and may not be replenished by weathering. For instance, the excessive soil loss, a rate that would impair long-term crop productivity, is estimated at about 25.4Gtyr- in around 1980.

Erosion from pastures is commonly less intensive than that from ploughed fields. However, soil losses have been greatly increased by overgrazing, which now affects more than half (i.e., at least 1720 x 106 ha) of the world's permanent pastures with a high erosion rate of 15 tha-1 each year. This leads to 25.8 Gtyr-1 of the soil loss from overgrazed pastures. Together with the amount of soil loss from cultivated grassland, the world's permanent pastures lose their topsoil at an annual rate of 34.4 Gt yr-1. Adding the losses from cropland and pastures, the world soil erosion from agricultural areas amounts to 72.9 Gtyr-in total, or 15 t ha-1 yr-1 on average. This is similar to previous estimates as discussed above.

Allowing for the poor condition of topsoil in developing countries, it might be appropriate to assume that the global phosphorus content in topsoil averages about 0.5 kg P t , or 1.0tPha . This gives the world phosphorus losses at 19.3 and 17.2 MMT Pyr-1 from cropland and pastures, respectively, as shown in Table 2.

The surface runoff loss of applied inorganic phosphate fertilizer varies significantly with a number of agronomic factors. Typical runoff rates of phosphorus in European

Table 2 Global soil erosion and phosphorus losses from agricultural land (2003)

Permanent pasture

Cropland Overgrazed Ordinary

Table 2 Global soil erosion and phosphorus losses from agricultural land (2003)

Cropland Overgrazed Ordinary

Total area (million ha)

1540

1720

1720

Soil erosion

Erosion rate

25

15

5

(tha-V-1)

Erosion quantity

38.5

25.8

8.6

(Gtyr1)

Phosphorus loss

P content in topsoil (kg

0.5

0.5

0.5

Pha-1)

P loss (MMT Pyr"1)

19.3

12.9

4.3

countries range from 0.2% to 6.7%, an average of 3.5%. Worldwide, the maximum rate can reach 10% under certain soil characteristics and climatic condition. Roughly, the world total phosphate fertilizer application can lead to a loss of 0.5 MMT P yr-1 in surface runoff.

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