Origin of Paddy Fields and Functions for Environmental Remediation in the Urbanized Areas of Yangtze River Delta - Environmental Contamination: Health Risks and Ecological Restoration - page 405

WaterandSoilConservationand
PreventionofNonpointPollution
From.1999.to.2004,.research.was.conducted.on.the.migration.of.substances.in.
the.soil-water.systems.of.Taihu.Lake.region,.which.demonstrated.that.irri-
gated.rice.ields.play.an.important.role.in.the.conversation.of.soil.and.water.
in.the.Yangtze.River.Delta.(Boo.et.al..2003,.Cao.et.al..2002,.2005,.Cao.and.Lin.
2006,.Cao.and.Zhang.2004,.Cao.and.Zhou.2008,.Xu.et.al..1998,.Zhang.et.al..
2003a,b)..This.is.due.to.the.fact.that.it.is.surrounded.by.15-20.cm.high.ridges,.
covered.with.7-10.cm.of.surface.standing.water.as.well.as.with.a.plow.pan.
under.a.15-20.cm.layer.of.top.soil;.therefore,.it.is.a.relatively.closed.system.for.
runoff.(Figure.18.8)..According.to.the.measurements.from.ield.experiments.
at.four.different.sites.during.1999.to.2004.in.paddy.ields.of.this.region,.only.
one.or.two.events.of.run.off.low.were.recorded.with.less.volume.and.carried.
much. fewer. solid. particles. as. compared. with. runoff. low. in. upland. ields.
(Cao.et.al..2005,.Cao.and.Lin.2006,.Cao.and.Zhang.2004,.Zhang.et.al..2003a,b)..
On.the.other.hand,.NH 4 + -N.is.the.major.form.of.nitrogen.(N).that.is.not.eas-
ily.lost.in.the.runoff.low.of.paddy.soils..Due.to.blockage.by.the.plow.pan,.
there. was. minimal. leaching. of. a. small. amount. of. nitrate. (NO 3 − -N). found.
in. paddy. soils,. resulting. in. minor. N. losses. via. paddy. soil. runoff. (Figure.
18.9.and.Table.18.3).(Cao.and.Lin.2006,.Xing.and.Zhu.2002,.Zhu.et.al..2000)..
Hence,.the.N.losses.through.runoff.or.leaching.from.paddy.soils.to.the.water.
body.are.not.the.main.contributors.responsible.for.the.eutrophication.of.sur-
face.water.body.(Table.18.3).
Phosphorus. (P). content. in. water. is. the. key. factor. in. controlling. shallow.
water. eutrophication. (Cao. 2003,. Cao. and. Lin. 2006,. Foy. and. Withese. 1995,.
Lagreid. et. al.. 1999,. Schaffner. et. al.. 2009,. Sharpley. et. al.. 1994),. where. once.

140

P losses via runoff g P/kg soil

120

100

15

12.3

80

NO 3 -N conc. (ng/L)
N leached (kg N/ha)
Ratio of N leached
to N applied (%)

10

60

40

05

4.3

1.3

20

0 0

Rice Mulberry Vegetable

Paddy field
Wheat field

Vegetable field

(a) (b)

FIGURE 18.9
Phosphorus.(P).(a).and.nitrogen.(N).(b).losses.via.runoff.and.leaching.from.various.land.use.in.
the.Taihu.Lake.Region.(1999-2004).