Hatfield J.L., Gandini A. (eds.) Nitrogen in the Environment

Nitrate Losses to Surface Water Through Subsurface, Tile Drainage 163

respectively for spring preplant application compared with a late October appli-

cation ( Table 8 ). A 4-year (2000–2003) study conducted on similar soils in Iowa

found nitrate-N concentrations in subsurface drainage to not be different between

fall and spring applications of aqueous ammonia with and without N-Serve under

slightly dry to normal precipitation conditions ( Lawlor et al., 2004 ). Although timing

and method of N application may be important, the authors concluded that applying

the correct amount of N was perhaps the most important factor.

Table 8.

Effect of time of N application and nitrapyrin on nitrate-N concentrations and

losses in subsurface drainage water in a corn-soybean rotation.

Nitrate-N Concentration

(mg/L)

Nitrate-N Loss (kg/ha/year)

N Treatment

1

1990–1993 1994–2000 1990–1993 1994–2000

Fall 16.6 12.3 54 18

FallNitrapyrin 13.7 10.5 47 18

Spring 13.8 10.8 45 14

SpringNitrapyrin – 11.2 – 15

Split 14.5 – 47 –

Adapted from Randall et al. (2003) and Randall and Vetsch (2005) .

1

Anhydrous ammonia applied 25 October (fall), 1 May (spring), or 1 May  30 June

(split).

Split application of N should theoretically result in increased N efficiency and

reduced nitrate losses because of greater synchronization between time of applica-

tion and crop uptake. However, evidence in the literature to support this concept

is mixed. Baker and Melvin (1994) reported losses of nitrate-N to be greater for

a split application compared to a preplant application for continuous corn. Losses

with split application for the corn-soybean rotation were lower in the year of appli-

cation but tended to be higher in the following year when soybeans followed corn.

Similar results were found in Minnesota ( Randall et al., 2003 ). In another Iowa

study, Bjorneberg et al. (1998) concluded that combining a split N fertilizer man-

agement strategy based on the pre-sidedress nitrate soil test (PSNT) with no-tillage

practices can have positive environmental benefits without reducing corn yields in a

corn-soybean rotation.

3.6 . Nitrification Inhibitors

Nitrification inhibitors are sometimes added to ammonium fertilizers [anhy-

drous ammonia (AA) and urea] to retard or slow the conversion of ammonium to

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164 Nitrogen in the Environment

nitrate after fertilizer application. Nitrapyrin has been the most commonly used

nitrification inhibitor in the United States and has been a component in many

N research studies. Many of these studies have focused on the agronomic effects of

nitrapyrin coupled with leaching conditions. Anhydrous ammonia with and with-

out nitrapyrin was applied in late October and spring preplant for corn in rotation

with soybeans in Minnesota ( Randall and Vetsch, 2005 ). Nitrate concentrations

and losses were reduced 16% and 10%, respectively, when nitrapyrin was used in

the fall. However, spring application of nitrapyrin increased nitrate concentrations

and losses by 4% and 7%, respectively. These increased concentrations of nitrate

in the drainage water with spring-applied nitrapyrin are similar to results with split-

applied N (spring  sidedress) described earlier ( Table 8 ).

Response to nitrapyrin may be particularly dependent on time of N applica-

tion. Quesada et al. (2000) reported the agronomic and economic effects of nitrapy-

rin applied with ammonia in the spring during a 10-year period in Iowa. Grain yield

responses occurred with nitrapyrin in 1 year for continuous corn but did not occur for

corn in rotation with soybean. These data suggest that nitrapyrin would not likely be

beneficial in reducing nitrate losses to tile drainage when applied with ammonia in

the spring.

3.7 . Tillage

Numerous studies have been conducted in Canada, Europe, and North America

to determine the effect of tillage on nitrate losses to subsurface drainage water. The

effect of tillage depends largely on the cropping system and the time of year when

most of the drainage occurs. Corn and soybean systems in the Corn Belt and south-

ern Canada generally show greater amounts of drain flow for no-tillage and other

greatly reduced tillage systems, whereas nitrate-N concentrations tend to be greater

with the more conventional tillage systems, for example, moldboard plow (MP)

and chisel plow ( Randall and Iragavarapu, 1995 ; Bjorneberg et al., 1996 ; Patni

et al., 1996 ). Multiple tillage systems were compared in a continuous corn and a

corn-soybean rotation for a 3-year period in Iowa ( Table 9 ) while MP and no-till

(NT) systems were compared for continuous corn in Minnesota (11 years) and

Ontario (40 months) ( Table 10 ). Nitrate-N losses among the tillage treatments in all

three studies were not statistically significant, leading to the general conclusion that

tillage does not affect total nitrate flux in subsurface drainage in these corn and soy-

bean systems. However, a 3-year study with continuous corn in Ontario ( Drury et al.,

1993 ) did show slightly decreased concentrations and losses of nitrate-N for ridge

tillage and no-tillage compared to MP tillage.

Cereal production systems in Europe, where drainage during the late fall and

winter predominates, have shown significant effects of tillage on nitrate losses in

subsurface drainage. In the United Kingdom, annual losses through mole and tile

drainage were smaller from direct drilled (NT) wheat than from plowed plots on

a clay soil ( Goss et al., 1988, 1993 ). Nitrate losses during the 6-year period were

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Nitrate Losses to Surface Water Through Subsurface, Tile Drainage 165

Table 9.

Drain flow, nitrate-N concentrations, and nitrate-N losses in subsurface tile

drainage as influenced by tillage in three Iowa cropping systems.

Three-year Average

Nitrate-N

Tillage Drain flow (mm) Concentration (mg/L) Loss (kg/ha)

Continuous corn

Moldboard plow 129 38 47

Chisel plow 194 32 65

Ridge tillage 207 25 54

No tillage 263 23 63

Corn after soybean

Moldboard plow 131 20 28

Chisel plow 178 20 35

Ridge tillage 145 17 24

No tillage 132 16 24

Soybean after corn

Moldboard plow 152 21 32

Chisel plow 182 20 35

Ridge tillage 168 16 25

No tillage 168 14 25

Adapted from Bjorneberg et al. (1996) .

Table 10.

Effect of tillage for continuous corn on tile flow and nitrate-N losses in subsurface

drainage in Minnesota and Ontario.

Minnesota Ontario

Parameter MP NT MP NT

Annual drainage (mm) 279 315 119 174

Mean nitrate-N concentration (mg/L) 15 13 25 21

Annual nitrate-N loss (kg/ha) 43 41 32 36

N lost as a % of N applied 21 20 21 24

Adapted from Randall and Iragavarapu (1995) and Patni et al. (1996) .

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166 Nitrogen in the Environment

reduced by 18% with direct drilling compared to plowing ( Table 11 ). Tillage prior

to planting enhanced mineralization providing a larger pool of nitrate in the fall,

which along with the 20 kg/ha application of fertilizer N leached into subsurface

drainage during the winter. Nitrate losses after the application of 157 kg N/ha in the

spring totaled only 15–25% of the annual loss and were not different between the

two tillage systems. Reducing the depth of autumn tillage from 200 to 100 mm or

incorporating harvest residues compared to burning did not significantly decrease

nitrate leaching in 3 or 4 years in a continuous winter cereal cropping system ( Goss

et al., 1998 ). Nitrate leaching in spring sown small grain crops (barley, oats, and

wheat) was 24 kg N/ha greater with early fall (September) MP tillage compared to

spring tillage during a 3-year period in Sweden ( Stenberg et al., 1999 ). Based on the

results from these European studies, it seems likely that fall tillage in the southern

and eastern region of the Corn belt may also stimulate over winter losses of nitrate,

especially following soybeans.

Table 11.

Nitrate-N loss in subsurface drain flow as affected by tillage for fall sown cereals in

England from 1981 to 1986.

Six-year Average Nitrate-N Loss

Tillage system Winter (kg/ha) Spring (kg/ha) Annual (kg/ha)

Plow 33 6 39

Direct drill (NT) 24 8 32

Adapted from Goss et al. (1988) .

3.8 . Drain Tile Spacing and Depth

The optimum spacing between subsurface, tile drains is generally a function

of the hydraulic properties of the soil, annual precipitation, rate or speed of

drainage desired, depth of placement, and installation cost. As farms and farm

equipment get larger, the importance of timeliness and suitable soil moisture con-

ditions for conducting field operations becomes more critical for crop producers.

Soil compaction is likely reduced and trafficability enhanced with closer drain

spacings that remove excess water more quickly. Lower yields are often thought

to be due to inadequate drainage in these poorly drained soil landscapes. Conse-

quently, many farmers install additional drain tile to narrow the spacing between

tiles with the expectation that crop yields will be improved because of enhanced

drainage.

A 15-year study on a poorly drained Clermont silt loam soil in Indiana showed

drain spacing to markedly affect nitrate loss in subsurface drainage water ( Kladivko

et al., 1991, 1999, 2004 ). Annual nitrate-N losses averaged across the 15-year study

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Nitrate Losses to Surface Water Through Subsurface, Tile Drainage 167

were 23, 30, and 41 kg/ha for the 20-, 10-, and 5-m drain spacings, respectively.

These differences were largely affected by drain flow which averaged 135, 166,

and 231 mm/year for the three spacings, respectively. Nitrate-N concentrations

were affected very little by drain spacing. Somewhat similar results were found

in a 6-year Indiana study on a poorly drained Mollisol (Drummer silty clay loam)

( Hofmann et al., 2004 ). Annual nitrate-N losses averaged 23, 27, and 30 kg/ha for

the 30-, 20-, and 10-m drain spacings, respectively. Subsurface drainage character-

istics during a 15-year period on a Webster clay loam in Minnesota showed  50%

of the annual nitrate loss occurred in 10% and 18% of the days drainage occurred

when the tile spacing was 15.2 and 27.4 m, respectively ( Randall, 2004 ). Moreover,

the drainage hydrograph was flashier for the 15.2 m spacing. Recent modeling

studies, however, suggest nitrate losses in subsurface drainage are not reduced as

much with changes in tile spacing compared with reductions in rate of N applied.

Davis et al. (2000) reached this conclusion after conducting long-term (80 years)

simulations using the ADAPT model. Similar conclusions were drawn from the

DRAINMOD-N model using climatic variation during a 24-year period ( Zhao et al.,

2000 ). Additional research is needed to more clearly define the agronomic and envi-

ronmental influence of tile spacing in subsurface drainage systems.

Controlled drainage is a management practice that adjusts or controls water

height within a subsurface, tile drainage system ( Gilliam et al., 1999 ). Controlling

the drainage by some mechanical means allows timely drainage but also maximizes

storage of water within the field for utilization by the crop. Field studies in North

Carolina and Ontario have shown substantial reductions in losses of nitrate from

subsurface drainage where controlled drainage has been employed.

Nitrate reductions in controlled drainage systems result from two factors: (i) vol-

ume of drainage water leaving a field is reduced and (ii) denitrification within the

soil profile is promoted because a higher water table level is maintained. Water table

depth in a controlled drainage system should be no lower than 90 cm below the soil

surface to reduce nitrate loss ( Gilliam et al., 1997 ). In North Carolina, reductions

in nitrate loss were due to less water leaving the field via tile flow in moderately

well-drained soils ( Gilliam et al., 1979 ). In Ontario, nitrate loss reductions resulted

from both less flow and lower nitrate concentrations because of enhanced denitrifica-

tion. During a 3-year period, Drury et al. (1996) found controlled drainage to reduce

flow volume by 24%, nitrate concentration by 25%, and nitrate loss by 43%. In a

2-year study, Mejia and Madramootoo (1998) found nitrate in effluent from control-

led drainage at 50- and 75-cm depths to be reduced by 84% and 77%, respectively,

in 1995 and 61% and 52%, respectively, in 1996. Nitrate loads to the receiving lake

were reduced 95% and 30%, respectively, in 1996 compared to free drainage. Nitrate

losses from a corn-sugar beet system were reduced 46–63% by controlled drainage

compared to free drainage in a 3-year study in Italy ( Borin et al., 2001 ). Results from

these studies demonstrated that controlled drainage is an effective method of reduc-

ing nitrate losses from subsurface drainage in relatively flat landscapes favorable to

their installation.

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168 Nitrogen in the Environment

4 . SUMMARY

Field studies have been conducted in Europe since the mid-1800s and in North

America for the last 40 years to assess the role of subsurface tile drainage on pro-

moting nitrate losses from agricultural production areas to surface waters. Generally

speaking, factors influencing nitrate losses in subsurface drainage can be divided

into two categories – uncontrollable and controllable. Uncontrollable factors consist

of precipitation and soil mineralization which can be manipulated to some degree.

Controllable factors are those management practices used by crop producers to best

fit the needs of their enterprise.

The following summarizes the effects of many individual factors on nitrate

losses in subsurface drainage from agricultural production systems:

● Precipitation has a huge effect on drainage flow volume and nitrate losses.

Annual nitrate-N losses can range from 0 in dry years to  100 kg/ha in years

with much above-normal precipitation. Losses are also affected by dry and

wet climatic cycles with greatest losses occurring in wet years following dry

years. The temporal distribution of precipitation (fall versus winter versus

spring versus summer) can affect some N and crop management decisions

(i.e., time of N application, tillage, etc.). A substantial proportion of the

annual nitrate loss may occur within only a few days when soils are saturated

and very large precipitation events happen.

● Soil mineralization contributes nitrate that is susceptible to loss in subsurface

drainage. Nitrate losses can be very high, especially following dry years, if

crop uptake is not synchronized with availability of the mineralized nitrate.

● Nitrate losses from the landscape are highly related to cropping system. Row

crops (e.g., corn and soybean), yield much greater drainage volumes and nitrate-

N concentrations in the drainage water than do perennial crops (i.e., alfalfa and

CRP grass/legume mix). Nitrate-N losses can be 30 to 50 times higher from

these row crops compared with these perennial crops. Small-grained cereal pro-

duction systems in Europe also yield greater nitrate-N concentrations and losses

in drainage water than do unfertilized grass by crops. This is especially true for

spring-planted cereals when a winter crop has not been established.

● Cover crops, including rye, ryegrass, or winter wheat established in the fall, can

reduce nitrate concentrations and losses substantially in the winter and spring,

but may falter when establishment is difficult in the Northern latitudes or break-

down of the residues is not synchronous with demands of the following crop.

● Rate of N application affects nitrate losses in drainage more than any other nutri-

ent management decision. Nitrate losses increase as the rate of N application

increases. At application rates greater than needed by the crop, the excess N will

likely remain in the soil profile where it is highly susceptible to loss, especially

in wetter-than-normal years. Nitrate losses are not different among N sources as

long as the rate of application is similar.

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Nitrate Losses to Surface Water Through Subsurface, Tile Drainage 169

● Nitrate losses to drainage as affected by time of N application are greatly influ-

enced by temporal distribution of precipitation, evapotranspiration demand,

leaching, and source of N. Fall application of N is particularly risky if leach-

ing is significant between application and crop uptake. If over-winter leaching

is absent due to frozen soils and spring conditions are dry, losses from fall-

applied N will not be different from spring-applied N. Anhydrous ammonia

and manure are likely to be less affected by time of application due to slower

nitrification than those fertilizers that nitrify more quickly. Sidedress and late

split applications of N to corn tend to give greater nitrate concentrations in

drainage in the succeeding year compared to preplant applications.

● Nitrification inhibitors added to fall-applied AA can reduce nitrate losses and

improve corn yields in those years when leaching is synchronous with persist-

ence of the inhibitor. If leaching occurs after the inhibitor has degraded, pro-

tection against nitrate losses will no longer be provided. Spring application of

nitrification inhibitors has generally not been beneficial on poorly drained soils.

● Tillage systems for row crops in the Northern US generally do not affect the

amount of nitrate lost in drainage. Drainage flow volume is often greatest for

greatly reduced tillage systems while nitrate concentrations are greatest for the

conventional, plow-tillage systems. In cereal production systems in Europe,

where over-winter drainage dominates, fall tillage increases nitrate losses com-

pared to no-tillage. This again shows the need to consider climate and time of

leaching when making crop and nutrient management decisions to minimize

nitrate losses.

● Limited field studies show greater nitrate losses as spacing between tile lines

is reduced. However, modeling studies suggest a lesser effect of drain spac-

ing and depth compared to rate of N applied. Controlled drainage systems

that manage water table depth throughout the season can significantly lower

nitrate losses in drainage water.

● Long-term, subsurface drainage research, which integrates the effects of cli-

matic variability, soil properties, and various cropping systems, is vital to our

understanding of nitrate losses to subsurface drainage. Educators and pol-

icy makers must consider this research as they deal with the occurrence of

nitrates from agricultural production systems in surface waters.

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