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

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132 Nitrogen in the Environment

NT farmers are applying a fertilizer program for the whole cropping system, which

could last at least 2 years, instead fertilizing each single crop. In this case, the total

nutrient exportation by grain harvested in the cropping system should be replen-

ished at the end of crop rotation cycle. NT plays an important role in this process by

promoting erosion control, increasing nutrient cycling, and reducing nutrient losses

by leaching and runoff, and allowing soil fertility buildup.

4. MAIN N FERTILIZER SOURCES

In Southern Brazil and Paraguay, most farmers use dry N fertilizer sources in

grain production systems. The use of liquid N fertilizer is still very limited. The

most commonly used dry N fertilizer for topdressing is urea, mainly due to com-

mercial (price and availability) and traditional reasons (farmers are familiar with

this N source and farm machinery availability). Other important N fertilizer sources

used are ammonium sulfate and ammonium nitrate. Composed fertilizer as DAP

and mono-ammonium phosphate (MAP) are mainly used at pre-plant stage espe-

cially in center pivot irrigation farms. NT corn farmers are increasing the amount of

starter N (25–30 kg/ha) at pre-planting using N-enriched formulas and also antici-

pating N topdressing. Typical rates of mineral N fertilization in commercial farms

are 60–120 kg/ha of N in corn and 30–60 kg/ha of N in wheat. When corn is cropped

under center pivot irrigation, the N fertilization goes up to 180 kg/ha of N.

Percentage of total

Brazilian fertilizer consumption

1990 1995 2000 2005

Years

Soybean

Maize

Rice

Wheat

Figure 5 . Relative of main grain crops in Brazilian fertilizer consumption. Adapted

from Diaz-Zorita et al. (2006) .

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Nitrogen Management in Field Crops of the Southern Cone of Latin America 133

The smallholder farmers from Southern Brazil and Paraguay because of limited

income, look for organic and green manure as alternative N sources, combined with

low rate of mineral N fertilization. The Southern Brazil has significant availability

of poultry and swine organic fertilizers. Typical rates of mineral N for smallholder

farmers are 0–50 kg N/ha in corn.

5. NITROGEN MANAGEMENT UNDER NT IN PARAGUAY AND

SOUTHERN BRAZIL

Nitrogen is the nutrient that most frequently limits the nonlegumes crop yields in

Brazil and Paraguay because, in general, the soil N supply is not enough for achieving

high yields, and as a consequence, the supplement of organic or mineral N fertilizers

sources are required. Biological N fixation process plays one important role in South

America agrosystems, mainly for soybean, legume cover crops and black beans.

As a consequence of an expected higher N immobilization, during the first

years of NT adoption, the early application of N fertilizers is recommended for the

production of corn ( Sá, 1999 ). Nitrogen fertilization at pre-planting interacts with

weather conditions and soil texture that should be considered to avoid N leaching.

In wet seasons and in sandy soils, the traditional strategy for corn fertilization is to

apply one-third of the N rate at planting and two-third topdressing at the V4–V6

growing corn stage, split in two fertilizations. High N volatilization loss from

broadcasted urea applications are described under the warm and dry conditions pre-

vailing in the Cerrado region – Central Brazil ( Cabezas et al., 1997 ). However, in

Southern Brazil, under wet and cooler conditions, this process apparently is less

significant achieving around 5% ( Wiethölter, 2002 ), and it also has been observed

that mixing the dry N fertilizer into 5 cm deep soil layer or applying the fertilizer

after a light rainfall sharply reduced the loss of N through volatilization.

The long-term use of legume cover crops increases the total soil N stock and

as a consequence, the soil N availability ( Amado and Mielniczuk, 1999 ). The mix

of cover crop species with different C/N ratios would be an efficient strategy to

provide both nutrient cycling and soil protection. Amado et al. (2002) developed

a N fertilization recommendation system for NT corn based on the consideration

of SOM content, corn yield goal and aboveground residue quantity and quality of

previous cover crop. Based in this cropping system approach, when legume cover

crops were used before corn, it is possible to reduce approximately 50% of the min-

eral N fertilizer rate requirement without yield reduction. The interaction between

previous cover crop and corn N fertilizer rate is shown in Table 4 . Wiethölter (2002)

showed that wheat N fertilization requirement can be reduced approximately by

20 kg/ha of N, when the previous crop is soybean rather than corn.

Means followed by the same capital letter across columns or same small letter

down rows are not significantly different using Tukey test at P  0.05.

The increase in N fertilizer efficiency under NT is still a challenge, because the

N management is more complex than under tillage systems. It is mainly because

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134 Nitrogen in the Environment

the large amount of residue left on soil surface increases the processes of immobi-

lization and mineralization, increases the potential N volatilization lost from broad-

casted urea under dry and warm environments and increases the N requirement to

build up SOM levels under NT.

As crop rotation is essential in tropical and subtropical environment to break

the cycle of pests and diseases that could increase under NT, it is also possible to

select cover crops to improve the input of symbiotic N

fixation from the atmos-

phere or that improves capturing and recycling leacheable nitrate and improve the

N balance. However, one of the most common immediate N effects of NT is the

potential of the residue cover to restrict N availability. Residues with a large C:N

ratio, such as black oat ( Avena strigosa Schreb), wheat, maize, sorghum, and rye-

grass pasture ( Lolium multiflorum Lam) induce high N immobilization in soil sur-

face strata during residue decomposition process. Nevertheless, the magnitude of

this effect is dependent on residue quantity and quality, as well as the mineral status

of the soil ( Bollinger et al., 2006 ). Sá (1999) suggests that in Southern Brazil, the

N immobilization process is more intense during the first 5 years of NT, after that,

it gradually diminishes due to the restoration of SOM at 0–5 cm depth. Since NT

especially increases the particulate SOM stock, which is strongly correlated to

potentially mineralizable N, thus soil N availability increases over time decreasing

the N requirement (Sá et al., 2001; Amado et al., 2006 ).

Legume cover crops previous to corn can decrease N requirement by about

40–90 kg/ha ( Calegari, 1995 ; Sá, 1999 ; Amado et al., 2000 ; Acosta, 2005 ). Amado

et al. (1999) found that the apparent equivalent mineral N fertilizer of common vetch

( Vicia sativa (L.) Walp.) was 55 kg/ha and the consortium black oat  common vetch

was 38 kg/ha ( Figure 6 ). In tropical and subtropical Brazil and Paraguay, legume

residues left on the soil surface decompose rapidly and provide a prompt N release,

sometimes so fast that it causes asynchronies with corn demand ( Giacomini, 2001 ;

Vinther, 2004 ; Acosta, 2005 ). Common vetch residue left on soil surface in Santa

Table 4.

Corn yields under long term NT for two cropping systems and two N fertilizer

rates. Eldorado do Sul, Rio Grande do Sul state, Brazil.

Cropping systems Grain yields (Mg/ha)

Winter cover crops Summer 0 kg/ha N 120 kg/ha N

Black oat Corn A 2.0 a B 7.1 a

Black oat  common vetch Corn  cowpea B 6.6 b B 7.6 a

Black oat  clover Corn B 5.4 b B 7.0 a

Corn  pigeon pea B 5.4 b B 7.2 a

Fallow Maize A 1.1 a B 6.5 a

Source : Adapted from Burle et al. (1997) .

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Nitrogen Management in Field Crops of the Southern Cone of Latin America 135

Maria (Rio Grande do Sul), for example, released 60 kg/ha of N in only 2 weeks

after management with knife roller ( Acosta, 2005 ). Acosta (2005) found that N

recovery by corn under such conditions is not more than 25% N derived from com-

mon vetch residues. Acosta (2005) , using N

, found that common vetch symbioti-

cally fixed 50–90% of its N requirement in a NT experiment carried out in Santa

Maria. Sisti et al. (2001) found that legume cover crops under NT system symbioti-

cally fixed a higher proportion of their N requirements compared to legumes grown

under plowed soil, which is presumably a consequence of the lower rates of soil N

mineralization under NT.

Under NT corn production in Southern Brazil, variations in traditional mineral

N fertilization strategies have been tested in last years. The use of part corn N ferti-

lization in the black oat cover crop, during the winter, had a positive effect in terms

of increasing black oat residue quantity and quality (lower C:N ratio), but this in

turn had a fairly limited effect on N supply to the following corn ( Figure 7 ) ( Amado

et al., 2003 ). Similar to this strategy, the N fertilization in wheat (winter cash crop)

can increase the following soybean yield, probably due to increased residue input

and soil moisture conservation. Results in Table 5 also suggest that there was a

positive sulfur residual effect on soybean yield. In Paraguay, wheat had a positive

response to residual corn N fertilization. Wendling et al. (2007) found as average

Corn yield (Mg/ha)

090180

N fertilizer rate (kg / ha)

Cropping system

Black oat/corn Black oatcommon vetch/corn

Common vetch/vetch

Figure 6 . Nitrogen fertilizer (urea) effect on corn yield in different cropping sys-

tems in Rio Grande do Sul, state. ( Amado et al., 1999) .

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136 Nitrogen in the Environment

of five experiments, wheat yields of 1,800 kg/ha with 0 kg/ha of N in previous corn

and 2,300 kg/ha with 213 kg/ha of N in previous corn.

The use of total rate of corn mineral N fertilization at cover crop termination

(approximately 15 days before seeding the corn) or at corn seeding time rather than

apply in topdressing has been evaluated in Southern Brazil under NT systems. In this

Corn N uptake (kg/ha)

0 40 80 120 160 200 240

Previous black oat N fertilization (kg / ha)

1999 y31.9 0.160.x r

0.92

1998 y25.9 0.086.x r

0.91

Figure 7 . Effect of N fertilizer applied in black oat (cover crop) in corn N uptake.

( Amado et al., 2003) .

Table 5.

Effect of wheat N fertilization in soybean yield.

N fertilization in wheat Rate kg/ha

Soybean yield

kg/ha Relative (%)

Check plot 0 2,706 100

Urea 50 3,144 116

Urea 100 3,187 118

Ammonium sulphate 50 3,904 130

Ammonium sulphate 100 4,360 151

Source : Adapted of Vitti and Trevisan (2000) .

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Nitrogen Management in Field Crops of the Southern Cone of Latin America 137

case, the assumption is that the residue mulch decomposition will temporarily trap

added N and thereby partially prevent leaching losses of N, as this eliminates the

need for an additional field operation, especially when corn is planted with narrow

rows ( Bollinger et al., 2006 ). However, in terms of corn yields, this strategy only

was efficient in years without heavy rainfall during corn growth ( Sá, 1999 ; Basso

and Ceretta, 2000 ; Pottker and Wiethölter, 2002 ). In years with high rainfall the tra-

ditional strategy of applying the higher amount of corn N fertilization at top dress-

ing at V4–V6 was more efficient ( Ceretta et al., 2002 ). Applications of N at late

corn growth stage have been evaluated with promising results, although farm oper-

ation issues need to be addressed. In center pivot irrigation systems, some farmers

are using N fertigation with the objective to increase N use efficiency (NUE).

6. DIAGNOSIS OF N NEEDS AND FERTILIZER RECOMMENDATION

FOR MAIN CROPS IN SOUTHERN BRAZIL AND PARAGUAY

Currently, a research NT program for the adequate diagnosis and recommen-

dation of fertilization practices in grain crops is under development in the main

Paraguay agroecologic zones supported by CAPECO, a soybean farmers associa-

tion ( Cubilla, 2005 ). It is a net experiment carried out in four departments (states)

of Paraguay (Alto Parana, Itapúa, Amambay, and Missiones), comprising the more

important agriculture soils (Oxisols, Inceptisols, and Ultisols). This research is

coordinated by Federal University of Santa Maria (Brazil) with the cooperation of

Paraguay agriculture cooperatives, universities, and agriculture research institu-

tion (Agriculture Ministry). Five rates of N fertilization were used in wheat after

corn and soybean, and also the residual effect of N applied on the previous corn

was evaluated. Wheat presented a positive economical response to the rate of

35 kg/ha of N after soybean which yields around 3,100 kg/ha ( Figure 8 ). After corn,

wheat responded economically up to 30 kg/ha of N, reaching yields of 2,100 kg/ha

( Wendling et al., 2007 ). In this research, besides the N effect, it was possible to

check the crop rotation effect, since the wheat yield after soybean was around

1,000 kg/ha higher than after corn. Soils with a content of SOM matter higher than

4% had the capacity of supplying sufficient quantity of N, without N fertilizer, to

achieve a wheat yield up 2,500 kg/ha.

The corn response to mineral N fertilization also was evaluated in this research.

The main results are showed in Table 6 . In 2004–2005, there was a regional sum-

mer drought that influenced corn response to N fertilization. Itapúa 2 was the

department with the most severe drought effect, and as a consequence, corn yield

was very low and without economic response to N fertilization. On the other hand,

in Naranjal (Alto Parana department), where the high-yielding farms are located

in Paraguay, the highest yield was reached even without N fertilization. It is not

a usual result, but it should be stressed that this soil has high organic matter con-

tent and it is under NT for a long time. The other places (Missiones and Itapúa 1)

showed a typical corn response to N fertilization.

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138 Nitrogen in the Environment

1000

2000

3000

4000

Wheat yield (kg / ha)

0 30 60 90 120

Nitrogen fertilizer (kg/ha)

Wheat/soybean y2.683  15.21N  0.1007N

0.97

Wheat/maize y1.828  13.87N  0.0982N

0.87

Figure 8 . Effect of N fertilizer in wheat yield under NT in Paraguay. ( Wendling

et al., 2007) .

Table 6.

Corn response to mineral N fertilization (urea) under NT in Paraguay.

Departments

in Paraguay Equation adjusted r

Mineral N fertilizer (kg/ha)

Highest corn

yield

Maximum

economic

efficiency

Missiones 1 Y  3,126  35.6N  0.13N

0.89 5,650 95

Missiones 2 Y  1,986  39.5N  0.10N

0.98 5,739 135

Itapúa 1 Y  1,427  54.4N  0.12N

1.00 7,497 178

Itapúa 2 Y  2,075  9.2N  0.02N

0.85 3,137 0

Alto Parana Y  8,831  5.0N 0.94 10,033 0

Source : Adapted from Wendling (2005).

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Nitrogen Management in Field Crops of the Southern Cone of Latin America 139

7. IMPROVEMENT IN DIAGNOSIS OF N NEEDS

The most common strategies for diagnosis of N evaluate SOM content and

plant analysis. Yield records are also important to define the goal yield and, as a

consequence, the N requirement. The advance of precision farm in Brazil is allowing

improved yield records basis. The mineral N soil available is only used for research

purposes and it is strongly influenced by the heavy summer rainfall. The equations

comprising balance of N have been investigated ( Amado et al., 1999 ; Wiethölter,

2003), but until now its farm use is very restricted. Most of wheat and corn N ferti-

lizer recommendations developed in Southern Brazil and Paraguay take in account

the cropping system and the previous cover crop discussed as earlier.

New strategies of diagnosis of N have been evaluated recently. The chlorophyll

meter has been calibrated in research plots for corn and wheat. The critical levels

proposed by Argenta et al. (2001) are shown in Figure 9 . The critical level increases

with the corn growing stage. To decrease the variability induced by genotype materi-

als, soil, and climate, the results should be compared with nonlimited N plots.

Chlorophyll meter reading (SPAD)

Days after emergence corn development stages

3–4 leaves 6–7 leaves 10–11 leaves Silking

45.4

52.1

55.3

58.1

21 35 53 67

Y  33.928  0.654x  0.00446x

0.98*

Figure 9 . Critical levels of chlorophyll meter readings (Minolta SPAD 502 ® ) for corn

in Rio Grande do Sul state, Brazil. (Argenta (2001) cited by Silva and Rambo, 2004) .

Acosta (2007), in his doctorate program at the Federal University of Santa

Maria, is evaluating new strategies of N diagnosis as chlorophyll meter and normal-

ized difference vegetation index (NDVI) compared to SOM and yield record maps

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140 Nitrogen in the Environment

in corn under center pivot irrigation. In this experiment long strip treatments have

been used (20  500 m) and variable N fertilizer rate is used along the strip fol-

lowing the N status. All the treatments evaluated had the same total amount of N

applied, 180 kg/ha, except the check plot (0 kg/ha). The variable N rate at topdress-

ing was determined using the criteria showed in Table 7 . When the subarea of the

strip had high N status, part of N fertilizer is reallocated to low N status subareas.

When the subarea of the strip had medium N status the flat rate of N (180 kg/ha) is

used. One reference strip had uniform flat N rate fertilization along the strip. The

topdressing N was split in two fertilization.

The results of this experiment show that it is possible to increase the corn N

uptake efficiency by improving the N diagnosis using new strategies ( Table 8 ).

Table 7.

Parameters used for N diagnosis and variable N rate fertilization. Rio Grande do

Sul state, Brazil.

Parameters Low Medium High

Chlorophyll meter reading 1st  42.0 42.0–43.2  43.2

(SPAD) 2nd  57.1 57.1–58.0  58.0

Soil organic matter (%)

 3.6 3.6–4.2  4.2

Record of yields (% average of yields

from previous years)

 95 95–105  105

NDVI   0.44 –0.44–0.38  –0.38

Source : Acosta (2007) (data not published).

Ranges determined basis on the cropland variation.

Table 8.

N diagnosis and variable N fertilization effect on corn nitrogen uptake efficiency.

Panambi, Rio Grande do Sul State, Brazil.

Treatments

Dry mass

(Mg/ha)

N concentration

(%)

N uptake

(kg/ha)

N uptake

efficiency (%)

Chlorophyll meter

reading (SPAD)

13.4 1.48 198.7 62

Soil organic matter 13.3 1.44 191.6 61

Record of yields 11.9 1.40 164.2 45

NDVI 11.8 1.42 167.7 45

Uniform rate 9.8 1.47 144.4 30

Check plot 9.0 1.03 90.8 –

Source : Acosta (2007) (data not published).

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Nitrogen Management in Field Crops of the Southern Cone of Latin America 141

8. FINAL COMMENTS

The Southern Brazil and Paraguay region has increased soybean, corn, and wheat

yields due to improvement in crop and soil management and higher fertilizer con-

sumption. NT has been adopted in more than 80% of the agriculture land of these

regions and plays one important role in this yield improvement. Nitrogen is the nutri-

ent that most frequently limits the corn and wheat yields. However, the rates of min-

eral N fertilization used are still low compared to other Southern Cone countries. In

addition, the NUE is lower than 50%. The high amount of residues left on soil surface

under the NT system increases the complexity of N dynamic in the agro-ecosystem.

Improvement in N diagnosis and N recommendations are necessary to increase NUE

and, as a consequence, the crop yields. Combining different strategies of N supply

such as legume cover crops, crop residue N, native soil N, and mineral fertilizer N is

necessary to supply the high demand of N to support competitive yields in a scenario

of limited capital, prevalent in Brazil and Paraguay. The improvement in N manage-

ment under NT is also a demand for environment protection.

REFERENCES

Acosta, J.A.A. (2005). Improving the fertilizer recommendations for nitrogen in maize,

adapted for use in the crop production systems of conservation agriculture. Alban

Programme Final Report, Copenhagen, Denmark, 75p.

Acosta, J. A. 2007. Dinâmica do nitrogênio em sistema plantio direto e parâmetros para o

manejo da adubação nitrogenada no milho. Exame de qualificação. Programa de Pós-

Graduação em Ciência do Solo, UFSM. 84p. (not published).

Amado , T.J.C. and J. Mielniczuk . 1999 . Plantio direto e rotação de culturas com legumino-

sas: uma excelente combinação para promover o incremento da capacidade produtiva do

solo . R. Pl. Direto 50 : 23 – 27 .

Amado , T.J.C. , J. Mielniczuk , S.B.V. Fernandes , and C. Bayer . 1999 . Culturas de cobertura,

acúmulo de nitrogênio total no solo e produtividade de milho (in Portuguese, with English

abstract) . R. bras. Ci. Solo 23 : 679 – 686 .

Amado , T.J.C. , J. Mielniczuk , and S.B. Fernandez . 2000 . Leguminosas e adubação mineral

como fontes de nitrogênio para milho em sistemas de preparo do solo (in Portuguese,

with English abstract) . R. Bras. Ci. Solo 24 : 179 – 189 .

Amado , T.J.C. , J. Mielniczuk , and C. Aita . 2002 . Recomendação de adubação nitrogenada

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direto (in Portuguese, with English abstract) . R. bras. Ci. Solo 26 : 241 – 248 .

Amado , T.J.C. , A. Santi , and J.A.A. Acosta . 2003 . Adubação nitrogenada na aveia preta. II

Influência na decomposição de resíduos, liberação de nitrogênio e rendimento de milho

sob sistema plantio direto (in Portuguese, with English abstract) . R. Bras. Ci. Solo 27 :

239 – 251 .

Amado , T.J.C. , C. Bayer , P.C. Conceição , E. Spagnollo , B.C. Campos , and M. V e i g a . 2 0 0 6 .

Potential of carbon accumulation in zero tillage soils with intensive use and cover crops

in Southern Brazil . J. Environ. Qual. 35 : 1599 – 1607 .

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