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Nitrogen Fixation in Pasture Systems

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Biological nitrogen fixation plays an essential role in the improvement of agricultural sustainability, particularly with regard to the contribution of pasture legumes. In fact, pasture legumes are among the most efficient leguminous plants in terms of nitrogen fixation and, depending on adequate management and on the establishment of effective symbioses with adequate rhizobia, they may contribute with high input rates of fixed-nitrogen into the soil (Materon 1988). A successful example of the BNF contribution in pastures is given by a particular agrosilvopastoral system in the Mediterranean area of Southern Iberian Peninsula. This system,which is designated “montado” in Portugal or “dehesa” in Spain, represents the most extended agroforestry system in Europe, covering more than 3.5 million hectares over the west, south-west and central parts of the Iberian Peninsula (Olea and San Miguel-Ayanz 2006; Trujillo and Mata 2001). The “montado” has been developed for a long time on poor or non-agricultural land, based on extensive livestock production associated with the exploitation of cork and holm oaks. Both natural and sown pastures are implemented among scattered oak trees and support the direct grazing by cattle and sheep. The Mediterranean climate of the “montado” is characterized by hot dry summers, usually lasting for several months, and cool winters with irregular, often scarce rainfall; the same type of climate is found at middle latitudes in all continents, including large areas of West Asia, Australia and North Africa (Saxena 1988). Desertification is a common situation, particularly in regions where the precipitation regime is more inconsistent, resulting in progressive degradation of the vegetation cover and erosion of surface soil. As a consequence, soils in “montado” are generally poor, deficient in phosphorus and calcium, and contain low levels of organic matter, making arable and intensive farming unsustainable. By using a strategy founded on the efficiency and diversification of structures, the “montado” represents an extremely rational form of land use in these environments, taking advantage of every natural resource with minimum inputs of energy and materials (Joffre et al. 1988; Olea and San Miguel-Ayanz 2006). When properly implemented, this multifunctional and versatile system ensures the optimization of available energetic resources through biomass production, circulation of nutrients, conservation of soil and water, and preservation of biodiversity, also contributing to climate bio-regulation or microclimate stability (Trujillo and Mata 2001). Pastures are an essential component of the “montado”, as main source of fodder for livestock. However, due to low soil fertility associated with a diverse but little productive native flora, natural pastures in the Portuguese “montado” are mostly poor and managed with low animal stocking rates. A recent study investigated the role of biological nitrogen fixation on a range of long term natural pastures in the “montado” ecosystem of Southern Portugal, covering different edaphoclimatic environments (Ferreira and Castro 2011). Legume yields and biological nitrogen fixation in field conditions were evaluated in 36 sites, using the isotopic 15N-dilution technique to access the amount and percentage of nitrogen derived from biological fixation. Although the amounts of fixed nitrogen were highly variable among sites, the results showed that nitrogen fixation was closely linked to the legume biomass production (Table 2). On average, nitrogen fixation contributed with 25 kg of nitrogen per 1000 kg of shoot dry biomass, a value that is similar to other field measurements undertaken at Mediterranean-type environments in Australia (Baldock and Ballard 2004; Peoples et al. 2001). The percentages of nitrogen derived from BFN were generally high (87-89%) and similar among sites, despite the large diversity of native legumes and the differences in edaphoclimatic conditions. It was concluded that biological nitrogen fixation in these natural ecosystems provides almost all the nitrogen present in legumes, indicating that the natural symbioses are well adapted to these environments. Nevertheless, the study also confirmed that the legume productivity in these natural pastures is very low, as the result of poor natural flora. In this context, the introduction of improved legumes with higher yield potential and previously inoculated with specific and highly effective rhizobia strains represents an efficient way of increasing productivity.

A model for improving pastures in the “montado” started to be developed in Portugal in the late 1960s and has been largely diffused since then, spreading throughout similar Mediterranean environments in southern Europe. The strategy is based on the establishment of biodiverse permanent pastures rich in legumes, by sowing a diversity of selected and improved species, in which inoculated legumes are preponderant. These biodiverse legume-rich mixtures provide better productivity than the natural flora and are able to renew themselves on a permanent basis (Crespo 2006). At least 30% of the sown mixtures are made up of hard seed legumes, including a range of annual clovers and annual medics, yellow serradella and biserrula. Inoculation of the legume seeds with specific and effective rhizobia ensures enhanced symbiotic nitrogen fixation in the pasture. This approach has demonstrated marked improvements on soil fertility and rapid build-up soil organic matter through carbon sequestration, offering superior pasture productivity and animal carrying capacity. Nowadays, it is considered as a powerful management tool for improving pastures yield in the “montado” ecosystem, in which legumes are important components of the strategy for increasing productivity and sustainability, using symbiotic nitrogen fixation as a major process of providing nitrogen to the soils.

Table 2. Legume yields and amounts of fixed nitrogen in long term natural pastures in the “montado” ecosystem of Southern Portugal.

Location

Soil origin/ site number

Legume shoot yield

Amount of fixed-N2

Location

Soil origin/ site number

Legume shoot yield

Amount of fixed-N2

   

(kg/ha)

(kg/ha)

   

(kg/ha)

(kg/ha)

Castro Verde

Schist 1

42

1.0

Portalegre

Granite 16

732

21.9

Castro Verde

Schist 2

724

16.6

Portalegre

Granite 17

100

2.7

Castro Verde

Schist 3

720

24.1

Portalegre

Granite 18

772

17.3

Castro Verde

Schist 4

107

3.1

Portalegre

Granite 19

71

1.6

Castro Verde

Schist 5

211

6.4

Portalegre

Granite 20

296

6.9

Castro Verde

Schist 6

14

0.4

Portalegre

Granite 21

919

20.4

Ourique

Schist 7

0

0.0

Monforte

Granite 22

178

5.3

Ourique

Schist 8

7

0.1

Monforte

Granite 23

712

22.5

Ourique

Schist 9

139

2.6

Monforte

Granite 24

302

6.4

Serpa

Schist 10

1346

28.6

Crato

Granite 30

42

1.2

Serpa

Schist 11

319

6.8

Crato

Granite 30A

125

2.7

Serpa

Schist 12

548

16.4

Alter

Granite 31

90

2.5

Sousel

Schist 25

6

0.1

Alter

Granite 32

30

0.8

Sousel

Schist 27

88

2.5

Alter

Granite 33

68

1.8

Crato

Schist 28

42

1.2

Crato

Granite 34

561

12.8

Crato

Schist 29

125

2.7

Crato

Granite 35

1003

26.4

Nisa

Schist 37

281

7.7

Crato

Granite 36

1632

33.5

Nisa

Schist 38

252

6.7

       

Nisa

Schist 39

242

6.6

       

Mean

 

274

7

   

449

11

Location

Soil of origin

Legume shoot yield

Amount of fixed-N

   

(kg/ha)

(kg/ha)

Castro Verde

Schist 1

42

1.0

Castro Verde

Schist 2

724

16.6

Castro Verde

Schist 3

720

24.1

Castro Verde

Schist 4

107

3.1

Castro Verde

Schist 5

211

6.4

Castro Verde

Schist 6

14

0.4

Ourique

Schist 7

0

0.0

Ourique

Schist 8

7

0.1

Ourique

Schist 9

139

2.6

Serpa

Schist 10

1346

28.6

Serpa

Schist 11

319

6.8

Serpa

Schist 12

548

16.4

Portalegre

Granite 16

732

21.9

Portalegre

Granite 17

100

2.7

Portalegre

Granite 18

772

17.3

Portalegre

Granite 19

71

1.6

Portalegre

Granite 20

296

6.9

Portalegre

Granite 21

919

20.4

Monforte

Granite 22

178

5.3

Monforte

Granite 23

712

22.5

Monforte

Granite 24

302

6.4

Sousel

Schist 25

6

0.1

Sousel

Schist 27

88

2.5

Crato

Schist 28

42

1.2

Crato

Schist 29

125

2.7

Crato

Granite 30

42

1.2

Crato

Granite 30A

125

2.7

Alter

Granite 31

90

2.5

Alter

Granite 32

30

0.8

Alter

Granite 33

68

1.8

Crato

Granite 34

561

12.8

Crato

Granite 35

1003

26.4

Crato

Granite 36

1632

33.5

Nisa

Schist 37

281

7.7

Nisa

Schist 38

252

6.7

Nisa

Schist 39

242

6.6

Average

 

353

8.9

Location

Soil of origin

Legume shoot yield

Amount of fixed-N

Location

Soil origin/ site number

Legume shoot yield

Amount of fixed-N2

   

(kg/ha)

(kg/ha)

   

(kg/ha)

(kg/ha)

Castro Verde

Schist 1

42

1.0

Monforte

Granite 22

178

5.3

Castro Verde

Schist 2

724

16.6

Monforte

Granite 23

712

22.5

Castro Verde

Schist 3

720

24.1

Monforte

Granite 24

302

6.4

Castro Verde

Schist 4

107

3.1

Sousel

Schist 25

6

0.1

Castro Verde

Schist 5

211

6.4

Sousel

Schist 27

88

2.5

Castro Verde

Schist 6

14

0.4

Crato

Schist 28

42

1.2

Ourique

Schist 7

0

0.0

Crato

Schist 29

125

2.7

Ourique

Schist 8

7

0.1

Crato

Granite 30

42

1.2

Ourique

Schist 9

139

2.6

Crato

Granite 30A

125

2.7

Serpa

Schist 10

1346

28.6

Alter

Granite 31

90

2.5

Serpa

Schist 11

319

6.8

Alter

Granite 32

30

0.8

Serpa

Schist 12

548

16.4

Alter

Granite 33

68

1.8

Portalegre

Granite 16

732

21.9

Crato

Granite 34

561

12.8

Portalegre

Granite 17

100

2.7

Crato

Granite 35

1003

26.4

Portalegre

Granite 18

772

17.3

Crato

Granite 36

1632

33.5

Portalegre

Granite 19

71

1.6

Nisa

Schist 37

281

7.7

Portalegre

Granite 20

296

6.9

Nisa

Schist 38

252

6.7

Portalegre

Granite 21

919

20.4

Nisa

Schist 39

242

6.6

6. CONCLUDING REMARKS

The dependency of agriculture on nitrogen fertilizer inputs and the associated environmental costs, underscore the importance of biological nitrogen fixation by rhizobia in symbiotic association with legumes, mainly due to the advantage of being environmental friendly and ideal for sustainable agriculture. Several issues in this matter are important for understanding research on BNF in the present and in the future. However a concerted effort should be done to put knowledge into practical application.

Definitely, understanding the ecology of different rhizobial groups will further enhance the knowledge of rhizobia and will help to predict the environmental responses of rhizobial groups, bringing a more practical meaning to rhizobial classification and diversity.

Also, soil populations of rhizobia are genetically diverse and could represent a pool of traits (such as plant growth-promoting activity, tolerance of soil acidity or salinity, or the ability to degrade pollutants) that can be readily exploited via selection or genetic manipulation for optimizing legume crop productivity.

Another important issue concerns the legume breeding programs, which must give greater emphasis to the symbiosis between host and rhizobia. Practical approaches to enhanced nitrogen fixation and improved tolerance to edaphic constraints would also permit lower costs and a more sustainable form of agriculture. Despite many decades of progress and the acquisition of a large amount of information, the physiological and molecular bases for the tolerance of legume rhizobia symbiotic systems to environmental stress remains largely unknown.

Finally, a better understanding of the diversity and dynamics of soil populations of rhizobia and how they affect the establishment of inoculant strains in nodules of the host plant is also required for the development of highly competitive commercial strains. The impact of agricultural practices, including the introduction of inoculants, on rhizobial diversity is little understood and there is a need for research in this area. In conclusion, the combination of a biotechnological approach (microbial inoculation) with a low-input technology could be a sustainable practice to facilitate the nutrient supply to plants.


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