**Analytical Interpretation of the Beneficial Interaction Between Microorganisms and Grasses Between Microorganisms and Grasses**

**Analytical Interpretation of the Beneficial Interaction** 

DOI: 10.5772/intechopen.69272

Rafael Goulart Machado Rafael Goulart Machado Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.69272

#### **Abstract**

Soil microorganisms compose ¼ of the biodiversity of our planet and are responsible for important processes such as the decomposition of organic residues and the transforma‐ tion of the nutrients contained in these residues into nutrients for plants. The micro‐ organisms also aid the grasses implantation, increasing the grasses yield by means of several mechanisms of plant growth promotion. These mechanisms of growth promo‐ tion of grasses can be direct, or indirect. In this chapter, we discuss the main mecha‐ nisms of growth promotion of grasses by soil microorganisms. It will be explained how the microorganisms in the soil act favoring the growth and development of cultivated grasses. For this, there will be clarified the importance of soil microorganisms in nutri‐ ent cycling, the mechanisms of nutrient capture, the production of phytostimulant sub‐ stances by microorganisms, and the mechanisms of soil pathogen suppression.

**Keywords:** nutrient cycling, biological nitrogen fixation, interaction between plants, microorganisms

## **1. Introduction**

The microbial population inhabiting the rhizosphere consists of a wide range of organisms, which together interact directly and indirectly with the cultivated plants. Only with regard to the number of bacteria, it is estimated that there are about 2 billion cells per gram of soil [1]. These microorganisms become interesting to the human species, as they interfere in the yield of the cultivated plants, by means of several mechanisms.

A microorganism is considered a plant growth promoter when it is capable of increasing the yield of the crops of interest. To measure this capacity, the interaction of a given microorganism with some plants of interest must first be evaluated under axenic conditions and in comparison

with cultivated plants. It is fundamental that this initial stage is studied in isolation of the interaction of the plant with the organism, thus isolating the interaction of other factors such as climate, environment, and other edaphic or epiedaphic macro or microorganisms to make sure that the effect on the yield of the plants of interest is solely and exclusively due to the inoculated microorganism. Without this initial screening under axenic conditions, it would be impossible to certify and prove that the positive effect observed in the studied plant is due to the microorganism of interest.

the low ratios of C/N, C/P, lignin/N, polyphenols/N, and (lignin + polyphenols)/N, and difficult

N must be transformed into a mineral nutrient so that plants can absorb it which depends on the C/N ratio of residue added to the soil. When the C/N ratio is greater than 30/1, the decomposition process is slower than usual, with accumulation of plant residues, as micro‐ organisms cannot easily degrade them. Since the microbial population of the soil lacks nutri‐ ents, it competes with plants for N, thus causing a temporary immobilization of N. The C/N ratio greater than 70/1 in grass straws makes the decomposition process more difficult to the

Conversely, when the C/N ratio of plant residues is less than 25/1, N is released [10], thus mineralizing this N present in the soil, which consists in the release of nutrients from the

+

ratio less than 20/1 during the flowering stage. Therefore, after being cut and incorporated into the soil, the legume tissue is a rich source of N to microorganisms which will transform it into a mineral nutrient contributing to the nutrition of grasses and other cultivated plants. As a consequence, part of the mineral N fertilizer can be suppressed in the cultivation of grasses

Under good drainage conditions, less oxidized forms of N present in the soil, such as ammo‐

atile N compounds from microbial activity in poorly drained environments return to the atmo‐

diazotrophic bacteria. This subject will be discussed individually due to its great importance.

Soil bacteria are capable of presenting beneficial effects on cultivated grasses. Several mech‐ anisms make bacteria to promote cultivated grasses, providing significant benefits to the

harmful to mankind. On the other hand, there are in the soil bacteria capable of transforming

) into nitrogen assimilable by plants (NH<sup>3</sup>

−), which is stable and easily absorbed by grasses and other plant families [10].

the same environmental conditions, *Nitrobacter* sp. transforms volatile nitrite (NO<sup>2</sup>

+

) or Nitrate (NO<sup>3</sup>

Analytical Interpretation of the Beneficial Interaction Between Microorganisms and Grasses

−

. The legume tissue generally presents a C/N

is absent in the soil, some microorganisms

O) [12]. N<sup>2</sup>

) can be fixed in the soil through biological N fixation by

+

), are transformed into more oxidized forms. Nitrifying bac‐

). In order to achieve that,

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http://dx.doi.org/10.5772/intechopen.69272

−). Fortunately, under

O and other vol‐

− present in their respiratory

. This gas is neither good nor

). The enzyme of N‐fixing

−) into

because of high levels of lignin and polyphenols.

Plants can absorb N either as Ammonium (NH<sup>4</sup>

plant residues that plants can absorb as NH<sup>4</sup>

) and ammonia (NH<sup>3</sup>

Under flood conditions, when the supply of O<sup>2</sup>

sphere as gases. The dinitrogen gas (N<sup>2</sup>

plants, mainly regarding nutritional aspects.

atmospheric nitrogen (N<sup>2</sup>

+

carry enzymes capable of consuming the oxygen from the NO<sup>3</sup>

chain as an electron acceptor, transforming it into nitrous oxide (N<sup>2</sup>

**3. Beneficial interaction between grasses and bacteria**

**3.1. Beneficial interaction between grasses and nitrogen fixing bacteria**

About 78% of the Earth's atmosphere gases are composed of N<sup>2</sup>

teria of the genera *Nitrosomonas* sp. transform N into volatile nitrite (NO<sup>2</sup>

soil's microorganisms.

in succession to legumes [11].

+

nium (NH<sup>4</sup>

nitrate (NO<sup>3</sup>

Only after the positive effect of the microorganism on the plant has been proven, this interac‐ tion will be tested under conditions of greater interference, such as greenhouse, fertilization, or soil conditions with an original field microbial population (nonsterile soil). Under these conditions, the resistance of the interaction to various interference factors will be tested. Once approved in tests conducted under controlled conditions, the microorganisms are tested under field conditions.

Several mechanisms are the mechanisms by which microorganisms act on the yield of plants and can act directly through the production of hormones [2] or nutrient supply, such as nitrogen [3], or indirectly by the suppression of pathogens [4]. The most well‐known mechanisms are bio‐ logical nitrogen fixation (BNF), where symbiotic or associative bacteria can capture atmospheric nitrogen under microaerobic conditions and through the enzyme nitrogenase, to convert it to forms assimilable by plants. Other mechanisms known are involved in the production of phy‐ tostimulatory substances, such as auxin group hormones [5], cytokinins [6], and gibberellins [7].

The constant selection and verification of the effect of plant growth promoting bacteria on spe‐ cies of agronomic interest is necessary for the indication of infective and efficient organisms in the composition of microbial inoculants. Thus, by means of periodic inoculations, it is possible to alter the diversity of the microbial populations interacting with the plants in the rhizosphere, favoring the infection of the roots by efficient and selected microorganisms. With respect to soybean cultivation, for example, in the Brazilian states producing this grain, the reinoculation of the crop induced positive results, compared to the nonreinoculated controls, and in some experiments, increases of up to 23% in yield and up to 25% in the N content of the grains [8]. This contribution favors the economics of mineral fertilizers.

In this chapter, we will discuss the interaction of grasses with soil microorganisms, explain how these microorganisms can benefit the growth and development of grasses, and also elu‐ cidate the main forms of interaction between grasses and soil microorganisms.
