**2. Soil microbial nitrogen (N) transformations**

Soil microorganisms, including bacteria, fungi, and protozoa, are responsible for the decomposi‐ tion of cultural residues such as leaves, stems, and roots, which release significant elements for plant nutrition from organic residues to the mineral phase absorbed by plants. The transfor‐ mation of nitrogen (N), sulfur (S), and phosphorus (P) is called nutrient cycling. Some simpler composition residues presenting higher concentration of N and P can be easily decomposed. According to Ref. [9], it is related to the chemical composition of the residues, being facilitated by the low ratios of C/N, C/P, lignin/N, polyphenols/N, and (lignin + polyphenols)/N, and difficult because of high levels of lignin and polyphenols.

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

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

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].

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‐

Soil microorganisms, including bacteria, fungi, and protozoa, are responsible for the decomposi‐ tion of cultural residues such as leaves, stems, and roots, which release significant elements for plant nutrition from organic residues to the mineral phase absorbed by plants. The transfor‐ mation of nitrogen (N), sulfur (S), and phosphorus (P) is called nutrient cycling. Some simpler composition residues presenting higher concentration of N and P can be easily decomposed. According to Ref. [9], it is related to the chemical composition of the residues, being facilitated by

cidate the main forms of interaction between grasses and soil microorganisms.

This contribution favors the economics of mineral fertilizers.

**2. Soil microbial nitrogen (N) transformations**

the microorganism of interest.

14 Grasses - Benefits, Diversities and Functional Roles

field conditions.

Plants can absorb N either as Ammonium (NH<sup>4</sup> + ) or Nitrate (NO<sup>3</sup> − ). In order to achieve that, 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 soil's microorganisms.

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 plant residues that plants can absorb as NH<sup>4</sup> + . The legume tissue generally presents a C/N 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 in succession to legumes [11].

Under good drainage conditions, less oxidized forms of N present in the soil, such as ammo‐ nium (NH<sup>4</sup> + ) and ammonia (NH<sup>3</sup> + ), are transformed into more oxidized forms. Nitrifying bac‐ teria of the genera *Nitrosomonas* sp. transform N into volatile nitrite (NO<sup>2</sup> −). Fortunately, under the same environmental conditions, *Nitrobacter* sp. transforms volatile nitrite (NO<sup>2</sup> −) into nitrate (NO<sup>3</sup> −), which is stable and easily absorbed by grasses and other plant families [10].

Under flood conditions, when the supply of O<sup>2</sup> is absent in the soil, some microorganisms carry enzymes capable of consuming the oxygen from the NO<sup>3</sup> − present in their respiratory chain as an electron acceptor, transforming it into nitrous oxide (N<sup>2</sup> O) [12]. N<sup>2</sup> O and other vol‐ atile N compounds from microbial activity in poorly drained environments return to the atmo‐ sphere as gases. The dinitrogen gas (N<sup>2</sup> ) can be fixed in the soil through biological N fixation by diazotrophic bacteria. This subject will be discussed individually due to its great importance.
