Legume plant physiology

2.1.3. Factors limiting biological nitrogen fixation
The process of N2 fixation depends on the physiological state of the host plant. In this respect, it is important to place emphasis on the fact that the natural environmental stresses the legume is exposed to nodules. As a result, their symbiotic partner (Rhizobium) may include photosynthate deprivation, water stress, salinity, soil nitrate, temperature, heavy metals, and biocides. This stress may multiple effects, including such effects as salinity, which may act as a water stress, which affects the photosynthetic rate, or may affect nodule metabolism directly. At the same time, there are extremely challenging environment, which are rhizobia. Rhizobia are marginal lands with low rainfall, extremes of temperature, acidic soils of low nutrient status, and poor water-holding capacity. Populations of Rhizobium and Bradyrhizobium species have different tolerance to major environmental factors influencing their development and growth. In such a situation, screening for tolerant strains is needed. Biological processes (e.g., N2 fixation) leading to the improvement of agricultural productivity while minimizing soil loss and ameliorating adverse edaphic conditions are essential (Zahran, 1999).

N and P cycling
In the course of the growth of microorganisms, the removal of NH4+ and NO3- from the soil’s inorganic, available-nitrogen pool occurs. As a result, nitrogen pools converts to organic nitrogen. This process is called immobilization. After the death of these organisms and their decomposition by others organisms and environmental factors, excess NH4 + can be released back to the inorganic pool in the course of mineralization.

Specialists distinguish four significant loss mechanisms, including leaching, denitrification, volatilization, and crop removal.
Leaching is the process in the course of which nitrogen accumulated in the soils is removed under the impact of water. Nitrates, nitrites and ammonia are all water soluble meaning. As a result, they dissolve fast and easily into water (Mosier et al., 1998).

Another important loss mechanism is denitrification, in the course of which some bacteria called Pseudomonas uses nitrate as an electron acceptor instead of the oxygen during respiration. In such a way, they use nitrate, inhaling it and exhaing nitrogen gas back into the atmosphere ( Ward, 1996). In the course of volatilization, Ammonium ions are basically anhydrous ammonia (NH3) molecules with an extra hydrogen (H+) attached. When the extra H+ parts from the NH4 ion under the impact of another ion such as hydroxyl (OH-), NH3 molecule is formed. This molecule can evaporate from the soil. Crop removal leads to the loss of nitrogen because nitrogen in the harvested crop plants is removed from the field completely. The nitrogen contained in crop is further recycled back into the system and specialists argue that it passes through the stage of immobilization rather than removal a considerable part of nitrogen is mineralized that means that this nitrogen may be later reutilized by a crop.

Phosphorus is a part of DNA-molecules, which store energy (ATP and ADP) . Phosphorus is also a part of molecules, which form blocks of certain parts of the human and animal body, such as the bones and teeth. At normal natural conditions, temperatures and pressures, phosphorus has a liquid form. Phosphorus is scarce in soil and often it limits the plants’ growth. Hence, people often apply phosphate fertilizers in farming. Phosphorus cycles through plants and animals much faster than it does through rocks and sediments. After the death of animals and plants, phosphates return back to the soils or oceans in the course of decay. After that, phosphorus ends up in sediments or rock formations again. Phosphorous may remain there for millions of years. Eventually, phosphorus is released again through weathering and the cycle starts over and over again.


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