Made by: Madiha Siddiqui

SOIL MICROBIOLOGY & NITROGEN CYCLE

Soil microbiology is the study of organisms in soil, their functions, and how they affect soil properties

Normal, fertile soils teem with soil microbes. In fact, there may be hundreds of millions to billions of microbes in a single gram . The most numerous microbes in soil are the bacteria (unicellular cells lacking a true nucleus) followed in decreasing numerical order by the actinomycetes, soil algae, cyanobacteria and soil protozoa (unicellular soil organisms that decompose organic materials as well as consume large numbers of bacteria).

Numbers of Microbes in Soil

Microbial Group No./Gram of Soil

Bacteria 100,000,000 - 1,000,000,000Fungi 100,000 - 1,000,000Algae and 1000 - 1,000,000Cyanobacteria Protozoa 1000 - 100,000

WHAT FACTORS CONTROL THE RATE OF GROWTH AND ACTIVITIES OF SOIL MICROBES ??

This is an excellent question because an understanding of what it takes to support the growth and activity of soil microbes enables one to make decisions about soil management. In general, microbes need what all living things need to prosper: air (oxygen), water, food and a suitable habitat to live in 

• Organic carbon

• Moisture

• Aeration

• pH

• Temperature

• Inorganic nutrients

• Grass clipping, crops residues, organic wastes etc

• 50-6-% water holding capacity

• Balance of air and water filled pores

• Near neutral

• 10-40 C

• Adequate N, P, K, S etc and trace element

PRINCIPLE ENVIRONMENTAL FACTORS AFFECTING SOIL MICROBES

• Bacteria

• Fungi

• Algae

• Protzoa

• Actinomycetes

TYPES OF SOIL MICROBES

SOIL MICROBES AND SOIL STRUCTURE • Soil structure dependent on the stable aggregates of

soil particles

• Soil organisms play an important role in soil aggregation

• Constituents of soil are organic matter, polysaccharides, gums, lignins synthesized by soil microbes play an important role in cementing of soil particle

• Cells and mycelial strands of fungi and actinomycetes play an important role in soil aggregation

CONT’D

• Different soil microorganisms, having soil binding properties are graded in the order as:

fungi > actinomycetes > gum producing bacteria > yeasts

• EXAMPLES:

bacteria like Rhizobium, Bacillus, Azotobacter

fungi like Rhizopus, Mucor

SOIL MICROBES AND PLANT GROWTH• Best medium for plant growth

• Convert complex organic nutrients in to simpler inorganic forms which are readily absorbed by the plant for growth

• Produce variety of substances like IAA, GIBBERLINS, ANTIBIOTICS etc which directly or indirectly promote the plant growth

NITROGEN CYCLE

The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This

transformation can be carried out through both biological and physical processes. 

Important processes in the nitrogen cycle include:

• fixation• ammonification

• nitrification• denitrification

PROCESS OF NITROGEN CYCLE

NITROGEN FIXATION

• The nitrogen molecule (N2) is quite inert. To break it apart so that its atoms can combine with other atoms requires the input of substantial amounts of energy.

• Three processes are responsible for most of the nitrogen fixation in the biosphere:

• atmospheric fixation by lightning

• biological fixation by certain microbes — alone or in a symbiotic relationship with some plants and animals

• industrial fixation

NITRIFICATION• Ammonia can be taken up directly by plants — usually through their roots.

However, most of the ammonia produced by decay is converted in nitrates This is accomplished in two steps:

• Bacteria of the genus Nitrosomonas oxidize NH3 to nitrites (NO2−).

• Bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO3−).

• These two groups of autotrophic bacteria are called nitrifying bacteria. Through their activities (which supply them with all their energy needs), nitrogen is made available to the roots of plants.

• Both soil and the ocean contain archaeal microbes, assigned to the Crenarchaeota that convert ammonia to nitrites. They are more abundant than the nitrifying bacteria and may turn out to play an important role in the nitrogen cycle.

• Many legumes, in addition to fixing atmospheric nitrogen, also perform nitrification — converting some of their organic nitrogen to nitrites and nitrates. These reach the soil when they shed their leaves.

DENITRIFICATION• The three processes above remove nitrogen from the atmosphere

and pass it through ecosystems.

• Denitrification reduces nitrates and nitrites to nitrogen gas, thus replenishing the atmosphere. In the process several intermediates are formed:

• Nitric oxide (NO)

• Nitrous oxide (N2O)(a greenhouse gas 300 times as potent as CO2)

• Nitrous acid (HONO)

• Once again, bacteria are the agents. They live deep in soil and in aquatic sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for the final electron acceptor in their respiration .

ANAMMOX• Anammox is abbreviated as anaerobic ammonia oxidation

• Under anaerobic conditions in marine and freshwater sediments, other species of bacteria are able to oxidize ammonia (with NO2−) forming nitrogen gas.

• NH4+ + NO2− → N2 + 2H2O

• The anammox reaction may account for as much as 50% of the denitrification occurring in the oceans.

• All of these processes participate in closing the nitrogen cycle.

NITROGEN CYCLE IS VERY IMPORTANT IN MAINTAINING OUR ENVIRONMENTAL BALANCE

THANK YOU!