Nitrogen cycle
HOW ARE you all? I hope this week has found you in excellent health and spirits, willing and able to take on the world!
Last week, we began our study of nutrient cycles by taking a look at the carbon cycle. This week, we will be looking at the nitrogen cycle but, first, some recap. Do you remember how we arrived at this point? Of course, you do! We started out looking at soils and then discovered that soils had, literally, millions of organisms living in them; so then we went on to look at the organisms living in the soil. We found out that these organisms fell into two groups: the detritivores and the decomposers.
I will pause here to let you know that some students have a difficulty in making a distinction between the two but you, my readers, can free yourself from this confusion by bearing in mind that the detritivores are those organisms such as earthworms and woodlice that feed on partially decaying plant and animal material known as detritus. These detritivores feed on the organic matter, breaking it down further and, in so doing, they increase the surface area of the material, enabling the next group, the decomposers, to complete their portion of the breakdown more easily. In other words, the decomposers complete the breakdown of the remnants of the organic compounds of dead and decaying organisms as well as their wastes to inorganic compounds, forming simple compounds that can be taken up by plants in the process. By doing this, the decomposers: Allow for the reuse of certain elements and compounds. Reduce the chances that these elements will be depleted. Prevent the accumulation of substances which might become toxic or cause pollution. Do you remember the types of organisms that fall into the category of decomposers? Did you say bacteria and fungi? If you did, then you are correct and are keeping up with the lessons! Remember that they are also known as saprophytes. What makes decomposers suitable for their role? Can you figure that out by yourself using the knowledge that you have gained so far? Let me help you to organise your thoughts. They do not have chlorophyll, so they cannot manufacture their own food. They have the ability to secrete enzymes which they use to digest the decaying material. Remember that the digestion is done externally and the end products of digestion then diffuse into the organism.
They can respire anaerobically, hence can live in conditions where there is a limited amount of oxygen.
Last week, we looked at the carbon cycle. This week, we will be looking at the nitrogen cycle. The nitrogen cycle is a bit more complex than the carbon cycle and tends to be the focus of both multiple-choice and essay-type questions in your external exams. Students are often asked to discuss the role of decomposers in the cycle. You will find that different types of bacteria are involved in the cycle, but not all of them are decomposers.
THE NITROGEN CYCLE
Plants and animals need nitrogen to make proteins. Although nitrogen gas constitutes about 80% of the air around us, this nitrogen cannot be used by plants and animals; it has to be converted to usable forms. This is in the form of ammonium compounds or nitrates. Some bacteria have the ability to convert atmospheric nitrogen to nitrates. These bacteria are known as nitrogen-fixing bacteria. Plants use the nitrates to make proteins, and animals get the proteins when they eat the plants. Figure 1 shows you the nitrogen cycle in the form of a diagram. Get familiar with the diagram; you never know when you might see it again!
THE NITROGEN CYCLE CAN BE DIVIDED INTO THREE STAGES:
Nitrogen in the air is made available to plants by nitrogen fixation. Nitrogen-fixing bacteria convert atmospheric nitrogen into nitrates. These bacteria include rhizobium in root nodules of leguminous plants, and azotobacteria found in aerated soil.
Ammonium ions from organic waste and dead organic remains can be converted into nitrites and, finally, nitrates by nitrifying bacteria such as nitrosomonas and nitrobacter.
Nitrate ions are absorbed by the plant roots. Some get leached into the ground water or can be converted back to nitrogen gas by denitrifying bacteria. Denitrifying bacteria are found in waterlogged soils under anaerobic conditions.
Notice from the diagram that the role of the decomposers remains the same; the decomposers break down dead plant and animal remains, along with waste material, into ammonium ions, which can be taken up by the plant or converted to nitrates by the nitrifying bacteria. They, in turn, incorporate some of this material into their bodies. When they respire, they return carbon in the form of carbon dioxide to the air and when they die, their remains also contribute to the nitrates in the soil. Interesting, isn’t it? See you next week!