18.2 Biogeochemical Cycles

18.2 Biogeochemical Cycles

Energy crosses an ecosystem and leaves as heat. The atoms do not leave for good. A biogeochemical cycle is the path of water or of a chemical element through living organisms, the solid Earth, and the air or water. The cycles to know are water, carbon, nitrogen, and phosphorus. Each has reservoirs, the places where the substance sits, and processes that move it. Two traps show up again and again. Not every cycle has a major atmospheric gas. Ordinary plants do not fix nitrogen gas with their own enzymes.

The water cycle

Water moves through the environment without being created or destroyed. Evaporation lifts water vapor from oceans, lakes, and wet soil. Transpiration is the loss from plants: water travels up through xylem and exits as vapor through stomata. Evaporation and transpiration together are often called evapotranspiration. Condensation turns that vapor into cloud droplets. Precipitation returns it as rain or snow. On land, some water runs over the surface as runoff and heads toward rivers and the sea. Some soaks in and becomes groundwater, which may reappear at a spring or be pulled up by roots. A molecule may finish these steps in days or sit in groundwater for centuries. Much of the vapor over a forest left through leaves.

The carbon cycle

Carbon atoms move among carbon dioxide in the air, organic molecules in organisms, and stored reservoirs in soil, ocean water, and rock. Photosynthesis is the biological intake. Producers take carbon dioxide and build sugars and other organic molecules. That is how inorganic carbon becomes wood, a leaf, and the body of an animal that eats the leaf. Cellular respiration is the biological return. Producers, consumers, and decomposers all respire, and respiration releases carbon dioxide. Combustion is the return that does not require a living cell. Burning wood, coal, oil, or natural gas oxidizes carbon and sends carbon dioxide back to the air.

Fossil fuels are stored carbon. Coal, oil, and natural gas are the remains of ancient organisms, buried before decay could finish and then changed by heat and pressure. They sit outside the fast loop until they are burned.

The nitrogen cycle

The atmosphere is about 78 percent nitrogen gas, N2. That abundance is misleading. A triple bond holds the two atoms together, and most organisms cannot break it. Animals cannot use N2 as a nutrient. Ordinary plants cannot either. Usable nitrogen is already fixed: ammonia, nitrite, nitrate, or organic nitrogen built into amino acids and nucleotides.

Nitrogen fixation converts N2 into a form organisms can use. Bacteria and archaea that possess nitrogenase make ammonia. Some are free-living. Others, such as Rhizobium in the root nodules of beans, peas, and clover, live with plants. The bacterial enzyme does the fixing. The plant does not fix N2 with its own enzymes in the ordinary case. Lightning also fixes nitrogen: the strike forms nitrogen oxides that become nitrate in rainwater.

Nitrification converts ammonia to nitrite and then nitrite to nitrate. Nitrifying bacteria do this in the presence of oxygen. Bacteria such as Nitrosomonas carry out the step to nitrite, and bacteria such as Nitrobacter carry out the step to nitrate. Other genera share the work. Plants can take up both ammonia and nitrate. Nitrate moves readily with soil water.

Assimilation builds organic nitrogen. A plant incorporates absorbed ammonia or nitrate into amino acids, proteins, and nucleic acids. An animal assimilates nitrogen by eating those molecules. Assimilation does not crack N2. It starts from nitrogen that is already fixed.

Ammonification, also called mineralization, returns ammonia from dead matter and wastes. Decomposer bacteria and fungi break proteins and nucleic acids and release ammonia into the soil. That ammonia can be assimilated again or passed to nitrifying bacteria.

Denitrification returns N2 to the air. Denitrifying bacteria, working where oxygen is scarce, convert nitrate to nitrogen gas. A flooded, oxygen-poor soil can lose fixed nitrogen this way. The field may still look dark and fertile while the nitrate is leaving to the atmosphere.

Bacterial steps to memorize

Fixation, nitrification, ammonification, and denitrification are the bacterial steps, and they need not run in one order in every soil. Assimilation sits beside the table: organisms build organic nitrogen from nitrogen that is already fixed.

The phosphorus cycle

Phosphorus does not cycle mainly as a gas. It has no major atmospheric gas phase, unlike carbon dioxide and nitrogen gas. The long reservoir is rock. Weathering releases phosphate into soil and water. Producers take up phosphate and build it into ATP, nucleic acids, and phospholipids. Animals also put phosphate into bone and teeth. Consumers obtain it by eating. Decomposers return phosphate from dead organic matter to the soil. Some phosphate washes to lakes and the ocean, settles into sediment, and returns to land only on the slow schedule of geological uplift. That is why this cycle is often the slow one.

Phosphorus can limit freshwater systems. Phosphate from fertilizer or sewage can remove that limit and fuel an algal bloom. The bloom does not mean phosphorus traveled as a gas.