The other bio-geochemical cycle we will exam in detail is the nitrogen cycle. I chose this because it is heavily modified by deliberate and inadvertent human action (compared, say, to the oxygen cycle, which we skipped).

The basic elements:

Storage is mostly in the atmosphere (78% of air is nitrogen)

Flux: main exchange is between air and soil.

Absorption is by "nitrogen fixation" by soil biota, especially bacteria that live on many plant roots (you will have heard that, say, legumes are good in some rotation in your garden because they fix nitrogen in the soil). They extract nitrogen from air, transform it into ammonia, that plants can us to build amino acids and proteins.

This gets nitrogen into the food chains. It also returns or stays in the soil.

Flux back to atmosphere (denitrification, by denitrifying bacteria that cause decay of organic matter on surface and in the soil)

Would be "balanced" were not for human introduction of nitrogen (most commonly in industrial agriculture as a form of anhydrous ammonia, a fertilizer created from fossil fuels, mostly natural gas.

Earth systems or cycles are the movement of energy and mass, in various forms. They interact in complex ways.

Let’s take a quick look at how "systems" operate.

Systems include elements that interact.

You can think in terms of storages, flows, and transformations (e.g., nitrification). You cn also think about relationships among elements of systems:

Primary relationships: the individual links in the chains of cause and effect thru a system (temperature and light, algae, fish, fish-eating birds, etc.)

Secondary relationships: these are the "feedbacks" in a system. Fig. 5.13 illustrates a
feeback of algal growth. The growth blocks light, which cools the water, and thus reduces the further growth of algae. This creates an "equilibrium" or range of light and temperature conditions that keep the algae growing but not running out of control and covering the pond.

Types of feedbacks:

Negative: the algae example, but also one from the global climate: solar radiation is absorbed at the surface and turned into heat, which warms the earth system (measured as surface temperature). Any object above absolute zero emits its own radiation, and thus cools. The rate of radiation is proportional to its temperature, so a warmer eath radiates more energy back to space, thus acting as a cooling factor that keeps the warming from continuing. This is a form of "homeostasis"----a set of system inter-relationships that tend through their interactions, to maintain some system property (global temp in this case) within some range, even when fluxes initially change 9e.g., solar radiation increases, or terrestrial radiation is decreased).

Positive: inter-actions of elements of systems that tend to magnify any trend (e.g., warming or cooling; storage of carbon, etc.) over time. Some climatologists think this created the ice ages, when during an initial period of cooler climate snow builds up at the poles, increases reflectivity (decreases the absorption of solar radiation at the surface) which cools the temp, and thus allows more snow to hang around, further reducing absorption and temp. Snow builds up into glaciers and they spread to cover par of the globe, thus making it even colder. The great question then becomes: when does the positive feedback cycle break and return to negative feedback, thus allowing warming and the retreat of the glaciers.

Homeostasis : When complex feedbacks (both positive and negative) work to tend to make a system, and measurements of it status, rather stable over time. In the cas eof ice ages, and warm intervals, the earth maintained an average temp that still maintained life, there was no "runaway" cooling or warming, the climate did not come eventually to be like that of Mars or Venus. Why not? But, changes can be introduced by an outside force (me adding light and heat to my daughter’s fish tank) or by natural internal fluctuations that sometimes get large enough to shift to new system states (chaos theory would apply here, but we won’t go into it).

Gaia: a hypothesis first proposed by British biologist James Lovelock: the whole earth system works to maintain a homeostatic balance in order to maintain the conditions conducive to life. Controversial: it seems to imply that life or the biosphere "knows" how to dampen trends that threaten its existence (e.g., runaway warming). Here’s another example: carbon in the atmosphere increases (due to humans burning fossil fuel), this increases photoynthesis and carbon flux from air to plants. One could say that the plants are combing carbon from the air so that the climate does not change and threaten their existence. But, what are it weaknesses of this model?