The Amazonian rain forest has a very different climate than arctic tundra. The rain forest is moist and warm, with lush growth of countless varieties of plants; the tundra is dry and cold, with sparse growth of far fewer plant species. Clearly, the climate of an area, especially its prevailing temperature and precipitation, affects the biomass and the type of vegetation that grows there. Climate is also one of the main factors influencing variation in primary production (Figure 7.1). As any gardener knows, most plants grow best when they have plenty of water and sunlight, so long as temperatures are not so hot that the plants wilt or so frigid that they freeze.

Natural ecosystems add layers of complexity atop these environmental variables. More specifically, they contain a variety of plant species that interact with one another, and scientists have long suspected that a link exists between the number of species and the productivity of an ecosystem—the idea being that each plant species has slightly different growth requirements and strategies. Together, they can take advantage of every beam of sunlight, drop of water, and soil nutrient.

To test this hypothesis, in the early 1990s, an ecologist named David Tilman prepared 147 plots—10 feet by 10 feet—in the Minnesota prairie. He and his colleagues seeded these plots with anywhere from 1 to 24 species of native grasses. As predicted, they found that plots with more species had higher primary production levels. In fact, a long-term study by Tilman’s research group found that primary production in the most diverse study plots was over 340% higher than in plots with a single species.

Plant growth patterns and physiology suggest causal mechanisms for these findings. Some root systems plunge deep into the soil, whereas others crawl along just under the surface, which means that each plant takes up nutrients and moisture from different parts of the soil column. Also, some plant species make the environment more favorable for other species by, for example, adding nitrogen to the soil or providing shade for species that prefer the community understory. These positive effects can facilitate greater productivity where multiple species grow together.

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Although climate and plant diversity have substantial effects on levels of terrestrial primary production, gardeners also know that the amount of fruits and vegetables they harvest is affected by soil fertility. Soil fertility is especially connected to levels of certain key elements, such as the availability of nitrogen, the soil nutrient that most commonly limits terrestrial primary production. Consequently, retaining nutrients in soil is critical for sustaining primary production. During one of the field experiments conducted by Tilman and his colleagues, they learned that study plots with more plant species were better able to take up and retain nitrate, a chemical form of nitrogen useful to plants but subject to leaching through the soil by water. More plant species on a single plot retained more nitrogen, and therefore overall primary production was higher (Figure 7.2).

Nitrogen, like other essential nutrients on Earth, cycles from soil to water to air. The chief reservoir of nitrogen, an essential component of protein and nucleic acids and thus critical for life, is the atmosphere, where elemental nitrogen, N₂, makes up 78% of atmospheric gases. However, most organisms cannot use elemental nitrogen to make essential nitrogen-containing compounds. As a consequence, they are dependent on nitrogen-containing compounds in soils and water that are released during the course of the nitrogen cycle.

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Six major processes drive the nitrogen cycle (Figure 7.3). Elemental nitrogen from the atmosphere enters the cycle through the process of nitrogen fixation, during which specialized nitrogen-fixing bacteria convert N₂ to ammonia, NH₃, which is incorporated into organic molecules such as amino acids, the building blocks of proteins. A small amount of nitrogen is also fixed by lightning. Nitrogen fixation is a critical link between the atmospheric pool of nitrogen, which cannot be used by most organisms, and soil and aquatic pools of nitrogen.

Ammonia and ammonium ions are released from decomposing plant, animal, and microbial biomass in a process called (2) ammonification. Some ammonium and ammonia are converted to nitrates through (3) nitrification, a two-step process in which bacteria first produce nitrites (NO₂⁻) and then nitrates. Primary producers can take up the nitrates and ammonium in terrestrial or aquatic ecosystems.

Once within the plant or alga, nitrate and ammonium enter a process called (4) nitrogen assimilation, in which they are incorporated into essential nitrogen-containing organic compounds, such as DNA and amino acids. When plants, or the consumers that eat them, decompose, the nitrogen in their bodies returns to soil or water, completing the nitrogen cycle.

Nitrogen can also be lost from ecosystems through the process of (5) denitrification. Denitrification takes place in poorly drained, poorly aerated soils, or in low- or no-oxygen environments in lakes and marshes, where denitrifying bacteria convert nitrates into elemental nitrogen gas. If nitrogen is not replenished by nitrogen fixation, denitrification may deplete the available nitrogen within an ecosystem. In ecosystems developing on nitrogen-rich sedimentary rocks, (6) weathering can make significant contributions to the amount of cycled nitrogen.

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