Nitrogen Fixation Farms Learning From the Forest

Most farms today use man made fertilizers. They can get into the water system and cause problems for lakes. Perhaps farmers could learn from the forest.

Nitrogen fixation: Nitrogen fixation is an essential function of microbes in forests. Without bacteria which are capable of converting gaseous nitrogen into nitrates and nitrites which plants can utilize, rainforest soils would rapidly become depleted of this essential mineral in usable form. Many million tons of nitrogen are converted annually and added to the soil by these organisms. In the many tropical soils which are nutrient-poor, only nitrogen-fixing bacteria allow plants to survive.

Tropical rain forests are canopied biomes situated at and around the equator, specifically between the Tropic of Cancer and the Tropic of Capricorn. A key feature of tropical zones is a fluctuation between dry and wet seasons, although tropical rain forests arguably have no distinct changes in rainfall and therefore have no seasons [1]. A unique population of microbial species resides in tropical rainforests, many of which cannot be found in any other biome. Nitrogen is critical to the growth of all organisms. The genomes of nitrogen cycling microbes encode a diverse array of metabolic enzymes to make soil nitrogen available to all organisms, including themselves. In addition to biological growth, nitrogen metabolism is an extremely encompassing macrocosm widely applicable in waste management, environmental control [1], and preservation of natural resources.

Observations of the tropical nitrogen (N) cycle over the past half century indicate that intact tropical forests tend to accumulate and recycle large quantities of N relative to temperate forests, as evidenced by plant and soil N to phosphorus (P) ratios, by P limitation of plant growth in some tropical forests, by an abundance of N-fixing plants, and by sustained export of bioavailable N at the ecosystem scale. However, this apparent up-regulation of the ecosystem N cycle introduces a biogeochemical paradox when considered from the perspective of physiology and evolution of individual plants: The putative source for tropical N richness—symbiotic N fixation—should, in theory, be physiologically down-regulated as internal pools of bioavailable N build. We review the evidence for tropical N richness and evaluate several hypotheses that may explain its emergence and maintenance. We propose a leaky nitrostat model that is capable of resolving the paradox at scales of both ecosystems and individual N-fixing organisms.

One of the most vexing problems lies in the resolution of the nitrogen (N) cycle across this vast biome. Although tropical forests are quite variable in biotic composition and functional properties, it is often assumed that humid lowland tropical forests generally are rich in N relative to other nutrients such as phosphorus (P) or calcium (Ca). This assumption is supported by evidence indicating that at least some tropical forests possess the capacity to build up, recycle, and export (via leaching and denitrification) very large quantities of N (e.g., Davidson et al. 2007, Hall & Matson 1999, Hedin et al. 2003, Houlton et al. 2006, Jenny 1950, Martinelli et al. 1999, Vitousek 1984, Vitousek & Sanford 1986). This capacity introduces a stark and fundamental contrast in our conception of forests as biogeochemical systems: Temperate forests are seen as subject to strong and persistent N limitation, whereas lowland tropical forests are viewed as possessing the exceptional capacity to develop abundant supplies of N. Is such a biome-scale dichotomy in N cycling real and, if so, what maintains it?

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