# 7 Nutrients

## 7.1 What is a nutrient?

Nutrients are the substances that help life survive, grow, and reproduce. These can be complex carbohydrates used for energy like starches or amino acids used to build proteins, or we can think of nutrients as the elements that make these complex life-building blocks. Frequently when talking about life, we focus on a group of elements that are most common in almost all biologically relevant molecules. This group is called CHNOPS for Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur. Together these elements make up 98% of living tissues. The video below does a good job explaining why these elements (and many others) are vital to life:

The elements vital to life are constantly cycling in and out of life forms and into abiotic forms. The carbon molecules in a peach are converted to CO$$_2$$ when we eat the peach, break it down into it’s component parts, and respire the sugar, using the energy to maintain ourselves. The movement of this carbon molecule into our bodies, and then into the atmosphere, and, eventually, back into another plant is called a biogeochemical cycle. A full discusiong of biogeochemical cycling is beyond the scope of this course but this is a great book if you are interested. Instead, we will focus on a subset of biogeochmical cycles called nutrient cycles, in particular the Nitrogen and Phosphorus cycles, which have a heavy influence on water quality.

## 7.2 Interacting nutrient cycles

### 7.2.1 Liebig’s law of the minimum

Nutrient cycles interact in a variety of ways, but some of our earliest concepts of how nutrient cycles depend on each other was from ideas from agriculture. One such concept is called "Liebig’s law of the minimum which states that plants grow not in response to the total amount of resources they have (water, nutrients, light, etc…) but in proportion to the most scarce resource (the limiting nutrient). So if a plant has all the N it needs but not enough P it will only grow to the point where it runs out of P. While we have a more rich understanding of coupled nutrient cycling, this concept is still a useful way for starting to think about how one nutrient can impact others.

### 7.2.2 Redfield ratio

Another critical way we understand nutrient cycling is by understanding ecological stoichiometry, which is the study of how lifeforms require elements in certain ratios and how they change in order to meet these requirements. The idea that lifeforms have specific ratios of elements they need to maintain comes from early research by Alfred C. Redfield in 1934. Through oceanic surveys of C, N, and P content in marine biomass Redfield found that there was a consistent ratio of these elements to each other, across the world’s oceans. We now call this ratio the redfield ratio which as originally stated was 106 C:16 N:1 P. Where there is 106 moles of carbon, for every 16 moles of Nitrogen, for every 1 mol of Phosphorus in the biomass of marine life. This ratio varies across different lifeforms, but it is another way of thinking about how nutrient cycles might interact.

## 7.3 The Nitrogen and Phosphorus cycles

Nitrogen and Phosphorus are vital elements for life and their biogeochemical cycles have been heavily altered by people, with severe consequences for water quality.

## 7.4 Excess nutrients cause water quality problems.

In order to sustain the world’s growing population, we need to consistently supply nutrients to our croplands so that our crops can grow. Before 1900, most of this fertilizer was simply animal waste or manure direclty applied to land. With limited supplies of manure, there was a limited apacity for agriculture to maximize yields on croplands. Until the Haber-Bosch process was invented, which dramatically increased our capacity to deliver Nitrogen compounds directly to plants for croplands.

### 7.4.1 Haber-Bosch

The Haber-Bosch process coupled with the globalized mining of Phosphorus has made it so that in many parts of the world these nutrients no longer limit plant growth.

### 7.4.3 Excess nutrients

The readily available N and P generated from Haber-Bosch and global phosphate mining has helped feed a growing global population, but frequently fertilizers end up running off into adjacent water bodies. Causing algae blooms and subsequent eutrophication in lakes, estuaries, and the near-coast environment.

## 7.5 Modern nutrient water quality problems

The problem of excess nutrients in freshwaters is a global problem that exists in almost every nation. Lots of modern research efforts are aimed at assessing the extent of excess nutrients in the world’s rivers, lakes, and coastal margins. Some of my favorite papers or summaries of papers on the subject are here:

http://science.sciencemag.org/content/342/6155/205

http://science.sciencemag.org/content/360/6387/427

https://www.jstor.org/stable/1743244?seq=1#page_scan_tab_contents

And many many more.

## 7.6 Redox

For biogeochemical cycles generally, and for nutrient cycling especially, redox reactions are the chemical reactions that govern the transformations, transport, control, and impact of nutrients on waters.