By now, most of us have heard about at least one water quality crisis within the country. These events receive especially abundant media attention whenever water quality reaches a point where citizens can no longer utilize their nearby water source. Recently, two events in particular have generated substantial amounts of press: Lead contamination of Flint, MI, and harmful algal blooms in Toledo, OH. The latter of these two events is the result of excessive nutrient loading. But what does that mean? From a dietary perspective, aren’t nutrients a good thing? Furthermore, what nutrients are “bad?” All of these questions do not come with easy, black-and-white answers, but over the next few weeks, we plan to release an informative series about the way one nutrient in particular, Phosphorus, moves through an ecosystem. In doing so, we will discuss how Phosphorus is generally a good (great, rather) thing, and we’ll also cover what conditions need to exist for this “great thing” to turn so bad that can deprive humans of one of the most important resources essential for life: clean, safe drinking water.
Phosphorus (P) primarily originates in rocks as the mineral apatite, which comprises approximately 0.1% the mass of the Earth’s crust. Rocks weather over time and form soil containing nutrients from this rock, including P. Phosphorus is also a plant essential nutrient that,once weathered and solubilized in the soil solution, is available for uptake by plants and microbes. Within the plant, P is central to energy transfer by adenosine tri-(di-, and mono-) phosphates (ATP, ADP, and AMP, respectively). Phosphorus is also a component of nucleic acids (DNA and RNA) as a phosphate-sugar “backbone.”
The total amount of P in an ecosystem includes many different forms, some which can be easily taken up by plants or microbes and some which cannot be utilized as readily (within the next several growing seasons). Available forms include phosphates, soluble and easily dephosphorylated organic P compounds (ATP and phosphate sugars), and P that is very loosely bound to soil or sediment surfaces. Unavailable forms include P which is tightly bound to mineral surfaces, unweathered P still existing in minerals, organic P in recalcitrant(resistant to decomposition) compounds, and P that is physically segregated or buried. Over time P can become available through dissolution, decomposition, ligand exchange (the exchange of one molecule for another), and upheaval. Available forms can also be converted to unavailable forms through precipitation reactions, inner sphere complexation, and burial. Internal cycling of P is dependent on the microbial communities which decompose the organic matter and release ligands to exchange for phosphate, chemical equilibrium of the different forms of P, and the weathering rates of primary minerals containing P. In this soil environment, plants and microbes must aggressively compete for the small amounts of available P resources.
From this point, P can mobilize through runoff, erosion,leaching through the soil profile,or it be directly removed from the site through harvest or consumption. Inputs to most natural systems are very minor and include dust settling, run-on from adjacent sites, and direct inputs from animals. Because these inputs are small relative to the loss of P (dominated by erosion and run-off), over time TP will likely decrease. This is in truth what is observed in most sites throughout the world. As a site develops and ages, there is a gradual loss of P from the system. The rate of loss depends heavily on the climate, parent material, organisms, and topography. No matter the rate, most systems tend toward loss of total P with the most developed and weathered sites being more P deficient.
As P decreases or available sources begin to deplete, deficiency symptoms in plants can be observed. When P is in low supply, gene expression and protein synthesis suffer and,as a result,plant growth slows or dieback occurs. As the nutrient capital of the site diminishes, only stunted plants will remain with off-colored foliage and smaller-than-normal fruiting bodies. Very developed and old locations with low levels of macronutrients such as P can develop unique communities of plants specifically adapted to low nutrient concentrations. These plants must access nutrients through mycorrhizal symbiosis (a relationship between plants and fungi that is beneficial for both), complex or deep rooting systems, or by employing carnivory to provide nutrients(like in the Venus fly trap). Many other plants in these systems simply grow very slowly, similar to bonsai trees. While these systems lack the growing vigor and biomass of younger sites, they still represent beautiful, diverse, and important ecosystems.
So what happens with the Phosphorus that gets carried away from one ecosystem to the next? And if many systems are Phosphorus deficient, then why is everyone talking about Phosphorus-rich systems? Both of these questions will be answered in our next blog, where we will examine the fate of Phosphorus which has left a site through erosion and the effects of P loading into eutrophic waters.