A classic paradigm in ecosystem science is the concept of nutrient limitation. Simply put, nutrient limitation theory states that the total plant productivity of an ecosystem (ie, plant biomass produced via photosynthesis) is always limited by the scarcest nutrient. This concept makes intuitive sense- plants require a suite of different nutrients in specific proportions to produce new tissue (animals including humans do, too, we just are fortunate enough to eat other living organisms whose tissue contains all the nutrients we require). For plants, the “big three” nutrients are carbon, nitrogen and phosphorus. Organic carbon, produced during photosynthesis using light energy and CO2 from the atmosphere, is the most important in terms of total amount required, but it is actually N and P that plants are believed to be limited by most often.
It turns out that the question of which nutrient, N or P, is going to be more limiting in a particular environment is not a trivial one.There is a long-held notion that temperate forests are more N-limited, while tropical forests are more P-limited. The reason for this is that the primary source of phosphorus for plants is found in rocks- their slow decay into soil releases phosphorus in a form that plants can readily take up and use. Tropical forests sit on ancient soils and even more ancient rocks that have had nearly all the phosphorus sucked out of them millions of years ago. By contrast, temperate forest soils are quite young, having only started developing at the end of the last glacial period ~11,000 years ago, and are relatively rich in rock-derived P. The lack of P in tropical forest soils has also led scientists to conclude that these forests are less resilient to human disturbance than their temperate counterparts. A patch of Amazon that is converted into cattle pasture has scant levels of essential nutrients in its soil to begin with, and typically erodes into desert in a matter of years.
This simple paradigm of temperate forest N-limiation and tropical forest P-limitation makes good sense until you start going out in the forest and taking measurements. On the island of Puerto Rico, soils are at least 10 million years old and up to 30 meters deep, making the forests that grow upon them perfect candidates for P-limitation. Scientists were thus surprsised several decades ago to find that there is abundant P in both the soils and plant biomass of Puerto Rican forests. Moreover, the forests of Puerto Rico experience frequent natural disturbances- hurricanes that occasionally flatten an entire landscape- and yet, within a growing season or two, things start returning to normal. Clearly, these forests are highly resilient, which begs the question of whether our classic model of tropical P limitation is completely off base.
As is often the case in science, it appears that the devil may be in the details. Puerto Rican forest soils may contain a lot of phosphorus, but they contain even more iron, a “garbage” element that’s found abundantly in soils that have long since had most other mineral-derived elements consumed or eroded away . It turns out that mineral-derived iron loves to bind P and make it inaccessible to plants. This much has been known for a long time. In fact, iron-phosphorus binding is often cited as auxillary evidence for tropical P limitation. However, recent research indicates that a special group of bacteria know as iron reducers can actually use iron in soil for metabolic energy, much as we and other aerobic organisms use oxygen to fuel our metabolism. In the process of “breathing iron”, these bacteria release iron-bound phosphorus, which plants are then free to take up.
So, are iron-reducing bacteria saving Puerto Rican, and perhaps other tropical forests from being P-limited? The answer appears to be yes, but only sometimes. Iron reduction in soils is a rather specialized process that only occurs under conditions of anoxia (no oxygen), which in turn only occurs when a soil is completely inundated with water. Sounds like just the type of scenario that we’d see when a hurricane strikes. In flattening a forest, hurricanes may not be wreaking havoc at all, but simply playing their part in a natural cycle that provides just the right conditions for an essential but plant-limiting nutrient to be pulsed into the soil.
So, P limitation? Still an open debate, but in my view forests are incredibly complex, self-regulatory systems that have evolved to attain everything they need from their environment. Furthermore, nature is messy and doesn’t like to fit into nice little paradigms. I’ll give it a “maybe” and a “sometimes” and call it a day.