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Who's in Control of the Nutrients That Sustain Tropical Forest Growth, the Living or the Dead?

Lydia Olander
Biological Sciences
Stanford University
June 2001

My research looks at how living microbes and non-living soil minerals and organic matter interact to control the availability of two important nutrients, nitrogen and phosphorus, in tropical soils.

Nitrogen and phosphorus are essential for the growth of plants and microbes. Their availability can control whether ecosystems, such as forests, grasslands, and agriculture, continue to grow or go into decline. Understanding what controls the availability of these nutrients, and thus what controls ecosystem health, is an essential element in determining what will happen to vegetated ecosystems as human activities continue to intensify land use, climate change, and pollution.

Soils are made up of minerals formed from the original rock and soil organic matter formed from decaying leaves and other living organisms. These minerals and organic matter undergo abiotic (inanimate) chemical reactions with nutrients such as nitrogen and phosphorus. Some abiotic chemical reactions bind the nutrients loosely, leaving them accessible to living organisms, while others bind them more strongly, in forms which may be permanently inaccessible to plants and microbes. Very few experiments have been done to test whether scientists' expectations are true concerning how abiotic reactions and living organisms interact to control nutrient availability. If my experiments show the unexpected, it could fundamentally change the way ecologists model and describe nutrient availability. If not, they will help support our expectations.

Complex global circulation models (GCMs) are currently being used to represent the functioning of the Earth’s system (oceans, atmosphere, land, and biota). They help explain how the Earth’s system works and predict how natural fluctuations and human actions change the system. For example, GCMs predict the magnitude of future global climate change and how it will affect the frequency and size of storms and droughts, and the growth of vegetated ecosystems. Biogeochemical models, one sub-unit of GCMs, describe how elements and nutrients like nitrogen and phosphorus flow through the Earth's system.

The models appear to do a fairly good job of representing temperate ecosystems where nitrogen is the primary element driving the system. However, there are still some uncertainties about how nitrogen gets stored in soils. Research in tropical ecosystems lags behind the study of the temperate zone where intensive agriculture and forestry have pushed research agendas for centuries. For tropical ecosystems where phosphorus, and possibly other elements like calcium and potassium, is more important than nitrogen, the models are much weaker. Results from temperate research cannot be directly applied to the tropics, not only because of differences in nutrient availability, but also because soil minerals and chemistry are significantly different.

Soils that are found throughout the tropics contain minerals that strongly bind phosphorus. It is commonly thought that this bound phosphorus is relatively unavailable to the living system, but this theory has not been directly tested. When biogeochemical models are run using the assumption that bound phosphorus is unavailable, they don't work. Obviously, we're missing something important. On the other hand, nitrogen is known not to bind tightly to soil minerals, thus it is more available to living organisms and also more easily lost from the system. Chemical binding of nitrogen to organic matter, however, may be important in both temperate and tropical soils. Several studies have observed that a significant amount of nitrogen added to soils rapidly disappears from the available pool. While some scientists suggest this is microbial consumption, a few studies indicate nitrogen may bind chemically with soil organic matter.

For my research I use soils from three Hawaiian forests, which are similar in elevation, temperature, rainfall, vegetation, angle of the slope they grow on, and the type of rock from which the soils were formed. The critical difference among these forests is soil age. Over time, chemicals and minerals in soils dissolve and are washed away forming new minerals. As a result, the soils contain different minerals and different amounts of organic matter and this affects their ability to bind nitrogen and phosphorus. In my experiments, I add isotopically labeled nitrogen and phosphorus to each soil to trace how much of the nutrients are consumed by the soil microbial pool versus bound to the soil mineral and organic matter pool. An isotope of an atom has a different number of neutrons, thus it has a different mass and can be distinguished from other isotopes. Most nitrogen has an atomic mass of 14, but I use nitrogen with an atomic mass of 15 and can trace its distribution into different pools in the soils. For phosphorus, adding an extra neutron creates an unstable atom that is radioactive, meaning it emits energy as it decays to a stable atomic form. This energy can be detected and can be used to trace phosphorus into different soil pools.

After adding isotopically labeled nitrogen and phosphorus to the soils, I wait a set period of time and see how much of the tracer is found in the microbes versus how much is bound to the mineral and organic matter soil pool. From this I can tell how differences in the three different soil types may affect the partitioning of nitrogen and phosphorus between the living microbes and the non-living soil minerals and microbes. In my experiments I also add simple carbon to a set of soil samples. Simple carbon is like “junk food” for the microbes. It will make them grow and increase their activity, enhancing their need for nitrogen and phosphorus. This will tell me if the microbes can access the nutrients already bound in the non-living soil pool if they need it.

If a lot of nitrogen turns up in the non-living soil pool, or if microbes can easily access phosphorus from the mineral soil pool, it will fundamentally change the way ecologists and the models describe the role of chemical reactions in controlling nitrogen and phosphorus in soils. Even if my results do not show anything profoundly different than theories predict, it will be the first attempt to actually determine the amounts and rates of nitrogen and phosphorus distributed into the different pools in tropical soils.

Actual numbers (rates and proportions) are required for modeling. With the help of another student in my lab, I will build a simple model of nitrogen and phosphorus cycling in my soils. I can then compare this with the more general biogeochemical models and see how and where they differ. With only one study and three soil types examined, my thesis research alone will not be enough to completely rework our biogeochemical models for tropical systems. However, it may provide a new mechanism to try in our models and ballpark figures to constrain the numbers currently plugged into the models. Also, it will provide an example of how these numbers can be determined experimentally in other soils and used to further refine the models.