Using Mathematical Models to Figure Out Which Nutrient Limits Plant Growth and Carbon Storage
The Franz Josef Glacier in New Zealand
Climate change is a daunting problem, and we need to know how much help we can expect from nature in solving this problem. Carbon dioxide (CO2) in the atmosphere is the leading cause of climate change, so understanding how it is removed from the atmosphere is critical to solving the climate problem.
Plant growth (via photosynthesis) is the main way CO2 leaves the atmosphere, and plant growth is often controlled by the amount of nutrients in the soil (which is why we spend so much money on fertilizer. In fact, two nutrients - nitrogen (N) and phosphorus (P) - seem to be the two nutrients that are most important. As a forest soil develops from bare rock (after a volcanic eruption, glacier recession, or any other disturbance that opens up bare rock), it seems that first N, then N and P together, then finally P alone control plant growth. This makes sense because rocks contain P but almost no N, whereas the atmosphere contains N but virtually no P. Therefore, N is in short supply until enough has entered the soil from the atmosphere (from rainfall and through plants and bacteria), whereas the P supply runs out as the rock is weathered away. Rock weathering is slow, so this process takes thousands to millions of years. However, we do not yet have an easy way to figure out when a particular forest will switch from being controlled by N to P. Alternatively, even if we know the soil age, we do not yet have an easy way to say whether a given forest is likely to be controlled by N or P (without dumping fertilizer and measuring the trees in a few years).
My project aims to develop a way to tell which forests are limited by N, P, or both, given the soil age and a few other easily-measured properties of the ecosystem, such as the amount of rainfall, the rock type, and the plants in the forest. I will do this by building mathematical models that describe carbon, N, and P transfers between plants, soils, and the atmosphere, analyzing these models to see which properties are most important, and testing them with existing data from well-studied forests in Hawaii and New Zealand. Because we are emitting lots of N into the atmosphere (along with CO2), forests that are controlled by N will grow more and take up more CO2, whereas P-limited forests will not. This project, which aims to provide an easy way to figure out whether N, P, or both control plant growth, will help us figure out how much help plants will give us with the climate problem.