Investigating Physical Controls of Nitrogen Isotopes in Terrestrial Ecosystems

The news is full these days with stories of increasing droughts and floods. These events, which will increase as climate change continues, have clear economic impacts.

As an environmental scientist however, I am interested in how climate change may impact other natural systems. One such system is the nitrogen cycle, which describes the transfer of this nutrient to various phases of the environment (Figure 1). Nitrogen is an essential nutrient for all forms of life. On an individual level, it makes up proteins and DNA. On an ecological level it acts as the major limiting factor for plant growth, impacting the vegetation and all sorts of system dynamics. The main goal of this thesis is to explore if climate has any effect on nitrogen cycling, so that further studies on climate change may be possible.

To understand if climate exhibits control over nitrogen cycling, nitrogen cycling must be quantified. This is done using nitrogen isotopes, which are two atoms of nitrogen that differ in the atomic mass. Using specialized instruments, we can closely measure the ratio of heavy and rare ¹⁵N to light and abundant ¹⁴N (expressed as δ¹⁵N). δ¹⁵N changes as nitrogen undergoes different reactions in the soil. Therefore, if I see δ¹⁵N vary predictably with climatic variables (precipitation, temperature, growing season), I can conclude that climate exhibits some control over nitrogen cycling.

In addition to the climatic variables, I also included physiochemical variables, namely pH and organic carbon content. These physiochemical variables are more local descriptors, as opposed to the regional descriptors of climatic variables. I included them in the analyses to compare to the climatic variables. I am predicting that climate’s effect on nitrogen content in soils will be indirect, where climate acts on vegetation and soil processes, which both directly affect δ¹⁵N (Figure 2). pH and soil carbon are more indicative of the direct links to δ¹⁵N. Previous studies have been able to construct predictive models of δ¹⁵N on a regional scale using climatic AND physiochemical variables. I had hoped to recreate such a model using multiple regression analysis on my dataset of the United States.

This exploration is possible due to the establishment of the National Ecological Observatory Network (NEON). NEON is an exciting database established in 2019 which collects data across the United States and opens up the doors for all sorts of ecological studies.

Moving to the results, I saw that that climate and δ¹⁵N had weak relationships. In order of strength of relationship with δ¹⁵N (determined via bivariate regression) we have length of growing season (highest), mean annual temperature, and mean annual precipitation (lowest).

Relationships between pH and δ¹⁵N as well as organic carbon content and δ¹⁵N were much higher than the climatic variables. This is not entirely unexpected, as I described that these physiochemical variables are more of a local predictor and may play a more direct role in controlling the soil nitrogen content. We can visually see how pH has a stronger relationship with δ¹⁵N than mean annual temperature and δ¹⁵N (Figures 3 and 4). This is confirmed with the R² values, which quantify the strength of the relationship. Interestingly the association of pH and δ¹⁵N is so high, that it is the only variable which contributed to the predictive model I had hoped to establish using a multiple regression.

Overall, the analyses show that climate has some control on δ¹⁵N, but the strength of these controls was not as high as I had predicted. If we look back at the conceptual model, we can understand that climate is rather far removed from nitrogen cycling, likely due to the fact that climate’s effects are all mediated through vegetation and microorganisms. This brings up more questions, however. As the climate changes how would nitrogen cycling respond? This brings up further questions of response time. This of course requires further studies, but really showcases how exciting and complex this topic can be. As NEON continues to gather data, further studies on such an important and fascinating topic will be possible.

 

Nico Lovinello is a graduating senior studying environmental science at the O’Neill School of Public and Environmental Affairs.