Impacts of urban soil nitrogen availability and herbivory on primary production of herbaceous annual plants in an arid ecosystem

Background/Question/Methods

Net primary production is influenced by both bottom-up and top-down factors in ecosystems. Urbanization, however, changes nutrient availability and consumer populations in and around cities. In general, urbanization increases nutrient availability, such as soil nitrogen (N), and changes herbivore populations relative to more remote areas. Therefore, both bottom-up and top-down forces are altered in urban environments. In this study, we asked how soil nitrogen pools and the distribution and abundance of herbivores impact aboveground biomass of annual plants in and around Phoenix, AZ, USA. It has been shown that urbanization both decreases predation and increases food availability for herbivores. Thus, we hypothesized that herbivory will have a greater influence than soil nitrogen on primary production inside the urban core. To test our hypothesis, we measured winter annual herbaceous biomass in plots with exclosures that exclude small mammals and birds in two different landscape regions: desert remnant parks (e.g. urban) and the surrounding desert (e.g. non-urban). Additionally, we measured soil nitrogen pools in the same locations using Plant Root Simulator (PRS) probes to determine the importance of nitrogen availability as a bottom-up factor of primary productivity.

Results/Conclusions

On average, soil nitrogen pools are greater in the desert remnant parks in comparison to the surrounding desert. However, annual herbaceous biomass does not increase as expected with increased soil inorganic nitrogen availability. We found no relationship between inorganic nitrogen availability and annual herbaceous biomass. We conclude that nitrogen availability is not a strong bottom-up driver of herbaceous plant growth in the Phoenix-metropolitan area. These results stress the importance of determining the biotic and abiotic regulatory forces that control primary productivity across an urban-rural gradient. Further results of this experiment will help us to understand how urbanization influences trophic dynamics between the primary producers and primary consumers in an arid urban ecosystem.