In arid communities, annual plants make up most of the plant diversity, support pollinators and wildlife, and are an important part of nutrient cycling, and so changes in annual communities may have a large impact on overall ecosystem function. In aridlands, precipitation and nutrient availability are important limiting factors on annual plant growth. Less is known about how precipitation and nutrient availability affect desert annual diversity over time, and therefore how changes in urban nutrient deposition and climate may affect diversity. We conducted a long-term (2006-2016) nutrient addition experiment to test how nutrient availability interacts with growing season (October-April) precipitation to affect desert annual plant community composition. We hypothesized that higher nutrient availability would only affect community composition and diversity in years with average to high precipitation, and that combined nitrogen and phosphorus addition would have the greatest effect. We applied nitrogen and phosphorus alone and in combination in 15 unmanaged Sonoran Desert preserves in the urban airshed of Phoenix, AZ. We quantified species composition and abundance of all winter annuals and compared diversity and composition in different nutrient addition treatments over time for sites within or outside the Phoenix metropolitan area.
We found a total of 70 species of annual forbs and grasses among all plots and treatments. Plot level species richness ranged from 1 to 24 species per 2 m2 sampling area. Treatments with added nitrogen had significantly lower species richness (5.9 ± 0.2) than control treatments (7.8 ± 0.3) or treatments with only phosphorus added (7.5 ± 0.3), and treatments with additional phosphorus did not differ from the control. However, community composition differed by year more than by nutrient addition treatment. Over the duration of our experiment, our sites experienced a range of winter precipitation amounts (40 - 267 mm), which influenced annual community composition and diversity and was therefore an important factor in yearly variability in composition. These results indicate that increased nitrogen availability in desert plant communities is likely to decrease annual diversity, with possible implications for pollinator and herbivore diversity as well. This is of particular concern for communities experiencing increased atmospheric nitrogen deposition from nearby urban areas, such as those considered in this study.