Nitrification is an important soil functions carried out by soil ammonia oxidizing microorganisms. Rates of nitrification can significantly impact primary productivity via availability of N for plant uptake, and environment health via loss of N to water and atmosphere. In the past 100 years, ammonia-oxidizing bacteria (AOB) was considered the dominant microorganism in ammonia oxidation, however, this perception totally changed after the discovery of ammonia-oxidizing archaea (AOA). Today, we know that these two microbes, which are completely different in physiological characteristics, are fundamental drivers of soil nitrification, and because of this, it is fundamental that we learn how to predict their distribution in our soils. For instance, we know that abiotic factors (e.g. pH, moisture) are important regulators of the abundance of both type of organisms, however, much less is known about the role of vegetation in determining the distribution of these organisms opening the door to improve our capacity to predict their distributions.
To address this research question and learn more about the role of vegetation in driving AOA and AOB communities, we focus on drylands - the largest terrestrial biome of Earth. These ecosystems covers around 45% of the Earth’s land surface and support over one third of the total global population, however, they are also highly vulnerable to global change drivers. Moreover, they also provide habitat to 20% of global plant diversity. In general drylands are heterogeneous ecosystems that are typically formed by a matrix of discrete plant patches surrounded in a matrix of open areas devoid of perennial vegetation. Such vegetation organization allow the existence of patches cover by either vegetation or bare soil devoid of vegetation within a single location, and allow us to identify the role of vegetation in controlling AOA and AOB communities.
This project is part of my Phd in the Hawkesbury Institute for the Environment, Western Sydney University at Australia where I worked evaluating the linkages between soil microbial functional diversity and soil processes (Trivedi et al. 2019), and also identifying the major ecological drivers of functional microbial communities. We used soils from 80 dryland sites located in 12 countries collected in patches cover by either vegetation or bare soil devoid of vegetation by Maestre lab (Maestre et al. 2015).
We found that the diversity and abundance of AOB were promoted under plant canopies, while AOA was enhanced in open areas devoid of perennial vegetation. Soil under plant canopies always had higher abundance and richness of AOB, suggesting that AOB and AOA bacterial communities prefer copiotroph and oligotrophic environments, respectively. These results have implications for the future of global drylands, as our study suggests that with predicted decline in vegetation under projected climate change, changes in the nitrifying composition will occur in such a way that could compromise the rate of nitrification, which is a crucial step in the nitrogen cycle.
Article link: https://www.nature.com/articles/s41396-019-0465-1
Trivedi, C. et al. 2019. Losses in microbial functional diversity reduce the rate of key soil processes. Soil Biology and Biochemistry, 135, pp.267-274.
Maestre, F.T. et al. 2015. Increasing aridity reduces soil microbial diversity and abundance in global drylands. Proceedings of the National Academy of Sciences, 112(51), pp.15684-15689.