Terrestrial N Cycling in an Endangered Oasis

In terrestrial arid ecosystems, one of the most limiting factors for productivity, following water, is thought to be nitrogen (N) availability. The N cycle can be summarized as an exchange of N forms between the atmosphere and the biosphere, mediated by the biological activity of microorganisms. Arid lands typically have a heterogeneous distribution of resources, with vegetated areas and microbial crusts having greater nutrient concentrations and microbial densities than bare soils. However, the contribution of each compartment to the entire N budget in these arid ecosystems is poorly understood. In this chapter, we summarize studies performed in the terrestrial component of Cuatro Cienegas Basin (CCB) regarding different aspects of the N cycle. We present selected results from two different studies that contrast microbial diversity and specific N transformations in (i) different moisture conditions and (ii) different temperatures. Although microbial crusts are important components of many desert ecosystems, there is very little evidence that the N fixed within them is in turn available to higher plants. Considering this, N fixers in the rhizosphere of plants could also be relevant N suppliers. In the last part of this chapter, we compare the potential composition of the microbial N fixers and denitrifier communities present in bare soils and in the rhizosphere of Agave lechuguilla, one of the most characteristic plant species in the Mexican arid regions. In general, these data suggest that environmental changes such as soil moisture reduction, changes in temperature, and vegetation removal could dramatically affect the terrestrial N cycle in CCB.