Contrasting above-belowground responses to long-term precipitation and nitrogen manipulations in a desert grassland
Ecosystem responses to long-term drought are modulated by nutrient availability in numerous ways. Nutrient availability may affect plant-species and microbial composition resulting in communities that are more or less sensitive to drought. In addition, high nutrient availability may increase the interannual variability of productivity as the ecosystem shifts from being limited by two factors (water and N) to just one factor (water), therefore amplifying the response to water availability. Here, we report on the results of an 8-year manipulation of rainfall and nitrogen availability in desert grassland in Southern New Mexico. Treatments included reduced incoming precipitation by 80%, control and enhanced precipitation by 80% crossed with a nitrogen fertilization of 5 g of N per m-2 per year applied twice during the growing season. Response variables encompassed plant and microbial components of the ecosystem. Aboveground, we measured primary production and plant-species composition. Below ground, we assessed the composition of the community of soil microorganisms via 454 sequencing and bioinformatics analyses of 16s rRNA genes. In addition, we also measured belowground biomass for the last year.
Aboveground, long-term drought affected differently the two major components of the plant community. Dominant grasses significantly reduced their productivity in drought treatments although not after a 3-year time lag. Shrubs, on the contrary, first did not respond to drought and then increased their productivity relative to controls. No effect of N fertilization was observed in aboveground primary production neither in species composition. Belowground root productivity responded in a parallel fashion to aboveground plant production. Surprisingly, belowground microbial community composition showed the opposite pattern than aboveground plant productivity. The identity composition of microorganisms did not change as a result of prolonged drought but significantly responded to nitrogen fertilization. Our results point out interesting differential responses of plant and microbial communities in terms of their response to water and N availability and contrasting responses of grasses and shrubs to drought. Ammonia oxidizing groups responded differently to changes in the availability of the species of N, which were modified by fertilization but they responded similarly to prolonged drought. Microbial species are apparently segregated by their unique roles in the N cycle and are specific to a type of substrate. On the contrary, the differential response of grasses and shrubs is associated with rooting patterns.