In drylands, key ecosystem processes such as primary production are limited by the availability of both water and nitrogen inputs. In dryland soils, biological soil crusts (biocrusts) and plant rhizospheres are hubs of microbial activity, and compositional changes in these communities can influence biogeochemical cycling of nitrogen (N) and carbon (C). The combined effects of manipulating water and N on soil microbes have not been extensively studied in drylands, but such research could help to improve predictions on ecosystem functioning under future climate and global change scenarios. We collected shallow soil and roots in a full factorial monsoon rainfall (“small” 5mm water pulses weekly and “large” 20mm water pulses monthly) and N addition (5g ammonium nitrate per m2 once yearly) experiment at the Sevilleta Long-Term Ecological Research (LTER) site to assess responses of biocrust bacteria and fungi associated with roots of black grama (Bouteloua eriopoda), a dominant Chihuahuan Desert grass species. We extracted DNA from biocrust soils for bacterial 16S rRNA sequencing and assessed roots for fungal colonization.
Results/Conclusions
Biocrusts were dominated by filamentous cyanobacteria, primarily Microcoleus vaginatus and species from the M. steenstrupii complex. The cyanobacteria community was most diverse under small, frequent water additions and was more diverse than either fertilized or unfertilized controls in plots with combined small water pulses and N addition (N X rainfall interaction, P < 0.005). Conversely, N addition alone lowered the relative abundance of cyanobacteria and reduced diversity at both the phylum and species levels (PERMANOVA, P < 0.05). Black grama roots were colonized more by melanized, septate hyphae than by aseptate or mycorrhizal hyphae; consistent with previous studies of this species. N addition alone did not alter root colonization, but when combined with small, frequent water pulses, N addition increased the ratio of septate to aseptate hyphae relative to controls (N X rainfall interaction, P < 0.05). Future work will additionally assess fungal community composition and diversity to further describe microbial responses to nutrient and water dynamics in this ecosystem. Overall, our results indicated that soil bacterial communities were sensitive to N addition independent of rainfall additions, and that both soil bacteria and root-associated fungi were stimulated by the combined resource increases of nitrogen and small, frequent water additions.