PS 40-96 - Direct and indirect effects of soil nitrogen fertilization on plant competition through altered microbial interactions

Abstract: Understanding the ecological consequences of nitrogen eutrophication, a pervasive component of global environmental change can improve predictions on future community dynamics. Eutrophication may alter species interactions, but these indirect effects have received less attention than the direct effects of nutrient additions. In dryland ecosystems, nitrogen is often the second most limiting resource and therefore has large potential to alter ecosystem processes by changing the outcome of lower-level species interactions. Here, we used a 15+year soil-nitrogen field experiment to evaluate whether nitrogen-induced changes to microbial assemblages in dryland biological soil crusts (Biocrust) could indirectly change the outcome of competition between co-dominant foundation plant species (black grama, Bouteloua eriopoda, and blue grama, Bouteloua gracilis). We asked, does long-term nitrogen fertilization alter the outcome of plant competition indirectly through changes in their interactions with biocrusts? Our experiments were motivated by the observation that nitrogen addition slowed the long-term take-over of blue grama by black grama, suggesting relatively greater benefits of soil nitrogen to blue grama. Blue grama biomass was 275% larger in fertilized plots than in control plots, and black grama biomass was 8% smaller. We planted greenhouse pots with field-collected biocrusts from control or chronically nitrogen-fertilized plots and manipulated plant competition by planting two individuals of the same species (intraspecific competition) or one individual each of the co-dominant species (interspecific competition). Replicate experiments altered pot size to intensify competition (low or high) and timing of plant addition to alter priority effects. Under low-intensity competition with no fertilization legacy, blue grama was 64% larger during interspecific competition than during intraspecific competition. However, in soils with a history of fertilization, blue grama was not larger than in control soils whether competing against black grama or with itself. In addition, under low-intensity competition and with no fertilization history, Black grama was not significantly smaller during interspecific competition than during intraspecific competition. Under high-intensity competition with live biocrusts from fertilized plots, blue grama was 400% larger during intraspecific competition than during interspecific competition. Under high competition, the combination of field nitrogen legacy and sterilization equalized inter and intraspecific competition for black grama, which grew 43% - 72% larger in interspecific competition than during intraspecific competition with live biocrust from plots with a legacy of nitrogen. These results were inconsistent with the takeover pattern observed in the field, suggesting that interactions with biocrusts did not explain how long-term nitrogen addition slowed black grama take-over of blue grama.