This project attempts to quantify resilience of prairie ecosystems to climate change in the Pacific Northwest (PNW). In this region, prairie ecosystems currently sustain ~1.3 million beef cows and cow-calf production costs are expected to increase to offset drought-induced plant productivity loss. Here, we investigate patterns of asymbiotic nitrogen fixation (ANF) and biogeochemical controls, that also influence plant community composition and prairie productivity, under experimental drought to address a major challenge for sustainable agriculture in the region. We hypothesize that the effect of drought on prairie vegetation cover increases soil asymbiotic N inputs by diminishing the dominance of symbiotic root-fungal networks. To test this hypothesis, we quantified the impacts of decadal drought stress on soil ANF using 15N-labeled dinitrogen (15N2) incubations of soils from high- and low-diversity prairies across a 520-km latitudinal gradient (i.e., southern Oregon-SOR, central Oregon-COR, and central Washington-CWA) representing increasingly severe Mediterranean conditions. We also quantified total soil organic carbon-C, total and available N, and available phosphorus-P and iron -Fe pools to better understand underlying mechanisms governing drought-induced changes in ANF. At each site, composite soil samples (n = 3) were collected from five co-located high- and low- diversity prairie plots under control (ambient) and drought (-40% precipitation) conditions.
Results/Conclusions
We found that soil ANF response to drought increased with the PNW Mediterranean drought intensity gradient; while ANF rates increased nearly twofold in the southernmost site (SOR), significant decrease in ANF was verified in the northernmost site (CWA). ANF response to drought also varied depending on plant diversity, where low-diversity prairies had more predictable response to drought than high-diversity prairies. For instance, ANF in SOR high-diversity prairies was suppressed but no change was verified in COR high-diversity prairies. Soil C and N contents were generally higher in high-diversity prairies whereas treatment had no significant effect across sites. Soil P availability, also affected by drought, and pH were the most important variables explaining ANF variability across vegetation types and sites. Based on our findings, low-diversity prairies in central WA may be those most severely impacted by increased climate-change induced drought stress. Our study highlights the importance of using soil-plant-atmosphere interactions to assess prairie ecosystem resilience to drought in the PNW.