Seasonal drought treatments impact plant and microbial uptake of nitrogen in a mixed grassland on the Colorado Plateau

Authors: FINGER-HIGGENS, REBECCA - Us Geological Survey; Hoover, David; KNIGHT, ANNA - Us Geological Survey; WILSON, SAVANNAH - Us Geological Survey; BISHOP, TARA - Us Geological Survey; REIBOLD, ROBIN - Us Geological Survey; REED, SASHA - Us Geological Survey; DUNIWAY, MICHAEL - Us Geological Survey

Submitted to: Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2024
Publication Date: 8/5/2024
Citation: Finger-Higgens, R., Hoover, D.L., Knight, A., Wilson, S., Bishop, T., Reibold, R., Reed, S., Duniway, M. 2024. Seasonal drought treatments impact plant and microbial uptake of nitrogen in a mixed grassland on the Colorado Plateau. Ecology. Article e4393. https://doi.org/10.1002/ecy.4393.
DOI: https://doi.org/10.1002/ecy.4393

Interpretive Summary: The impacts of drought on plants, such as reduced growth and mortality is well known. However, less is known about how drought impacts plants through changes in soil biogeochemistry. In this study, we experimentally imposed seasonal droughts (cool and warm season) and examined the impacts on both plants (shrubs and grasses) and soils (carbon, nitrogen, and microbes) in a semiarid rangeland in the Colorado Plateau. We found that drought had larger impacts on grasses than shrubs, with the largest declines in grass cover in the warm season drought treatment. In soils, we found that nitrogen cycling was sensitive to drought. On the other hand, carbon stocks and fluxes were most influenced by shrub cover. These results highlight the complex interactions between drought, soils, and plants in water limited ecosystems.

Technical Abstract: For many drylands, droughts cause declines in vegetation and changes in biogeochemical cycles, which can accentuate landscape heterogeneity at both temporal (e.g., role of seasonal patterns) and spatial (e.g., patchy plant cover) scales. Furthermore, declines in precipitation during certain times of the year can exacerbate drought conditions, which can further impact vegetation and microbes, thereby potentially altering the structure and function of these ecosystems. Here, our goals were to examine how changing seasonal precipitation dynamics might impact soil moisture, vegetation, and key biogeochemical cycles, particularly the cycling of carbon (C) and nitrogen (N) as we compare between different experimental seasonal drought treatments: a warm season drought (Apr.-Oct), a cold season drought (Nov.-Mar.), and ambient precipitation conditions over a four-year period. Soil moisture dynamics suggested that warm season experimental drought had longer and more consistent drought legacy effects (occurring two out of the four drought cycles) than either cool season drought or ambient conditions, even during the driest years. To account for landscape patterns, we focused on three dominant cover types, including a shrub, Ephedra viridis, a C3 bunchgrass, Achanatherum hymenoides, and interspace devoid of perennial plant cover, as representative members of the heterogeneous and patchy plant cover of the greater region. We found that E. viridis biomass remained consistent across treatments, while bunchgrass cover declined with time across all treatments, with the earliest declines noticeable in the warm season drought plots. We also found that N cycling, including extractable dissolved inorganic nitrogen (DIN) and microbial biomass N (MB-N), appeared to be more sensitive to drought conditions, with DIN increasing and MB-N decreasing with less soil volumetric water content, while C stocks and fluxes appeared to be most tightly connected to shrub cover. Additionally, we found that under E. viridis, there was a negative relationship between DIN and MB-N, suggesting that drought increases in DIN may be due to declines in N uptake from microbes and plants alike. This work builds upon previous findings that show how temporal and spatial heterogeneity interacts with overall drought effects to better understand plants and soil microbes respond to global change.