The invasive annual cheatgrass increases nitrogen availability in 24-year-old replicated field plots

Area of Science:

• Ecology
• Soil Science
• Plant Ecology

Background:

• Previous studies suggest invasive cheatgrass increases soil nitrogen (N) cycling, but correlative data cannot distinguish cause and effect.
• It remains unclear if cheatgrass drives N cycling changes or invades already N-rich sites.

Purpose of the Study:

• To conclusively determine if cheatgrass actively alters soil organic N pools and N cycling rates.
• To compare N cycling under established cheatgrass stands versus native sagebrush-perennial grass communities.

Main Methods:

• Utilized 24-year-old replicated field plots in a sagebrush-steppe ecosystem with different plant communities.
• Measured soil carbon (C) and N concentrations, net and gross N cycling rates (mineralization, nitrification), and soil respiration.
• Analyzed soil samples from 0-60 cm depths and surface soils (0-10 cm) over two years.

Main Results:

• Soil nitrate (NO3-), organic C and N, net N mineralization, net nitrification, and respiration were significantly higher under cheatgrass.
• Gross N mineralization, net N mineralization, and net nitrification rates were faster in surface soils beneath cheatgrass.
• Modeling indicated cheatgrass provides microbes with lower C:N substrates, explaining accelerated N cycling.

Conclusions:

• This long-term study provides conclusive evidence that cheatgrass creates larger soil organic N pools and stimulates N cycling.
• Accelerated N cycling beneath cheatgrass represents a positive plant-soil feedback, potentially promoting its continued dominance.
• Cheatgrass actively modifies soil N dynamics, independent of initial soil disturbance or fire history.