Root exudates and senescence contribute to nitrogen cycling in the rhizosphere of cheatgrass and crested wheatgrass

Background/Question/Methods

Cheatgrass promotes the accumulation of inorganic N in soils but the mechanism is not known. We used cheatgrass and crested wheatgrass in a ¹⁵N isotope tracer experiment, where ¹⁵N-labeled urea was supplied to plant shoots, to determine the contribution of root exudates to soil N pools. Plants were grown in mesocosms in a greenhouse and exposed to either moist or dry soil conditions to determine the contribution of soil drying to N pools. Ammonium ¹⁵N pool dilution was used to determine plant and soil moisture effects on soil N transformation rates.

Results/Conclusions

After 75 days of growth cheatgrass soil had more total N and organic C than crested wheatgrass soil. Soil moisture treatments affected soil N cycling rates more than plant species; however, during the 1-week ¹⁵N tracer experiment, cheatgrass roots exuded more than twice as much N (0.11 mg N kg^-1 soil d^-1) as crested wheatgrass roots (0.05 mg N kg^-1 soil d^-1) in both moist and dry soil treatments. We propose that exudation of high N content root exudates leads to the changes in soil N pool size and transformation rates commonly observed in soils under cheatgrass.