High-impact grazing increases soil nutrient supply and exotic plant invasion

The benefits of changing cattle grazing practices on native plant communities in the western United States remain controversial, due in part to a lack of experimentation. In 1997, we initiated an experimental study of two rangeland alternatives, cattle removal and short duration, high-impact grazing and compared grassland soil and plant responses with more conventional, moderate grazing practices.  Soil nutrient supply rates were measured with PRS™ - probes and the plant community was monitored in 2001-2003 in a high-elevation, semi-arid grassland near Flagstaff, Arizona.  The soil nitrate (NO₃^--N) supply rate responded to grazing treatments as well as to seasonal variation in moisture.  A lab incubation in 2001 showed that high-impact grazing had a 30% greater potential NO₃^- supply rate than the moderate grazing treatment.  We then buried the PRS™ - probes in the field during the fall season and the high-impact grazing treatment showed the greatest NO₃^- supply rate in 2 of the 3 years.  We also observed a trend of higher H₂PO₄^-/HPO₄^2- supply rate during the wet season in the high intensity grazing treatment, however, this was not statistically significant.  Increased availability of soil nutrients may help explain a two-fold increase in plant cover by exotic plant species in the high-impact grazing plots despite a treatment duration of only 24 hours.  In contrast, the cattle removal treatment showed a trend of decreased NO₃^-, but the plant community was not significantly different from the moderate-grazing treatment.  Soil nutrient supply may be an important predictor of exotic plant invasion and an indicator for livestock management.