Influence of grazing on soil nutrient supply measured using PRS probes in riverbank of Lower Little Bow River

Background/Question/Methods

Riverbanks are the critical interface linking agricultural land use on soils to water quality in the river.  Unrestricted grazing of native pastures by cattle that are located adjacent to rivers or streams may cause water pollution by fecal and urine deposition, as well as physical damage to riparian zone such as compaction, pugging, bare soil, and streambank erosion.  Streambank fencing is a possible beneficial management practice or BMP to protect rivers and riparian zones from cattle.  We used PRS (Plant Root Simulator) probes (anionic resin exchange membranes) to study the influence of grazing on nutrients (NO₃-N, P) and micronutrients (Cu, Fe, Mn, Zn) in soils on the riverbank of the Lower Little Bow River in southern Alberta, Canada.  Impacts of grazing treatment were determined in four time periods from 2012 to 2014.  We hypothesized that soil nutrient supply would be higher in unfenced than fenced reaches and higher at sites with high cattle activity.

Results/Conclusions

Streambank fencing reduced soil P, Fe and Mn, but did not impact soil nitrate. A strong positive correlation between P and Fe indicated that fencing reduced soil P supply by increasing redox potential rather than by reducing direct fecal deposition from cattle. Supply rates of Cu higher at sites with vegetation, which were attributed to rhizosphere effects.  The use of ion exchange membranes installed in situ was useful to evalautate impacts of streambank fencing and cattle activity on nutrient availability in riverbank soils.