Grazing impacts on soil nitrogen and phosphorus under Parkland pastures

Because intensive grazing is new to the humid western Canadian parkland (pr airies), there is little information available about its effects on soil N and P status. This study addressed the question of grazing intensity and pa sture species effects on soil macronutrient status in a Typic Haplustoll at Lacombe, Alberta. Paddocks of smooth bromegrass (Bromus inermis Leyss.), m eadow bromegrass (Bromus riparius Rhem.), and winter triticale (X Triticose cale Wittmack.), replicated 4 times, were subjected to 3 grazing intensitie s (heavy, medium, and light as defined by frequency and severity of defolia tion) using yearling beef heifers. Nitrogen (N), P and K fertilisers were b roadcast annually at 100, 22 and 42 kg ha(-1) during production years. The experiment was maintained on the same paddocks for 4 years. In the establis hment year and in the third and fourth production years, soil samples were taken randomly from each paddock to a depth of 60 cm. Concentrations of nit rate-N (NO3-N), ammonium-N (NH4-N), mineral-N (the sum of NO3-N and NH4-N), total Kjeldahl-N, and extractable-P were determined in the 0-15, 15-30, 30 -60, and 0-60-cm depths. Nitrate-N concentration was (1.7 to 2.4 times) gre ater for heavy than light grazed treatments for each soil depth increment a nd the amount of NO3-N in the 0-60 cm depth was 2.2 times greater than ligh t paddocks. More NO3-N was measured under perennials than triticale (22.2 v s 13.6 mg kg(-1), respectively) at the 30-60-cm depth. Ammonium-N amount (0 -60 cm) was greater in meadow bromegrass (30 kg ha(-1)) than in triticale ( 25 kg ha(-1)), but not smooth bromegrass paddocks for the 0-15-cm depth. Ex tractable-P concentration was greater in the 0-15-cm depth of heavy (154 mg kg(-1)) than in medium (138 mg kg(-1)) or light-grazed (127 mg kg(-1)) pad docks and was higher under meadow bromegrass than under triticale. Given th e large amounts of NO3-N in the heavy paddocks, there is potential for loss through both leaching and denitrification. Differences among treatments fo r NH4-N, and P concentrations are not of particular concern environmentally , but are important from a fertility management point of view.