Surface runoff response of native and introduced grasses under simulated rainfall in southern Alberta

Introduced perennial grassbased pastures (e.g., crested wheatgrass, Agropyron cristatum, and Russian wildrye, Elymus junceus) are promoted as desirable alternatives to natural grasslands (Stipa-Bouteloua-Agropyron spp.) for livestock production systems on the mixed-grass prairie of Alberta. A study was conducted on plots established in 1993 to examine the surface runoff response from natural grasslands, introduced grasses and spring wheat (Triticum aestivum L.) under simulated rainfall in response to pasture and soil characteristics. The grass plots were never grazed, but herbage growth was harvested on an annual basis, while wheat was seeded each year following tillage. Both wheat and crested wheatgrass were less able to retain applied water; with low initial abstraction (5.1-5.7 mm), runoff peaked earlier (14-26 min) and with higher intensity (65.1-68.8 mm h(-1)) giving larger values of the rising limb factor (0.61-1.10). Two grasslands dominated by native species showed more desirable runoff characteristics; with high initial abstraction (9.6-13.3 mm), runoff peaked later (32-54 min) and with lower intensity (32.7-45.8 mm h(-1)) giving smaller values of the rising limb factor (0.13-0.35). Russian wildrye had an intermediary response. A linear model identified that to reduce the rising limb factor and amount of runoff generated after 30 min, the antecedent conditions of ground cover (%), litter dry matter (DM) and dead herbage DM were singularly important variables and the native grasslands had significantly higher levels of these. Importantly, fine and medium litter fragments of natural grasslands had higher water-holding capacity (>3.0 g g(-1)) compared with litter of the introduced species(similar to 2.5 g g(-1)). However, when variables were considered in combination runoff decreased with higher ground cover and increased with higher soil water content. Levels of total N and total P in surface runoff were generally low (< 2 mg N L-1, <1 mg P L-1), but ammonium and reactive P export from the native species was greater than for the introduced species. Suspended sediment yield did not differ among the grassland treatments, but was significantly higher for wheat. Less runoff was generated on grasslands that had high amounts of litter, dead standing herbage and ground cover. Increasing the amount of litter in pastures by using grazing management may decrease the runoff response and so avoid loss of surface water and soil nutrients.