Nutrient Uptake Potential for Three U.S. Native Grasses and Tall Fescue

There is interest in using native grasses for the mitigation of runoff in vegetated buffers and grassed waterways.  However, little research has been conducted into the nutrient uptake potential of these grasses, particularly considering that runoff from some crop production operations may have high concentrations of N or P.  A greenhouse experiment was conducted in Riverside, CA to determine the N and P uptake potential of three U.S. native grasses: Leymus triticoides, Melica imperfecta, Vulpia microstachys, and the non-native tall fescue, Lolium arundinaceum.  Grasses were seeded into a soilless medium in 15-cm diameter pots to a rate of 1000 plants m^-2.  Nitrogen was applied with irrigation water at either 0, 33, 67, or 100 mg N L^-1, for a rates of 0–1380 kg•ha^-1 per twelve-week experimental period.  Water and P application rates were kept constant across treatments, with 26 L of water applied per twelve-weeks and a P concentration of 22 mg•L^-1.  Shoot and root material was collected, dried, and analyzed for N and P concentration.  Statistical differences (P < 0.05) were found among grass species for water uptake, N uptake, P uptake, and apparent nitrogen recovery, suggesting some species may be more effective in removing water, N, and P from runoff.  Least square mean water uptake ranged from 6.2 to 7.6 cm•wk^-1 across species.  Least square mean N uptake ranged from 35 to 60 kg•ha^-1•week^-1 across species, with Lolium arundinaceum having the highest N uptake, and Vulpia microstachys numerically the least.  Least square mean P uptake ranged from 5.5 to 11.2 kg•ha^-1•week^-1 across species, with Lolium arundinaceum having the highest uptake.  In general across species, as N application rate increased, N uptake also increased, though neither Vulpia microstachys nor Melica imperfecta showed any difference in N uptake between the 33 and 100 mg N L^-1 treatments, suggesting that these two grasses had reached their N uptake potential in this range of applied N concentrations, for the conditions of this experiment.  For all species, as N application rate increased, apparent nitrogen recovery decreased, suggesting that these grasses will exhibit a decreased efficiency in scavenging N when N concentrations in applied runoff are higher.  These observations suggest that nutrient uptake potential is limited for these grasses, and that flow paths through grassed buffers or waterways may need to be longer when nutrient concentrations in runoff are high to achieve effective mitigation of nutrients in runoff water.