Within the context of northern tall grass prairie restoration in the USA, a study was undertaken at the NDSU Albert Ekre Grassland Preserve, southeast North Dakota (46°33′N, 97°7′W), to identify the most efficacious seed mix and fertilizer addition combinations.
The experiment comprised 50 species mixtures fertilized with nitrogen (N) or phosphorous (P) at high (200 kg/ha/yr for N; 40 kg for P), or low levels (20 kg/ha/yr N; 4 kg P). N and P were applied in the early spring year as slow release prills.
In plots (3 x 3 m) 1, 2, 5, 10, or 20 plant species were hand broadcast and covered with a thin layer of soil (10 replications/treatment; total 400 plots). Most were sown in autumn 1999 (some in the following spring). Seeding rate was 400 seeds per m2 (equal amount/species).
Aboveground biomass by species, species foliar cover and number of species per plot was assessed in sample quadrats (2000-2004).
Overall, results indicate that invasion by unsown (and mostly undesired) species drastically declined as the number of species sown/established increased, and where there was greater functional form richness.
Overtime, aboveground biomass increased and year-to-year variation in productivity declined as species and functional form richness rose. Highest biomass occurred in the high N treatment, and next the low N. P addition had little effect, as expected, due to high available P in the soil prior to the experiment.