Native warm-season grasses of the tallgrass prairie are anticipated to be a significant source of lignocellulosic biofuel feedstocks for renewable energy, while delivering additional ecosystem services such as nitrogen (N) retention. Whether these services are realized will depend largely on how these perennial cropping systems are managed. Over two growing seasons, we compared biomass yields, plant tissue N concentrations, and plant-level N-use efficiency (NUE) under alternative N fertilizer rates and harvest timings of restored tallgrass prairie and monoculture Panicum virgatum (switchgrass) growing on prairie-derived soils in the upper Midwest. We used three different metrics to evaluate the N-use dynamics of the different cropping systems and management treatments: yield-NUE, apparent fertilizer-N recovery, and the ratio of yield to biomass N concentration (yield:[N]).
Results/Conclusions
Yield responses to N fertilizer additions were weak and inconsistent across cropping systems, years, and harvest timings. Nitrogen addition proved to be relatively inefficient, as 50 kg N ha-1 increased biomass by only 11.9 and 6.3 kg per kg of fertilizer-N in prairie and switchgrass, respectively. Regression of biomass yield on [N] showed biomass accrual in switchgrass is more dependent on increases in tissue N than in the prairie, where this relationship was highly variable. Regression tree analysis indicated that species composition, but not species richness, explained more variability in the yield:[N] than fertilizer or harvest management in prairie plots. Cover of C4 grasses and a prairie forb, Helianthus grosseserratus (sawtooth sunflower), were positively correlated to the yield:[N] signifying that this group of species were the most nitrogen-use efficient. These results suggest that selecting particular species assemblages for high NUE can be as effective as applying exogenous N, when growing and harvesting restored tallgrass prairie plants in highly productive soils.