Development of sustainable, native grass-based bioenergy production systems in the prairie region of Minnesota: Biomass production and plant community response to fertilizer and harvest treatments

Background/Question/Methods

Native perennial plants are emerging as an alternative, low-carbon, bioenergy feedstock.  Land restored from crop monocultures to diverse, native plantings has the potential to provide a host of ecological services, as well as farm income.  However, best management practices for maintaining a diverse, healthy stand while promoting biomass production and producing income have yet to be developed.  In this on-farm project, we established a native grass-forb mix, suitable for biomass production and grazing, in a 4-acre (1.6 ha), randomized split-plot block design in 2008.  The mix contained seven native grasses (Andropogon gerardii, Sorghastrum nutans, Schizachyrium scoparium, Panicum virgatum, Elymus canadensis, Bouteloua curtipendula and Pascopyrum smithii) and three forbs (Dalea purpurea, Dalea candida and Ratibida pinnata).  We applied fertilizer treatments (zero, composted cattle manure, half rate (30-10-30 NPK) and full rate (60-20-60 NPK)) to 16 whole plots, with first fertilization occurring in June of 2011.  Harvest of split plots occurred in fall of 2010, 2011 and 2012.  In late summer of each year, we estimated the cover of all plant species, and sampled aboveground primary productivity, separating legumes from other plants within each experimental plot.  Cover data were used to calculate species frequency for each fertilizer x harvest treatment.        

Results/Conclusions

Use of NPK fertilizer increased biomass marginally, but not significantly, and nearly eliminated legumes when applied at the full rate.  The frequencies of S. nutans and B. curtipendula were reduced by application of commercial fertilizer but other grass species were unaffected by fertilizer treatments.  Harvested plots produced significantly more total biomass than unharvested plots across all fertilizer levels.  Harvest significantly increased the frequency and cover of native legumes, but its effect varied by grass species.  Non-native Melilotus spp. significantly increased in frequency and cover in harvested plots.  Inconsistent patterns in fertilizer x harvest interactions for other species makes interpretation complex.  These results suggest that applying fertilizer is counterproductive for maintaining stand integrity, and might reduce net income because of input costs.  In contrast, harvesting (which is necessary to produce farm income) helps maintain desired community structure by fostering the presence of native legumes (which can provide a free source of nitrogen when present) and grass species diversity.  Continued monitoring will determine if these patterns persist.