Nitrogen conservation decreases with fertilizer addition in two perennial grass cropping systems for bioenergy

Abstract

Warm-season prairie grasses are promising bioenergy crops that exhibit conservative nitrogen use and cycling, which promises to reduce expensive N amendments and contributions to environmental N pollution associated with annual cropping systems. However, efforts to maximize crop yields with fertilizer may reduce N conservation in these systems. We used two perennial grass systems of differing diversity levels-a restored tallgrass prairie and a Panicumvirgatum(switchgrass) monoculture-to determine the effects of N fertilizer level and harvest timing on biomass yields, N concentrations and N removal at harvest. To address plant N conservation, we measured N resorption efficiency (proportion of N resorbed), resorption proficiency (minimum N level attained after senescence), and timing of resorption under different N fertilizer rates. Yield responses to N fertilizer were not consistent between cropping system, year, or harvest timing, and were generally weak, resulting in an average of only 1.27 times more biomass compared to unfertilized plots. In contrast, fertilized plots removed 1.67 times more N relative to unfertilized plots, as N removal was largely driven by increases in biomass N concentrationsrather than increases in yield. N resorption was affected by fertilizer in switchgrass, but not in selected prairie species. Fertilized switchgrass plants took longer to reach their maximum resorption levels and had reduced resorption proficiency, despite higher resorption efficiencies. Our results suggest that striving to increase biomass yields with N fertilizer may be an imprudent approach to sustainable bioenergy production, because yield responses are highly variable and N conservation mechanisms are compromised.

Cellulosic biofuel contributions to a sustainable energy future: Choices and outcomes

“…(ii) Relatively little N is removed in harvested biomass; unlike annual grain crops that are bred for protein-rich seeds, most cellulosic crop biomass is harvested as lower-N vegetative tissue or wood. (iii) If harvest is properly timed to post-senescence, much of the N in vegetative tissues will have been translocated belowground (75). (iv) Without annual tillage, perennial crops accumulate N in soil organic matter (22).…”

Distribution of switchgrass (Panicum virgatum L.) aboveground biomass in response to nitrogen addition and across harvest dates

“…Carbohydrate retranslocation can decrease switchgrass yield by 15% over a two-month period [44]. Similarly, 20 to 60% of peak N is retranslocated in switchgrass [16,24,31,45]; however, fertilization decreases N retranslocation, suggesting that N addition may compromise the N conservation strategies of perennial grasses [24]. However, much more information is needed to understand the effects of environmental and fertility factors on the timing and rates of retranslocation in switchgrass [46,47].…”

“…Nitrogen (N) addition is recommended to increase and maintain switchgrass yields [1], however, highly variable responses have been reported [13,[22][23][24] and the response to N is uncertain for many perennial grass bioenergy crops [25]. Because exogenous N is a major source of agricultural pollution [26,27], N conservation should be a primary sustainability concern for bioenergy production.…”

“…However, trade-offs among yield, crop quality, and environmental cost or benefit are inherent in decisions about the timing of switchgrass harvest, with economic and environmental implications. Although delaying harvest maximizes nutrient retranslocation, delays also reduce yield [24,[30][31][32][33][34][35] and can result in significant economic loss for farmers [16].…”

Prairies Thrive Where Row Crops Drown: A Comparison of Yields in Upland and Lowland Topographies in the Upper Midwest US

“…Therefore, while harvest efficiency is generally greater in graminoids than forbs, the precise difference in harvest efficiency between graminoids and forbs may depend upon the individual plant species and harvest methods. Earlier harvest may help to improve harvest efficiency, but the yield gain must be weighed against the potential for greater removal of nitrogen in biomass, as plant nitrogen resorption may be incomplete [42]. Additional research is necessary to address this tradeoff in prairie biomass cropping systems.…”