Nitrogen use by warm-season grasses for biomass production

Perennial, warm-season grasses are being evaluated as potential renewable energy crops. These species are well-suited for the production of biomass for energy applications because they utilize C4 photosynthesis and are perennial. Grasses that employ the C4 photosynthetic pathway use water, nitrogen (N), and solar radiation more efficiently than plants having the C3 pathway, and therefore are generally more productive per unit land area and resource input relative to other potential energy crops. Perenniality also confers important advantages to energy crops, including the ability to cycle nutrients seasonally between shoots and roots, thus improving feedstock quality and minimizing fertilizer requirements for sustained biomass production.

Despite high N-use efficiency by grasses possessing both C4 photosynthesis and a perennial lifecycle, numerous studies have demonstrated that significant fertilizer N inputs are required to optimize biomass production by these species when managed as forage crops.

Although few studies have assessed the effect of N fertilization on yield of perennial, warm-season grasses managed specifically as bioenergy feedstocks, there is emerging consensus that N fertilization requirements should be reduced for single-harvest feedstock management systems relative to multiharvest forage systems, as the latter are characterized by greater N removal as a result of harvest of immature, N-rich biomass.

Allocation of plant nutrients to roots before crop harvest is a desirable trait for energy crops, as a high mineral concentration negatively affects biomass quality for bioenergy applications, especially thermochemical processes.  Additionally, nutrients retained in roots can be recycled by the crop for future growth, thus reducing long-term fertilization requirements. The researchers indicate that previous research on switchgrass and eastern gamagrass found that N fertilization can result in increased shoot N concentrations, and therefore increased N removal with biomass harvest. However, the effect of N fertilization on concentrations of other plant macronutrients in shoots and on the partitioning of biomass and nutrients between shoots and roots is not well established. The researchers indicated that available information suggests that N fertilization is likely to have minimal impact on shoot concentrations of plant macronutrients other than N, and have little effect on root biomass, but potentially favor partitioning of nutrients to shoots over roots.

The objective of this group of researchers was to evaluate the effects of N fertilization on biomass and nutrient partitioning between above- and belowground crop components by four perennial warm-season grasses that show potential for biomass production in the central United States.

Grasses evaluated in the study included one locally adapted cultivar or population each of big bluestem, switchgrass, indiangrass, and eastern gamagrass. Based on previous studies, they hypothesized that yield would respond positively to N, but that optimal N input levels would be lower than those reported in forage-based studies. They also anticipated that N fertilization would alter biomass and nutrient partitioning between shoots and roots, but that yield optimal rates of N fertilization would not reduce the ability of grasses to retain nutrients in roots for future remobilization.

The researchers indicate that N fertilization alters patterns of biomass and nutrient partitioning by perennial warm-season grasses, but that specific N effects can differ quite markedly among species. They found that nitrogen fertilization had generally positive effects on yield, but yield gains beyond 140 kg N ha−1 were minimal for most grasses.

While N fertilization had positive effects on yield by all grasses, root biomass and nutrient content responses were more complex. For big bluestem and switchgrass, fertilizer inputs up to 140 kg N ha−1 consistently increased root biomass commensurate with shoot biomass, and maintained a high level of nutrient allocation to roots compared with unfertilized conditions. In contrast, for indiangrass and eastern gamagrass, root biomass was generally adversely affected by N fertilization, as was root nutrient allocation.

The researchers conclude that these findings indicate the management of perennial, warm-season grasses as biomass feedstocks will need to contend with the potentially conflicting effects that N can have on relevant performance criteria. They also believe that the identification of crops and management practices that optimize yield, and resource partitioning to roots at low to intermediate N input rates will promote the development of productive and efficient bioenergy systems by furnishing large quantities of high quality, low cost feedstocks.

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