Sustainability of Perennial Grasses as Bioenergy Feedstock for the Southeast

William F. Anderson1, Joseph E. Knoll1, Tim Strickland2, and Bob Hubbard2

1 USDA-ARS Crop Genetics & Breeding Research Unit, Tifton, GA; 2 USDA-ARS Southeast Watershed Research Unit, Tifton, GA

ABSTRACTWarm-season perennial grasses will be part of the biomass production system in the Southeast for the emerging bioenergy industry. Among the candidates for dedicated feedstocks are energy cane (Saccharum sp.), Miscanthus x giganteus, switchgrass (Panicum virgatum), and napiergrass (Pennisetum purpureum). Two studies have been conducted to assess yields, and assess soil carbon and nitrogen. One study was initiated in fall 2005 at Tifton, GA, to assess the performance of perennial grasses under rainfed conditions with no fertilizer inputs. The test consisted of four replications in a randomized complete block design, and included one energycane cultivar, two napiergrass genotypes, two giant reed clones, one Erianthus and two switchgrass genotypes. Study 2 was initiated in the fall of 2006 and consisted of napiergrass grown under three rainfed fertilizer treatments (no additions, poultry liter and inorganic fertilizer at approximately equivalent N, P, and K rates). Total shoot biomass was harvested and weighed each year in winter, and was analyzed for total carbon and nitrogen content. Soil samples were collected periodically to assess possible changes in soil carbon and nitrogen. Under no inputs dry matter (DM) yield was highest in the second year. Averaged over three years, DM yields of energycane, and napiergrass were significantly higher than switchgrass. Switchgrass had higher nitrogen use efficiency, based on the nitrogen content of the harvested plants. Poultry liter and inorganic fertilizer treatment of napiergrass resulted in similar yields each year and were 17% and 48% greater than the unfertilized control in the second and third year of growth, respectively. Carbon/nitrogen cycling and water quality were monitored as well.

MATERIALS AND METHODS

Test 1 consisted of four replications in a randomized complete block design. The entries were two giant reed clones, two napiergrass (Merkeron and N51), two switchgrass (GA Exp.), one energy cane (L79-1002) and one Erianthus clone. Plots were 4 m × 7 m, and were established in fall, 2005 at Tifton, GA. Total above-ground biomass was harvested annually in the winter (after senescence) and weighed fresh. A subsample was taken and weighed fresh, then dried and weighed again to determine % dry matter (DM). The dried samples were then ground to fine powder. C and N content of the samples was determined and nitrogen use efficiency (NUE) was calculated based on the actual amount of nitrogen taken up by the harvested crop. The soil at the test site is a Tifton loamy sand. Soil samples were taken from the surface (0-15 cm) in spring 2007 and 2008, and 1 m soil cores were taken in spring 2009. Total C and N content of the soil samples was also determined by dry combustion.Test 2 consisted of three replications of napiergrass grown under rainfed conditions with three fertilizer treatments. Napiergrass was initiated in the fall of 2006 on Tifton sandy loam soils. The three treatments were: 1) control (no fertilizer), 2) poultry litter (1:1:1 N:P:K ratio at 84 kg/ha available N), 3) inorganic fertilizer (simulating the poultry litter application amounts). Fertilizer treatments were applied each spring starting in 2007. Three 10’ x 10’ plot areas were harvested and weighed from each plot and averaged. Samples were processed for nitrogen uptake.

Figure 1. Average annual dry matter (DM) yield of bioenergy crops grown under rainfed conditions with no additional fertilization at Tifton, GA, for four years. Error bars represent one standard deviation.

Figure 2. Average annual DM yield/ kg nitrogen plant tissue (kg DM/kg N uptake) of bioenergy crops grown under rainfed conditions with no additional fertilization at Tifton, GA, for four years. Error bars represent one standard deviation.

CONCLUSIONSTest 1: Energycane, Erianthus and napiergrass were capable of producing large quantities of biomass in South Georgia under minimal inputs for two, or even three, years, but yields tend to decline if no fertilizer is applied. Switchgrass had the highest nitrogen use efficiency in this test (based on nitrogen content of the harvested biomass). Though slight changes in soil C and N were observed, individual entries did not significantly affect soil carbon or nitrogen content over the first three years. Test 2: Yields of napiergrass with no inputs significantly declined during the third year of growth. The application of either organic or inorganic fertilizer at a rate of 84 kg/ha appears not to be sufficient to maintain yields of napiergrass. There were some slope effects on yield within plots.

ACKNOWLEDGEMENTSWe would like to thank Freddie Cheek and Tony Howell for assistance in the field, Lorine Lewis for assistance with C/N analysis.

Test 1 site near the end of the fourth growing season. Giant reed is in the foreground. Behind is switchgrass and napiergrass.

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Figure 3. Average annual DM yield (Mg/ha) of napiergrass grown with no fertilizer inputs (Control), with 1:1:1 84 kg/ha (NH4)2SO4 :K2SO4:P2O5 (Inorganic), or with chicken litter with approximately same amount of nutrients (Organic)

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Test 2 - Variation of yields due to slope within treatments