1. CARBON LIFE CYCLE ANALYSIS OF BIOENERGY CROPS PLANTED IN THE MARGINAL SITES IN LOWER MISSISSIPPI ALLUVIAL VALLEY K.C. Dipesh1, Michael A. Blazier1, Hal O. Liechty2, Matthew H. Pelkki2, Montgomery W. Alison Louisiana State University AgCenter, 2Arkansas Forest Resources Center
ABSTRACT
The Lower Mississippi Alluvial Valley (LMAV) region of the southern US may be developed as an agricultural industry because of its long growing seasons and well-developed infrastructures. Short-rotation woody crops (SRWCs) and perennial grasses are high biomass yielding bioenergy crops and could be better-suited for the marginal sites of LMAV region than conventional agricultural crops as crop production on such sites is chronically sub-optimal. With increased production and carbon sequestration in biomass and soils, bioenergy crops may provide landowners with additional income along with ecological benefits. It is, however, important to better understand how conversion of these marginal soils for bioenergy crop production affects carbon pools. We planted eastern cottonwood (Populus deltoides Bartr), ‘Alamo’ switchgrass (Panicum virgatum L.) and soybean-sorghum (Glycine max – Sorghum bicolor) rotation in Arkansas and Louisiana in 2009 to quantify total carbon sequestration and emissions associated with each of the production systems. We carried out ‘cradle-to-grave’ life cycle analysis (LCA) approach in which we evaluated and quantified a complete carbon inventory for every field activity, total biomass (aboveground and belowground) produced, and soil in each production system from the start to the end of crop harvest in We found that soybean-sorghum produced largest biomass carbon. Overall, soybean-sorghum rotation cropping required more carbon release to the atmosphere, followed by cottonwood. We also found total soil carbon (top 30 cm) was higher in the end than at the beginning in Pine Tree and Rohwer, but not in Archibald.
Data
Total input data and output carbon data were recorded for the LCA analysis (Fig. 3). We recorded machine types, their engine horsepower, and their operation hours from the establishment phase to the end of the study in These data were used to estimate total carbon emitted during management. Total carbon emitted during the production of seeds/cuttings and fertilizers/herbicides were also taken into account based on previous studies. Oven-dried biomass of the crops were estimated each year of harvest except for cottonwood, whose aboveground biomass was estimated using the height and diameter at breast height at the end of the study. Belowground biomass and litterfall at the end of the study were also taken into account to estimate total biomass production. Samples of the representative crops were taken to the lab for carbon analysis. We used carbon percentage and biomass measurements to estimate total carbon sequestered by the crops. Change in soil carbon (top 30 cm) during the study period from the sites was also calculated from the soil samples taken in the beginning and at the end of the study.
The Lower Mississippi Alluvial Valley (LMAV) in the South Central United States provides and opportunity to explore the potential of bioenergy crops due to its longer growing seasons and well-developed agricultural industry. Growing the bioenergy crops may also increase carbon sequestration in biomass, accumulate carbon in soils, and with adequate markets, provide additional income to landowners. In 2009, we established a study at three locations to evaluate the potential of cottonwood (Populus deltoides Bartr.) , switchgrass (Panicum virgatum), and soybean-sorghum (Glycine max – Sorghum bicolor) rotation on the marginal lands of the LMAV. A component of the study involves carbon life cycle analysis (LCA) in which we quantified total carbon emitted and sequestered during the conversion of these marginal soils to bioenergy crop plantations. Here, we present the preliminary findings of our study.
INTRODUCTION AND OBJECTIVES
RESULTS
Fig 4. Graph showing total carbon released from different sources during crop establishment and total carbon sequestered by the crops by the end of the study period (AR = Archibald, PT = Pine Tree, and RO = Rohwer)
Soil carbon 2009 and 2013 (Mg/ha)
Study sites
The study was established in 2009 and includes 2 study sites in Arkansas (Pine Tree and Rohwer) and 1 in Louisiana (Archibald) along the LMAV (Fig. 1). Each block consists of three treatments: 1) cottonwood (Fig. 2a) , 2) switchgrass (Fig. 2b), and 3) conventional soybean-grain sorghum rotation (Fig. 2c). Cottonwood cuttings were planted at a density of 4495 per ha-1 in Seeds of ‘Alamo’ switchgrass were drilled in the soil at 11.2 kg ha-1 in 2009 with replanting in 2010 where needed. The row crop plots consist of soybean plantation in 2009, 2011, 2012, and 2014, and sorghum in 2010 and Soybean seeds were planted at 61 kg ha-1 in whereas sorghum seeds were planted at 7 kg ha-1.
METHODS
SCHEMATIC DIAGRAM OF LCA STUDY
Product I
-Biomass (+)
Establishment stage
-Site preparation (-)
-Plantation & seeding (-)
Mid-stage
-Competition control (-)
-Fertilzation (-)
Harvesting stage
-Biomass harvesting (-)
Pre-establishment stage
-Seed production (-)
-Cutting preparation (-)
Post-harvesting stage
-Transportation to the facility (-)
Product II
-Litterfall (+)
Product III
-Change in soil C (+/-)
Inputs
- Gasoline, diesel, lubricants (-)
-electricity (-)
Outputs
Fig 5. Graph showing total soil carbon at the beginning and at the end of the study period (AR = Archibald, PT = Pine Tree, and RO = Rohwer).
We would like to express sincere gratitude to Agriculture and Food Research Initiative of the National Institute of Food and Agriculture, Grant No We would also like to thank USDA Sustainable Agriculture and Research and Education program, Sun Grant, Arkansas Forest Resource Center, and Louisiana State University Agricultural Center.
ACKNOWLEDGMENT
Crops from all sites showed gain in net carbon at the end of the study except switchgrass in Rohwer (Fig. 4). Switchgrass cultivation was not a success in Rohwer for the first three years, and was harvestable in 2013 and 2014 only. However, switchgrass had the highest carbon sequestration at Pine Tree, where soil conditions for switchgrass were better unlike Archibald (ponding problems) and Rohwer (Sharkey clay-shrinking and swelling problems).
Overall, soybean+sorghum sequestered more carbon than the others. The soil conditions did not seem to affect soybean+sorghum as they did to other crops.
Soil carbon in the top 30 cm depth at the end of the study was higher at the end of the study than at the beginning except Archibald (Fig. 5), probably because Archibald was being used as a pasture for 5 years prior to the study.
We are also analyzing plant and soil respiration data along with soil nitrogen data. We will soon be estimating the carbon emission while converting the crop biomass to biofuel in our lab. These data will be used with our current findings for a complete carbon LCA study.
DISCUSSION AND CONCLUSIONS
Fig. 1. Study sites.
Fig. 2. Study site representing cottonwood (a),
switchgrass (b), and soybean-sorghum (c). a b c
Fig 3. System boundary along with variables considered for the study. The
-ve signs indicate carbon loss while +ve sign indicates carbon gain.