Soil Carbon Sequestration by Switchgrass and No-Till Maize Grown for Bioenergy

Abstract

Net benefits of bioenergy crops, including maize and perennial grasses such as switchgrass, are a function of several factors including the soil organic carbon (SOC) sequestered by these crops. Life cycle assessments (LCA) for bioenergy crops have been conducted using models in which SOC information is usually from the top 30 to 40 cm. Information on the effects of crop management practices on SOC has been limited so LCA models have largely not included any management practice effects. In the first 9 years of a long-term C sequestration study in eastern Nebraska, USA, switchgrass and maize with best management practices had average annual increases in SOC per hectare that exceed 2 Mg Cyear −1 (7.3 Mg CO 2 year −1 ) for the 0 to 150 soil depth. For both switchgrass and maize, over 50 % of the increase in SOC was below the 30 cm depth. SOC sequestration by switchgrass was twofold to fourfold greater than that used in models to date which also assumed no SOC sequestration by maize. The results indicate that N fertilizer rates and harvest management regimes can affect the magnitude of SOC sequestration. The use of uniform soil C effects for bioenergy crops from sampling depths of 30 to 40 cm across agro-ecoregions for large scale LCA is questionable.

Dedicated Energy Crops and Crop Residues for Bioenergy Feedstocks in the Central and Eastern USA

“…The Central-East RBRC, in partnership with the USDA National Institute for Food and Agriculture-funded CenUSA Bioenergy program, has evaluated the use of switchgrass, big bluestem, and indiangrass for producing perennial energy crops in an environmentally sustainable manner. Recent findings include (i) identifying that sufficient land exists in the US Corn Belt to support a cellulosic ethanol industry using both corn stover and switchgrass without impacting productive cropland [10], (ii) the potential of perennial bioenergy crops to sequester soil carbon [11], (iii) release of a bioenergy-specific switchgrass cultivar [12], (iv) documenting that switchgrass ethanol yield potential is similar to corn grain and stover ethanol potential on marginally productive cropland [13], and (v) documenting that existing field-scale and watershed-scale model refinements to address spatial and temporal environmental impacts on cropland conversion to perennial energy crops [9,14]. Further advancements in genomic selection and concurrent cost reductions likely will allow for accelerated rates of genetic gain (i.e., increased biomass production, increased winter hardiness) in perennial grasses [9].…”

“…The economically optimal N rate was >12 kg N ha −1 less with stover removal compared with no stover removal under conservation tillage [74]. Stover removal tends to increase early growing season soil temperature resulting in increased crop development rates [66,74] but also can lead to increased evaporation rates especially under drought conditions resulting in decreased yields [11,64]. Ongoing, long-term REAP sites are being monitored for grain yield and soil fertility changes from nutrient removal under continuous stover harvests.…”

From Tiny Microalgae to Huge Biorefi neries

“…All bioenergy cropping systems evaluated in our study had SOC sequestration rates exceeding 7.3 Mg CO 2 yr 21 (Table S4 in File S1), with over 50% of SOC sequestration occurring below the 0.3 m soil depth [15]. Soil organic C increased even with corn stover removal, indicating that removal rates were sustainable in terms of SOC and grain yield for this time period.…”

“…Average changes in total SOC (0-1.5 m) from 1998-2007 were used to estimate direct soil C changes. Further management practices and detailed soil property values from this study have been previously reported [15,20]. Summary of petroleum offsets (GJ ha 21 ), ethanol production (L ha 21 ), greenhouse gas (GHG) emissions (g CO 2 e MJ 21 ), net GHG emissions (Mg CO 2 e ha 21 ), and GHG reductions (%) for corn grain, corn grain with stover removal, and switchgrass are presented in Table S1 in File S1.…”

“…corn stover, dedicated perennial energy grasses) are expected to have lower GHG emissions than conventional gasoline or corn grain ethanol [9][10][11][12][13]. Furthermore, dedicated perennial bioenergy crop systems such as switchgrass (Panicum virgatum L.) have the ability to significantly increase SOC [14][15][16] while providing substantial biomass quantities for conversion into biofuels under proper management [17,18].…”