Biomass and Macronutrient Accumulation and Losses in Switchgrass During and After the Growing Season

Switchgrass is widely considered to be a likely crop to grow as a feedstock as the cellulosic bioenergy industry takes shape. It is a native, drought-tolerant, perennial grass with high yield potential on a wide range of soil conditions. Switchgrass is reputed to have high efficiency in converting fertilizer nutrients to harvestable biomass and low nutrient removal rates; however, there are very little data available on switchgrass nutrient use and fertilizer needs to substantiate that. Basic information is therefore needed on dry matter (biomass) accumulation and macronutrient uptake patterns during the growing season and their post-frost losses to construct useful nutrient balances.


Justification

Switchgrass is widely considered to be a likely crop to grow as a feedstock as the cellulosic bioenergy industry takes shape. It is a native, drought-tolerant, perennial grass with high yield potential on a wide range of soil conditions. Switchgrass is reputed to have high efficiency in converting fertilizer nutrients to harvestable biomass and low nutrient removal rates; however, there are very little data available on switchgrass nutrient use and fertilizer needs to substantiate that. Basic information is therefore needed on dry matter (biomass) accumulation and macronutrient uptake patterns during the growing season and their post-frost losses to construct useful nutrient balances. During late-season maturation and after frost, the unharvested plant undergoes natural drying and loss of macronutrients via translocation to perennating organs (roots, crown), leaf droppage, and leaching. Very little is known of the magnitude of such losses and their impact on nutrient balance and fertilizer needs. Such knowledge will help growers and bioenergy managers calculate the economically, energically, and environmentally most efficient levels of nutrient inputs. The proposed research is a start in quantifying plant needs of nutrients by constructing growth and nutrient uptake and loss curves and a simple nitrogen response study.


Objectives

1. Describe growing-season biomass accumulation and NPK uptake curves for switchgrass grown for biomass.
2. Characterize post-growing-season losses in biomass and macronutrients in delayed-harvested stands of switchgrass for biomass.
3. Determine nitrogen response curves for biomass yield for second- and third-year switchgrass stands.

Methodology

Location: Arkansas Agricultural Research and Extension Center (Fayetteville), University of Arkansas Division of Agriculture, on Captina silt-loam soils. This soil has depth and hydrology that would represent many upland Arkansas soils in the Ozark region that would potentially be used for growing switchgrass.

Experiment 1 (Growth and nutrient curves): Four replicate blocks will be seeded with cv. Alamo in 2008 (Year 1), and data will be collected in Years 2 (May 2009 to February 2010) and 3 (May 2010 to February 2011). The soil will have medium to medium-high levels of soil-test P and K, and no N will be applied in Year 1. In Years 2 and 3, there will be 12, 10’ x 23’ plots per replicate, and all plots will receive 75 lbs/acre of N as urea on April 1. Treatments will consist of 12 harvest dates, 6 before October 1 (in-season) and 6 from October 1 onward (post-season). The dates for the in-season phase will be May 1, May 22, June 12, July 3, July 31, and August 28 (expected date of maximum dry matter accumulation). Post-season harvest dates will occur every 28 days henceforth, ending on February 12. On February 12, the regrowth of all other plots will be cleared off the field. All harvests will include determination of plant moisture content, dry biomass yield, with sampling for N, P, and K concentrations. N, P, and K uptakes will be calculated by multiplying dry biomass yield by their respective concentrations. Growth and nutrient uptake curves and post-season losses will be determined through regression analysis. Soil will be grid-sampled in Year 1 and at the end of Year 3 to detect overall changes in total N and C, and Mehlich-3 extractable P and K.

Experiment 2 (N response): Six replicate blocks will be seeded with cv. Alamo in 2008 (Year 1), and data will be collected in Years 2 (2009) and 3 (2010). The soil will have medium-high levels of soil-test P and K, and no N will be applied in Year 1. In Years 2 and 3, there will be 5, 10’ x 23’ plots per replicate, each assigned a different nitrogen treatment. Treatments will consist of urea applied on April 1 at the rates of 0, 30, 60, 90, and 120 lbs/acre of elemental N. One harvest per year will be taken after the first killing frost (usually mid-October) for moisture content and biomass yield. Plant biomass will be sampled to determine N concentration and to calculate N removal. Regression analysis will be used to calculate the response functions of biomass and N removal to N application rate and apparent N recovery.