Biomass and Macronutrient Accumulation and Losses in Switchgrass During and After the Growing Season
IPNI-2008-USA-AR33
25 Feb 2010
2009 Annual Interpretive Summary
Two switchgrass field studies were established at the University of Arkansas in 2008. One describes growing-season biomass accumulation and NPK uptake curves and the other determines N response curves for biomass yield. The first study consisted of 12 harvest dates, ranging from May to February. Trends in cumulative growth and nutrient concentration were fitted to regression models as a function of day of year. The 2009 data showed that growth followed a typical S-shaped curve. Peak yield occurred at the August 28 sampling date. Yields were essentially constant from September 30 to October 27, and then gradually declined to February 17, 2010. Nitrogen removal increased throughout the season to 76 kg/ha on July 31. Data from later sampling dates are not yet available. Potassium uptake peaked on July 1 at 136 kg/ha and declined to 110 kg/ha by July 31. Phosphorus uptake increased gradually up to July 31 to a relatively low level of 15 kg/ha. Soil test P level was 30 to 35 ppm in the surface 10 cm, which is not considered deficient for switchgrass. Completing the 2009 sample analyses for N, P, and K will shed light on uptake patterns as affected by crop maturity and senescence.
The second study includes treatments of urea applied at 0, 35, 70, 105, and 140 kg N/ha. One biomass harvest was taken in early October. Linear regression analysis of biomass yield showed a slightly positive slope that was not significantly different from zero, and with a very low R2. The lack of significant response to N fertilizer was unexpected; however, switchgrass and other native grasses are known to be efficient in their use of plant available N and do not always show a response. There was inconsistent response of switchgrass growth to N application within the four replications, which were blocked across the field area. There was also variation in plant population density. Switchgrass was seeded in 60 cm spaced rows in early July 2008, and not all rows achieved solid stands. Therefore, the first production year, 2009, consisted of fairly young plants, with some gaps between them. It is expected that in 2010 the plants will tiller out more and fill in the gaps more evenly, resulting in less yield variation within treatment. AR-33