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

A switchgrass growth and composition trial was conducted in 2009 and 2010 at the University of Arkansas. The trial consisted of 12 sampling dates from early May to mid-February. For the 2009-10 growing season, peak yield (6.25 t/A) occurred at the August 28 sampling date. Yields were essentially level from September 30 to October 27, and then followed a gradual decline until February 17, 2010. Moisture content declined linearly in-season and curvilinearly post-season, attaining levels safe for storage of direct-chopped biomass in December onward (<20%). Cutting the stand crop at dates through November would require field curing before packaging and transport to safe storage because of excessively high moisture. Data for the 2010-11 harvest year trended similarly to that of 2009-10. In 2010, moisture content declined to around 50% by September 28. After October 26, moisture content declined more sharply, reaching 16% by December 20. As in 2009, crop moisture content was not safe for direct chopping and immediate storage before December. Nitrogen, P, and K concentrations were determined on sub-samples to determine uptake and removal in the harvested biomass. In 2009, N uptake exhibited a broad peak between July 3 and Sept. 27. The peak N removal was 71 lb N/A on August 28, the same day as peak biomass yield. The senescent period showed N removal rates reduced to about half the peak level, ending at 26 lb N/A in mid-February. Potassium uptake peaked on July 3 at 121 lb K/A (2 months before peak biomass yield) and declined to 20 lb K/A by February 17. There was substantially more K than N removed during June through August, but they declined to similar low levels by winter. Interestingly, peak dates of N and K uptake did not coincide. Phosphorus uptake increased gradually and at very low levels (14 lb P/A) to July 31, then declined to nearly zero by mid-February. Loss of all three nutrients in the latter half of the sampling year likely resulted from a combination of leaf and seed droppage (not measured) during senescence, re-mobilization of mobile nutrients for next year’s growth to roots and the crown, and by leaching from the leaves. AR-33