Nitrogen Dynamics in Sweet Sorghum with Winter Crop Rotations in Florida.

Nitrogen can be a major limiting factor for plant growth and yield, and nitrogen cycling plays an important role in crop nitrogen uptake and loss. In large-scale bioenergy cropping systems, nitrogen is a major input necessary to produce reasonable yields, and in Florida sweet sorghum (Sorghum bicolor L. Moench) fertilization requirements are typically 100-135 kg ha^-1 per crop. This demand is currently met almost exclusively by applied chemical fertilizer, which is readily lost from the rooting zone in the sandy soils common to much of Florida. However, rotation crops produced over the winter, with residues soil incorporated prior to sorghum planting, may be able to meet much of the demand for nitrogen in a sorghum crop. The current research focuses on the effects of multiple winter crops, including the dedicated bioenergy crops camelina [Camelina sativa] (low nitrogen input) and sugar beet [Beta vulgaris] (high nitrogen input), and traditional rotations of rye [Secale cereal] and red clover [Trifolium pratense], in rotation with sorghum produced under high and low chemical nitrogen fertilization over three summer growing seasons. Tissue nitrogen concentrations in each crop, as well as seasonal dynamics of nitrogen availability and release into the rooting zone through the use of in-situ measurements of nitrate availability by ion exchange resins, were measured. Total winter biomass yields were highest in sugar beet and clover, and lowest in camelina. Clover contained the greatest amount of N (~120 kg N ha^-1) returned to the soil, followed by beet tops. Release of tissue N in the rooting zone was also highest from clover, followed by beet tops. Sorghum yields under low input nitrogen fertilization were significantly higher following clover rotation than all other rotations, but were lower than sorghum produced under high inorganic N inputs, likely due to the rapid loss of nitrogen from decomposing biomass.