Phytoremediation of P-impacted sites through the production of forage on farm is a nutrient management practice that has potential to ameliorate excess soil P created by intensively managed operations like dairy farms. Bahiagrass (BG; Paspalum notatum), limpograss (LG; Hemarthria altissima), and stargrass (SG; Cynodon nlemfuensis) are prospective forage species that when managed for hay under intensive N fertilization are capable of producing large amounts of dry matter and removing residual soil P. An on-farm study to measure the effects of N fertilization rates (0, 50, 67, and 100 kg N ha-1 per harvest) on P-removal capacity was conducted for the above species using a randomized block design with four replicates for each forage type. Phosphorus removal in herbage tissue was greater for BG, intermediate for SG, and low for LG. N-fertilizer plant uptake efficiency decreased linearly with increasing N fertilization for LG and SG, but showed a quadratic trend for BG. Change in soil P (ΔP) was measured at the beginning and at the end of first year. Soil P decreased in the Ap horizon to 0 (BG and LG) and to -11 mg kg-1 (stargrass). In the Bh horizon soil P was reduced in 21, 12, and 17 mg kg-1 for BG, LG, and SG, respectively. Overall ortho-P concentration tended to decrease with increasing N rate. Although fertilizer N efficiency was reduced with N rate resulting in a linear increase in shallow-well-nitrate levels, nitrate concentrations in both run-off and deep-well samples were not affected by N rate. It is estimated that, in addition to enhanced potential P removal, increasing N rates may also improve forage quality and productivity.
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