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Growth and nutrient removal efficiency of duckweed in beef feedlot runoff in laboratory and greenhouse conditions 

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  2021 ASABE Annual International Virtual Meeting  2100934.(doi:10.13031/aim.202100934)
Authors:   Jacob L Richardson, Andrea M Harris, Mara Zelt, A. M. Schmidt
Keywords:   Beef Feedlots, Duckweed, Nutrient Efficiency, Runoff Treatment, Wastewater Treatment

Abstract. Across the US, runoff from open-air beef feedlots is confined in holding ponds prior to irrigation on nearby fields. However, nitrogen loses during storage due to volatilization, and inability to transport the runoff water far from the source contribute to local and regional imbalances in nutrient use. This project seeks to assess the possibility to incorporate duckweed (Lemnoideae) into runoff holding ponds in Nebraska as both a partial cover to mitigate volatilization and to transform nutrients in water into a potentially more mobile form. Two parallel studies were conducted to determine the viability of duckweed in feedlot holding ponds in Nebraska. A laboratory study was conducted to measure weekly dry mass production over a one-month period for three duckweed species (Lemna minor, Spirodela polyrhiza, and Lemna gibba) grown on holding pond effluent at 10, 15, 20, and 25 C to simulate weather conditions in Nebraska during a growing season. Two of the duckweed species (Lemna minor and Spirodela polyrhiza) were used in the second study to determine the effect a growing cover of duckweed would have on nutrient loss. The second (greenhouse) study established a completely randomized block design of three replicates of four treatment types - Lemna minor, Spirodela polyrhiza, a covered control, and an uncovered control. Nutrient concentrations were measured in the water and duckweed before and after an 8-week growth period and nitrogen losses were estimated by a mass balance. Preliminary results from the laboratory study indicate that Lemna minor had the highest average mass production ratio (final mass/initial mass) at higher temperatures 20 C+. However, only Lemna gibba grew significantly below 15 C, though both Lemna minor and Spirodela polyrhiza continued to maintain their live-plant mass at lower temperatures. Results from the greenhouse study indicate that nitrogen losses from water growing Lemna minor or Spirodela polyrhiza was not statistically different (α=0.05) from the control (uncovered) -- averaging 382, 375, and 403 mg/m2.d, respectively. However, nitrogen losses from water covered by a loosely with a plastic cover had a significantly lower average N loss of 286 mg/m2.d. Additionally, Lemna minor incorporated an average of 2.8 mg/m2.d N, and 2.5 mg/m2.d P into the plant biomass whereas Spirodela polyrhiza incorporated an average of 2.2 mg/m2.d N and 1.1 mg/m2.d P. Given the results of the two duckweed may have potential for incorporation into beef feedlot holding ponds in Nebraska to increase flexibility of nutrient form but was not very promising for reducing total N lost from the system. Future research efforts will look to determine effective harvest methods and potential uses for biomass produced prior to the establishment of any field-scale experiment.

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