Behavior of Duckweed as an Agricultural Amendment: Nitrogen Mineralization, Leaching, and Sorghum Uptake

Open Access
Author:
Kreider, Andrew Nathan
Graduate Program:
Environmental Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 13, 2015
Committee Members:
  • Rachel Alice Brennan, Thesis Advisor
  • Maryann Victoria Bruns, Thesis Advisor
  • Ray B. Bryant, Thesis Advisor
  • William D Burgos, Thesis Advisor
Keywords:
  • Duckweed
  • Organic Soil Amendment
  • Yield
  • Leaching
  • Runoff
  • Nitrogen
  • Phosphorus
  • Mineralization
  • Forage Sorghum
Abstract:
Excessive nitrogen (N) and phosphorus (P) in surface waters can promote eutrophication (algae-dominated, low oxygen waters) which decreases water quality and aquatic life. Duckweed (Lemna minor), a floating aquatic plant, rapidly absorbs N and P from water and its composition shows strong potential as a soil amendment. Therefore, it may be used to transfer N and P from eutrophic water bodies to agricultural fields. In this work, dried duckweed was incorporated into agricultural soil in microcosm, column, and field tests to evaluate biological nitrogen cycling, nutrient retention, and crop yield compared to compost, diammonium phosphate (DAP), and an amendment-free control. In microcosm tests, 25 ± 13% of duckweed N was mineralized, providing on average less mineral-N than DAP (107 ± 21%), but more than compost (11 ± 12%). In columns, duckweed treatments leached only 2% of the N added, which is less than the 60% of DAP-N that leached. Nearly 4% more phosphate was leached from DAP treatments than from duckweed treatments relative to the control. Duckweed and compost stabilized additional phosphate-P, likely due to their organic carbon content. Crop yield, as well as runoff N and P, were measured in field tests growing forage sorghum (Sorghum bicolor). Although less total N was applied to duckweed plots than to DAP plots (75 kg/ha vs. 130 kg/ha, respectively), duckweed was found to retain 30% more total mineral-N in a tilled agricultural field than DAP, while supporting a comparable yield. These tests indicate that duckweed may provide a sustainable source of N and P.