Recovery Of Waste Nutrients By Duckweed For Reuse In Sustainable Agriculture: Second-Year Results Of A Field Pilot Study With Sorghum

Between 62% and 92% of industrial and municipal wastewater in upper-middle, low-middle, and low income countries is discharged to the environment untreated, releasing valuable nutrients such as nitrogen (N) and phosphorus (P) into rivers, lakes, and oceans (Lipponen and Nikiforova, 2017). This, in addition to excess nutrients often present in agricultural runoff due to overuse and misuse of fertilizers, can lead to eutrophication, often causing irreparable damage to aquatic ecosystems. For these reasons, new techniques must be found to effectively recover waste nutrients and upcycle them into natural soil amendments that can be used to enrich soil quality and grow food for future generations while minimizing agricultural runoff. Duckweed is a small floating aquatic plant that can hyperaccumulate nutrients present in wastewater and agricultural runoff and then be harvested and reused to replace or supplement commercial soil fertilizers. As part of a two-year field trial, duckweed was tested for the second consecutive year in this study as a soil amendment in comparison to, and in combination with, commercial fertilizer for the growth of sorghum, a drought-resistant grain. Relative to fertilizer in all cases, soils amended with duckweed generated less ammonia and nitrate in surficial runoff. No differences in P in cumulative runoff were found among the different treatments (p = 0.509). Additionally, duckweed application produced sorghum grains with greater N and P content than other treatments (1.63 +/- 0.03% N (p = 0.001) and 0.35 +/- 0.0% P (p = 0.016)). Duckweed treatments also showed increased soil residue carbon and P after harvesting the crop. When normalized by germination rate, sorghum yield was similar across treatments. In agreement with first-year findings, the results indicated that duckweed may be a viable alternative to commercial fertilizer from an environmental and agricultural perspective, providing acceptable yields and contributing to the buildup of beneficial nutrients in the soil profile. Additional testing is needed to further evaluate potential germination inhibitors, greenhouse gas emissions (ex., N2O), and efficacy when applied to different crops and soil types.