Anthronutrients: Soil Amendments for Closing Urban to Rural Nutrient Cycles

Jessica Nicksy1, Brian Amiro1, Martin Entz2

1Department of Soil Science, University of Manitoba 2Department of Plant Science, University of Manitoba

Highlights

IntroductionNutrient export from agricultural land can lead to nutrient deficiency and system collapse over time, especially in organic systems where options for nutrient input are limited (Welsh et al. 2009). Phosphorus (P) deficiency is apparent on many organically managed farms on the northern Great Plains, particularly those that have been in organic production longest (Entz et al. 2001). Nutrients loss from the food system contribute to environmental problems like eutrophication, and increases the need for import of nutrients from outside the food system. Phosphate rock, used to make phosphate fertilizer, is a non-renewable mined resource, with some estimates indicating resource depletion in 70-140 years (Li et al., 2018). Diverting more food and human “waste” products back to farms, rather than allowing them to enter landfills or waterways, is necessary for long term food system viability and sustainability.

Amendments flowing through urban and peri-urban environments that contribute to cycling nutrients back onto farms are referred to as “anthronutrients” in this project, as shown in Figure 1.

Figure 1: Simplified food system nutrient flow model, highlighting how anthronutrients cycle nutrients within the food system.

Materials and Methods Results

StruviteNH4MgPO4 • 6H2OStruvite is a hydrated

ammonium magnesium phosphate mineral that

can be precipitated from municipal wastewater or animal waste, capturing

phosphorus before it enters waterways. This

study uses struvite precipitated from municipal waste.

Anaerobic Digestate

Anaerobic digestate is produced when organic

material is decomposed in the absence of oxygen,

generating methane which can be captured and used to generate electricity. The

digestate in this study is derived from municipal green waste and food

processing waste.

Black Soldier Fly Larvae Frass

Black soldier fly larvae can feed on a variety of organic

matter. The frass used in this study is the waste

product of these larvae grown on a diet of urban

food waste. The larvae are produced as a high protein

animal feed, while the frass can be used as a soil

amendment.

Anthronutrient Profiles

Wheat Field TrialA fully factorial wheat experiment with a randomized complete block design with 4 replicates was conducted in the summer of 2019 on severely P depleted soil (3ppm Olsen-P) near Libau, MB. Amendments, summarized in Table 1 were broadcast and incorporated in plots at a rate of 20kg/ha total P. Two wheat varieties were used: AAC Brandon, a popular variety in Manitoba, and BJ08-IG,

Amendment % P % N N:PStruvite 12.2 5.7 0.47

Frass 0.87 3.2 3.66Digestate 2.75 3.8 1.38Compost 0.23 0.71 3.12

MAP 22.72 12.2 0.54Unfertilized - - -

Ryegrass Pot Trial

a line selected by a MB farmer under low phosphorus organic conditions through the Participatory Plant Breeding program at the University of Manitoba. Above-ground biomass was sampled at stem elongation, anthesis, and maturity, and grain yield was determined by combining whole plots.Table 1: Phosphorus and nitrogen content

of amendment treatments.

As shown in Table 1, the amendments vary in nitrogen to phosphorus content, leading to variable rates of N applied. A fully factorial ryegrass pot study was conducted with all amendment treatments in the presence or absence of non-limiting nitrogen in the form of dissolved urea (+N vs. –N). Pots were filled with 1915g dry soil equivalent from the wheat field, amendments were added at 20kg/ha P based on pot surface area, 10 ryegrass plants were established per pot, and 90kg N/ha as dissolved urea was added to nitrogen positive pots. Above-soil biomass was harvested four times, approximately one month apart.

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Amendment Effect on Wheat Yield

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Conclusions

Variety and the amendment by variety interaction did not have a significant effect on grain yield, but amendment did significantly affect yield, as shown in Figure 2. Digestate and frass significantly improved yield over the unfertilized control, with frass yielding as well as the synthetic fertilizer. Figure 2: Wheat grain yield as affected by amendment.

Amendments with the same letter are statistically similar. Error bars are + and – one standard error.

The effect of N by amendment interaction was significant for cumulative ryegrass biomass, as show in Figure 3. All treatments without added N produced less cumulative biomass than pots with added N, but the frass treatment distinguished itself from other amendments by producing significantly more biomass than the control. Frass may have beenable to provide more N to the plants under the nitrogen limited conditions. In the presence of non-limiting N, all sources yielded more biomass than the control, likely due to P supply.

Frass and digestate showed improvement over the control in both field and +N pot study conditions, with frass increasing yields more than the digestate in both cases. However, struvite showed improvement only in the +N pot study condition. Factors such as water availability and nutrient placement may have influenced these differences.

Anthronutrient sources have the potential to supply phosphorus and other essential nutrients in organically managed systems to varying degrees. In these trials frass has shown the greatest potential, improving yields similarly to the soluble synthetic fertilizer in both the field and non-limiting N in the pot study. Digestate performed similarly to compost, the typical organic nutrient source, in the field. While struvite did not improve yield compared to the control in the field, it performed well in the greenhouse. Future work on this project will include replication of the wheat experiment in 2019, and testing of N and P tissue concentrations in all biomass samples will allow determination of nutrient supplying power.

ReferencesEntz, M.H., Guilford, R., and Gulden, R. 2001. Crop yield and soil nutrient status on 14 organic farms in the eastern portion of the northern Great Plains. Can. J. Plant Sci. 81: 351–354. NRC Research Press Ottawa, Canada.Li, B., Boiarkina, I., Young, B., Yu, W., and Singhal, N. 2018. Prediction of Future Phosphate Rock: A Demand Based Model. J. Environ. Informatics 31: 41–53.Welsh, C., Tenuta, M., Flaten, D.N., Thiessen-Martens, J.R., and Entz, M.H. 2009. High yielding organic crop management decreases plant-available but not recalcitrant soil phosphorus. Agron. J. 101: 1027–1035.AcknowledgmentsThe author would like to thank the Natural Systems Agriculture group & University of Manitoba Graduate Fellowship.

• “Anthronutrients” cycle nutrients from urban food and human waste back onto farms, a necessity for long term food system sustainability

• Three anthronutrient sources are evaluated: mineral struvite from municipal wastewater, anaerobic digestate from municipal food waste, and insect frass from urban food waste

• Anthronutrient sources have good potential to improve yields in P-depleted organic systems; frass produced the highest wheat yields, which were statistically similar to those from synthetic fertilizer

Figure 2: Cumulative ryegrass biomass by amendment and nitrogen treatment. Treatments with the same letter are statistically similar. Error bars are + and - one standard error.

Research ObjectiveThis research evaluates the agronomic potential of three sources of anthronutrients in phosphorus depleted organically managed systems. Typical organic and conventional sources, and an unfertilized control, are included for comparison.