Composting dairy manure with biochar: Compost characteristics, aminopyralid residual concentrations, and phytotoxicity effects

Authors: NETTHISINGHE, ANNESLY - Western Kentucky University; WOOSLEY, PAUL - Western Kentucky University; STRUNK, WILLIAM - Western Kentucky University; Agga, Getahun; Sistani, Karamat

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/29/2024
Publication Date: 5/1/2024
Citation: Netthisinghe, A., Woosley, P., Strunk, W., Agga, G.E., Sistani, K.R. 2024. Composting dairy manure with biochar: Compost characteristics, aminopyralid residual concentrations, and phytotoxicity effects. Agronomy. https://doi.org/10.3390/agronomy14050952.
DOI: https://doi.org/10.3390/agronomy14050952

Interpretive Summary: Aminopyralid is a weed killer widely used to control broad leaf weeds in pasture and hay fields. Since there is no post application withdrawal time for forage utilization, part of the chemical that is absorbed into the forage can be excreted through the feces of grazing animals. It can also contaminate the feed stocks such as hay, grass bedding material that are added to dairy manure for composting. Concerns have been raised regarding the residual effects of the weed killer on crops grown on soils fertilized with composts derived from aminopyralid contaminated feed stocks and manure. Biochar additive may affect the composting process and immobilize organic pollutants. In this study, aminopyralid was added at 10 parts per billion concentrations to dairy manure mixed with sawdust which was composted for six months with varying levels of biochar additive (0%, 2%, 4%, and 10%). Biochar addition decreased the organic matter and aminopyralid concentrations in a dose-dependent manner, with increasing effect as the biochar level increases. Composting with biochar concentrated higher amounts of nitrogen, phosphorous and potassium in the final product, essential elements for plant growth. Compost with biochar caused only mild injuries to tomato plants compared to a no-biochar compost. Increase of biochar rate from 0% to 10% increased the above ground biomass and mean height of tomato plants. In general, the use of biochar amendment in dairy manure composting can reduce plant damage and enhances the key fertilizer values of the compost.

Technical Abstract: Aminopyralid (2-pyridine carboxylic acid, 4-amino-3, 6-dichloro-2-pyridine carboxylic acid) is an auxin herbicide that has been used widely to control broadleaf weeds in pasture and hay fields. With no post-application withdrawal time, aminopyralid absorbed into forage material can contaminate compost feed stocks such as hay, grass bedding material, and manure. Composts derived from such feed stocks raises concerns about after-effect injuries to sensitive crops by residual aminopyralids. Biochar (BC) additive may affect the composting process and immobilizes organic pollutants. This study examined the effect of composting dairy manure/sawdust 1:1 mixture containing 10 ppb (wet) of aminopyralid with 0%, 2%, 4%, and 10% (w/w) BC levels on chemical and biological characteristics of compost, residual aminopyralid concentration, and intensity of plant injury to tomato(Lycopersicon esculentum L.) plants after composting in 140 L plastic rotary drum reactors for two 6-month cycles. Biochar addition decreased organic matter degradation and intensified reduction in residual aminopyralid levels in a dose-dependent manner. Composting with BC concentrated more N, P, and K, caused mild plant injuries, and increased the above ground biomass compared to the no BC incorporation. Addition of BC for composting aminopyralid-contaminated dairy manure can increase the phyto safety level of compost while enhancing the key fertilizer values.