Preferential flow of phosphorus and nitrogen under steady-state saturated conditions

Authors: MALHOTRA, KRITIKA - Auburn University; LAMBA, JASMEET - Auburn University; Way, Thomas - Tom; WILLIAMS, COLLEEN - University Of Wisconsin; KARTHIKEYAN, K - University Of Wisconsin; BUDHATHOKI, SUMAN - Virginia Tech; PRASAD, RISHI - Auburn University; SRIVASTAVA, PUNEET - University Of Maryland; ZHENG, JINGYI - Auburn University

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2024
Publication Date: 5/12/2024
Citation: Malhotra, K., Lamba, J., Way, T.R., Williams, C., Karthikeyan, K.G., Budhathoki, S., Prasad, R., Srivastava, P., Zheng, J. 2024. Preferential flow of phosphorus and nitrogen under steady-state saturated conditions. Vadose Zone Journal. 23:e20331. https://doi.org/10.1002/vzj2.20331.
DOI: https://doi.org/10.1002/vzj2.20331

Interpretive Summary: Water flow through soil is important for providing infiltration to deliver rain and irrigation water to crop roots, and for reducing runoff water. However, this flow can have detrimental environmental effects, as it allows nutrients and other solutes to be transported by subsurface water flow away from the field area to which they are applied. Soil macropores are cavities in soil larger than 75 micrometers, and they strongly affect water flow through soil. Subsurface water flow typically increases as the sizes and connectivity of macropores increase. Cylindrical soil samples (columns), 15 cm diameter and 50 cm depth, were collected from the fine sandy loam soil of a pasture field in northeastern Alabama. In the laboratory we broadcast-applied broiler litter, which is a mixture of chicken manure and a bedding material, on the soil surface of half of the columns. We simulated rainfall by sprinkling water on the soil surface of each column. Leachate water was collected after it flowed down through the soil columns. The concentration of phosphorus, which is present in broiler litter, in the leachate water was much greater for the columns to which litter was applied, than for the columns to which no litter was applied. These results are expected to be useful in analyzing the movement of water, nutrients, and other solutes in soil, and in promoting the accuracy and usefulness of computer modeling to improve agricultural productivity and reduce environmental contamination.

Technical Abstract: Broiler litter is a mixture of chicken manure and a bedding material. Over-application of broiler litter on agricultural lands can result in nutrient enrichment of the subsurface effluent when conditions promote nutrient leaching, such as preferential flow through soil macropores. Preferential flow via soil macropores strongly affects nutrient transport in no-till soils, although studies quantifying nutrient losses from intact columns are lacking. We conducted a laboratory leaching experiment using broadcast application of broiler litter on the soil surface of cylindrical soil columns. The intact soil columns (15 cm diameter and 50 cm length) were collected from the fine sandy loam soil of a pasture field in the Sand Mountain region in northeastern Alabama. The two treatments in the laboratory were broadcast-applied broiler litter applied at 5 Mg/ha on the soil surface and control (no litter applied). We used rainfall simulation and investigated the subsurface transport of nitrogen (N) and phosphorus (P). Leachate was analyzed for dissolved reactive P (DRP), total P (TP), and nitrite + nitrate-N. Bromide breakthrough curves (BTCs) and X-ray computed tomography (CT) were used to assess the preferential flow pathways and quantify the soil macropore characteristics. In all columns, we found a rapid arrival of tracer and asymmetrical shape of BTCs, indicating the existence of preferential flow in this pasture soil. The X-ray CT analysis also provided evidence of soil macropore networks. Without the application of litter, the average concentration of TP and DRP in leachate from all the columns did not exceed 0.023 and 0.013 mg/L, respectively. Litter application significantly increased leachate P concentrations, with the highest average TP and DRP concentrations being 1.8 and 1.67 mg/L, respectively. The nitrate-N concentrations in leachate exceeded the U.S. EPA drinking water standard of 10 mg/L for all columns to which litter was applied and for 67% of the columns receiving no litter.