An integrated connectivity risk ranking for phosphorus and nitrogen along agricultural open ditches to inform targeted and specific mitigation management

• Published in: Frontiers in Environmental Science
• Main Authors: D. G. Opoku, M. G. Healy, O. Fenton, K. Daly, T. Condon, P. Tuohy
• Format: Article in Journal/Newspaper
• Language: English
• Published: Frontiers Media S.A. 2024
• Subjects: water quality; nutrient loss; grassland; drainage management; connectivity pathways; North Atlantic Europe; Environmental sciences; GE1-350; North Atlantic
• Online Access: https://doi.org/10.3389/fenvs.2024.1337857; https://doaj.org/article/5b53f1d35d174ad7a7be25c617f2f39c

Introduction: On dairy farms with poorly drained soils and high rainfall, open ditches receive nutrients from different sources along different pathways which are delivered to surface water. Recently, open ditches were ranked in terms of their hydrologic connectivity risk for phosphorus (P) along the open ditch network. However, the connectivity risk for nitrogen (N) was not considered in that analysis, and there remains a knowledge gap. In addition, the P connectivity classification system assumes all source–pathway interactions within open ditches are active, but this may not be the case for N. The objective of the current study, conducted across seven dairy farms, was to create an integrated connectivity risk ranking for P and N simultaneously to better inform where and which potential mitigation management strategies could be considered.Methods: First, a conceptual figure of known N open ditch source–pathway connections, developed using both the literature and observations in the field, was used to identify water grab sampling locations on the farms. During fieldwork, all open ditch networks were digitally mapped, divided into ditch sections, and classified in terms of the existing P connectivity classification system.Results and Discussion: The results showed that not all source–pathway connections were present across ditch categories for all species of N. This information was used to develop an improved open ditch connectivity classification system. Farmyard-connected ditches were the riskiest for potential point source losses, and outlet ditches had the highest connectivity risk among the other ditch categories associated with diffuse sources. Tailored mitigation options for P and N speciation were identified for these locations to intercept nutrients before reaching receiving waters. In ditches associated with diffuse sources, nitrate was introduced by subsurface sources (i.e., in-field drains and groundwater interactions from springs, seepage, and upwelling) and ammonium was introduced through surface ...