The speciation of Iowa’s nutrient loads and the implications for midwestern nutrient reduction strategies

Abstract

Nitrogen (N) and phosphorus (P) are the primary drivers of eutrophication in Iowa, but the chemical species that comprise these nutrients have not been well-quantified. In this study, we modeled stream loads for three forms of N (nitrate [NO₃⁻], organic N [ON], and ammonia [NH₃]) and two forms of P (orthophosphate [OP] and particulate P [PartP]) at 46 Iowa sites from 1998 to 2022 to determine these forms’ contributions to overall nutrient loading. Statewide totals were determined by aggregating loads from 16 sites near Iowa’s border, and regional patterns were explored by examining yields at 41 watersheds throughout the state.

Iowa’s statewide total N yield was 22 kg ha⁻¹ y⁻¹, which consisted of 81% NO₃⁻, 18% ON, and 1% NH₃. Iowa’s total P yield was 1.5 kg ha⁻¹ y⁻¹—73% PartP and 27% OP. While no temporal trends were present among the annual statewide loads, the proportion of PartP that makes up the total P decreased. Yields in individual watersheds varied considerably. Watersheds in the hillslope-dominated plains of southern Iowa had the greatest ON and PartP yields, and ON was the dominant N form at five sites. Watersheds in the heavily tile-drained landscapes of north-central Iowa had the greatest NO₃⁻ and OP yields.

Concentrations of particulate nutrients (PartP and ON) were strongly correlated at individual sites, suggesting these nutrients have similar sources and transport pathways. While high NO₃⁻ and OP yields tended to coincide, their concentrations were not related, and these dissolved nutrients are likely entering Iowa’s waters under different seasonal and hydrologic conditions. Our findings may help inform remediation efforts by identifying which nutrient forms are most prevalent throughout Iowa.