Stream Nitrogen, Phosphorus, and Sediment Concentrations Show Contrasting Long‐term Trends Associated with Agricultural Change

Abstract

Stream water quality can be greatly influenced by changes in agricultural practices, but studies of long‐term dynamics are scarce. Here we describe trends over 21 yr (1994–2014) in nutrients and suspended sediments in three streams in a Midwestern US agricultural watershed. During this time, the watershed experienced substantial changes in agricultural practices, most importantly a pronounced shift from conventional to conservation tillage. In the 1990s and early 2000s, NH4, soluble reactive P, and suspended sediment concentrations (standardized for discharge and season) each declined significantly (>4–12% per year) in at least two of the three streams (P < 0.01), whereas NO3 changed relatively little. However, since the early 2000s, declines in NH4 and sediment concentrations have slowed, soluble reactive P concentrations have not declined and may actually have increased, and NO3 concentrations have declined sharply. The more recent lack of decline in soluble reactive P coincides with a plateau in the prevalence of conservation tillage and may be because of increased soil P stratification due to long‐term reduced tillage. The more recent decline in NO3 may be due to improved efficiency of N fertilizer use, increased soil denitrification, and/or declines in atmospheric N deposition. Our study shows that stream concentrations of N, P, and sediment can respond in contrasting ways to changes in agriculture, and that temporal trends can moderate, accelerate, or reverse over decadal timescales. Management strategies must consider contrasting temporal responses of water quality indicators and may need to be adaptively adjusted at scales of years to decades. Core Ideas Stream water quality in a Midwestern watershed responds to agricultural management. Conservation tillage and nutrient management appear to be major drivers of change. Nitrogen, phosphorus, and sediment show contrasting dynamics over decadal timescales. Management plans must anticipate temporally variable trends over multiple decades.

Nitrate, ammonium, and phosphorus drive seasonal nutrient limitation of chlorophytes, cyanobacteria, and diatoms in a hyper‐eutrophic reservoir

“…Acton has a surface area of 2.4 km 2 and a mean depth of 3.9 m (Vanni et al ). The lake has a relatively large watershed of 257 km 2 (watershed area: lake area = 111), 70–80% of which is in row crop agriculture, primarily corn and soybeans (Vanni et al , ; Renwick et al ). Therefore, the lake receives large nutrient inputs from the terrestrial landscape (Williamson et al ; Kelly et al ).…”

“…In years when P limits phytoplankton in summer, the cyanobacteria assemblage is dominated by taxa that do not fix N, but in years with N limitation, heterocystous N fixers are common (Hayes et al ). In general, external loading to Acton has a high N:P ratio; however, over the past decade, N concentrations and loads, as well as the load N:P, from tributary streams have been declining (Renwick et al ; Kelly et al ). Changes in agricultural practices over the past two decades, especially a pronounced shift to conservation tillage, may be the cause for this decline in N (Renwick et al ; Kelly et al ).…”

“…In general, external loading to Acton has a high N:P ratio; however, over the past decade, N concentrations and loads, as well as the load N:P, from tributary streams have been declining (Renwick et al ; Kelly et al ). Changes in agricultural practices over the past two decades, especially a pronounced shift to conservation tillage, may be the cause for this decline in N (Renwick et al ; Kelly et al ). As N:P ratios have declined, phytoplankton communities have become increasingly N‐limited in recent years (Vanni et al unpubl.…”

“…We estimated watershed tributary discharge and watershed nutrient loading to Acton Lake during the study period. A full treatment of these methods can be found in Vanni et al (), Renwick et al (), and Williamson et al (). We do not yet have stream nutrient data for 2018; however, we estimated loading rates of NO 3 , NH4, and soluble reactive phosphorus (SRP) using regressions of daily stream discharge vs. daily loads obtained for the years 2015–2017 (using only May–October dates) and 2018 daily stream discharge ( see Supporting Information for details).…”

Increasing the Effectiveness and Adoption of Agricultural Phosphorus Management Strategies to Minimize Water Quality Impairment

“…A meta-analysis comparing P losses of conservation versus conventional tillage demonstrated a 40% increase in both SRP concentration and load due to conservation tillage, although the amount varied based on rainfall (Daryanto et al, 2017). Other studies have demonstrated SRP increases from land under conservation tillage (Dodd and Sharpley, 2016;Jarvie et al, 2017;Renwick et al, 2018;Uusitalo et al, 2018). The increase in SRP with conservation tillage was relative to the reduction in sediment-attached P, which may actually increase degradation of water quality since SRP is more bioavailable.…”