Use of continuous monitoring to assess stream nitrate flux and transformation patterns

Abstract

Delivery of nitrogen from farmed fields to the stream network is an ongoing water quality issue in central North America and other parts of the world. Although fertilization and other farming practices have been refined to produce environmental improvements, stemming loss of nitrogen, especially in the soluble nitrate form, is a problem that has seemingly defied solution. The Iowa Nutrient Reduction Strategy is a policy initiative designed to implement conservation and other farm management practices to produce reductions in nitrate loading. The strategy does not focus on how the streams themselves may or may not be processing nitrogen and reducing downstream loading. We used continuous high-frequency nitrate and discharge monitoring over 3 years at two sites separated by 18 km in a low-order, agricultural stream in eastern Iowa to estimate how nitrogen is processed, and whether or not these processes are reducing downstream loading. We conclude that the upstream to downstream nitrate concentration decline between the two sites was not driven by denitrification. These data also show that nitrate concentrations are closely coupled to discharge during periods of adequate moisture, but decoupling of concentration from discharge occurs during dry periods. This decoupling is a possible indicator of in-stream nitrate processing. Finally, nitrate concentrations are likely diluted by water sourced from non-row crop land covers in the lower reaches of the watershed.

Iowa Statewide Stream Nitrate Load Calculated Using In Situ Sensor Network

“…Instream NO x ‐N processing over a three‐year period was quantified by Jones et al . () using two continuous sensors separated by 18 km in an eastern Iowa stream. Reynolds et al .…”

“…In the U.S., diurnal and seasonal NO x -N patterns in the Mississippi River have been quantified with continuous NO x -N sensors (Bark, 2010). Instream NO x -N processing over a three-year period was quantified by Jones et al (2017a) using two continuous sensors separated by 18 km in an eastern Iowa stream. Reynolds et al (2016) subsampled 15-min NO x -N in situ measurements to quantify uncertainties associated with conventional, laborintensive water sampling strategies, showing that manual and automated grab sampling do not capture spatial and temporal variability of NO x -N loads as accurately as continuous monitoring.…”

Groundwater monitoring at the watershed scale: An evaluation of recharge and nonpoint source pollutant loading in the Clear Creek Watershed, Iowa

“…Schilling and Zhang () indicated that NO 3 ‐N is particularly susceptible to leaching and export with baseflow when crop uptake is minimal or nonexistent (Dinnes et al, ). In‐stream removal rates of nitrate may be partially responsible for the nitrate depletion in baseflow, but this process is typically more prominent in reducing stream nitrate concentrations in September and October (Jones, Kim, & Schilling, ).…”

Inter‐ and Intra‐Annual Variability of Nitrogen Concentrations in the Headwaters of the Mero River

“…These factors are dynamic and highly variable in space and time, and their action can result in heavy discharges of nitrogen into the watercourses. The impact of natural factors on nitrogen losses can be modified by anthropogenic factors, including soil use and crop rotation, the application of slurry and other fertilizers, as well as the amount and method of application [4][5][6]. The diffuse pollution of water resources by nitrogen is a problem that affects the whole world, with agriculture being the main primary source of diffuse pollution of water resources [6,7].…”