Urea Release by Intermittently Saturated Sediments from a Coastal Agricultural Landscape

Abstract

Urea-N is linked to harmful algal blooms in lakes and estuaries, and urea-N-based fertilizers have been implicated as a source. However, the export of urea-N-based fertilizers appears unlikely, as high concentrations of urea-N are most commonly found in surface waters outside periods of fertilization. To evaluate possible autochthonous production of urea-N, we monitored urea-N released from drainage ditch sediments using mesocosms. Sediments from a cleaned (recently dredged) drainage ditch, uncleaned ditch, forested ditch, riparian wetland, and an autoclaved sand control were isolated in mesocosms and flooded for 72 h to quantify urea-N, NH 4 + -N, and NO 3 --N in the floodwater. Sediments were flooded with different N-amended-N) and incubated at three water temperatures (16, 21, and 27°C). Urea-N concentrations in mesocosms representing uncleaned and cleaned drainage ditches were significantly greater than nonagricultural sediments and controls. While flooding sediments with N-enriched solution had no clear effect on urea-N, warmer (27°C) temperatures resulted in significantly higher urea-N. Data collected from field ditches that were flooded by a summer rainstorm showed increases in urea-N that mirrored the mesocosm experiment. We postulate that concentrations of urea-N in ditches that greatly exceed environmental thresholds are mediated by biological production in sediments and release to stagnant surface water. Storm-driven urea-N export from ditches could elevate the risk of harmful algal blooms downstream in receiving waters despite the dilution effect. (Glibert et al., 2004). This phenomenon has drawn attention to sources of urea-N to surface water, and elevated urea-N concentrations issuing from agricultural watersheds have led some researchers to suggest a link to the land application of manures and urea-N synthetic fertilizers (Glibert et al., 2001(Glibert et al., , 2005(Glibert et al., , 2006Lomas et al., 2002;Thorén et al., 2003). Indeed, global urea-N fertilizer consumption ballooned 100-fold from the 1960s to the 2000s (Glibert et al., 2006), part of a trend of dramatically increasing fertilizer N use since the mid-twentieth century. Due in part to restrictions on inorganic fertilizers because of security concerns, urea-N comprises perhaps 60% of global N fertilizer use on an annual basis, and this proportion is expected to grow (Glibert et al., 2014). However, although small losses of agriculturally applied urea-N could be biologically important in receiving waters (Glibert et al., 2006), the export of untransformed urea-N fertilizer is likely minor. The rapid hydrolysis of urea-N in soils generally precludes substantial export from fertilizer applications (Fisher et al., 2016). Losses via leaching and overland flow are brief and typically small even under heavy rainfall conditions (Han et al., 2015;Kibet et al., 2016). In a synoptic watershed study on the Delmarva Peninsula, Tzilkowski (2013) found no evidence that storms in the weeks after poultry manure application led to increased ur...

Urea Fluctuations in Stream Baseflow across Land Cover Gradients and Seasons in a Coastal Plain River System

“…In Palmetto Branch, urea-N had similar directional associations with temperature and DOC: DON, but the relationships were weak and not statistically significant (Figure 7e,7f). In the drainage ditch, we found a strong positive effect of temperature on urea-N concentrations (Figure 7g; p < 0.01), which agreed with a recent microcosm study of urea-N production in ditch sediments from Maryland's Lower Eastern Shore (King et al 2017). In contrast, urea-N levels in ditches displayed a weak, negative relationship with DOC:DON (Figure 7h), which was directionally consistent with the other sites, but not statistically significant (p = 0.26).…”

“…Interestingly, all four sites showed some effects of temperature and DOC:DON ratios on urea-N concentrations, although some relationships were not statistically significant. In the drainage ditch, we found a strong positive effect of temperature on urea-N concentrations (Figure 7g; p < 0.01), which agreed with a recent microcosm study of urea-N production in ditch sediments from Maryland's Lower Eastern Shore (King et al 2017). In Palmetto Branch, urea-N had similar directional associations with temperature and DOC: DON, but the relationships were weak and not statistically significant (Figure 7e,7f).…”

“…Interestingly, urea-N concentrations in ditch drainage waters were similarly high during UREA FLUCTUATIONS IN STREAM BASEFLOW ACROSS LAND COVER GRADIENTS AND SEASONS IN A COASTAL PLAIN RIVER SYSTEM JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION these same periods when the ditch was hydraulically disconnected. Strong positive associations with water temperature indicated the potential for urea-N production in ditch sediments, as was demonstrated by King et al (2017) in a paired mesocosm and field study of drainage ditch sediments in Somerset County, Maryland. Thus, even though agricultural land cover did not emerge as a key predictor of urea-N concentrations in perennial streams, there was strong evidence to suggest that urea-N in agricultural ditches during low flows may be available for transport in stormflow by ensuing storm events.…”

Seasonal variation in effects of urea and phosphorus on phytoplankton abundance and community composition in a hypereutrophic hardwater lake

“…Comparison of mesocosm and bioassay findings suggested that urea lacked the toxic effects of observed in cool waters (Glibert et al., 2016; Swarbrick et al., 2019), in part because of limited hydrolysis of urea during spring and late autumn. Overall, growth stimulation by urea was consistent with previous short‐term laboratory (Belisle et al., 2016; Berman & Chava, 1999; Yuan et al., 2012) and mesocosm experiments (Bogard et al., 2020; Donald et al., 2011; Finlay et al., 2010), but showed that phytoplankton growth was strongly enhanced in spring, when urea influx is expected to be elevated (Bogard et al., 2012; Glibert et al., 2006) and temperature‐dependent enzymatic hydrolysis to NH 4 + is low (King et al., 2017; Siuda & Chrόst, 2006).…”

“…Preliminary research suggests that effects of urea on water quality in P‐rich lakes may vary among seasons (Belisle et al., 2016; Mitamura, Tachibana, Konda, Ueda, & Seike, 2010; Siuda & Kiersztyn, 2015). First, export of urea is expected to be highly seasonal (Glibert et al., 2006; King et al., 2017), particularly in northern regions where agricultural fertiliser application and release of wastewater lagoon effluents tend to occur in spring and autumn, when low temperatures coincide with reduced microbial and enzymatic activity (Siuda & Chrόst, 2006) and elevated runoff (Pham, Leavitt, McGowan, Wissel, & Wassenaar, 2009; Pomeroy et al., 2007). Together, these factors reduce the extent of urea hydrolysis and favour export to lakes (Di & Cameron, 2008; Silva, Cameron, Di, & Jorgensen, 2005; Swensen & Singh, 1997).…”