Seasonal variability of sediment controls of nitrogen cycling in an agricultural stream

Sophie A Comer-Warner,Sarah K Wexler,Jan Kaiser,Stefan Krause,Nicholas Kettridge,Luke Glover,Sami Ullah,Daren C Gooddy

doi:10.1007/s10533-020-00644-z

Abstract

Agricultural streams receive large inputs of nutrients, such as nitrate (NO3−) and ammonium (NH4+), which impact water quality and stream health. Streambed sediments are hotspots of biogeochemical reactivity, characterised by high rates of nutrient attenuation and denitrification. High concentrations of nitrous oxide (N2O) previously observed in stream sediments point to incomplete denitrification, with sediments acting as a potentially significant source of global N2O. We investigated the effect of sediment type and seasonal variation on denitrification and N2O production in the streambed of an agricultural UK stream. Denitrification was strongly controlled by sediment type, with sand-dominated sediments exhibiting potential rates of denitrification almost 10 times higher than those observed in gravel-dominated sediments (0.026 ± 0.004 N2O–N μg g−1 h−1 for sand-dominated and 0.003 ± 0.003 N2O–N μg g−1 h−1 for gravel-dominated). In-situ measurements supported this finding, with higher concentrations of NO3−, nitrite (NO2−) and N2O observed in the porewaters of gravel-dominated sediments. Denitrification varied substantially between seasons, with denitrification increasing from winter to autumn. Our results indicate highest NO3− reduction occurred in sand-dominated sediments whilst highest N2O concentrations occurred in gravel-dominated sediments. This suggests that finer-grained streambeds could play an important role in removing excess nitrogen from agricultural catchments without producing excess N2O.

Highlights

Large inputs of nutrients, such as nitrate (NO3-) and ammonium (NH4?), transported to freshwater ecosystems either directly or through subsurface flows, severely impact ecosystem health and functioning in many agricultural lowland streams (Krause et al 2009; Pinay et al 2015, 2018; Smith et al 1999)
The N2O concentrations in the sanddominated sediments of sites 1 and 2 were lower than previously observed in sandy sediments influenced by agriculture (Hinshaw and Dahlgren 2013; Pretty et al 2006), whereas the concentrations in the graveldominated sediments were similar to those found in coarse gravel sediments influenced by agriculture (Pretty et al 2006), but lower than those found in a gravel bar (Hlavacovaet al. 2005)
N cycling in the investigated agricultural lowland stream was strongly controlled by sediment type, with sand-dominated sediments characterised by higher rates of denitrification than gravel-dominated sediments

Summary

Introduction

Large inputs of nutrients, such as nitrate (NO3-) and ammonium (NH4?), transported to freshwater ecosystems either directly or through subsurface flows, severely impact ecosystem health and functioning in many agricultural lowland streams (Krause et al 2009; Pinay et al 2015, 2018; Smith et al 1999). The relative N2O contributions from streams and rivers in relation to their share of the Earth’s surface, * 0.15% (Allen and Pavelsky 2018), indicates that streams and rivers are disproportionality important in global N2O emissions It is essential, to understand drivers and controls of N2O emissions from aquaticatmospheric interfaces, especially as it is a GHG approximately 298 times more potent than CO2 on a mole per mole basis (Forster et al 2007), with a large ozone-depleting potential compared to other ozonedepleting compounds of anthropogenic origin (Ravishankara et al 2009)

Results

Discussion

Conclusion