Hydrological controls on DOC  :  nitrate resource stoichiometry in a lowland, agricultural catchment, southern UK

Abstract

Abstract. The role that hydrology plays in governing the interactions between dissolved organic carbon (DOC) and nitrogen in rivers draining lowland, agricultural landscapes is currently poorly understood. In light of the potential changes to the production and delivery of DOC and nitrate to rivers arising from climate change and land use management, there is a pressing need to improve our understanding of hydrological controls on DOC and nitrate dynamics in such catchments. We measured DOC and nitrate concentrations in river water of six reaches of the lowland river Hampshire Avon (Wiltshire, southern UK) in order to quantify the relationship between BFI (BFI) and DOC : nitrate molar ratios across contrasting geologies (Chalk, Greensand, and clay). We found a significant positive relationship between nitrate and BFI (p < 0.0001), and a significant negative relationship between DOC and BFI (p < 0.0001), resulting in a non-linear negative correlation between DOC : nitrate molar ratio and BFI. In the Hampshire Avon, headwater reaches which are underlain by clay and characterized by a more flashy hydrological regime are associated with DOC : nitrate ratios > 5 throughout the year, whilst groundwater-dominated reaches underlain by Chalk, with a high BFI have DOC : nitrate ratios in surface waters that are an order of magnitude lower (< 0.5). Our analysis also reveals significant seasonal variations in DOC : nitrate transport and highlights critical periods of nitrate export (e.g. winter in sub-catchments underlain by Chalk and Greensand, and autumn in drained, clay subcatchments) when DOC : nitrate molar ratios are low, suggesting low potential for in-stream uptake of inorganic forms of nitrogen. Consequently, our study emphasizes the tight relationship between DOC and nitrate availability in agricultural catchments, and further reveals that this relationship is controlled to a great extent by the hydrological setting.

Deconstructing the Effects of Flow on DOC, Nitrate, and Major Ion Interactions Using a High‐Frequency Aquatic Sensor Network

“…In streams and rivers draining developed catchments, however, diluted with higher flows (Figure ), suggesting that the dominant source of to streams shifts with watershed land use, from shallow groundwater, riparian soils, or throughfall in forested streams to high‐nitrogen groundwater sources in streams draining catchments with significant agriculture or human development. However, it is unclear to what extent this pattern is generalizable or specific to our study streams in New Hampshire, since positive relationships between and discharge have been reported in other agricultural catchments (Heppell et al, ; Oeurng et al, ), possibly due to flushing of accumulated from agricultural soils. In contrast to relationships with streamflow, our results suggest that DOC consistently flushes with flow and is transport‐limited in eight of our 10 study streams (Figure ).…”

Habitat‐specific invertebrate responses to hydrological variability, anthropogenic flow alterations, and hydraulic conditions

“…The Hampshire Avon is classified as a Special Area of Conservation (SAC) under the EU Habitats Directives (92/43/EEC), with areas of the catchment also being designated as Sites of Special Scientific Interest (Natural England, 1996). The catchment is primarily underlain by a chalk lithology (Heppell et al, 2017), a fine-grained limestone that exhibits a relatively low specific yield, although it can develop high transmissivities as groundwaters move through small fissures (Soley et al, 2012). As such, chalk is considered a highly F I G U R E 1 The location of the study sites within the Hampshire Avon.…”

“…Dark grey = highly productive aquifer; light grey = moderately productive aquifer; white = low productivity aquifer or rocks with essentially no groundwater (for classification, see British Geological Survey, 2018) productive aquifer (British Geological Survey, 2018; see Figure 1) and overlaying rivers typically convey seasonally consistent flow regimes as groundwater levels rise and fall in accordance with antecedent climatic conditions (Sear, Armitage, & Dawson, 1999). However, the Hampshire Avon is also underlain by bands of greensand (a moderately productive aquifer) and clay (possessing essentially no groundwater) in the west of the catchment (Figure 1 and British Geological Survey, 2018 for nomenclature), which facilitate quicker hydrological responses to rainfall (Heppell et al, 2017). The land use across the four sub-catchments studied is predominantly arable agriculture (although the Wylye exhibits a higher proportion of grassland coverage) with minimal urban coverage (see Table 1).…”

Spatio-temporal controls of C-N-P dynamics across headwater catchments of a temperate agricultural region from public data analysis

“…Conversely, in more homogeneous catchments  e.g. mostly undisturbed (Mengistu et al, 2014) or mostly rural (Heppell et al, 2017;Lintern et al, 2018)  "natural" controls such as topography, geology, and flow paths are more frequently highlighted as the main factors that explain spatial variability in C, N and P.…”