Characterising changes in fluorescence properties of dissolved organic matter and links to N cycling in agricultural floodplains

Abstract

A B S T R A C TSand and gravel aquifer systems are common features below and adjacent to river networks and are important in providing a hydrological link between terrestrial and aquatic ecosystems. They are often used for intensive agriculture and therefore provide a conduit for the transport of nutrients to aquatic systems. Understanding the biogeochemical cycles of C and N in such systems is essential in efforts to improve water quality, with a major link being the transfer and properties of DOM which drives microbial processing of nutrients. In this work undertaken in the Trent Valley, U.K., the differences in dissolved organic matter (DOM) properties in the soil, aquifer ground water and river water are examined using excitation-emission matrix fluorescence spectroscopy (EEM), in addition to chemical analysis. In the aquifer system studied the water table is $1.5 m below the land surface, suggesting that DOM and nutrient transfer would occur between the soil and groundwater. Nitrate levels in the ground water were $50 mg L À1 NO 3 -N. Soil DOM properties were measured in KCl extracts used to extract inorganic and organic N so that further information could be obtained regarding possible sources and properties of the DON and the biodegradability of the DOC pool. Within the soils tested, fulvic-like properties and the humification index (HI) were significantly (P < 0.05) higher in sandy soils suggesting that microbial accessibility to organic matter was greater. The applicability of assessing DOM characteristics in the different matrices were assessed and quantitative or qualitative comparisons made. Properties of DOM in both the soils and aquifer waters all showed a typical terrestrial source with principal component analysis (PCA) showing strong correlations between DOC, fulvic-like properties, the HI and tryptophanlike properties. A major finding was that the HI was generally higher in the groundwater, suggesting continued processing of DOM within the aquifer whilst qualitative comparisons showed the groundwater possessed a higher fluorescence index (FI). Whilst DON and the tyrosine-like proteins were correlated in the soil, the tryptophan-like proteins were strongly correlated to fulvic-like substances in both the soil and groundwater. This suggested that phenolic compounds were also being identified in this part of the EEM matrix, emphasising the need for additional compound analysis to fully understand the nature of DOM in these systems.

Chronic atmospheric reactive N deposition has breached the N sink capacity of a northern ombrotrophic peatbog increasing the gaseous and fluvial N losses

“…aquifers). Microbial processing of DOM has been shown to alter fluorescence-based indices of DOM composition, with increases in both humification and fluorescence index, indicative of continued microbial decomposition of DOM (Tye and Lapworth, 2016). Given the strong regression observed between lab DOC and DON concentrations and subsequently, between observed and modelled DON we are confident that the load estimation is robust, covering all flow conditions experienced across the period of study.…”

Macronutrient status of UK groundwater: Nitrogen, phosphorus and organic carbon

“…In the Penrith Sandstone fluorescence combined with CFCs suggested a rapid flow route to 60 m depth possible fracture flow and increasing recalcitrant fulvic acid content with depth in the rest of the profile. A recent study by Tye and Lapworth (2016) showed higher humification indices in groundwater compared to soils suggesting continued processing of DOM within the aquifer.…”

“…However, more labile protein-like dissolved organic matter (DOM) may also be present in detectable concentrations, particularly where rapid flow pathways in the subsurface are present, from surface sources such as landfills, rivers, urban waste water and agricultural inputs (e.g. Baker and Curry, 2004;Lapworth et al, 2009;Lapworth et al, 2008a;Rhymes et al, 2015;Tye and Lapworth, 2016).…”

Quantitative and Qualitative Responses of Soil Water-Extractable Organic Matter to Carbon and Nitrogen Management Practices in Loess Soil

“…Inamdar et al [63] found that the DOM fractions in superficial watersheds were composed of humic-like substances, while the DOM fractions from groundwater sources were isolated based on their protein-like fluorescence. Tye and Lapworth [64] revealed that groundwater DOM was generated by the DOM leached from soils in agricultural floodplains. Accordingly, we speculated that the proportions of C-rich organic substances in the soil were increased by N fertilization, mainly owing to the loss of N-rich organic matter via leaching.…”