Nitrogen and carbon stocks and species in soils, sediments and groundwater of a shallow floodplain aquifer in the Trent Valley

Abstract

This report largely concentrates on a geochemical survey of nitrogen and carbon stocks and their speciation in the different soil / sediment types and in the groundwater of a shallow river aquifer in the Trent Valley. The transect starts on a Triassic mudstone escarpment and follows the valley down onto a series of Pleistocene terraces and a modern floodplain, largely consisting of sand and gravel underlain by the Triassic mudstone, thus creating a shallow floodplain aquifer. In total 11 sites were sampled for soils / sediments down to depths ~2m. Total organic Nitrogen (N) and inorganic N were determined along with organic C concentrations. Results showed that organic Carbon (C) storage was between 5279 to 29624 g m-2 to depths of 120 cm. Total organic N storage was found to vary between 468 and 2199 g m-2 to a depth of 60cm. These figures are within the range expected for UK arable soils. Total inorganic N (NO3-N + NH4-N) in the alluvial sands and gravels to depths of 90 cm (water table is ~1m from the soil surface) varied from 2.00 to 5.01 g m-2. In the escarpment soils, total inorganic N varied between 2.96 -7.32 g m-2. Therefore the amount of inorganic N being stored in the top and sub soils equates to ~ 30 – 70 kg ha-1 N, roughly half of the yearly fertiliser N applications for typical arable crops. The groundwater of the shallow aquifer was sampled 3 times over a period of a year from a series of 13 piezometers sited within the study area. Results showed that average nitrate concentrations in the groundwater were ~ 60 mg L-1 (Range: 9-200 mg L-1). As a comparison this mean value would exceed the limits of 50 mg L-1 suggested within the EU Nitrates Directive. In some groundwater samples, concentrations of nitrate exceeded 150 mg L-1. Dissolved Organic Nitrogen (DON) was present in the groundwaters but at concentrations generally 10 years for the aquifer to lose much of the nitrate. The well oxygenated aquifer waters (> 2mg L-1 oxygen) would suggest that denitrification potential was low. Laser fluorescence (LF) spectroscopy was used to assess the characteristics of the Dissolved Organic Matter (DOM) in the top soil and groundwater. Results suggest that the Dissolved Organic Carbon (DOC) in the groundwater is more humified and more fulvic-like in origin than that in the top soil, an indication that the carbon source in the groundwater was older and more decomposed thus not being such an effective potential source of C for denitrifying bacteria. As the study area is representative of large parts of the Trent Valley in Nottinghamshire and Derbyshire, the results can be seen as an analogue for similar areas of the Trent Valley where intensive agriculture occurs on sand and gravel river aquifers