Spatial and bulked soil and herbage selenium concentrations in different pasture types in SW England

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The data included in this dataset are selenium (Se) concentrations of soil and herbage from a number of pasture fields, with additional soil, herbage and farm management data either available as part of this data package or in linked sources. For 20 fields, bulked soil samples and bulked herbage samples (not from the same locations) were taken to be representative of the fields. For three of these fields, additional sampling at point locations was undertaken, with both soil and herbage being taken from the same places.
For herbage and soil, total Se was measured. In addition, soil Se bioavailability was assessed through sequential fractionation. Fractions F0 and F1 (see below for methods) are assumed to be soluble Se; fraction F2 is exchangeable Se; fraction F3 is associated with Fe, Al and Mn oxides, carbonates and hydrolysable organic matter, and is assumed to be not readily available to plants; fraction F4 is the residual fraction that is elemental, insoluble and unavailable.
The experiment took place on the North Wyke Farm Platform (NWFP), a UK National Capability in SW England. The NWFP is split into a number of self-contained farms (‘farmlets’) that are managed according to different operation philosophies or practices. At the time of the experiment, there were three farmlets on the NWFP with different pasture management strategies. Permanent pasture (PP), a perennial ryegrass monoculture (HS) which was sown with a high sugar Lolium perenne cv. AberMagic, and a white clover/perennial ryegrass mix (WC) with the same ryegrass variety as the HS pasture. The PP and HS pastures received N fertilizer at a standard rate, but the WC pastures did not due to the inclusion of a legume. Fields within a farmlet are cut for silage and grazed by cattle and sheep.
The NWFP is highly instrumented and monitored, and core NWFP datasets are open and include in-situ water flow and chemistry taken at 15-minute intervals; 15-minute Met measurements; 15-minute soil moisture measurements; 30-minute GHG emissions; soils, crop and botanical field survey data; livestock and crop performance data; and farm operational activities, and contextual information is also available. See https://nwfp.rothamsted.ac.uk/. Registration is required, but all data is freely available for download. Further information about the NWFP can be found via https://doi.org/10.23637/rothamsted.98y1x.

Sample collection methods:
‘Field_’ samples (n = 20, collected March 2015): 25 soil samples and 15 herbage samples collected per field and bulked per sample type. Samples collected in a ‘W’ traverse pattern across the field, trying to ensure the sample was representative on the field scale. The soil and herbage samples were not taken from the same locations.
‘Point_’ samples (collected May 2015): For 3 fields, a further 10 samples per field were taken from point locations on a 50 m grid pattern, excluding areas near gateways, headlands, or in the immediate vicinity of hedges and trees. At each location, both an herbage and a soil sample were taken.
Herbage samples were taken from the top 1-4 cm of the plant, depending on the sward height, and obvious weeds were excluded. Herbage was freeze dried and finely milled. Soil samples were taken with a pot corer to 10 cm depth. Soils were air dried and sieved to < 2 mm.
Selenium analysis methods:
Herbage_total_Se method: 0.1 g herbage in a digestion vessel with 3 mL concentrated nitric acid and heated for one hour at 120 oC. 1 mL of hydrogen peroxide added and heated for a further hour at 120 oC. Cooled and diluted to 15 mL with deionized water. 15 mL concentrated hydrochloric acid added and heated for a further 45 minutes. Diluted to 50 mL with deionized water.
Soil_total_Se method: 0.5 g soil digested using aqua regia (2.5 mL concentrated nitric acid and 7.5 mL concentrated hydrochloric acid), heated at 120 oC for 3 hours. Diluted to 15 mL using deionized water. 10 mL concentrated hydrochloric acid added, heated for a further 45 minutes. Diluted to 50 mL with deionized water.
Soil selenium sequential fractionation: Based on Munier-Lamy et al., 2007.
• Soil_F0_Se: 2 g silica gel and 15 mL deionized water added to 6 g soil. Shaken for 30 minutes at room temperature to disperse soil particles. Centrifuged for 30 minutes at 4000 rpm. Supernatant filtered at 0.45 μm and stored at 4 oC.
• Soil_F1_Se: 15 mL 0.25 M potassium chloride added to the residue of the previous step. Sample shaken, centrifuged, filtered and stored as in the previous step.
• Soil_F2_Se: 30 mL 0.1 M monopotassium phosphate added to the residue of the previous step. Sample shaken, centrifuged, filtered and stored as in the previous step.
• Soil_F3_Se: 20 mL 4 M hydrochloric acid added to the residue of the previous step. Heated at 95 oC in a thermostatic bath for 45 minutes. Then centrifuged, filtered and stored as in the previous step.
• Soil_F4_Se: 1.5 g of residue from the previous step was digested in aqua regia (2 mL concentrated nitric acid and 6 mL concentrated hydrochloric acid). Followed the same procedure as for Soil_total_Se but with final dilution to 40 mL instead of 50 mL.
All solutions analysed for selenium concentration using hydride generation atomic fluorescence spectrometry (HG-AFS). Values blank corrected and checked against standards (5 – 20 ppb Se) prepared from sodium selenite, Na2SeO3. For each of the analyses, some samples were replicated to quantify precision. Values ranged from 1.80 – 13.34 % (mean precision value 6.20 %).
Soil nutrient status:
Soil phosphorus, potassium, magnesium and nitrogen were assessed according to the recommendations provided to farmers and land managers in the UK by the Agriculture and Horticulture Development Board. This nutrient management guide is known as RB209, and Section 3 is of relevance, referring to grass and forage crops. The version available at the time of sampling (2015) is no longer available, but the most recent version is identical with respect to assessing soil nutrient status.

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