Provenance & quality
Soil samples were taken by members of the sites. At North Wyke and Henfaes farm, staff from CEH Bangor helped taking the samples. Two sets of soil cores were taken. One was sent in cooling boxes on ice to CEH Bangor for measuring soil physicochemical properties. The second set was send to Rothamsted Research Harpenden for the analysis of nitrogen genes and microbial community composition (related dataset). The topsoil 0-15 cm were taken with the same type of corer across the sites, samples below 15 cm soil depth were sampled with corers available at the sites. Samples were wet sieved to 5 mm by staff from Bangor University and CEH Bangor. Subsamples of these were analysed for physicochemical and biological parameters.
Nitrate (NO3‒), Ammonium (NH4+), Dissolved Organic Carbon (DOC), Dissolved Organic Nitrogen (DON), amino acids, peptides and proteins: extraction with 0.5 M Potassium Sulphate) K2SO4, 5 g soil and 25 ml extractant, after sieving through 5 mm. The supernatant was stored at −20 degrees Celsius until analyses. (Bangor University)
Total dissolved organic carbon (DOC) and total dissolved Nitrogen (TDN) were measured using a Multi N/C 2100/2100 analyser (AnalytikJena AG, Jena, Germany). Dissolved organic Nitrogen (DON) was calculated by subtracting ammonium (NH4+) and nitrate (NO3−) from the TDN value. (Bangor University)
Ammonium in the supernatant was determined colorimetrically using the salicylate method of Mulvaney (1996) and NO3− following the salicylate method of Miranda et al. (2001) in an Epoch® microplate spectrophotometer (Bio Tek Instruments Inc., Winooski, VT). (Bangor University)
Microbial biomass carbon and nitrogen were calculated in a similar way than DOC and DON but the 5 grams of soil were kept for 7 days in a desiccator with chloroform (to kill the microorganisms). The difference between DOC, DON after these 7 days (and after extraction as explained before) and the initial values (calculated in the previous paragraph) between 0.45 for microbial C and 0.54 for microbial N were microbial C and microbial N, respectively. (Bangor University)
Amino acids and peptides were measured using a Cary Eclipse Fluorescence Spectrophotometer with a ProStar Solvent Delivery Module (Varian, USA). (Bangor University)
Texture. Particle size distribution was measured using a Beckman-Coulter LS 13320 laser diffraction particle size analyser. We manually quarter 0.5 g subsample from the soil, remove organic matter using Hydrogen peroxide (H2O2) and transfer the sample into 250 mL bottle, we added 5 mL of 5 percent Calgon and shaked overnight. The entire contents of the bottle was introduced to the laser analyser for measuring particles size distribution. (CEH Bangor)
Soil water content and loss on ignition (LOI). Soil initially dried at 25 degrees Celsius for 14 days, followed by weight a 10 gram subsample into a crucible and put into an oven at 105 degrees Celsius for 24 hours, the weight loss provides the soil water content. The same crucible with the dry soil is introduced into a Carbolite furnace at 375 degrees Celsius for 16 hours, this second weight loss is quantified and reported as Loss on ignition (LOI). (CEH Bangor)
pH and electrical conductivity (EC). In a beaker we add 25ml deionised water into 10g field moist soil; we allowed 30 minutes equilibration time with stirring at time=0 and 15 minutes, we measure pH using Corning 220 pH meter, VWR combination electrode 662-1805. EC is measured after pH in the same beaker using a Jenway 4510 with a 662-1805 electrode. pH in CaCl2 is measured after EC following addition of 2 mL 0.125 mole (M) Calcium Chloride (CaCl2). (CEH Bangor)
Permanganate oxidizable carbon (POXC). We manually quartered 2.5g of soil subsample and analyse for POXC using 0.2M KMnO4 following the method described in Weil et al., 2003. (CEH Bangor)
Citrate-P. We manually quarter 0.5 grams of soil and analyse for citrate extractable phosphorous following the method described in DeLuca et al. (2015). (CEH Bangor)
Aggregate size distribution. We sieved about 1 gram of air-dry soil through a stack of 2 mm and 1 mm sieves, the fraction collected into the 1 mm sieve was introduced into a Beckman-Coulter LS 13320 laser diffraction particle size analyser and measured for the size distribution. (CEH Bangor)
Total carbon and nitrogen was measured on a Vario EL which uses oxidative combustion followed by thermal conductivity detection (CEH Lancaster).
Total phosphorus was measured at CEH Lancaster. Total phosphorus is digested with a H2O2/Sulphuric Acid (H2SO4) based digestion reagent mix (along with selenium and lithium sulphate) and subsequent colorimetric analysis carried out on a Seal discrete analyser (CEH Lancaster).
All results were entered into Excel spreadsheets providing individual datasets for each set of N parameters. Data were exported from Excel as .csv files for ingestion into the EIDC.