Soil chemistry measurements from the Kew Wakehurst Ecosystem Observatory, UK, 2021-2024

Samples were collected using a standard manual soil corer. Loose leaf litter was removed before coring, and cores were extracted to the required depth. Where multiple depth bands were needed, cores were split at 15 cm in the field; where deeper samples (30-45 cm) were required, 15 cm cores were taken from a spade-dug hole at 30 cm depth. Samples were bagged, labelled, and dispatched to the laboratory within 48 hours of collection. Sampling designs varied by campaign. Annual baseline assessments and most research campaigns used random plot selection within habitat strata. Only the hedgerow and large tree surveys used purposive sampling to target specific landscape features.

All chemical analyses were conducted by NRM Laboratories (Cawood Scientific, UK), a UKAS-accredited laboratory. Carbon and nitrogen were determined by Dumas combustion. Soil pH was measured in a 1:2.5 soil:water suspension. Bulk density was measured gravimetrically from a known volume of dried, sieved soil. Dry matter was determined after drying at 105°C and organic matter by loss on ignition at 550°C. Available nutrients (sodium, calcium, magnesium, potassium, sulphate, phosphorus) were extracted using standard methods including ammonium acetate extraction and the Olsen method for phosphorus. Cation exchange capacity, electrical conductivity, and soil texture (sand, silt, clay fractions by sedimentation) were also determined. Carbon stocks (t C ha⁻¹) were derived from carbon concentration, bulk density, and sample depth interval.

Spatial coordinates were recorded using mobile phone GPS within the ESRI ArcGIS Field Maps application, with typical positional accuracy of ±5-10 m. Coordinates were verified against the Wakehurst estate boundary. Laboratory results were reviewed for anomalous values. Not all variables were measured for all samples, particularly soil texture and organic matter; missing values are coded as NA.