Data from: Environmental filtering explains variation in plant diversity along resource gradients

Floristic surveys were conducted on the 60 permanent 10 m × 10 m plots (10 plots in each of six chronosequence stages). Each plot was surveyed at least once between August 2011 and March 2012. To estimate canopy cover and number of individuals for each plant species within the 10 m × 10 m plots, seven randomly-located 2 m × 2 m subplots were surveyed within each plot. Within each subplot, all vascular plant species were identified, the corresponding number of individuals was counted and the vertically projected vegetation canopy cover was estimated.

Between 11–16 June 2012, seven soil samples (one sample per 2 m × 2 m subplot) were collected in each of the sixty 10 m × 10 m plots, for a total of 420 samples. Subsampling was done to obtain more accurate estimates of the mean and to estimate within-plot spatial variation in soil properties. Samples were taken at 0–20 cm depth using a 50-mm diameter sand auger, sieved (<2 mm) to remove roots and other large organic debris, homogenized, and air-dried prior to chemical analysis.Total soil [P] was measured by ignition (550 °C, 1 h) and extraction in 1 M H2SO4, with phosphate detection by automated online neutralization and molybdate colorimetry using a Lachat Quikchem 8500 (Hach Ltd, Loveland, CO, USA). A soil P fractionation procedure (46) was performed on bulked samples from the 60 plots. Phosphate extracted by anion-exchange membranes (1 cm × 4 cm; manufactured by BDH, Poole, UK, and distributed by VWR International, West Chester, PA, USA) was desorbed from the membranes by shaking for 1 h in 0.25 M H2SO4. All other extracts were centrifuged (8000 × g, 15 min) and an aliquot decanted for analysis. Each aliquot was neutralized using phenolphthalein indicator and dilute NaOH or H2SO4 (as appropriate) and analyzed for inorganic phosphate and total P. Extraction solutions were: 0.5 M NaHCO3, 0.1 M NaOH and 1 M HCl. Phosphate was determined by molybdate colorimetry at 880 nm with a 1-cm path length. Total P was determined by the same procedure following acid-persulfate digestion at 80 °C overnight in sealed glass tubes. In both cases, standards were prepared in the extract solution following identical neutralization and dilution steps. The detection limit for both procedures was approximately 0.6 mg kg-1. For each extract, organic P was calculated as the difference between total P and inorganic P. Residual P was calculated as the difference between the independent estimate of total P measured by ignition and the sum of all inorganic and organic P fractions measured from the sequential extractions. Total soil [N] was determined by combustion and gas chromatography on a Thermo Flash 1112 analyzer (CE Elantech, Lakewood, NJ, USA). Nitrate and ammonium were determined colorimetrically on a Lachat Quikchem 8500 using standard procedures from the manufacturer. Total dissolved N was determined by combustion and gas chromatography on a Shimadzu TOC-TN analyzer. Dissolved organic N was calculated as the difference between total dissolved N and the sum of nitrate and ammonium. Soil pH was determined in a 1:2 soil to solution ratio in 10 mM CaCl2 using a glass electrode. Exchangeable cations (Al, Ca, Fe, K, Mg, Mn, Na) were determined by extraction in 0.1 M BaCl2 with detection by ICP-OES on an Optima 7300 DV (Perkin Elmer, Inc, Shelton, CT). Units are: total [N], %; total [P], mg kg-1; effective cation exchange capacity (ecec), cmol(+) kg-1; dissolved organic [N] (don), mg kg-1; ammonium (nh4), mg kg-1; nitrate (no3), mg kg-1; resin [P], mg kg-1; all other organic and inorganic [P] fractions, mg kg-1; canola mass (plant-based index of soil fertility), g.