Data were collected in May 2018 at Salada Grande, Salada Pequeña and Salada de la Jabonera de las Torrazas, located south of the River Ebro and several kilometres west of Alcañiz in the Teruel Province of Spain.
On each salada, four pits up to 0.5 m deep were excavated at equal distances within the area of the soil moisture sampling grid. From each pit, up to five soil samples were collected from a range of depths, then air dried and bagged prior to analyses.
H+ ion activity was determined in a 1:2.5 soil-water mixture using a pH probe. Approximately 5 g of each sample was dried at 105 °C to a constant weight and heated at 430 °C for 16 hours in a muffle furnace to determine organic matter by loss-on-ignition. Bulk density was determined after weighing the oven dry mass of a soil sample collected from the subsurface using a stainless steel tube with an internal volume of 99 cm3.
An elemental analyser (vario PYRO cube, Elementar UK Ltd.) was used to determine the total C, N and S content of 10–30 mg sub-samples in tin capsules. C stable isotope measurements were made on the same sub-samples using a coupled mass spectrometer (visION, Elementar UK Ltd.). Organic C content and isotopic compositions were determined from separate sub-samples, after removal of inorganic C with 50 μL of 10% HCl in silver capsules and then overnight drying at 60 °C. Stable isotope data are reported as δ¹³CVPDB values, the proportion per thousand (‰) variation from the ratio of 13C/12C in V-Pee Dee Belemnite (VPDB). Isotope ratio measurements were calibrated to the VPDB scale using commercially available standard reference materials (B2205 EMA P2, B2153 low organic content soil, B2151 high organic content sediment and B2159 sorghum flour, all from Elemental Microanalysis, UK). Further elemental analysis on each sediment sample was performed with a Nito XL3t 950 GOLDD+ portable X-ray fluorescence spectrometer (pXRF).
To quantify CO2 efflux, respiration chambers were positioned at 12 locations in a grid across the water-free surface of each salada. Maximum distances between chambers were c. 50 m on Salada Pequeña, c. 120 m on Salada Grande and c. 70 m on Salada Jabonera. Chambers are made from white uPVC and comprise two parts: i) a lower chamber that when pushed 3 cm into the surface forms an air-tight seal; and ii) a screw-on lid that enables soil gases to accumulate inside the chamber and to be sampled during measurement cycles. The chamber lids contain a sampling port covered with a Suba seal for gas extraction and a two-way valve to ensure any pressure differences between the chamber and atmosphere are minimal and rapidly equilibrated. Chamber surface area is 83 cm2 and in this study chamber volume ranged from 0.48 to 0.52 litres depending on insertion depth. Heat sinks mounted through the chamber walls ensured the internal air temperatures were not elevated above ambient. Three of the 12 chambers were equipped with a sensor (USB502, Adept Science, UK) to record the air temperature and humidity inside the chamber at 2 minute intervals.
Measurements were taken three times each day to capture a range of temperature and light conditions, giving a total of 36 measurements at each of the three saladas. To determine soil CO2 efflux, the lid was placed on the chamber and 12 ml of gas was immediately extracted through the sample port using a syringe and hypodermic needle secured with a lock. After approximately five minutes, another syringe was gently pumped to mix the air within the chamber before a second sample was collected. Both sample CO2 concentrations were determined immediately after each of the three measurement cycles using an EGM-4 infrared gas analyser (PP Systems, Amesbury, USA). Mass CO2 flux in mg m-2 hr-1 was determined from the changes in CO2 concentration normalised to mean temperature and pressure during measurement. To correct for the effect of any diffusion suppression owing to the accumulation of CO2 inside the chamber, a diffusion correction factor was applied. At the time of sampling, surface soil temperature and moisture adjacent to each respiration chamber were determined (n = 3) using an infra-red thermometer and a soil moisture probe (SM150, Delta-T Devices Ltd., Cambridge, UK).
European Union's Horizon 2020 Research and Innovation Programme under a Marie Skłodowska-Curie Grant (No. 663830)
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