Nah14co3

Two weeks after ³³P and ¹⁵N tracer additions, plants were prepared for ¹⁴CO₂ labelling, to allow movement of carbon from plant to fungus to be quantified. A 110 µl solution of NaH¹⁴CO₃ (Perkin Elmer) containing 1.0175 MBq ¹⁴C (specific activity = 1.621 GBq/mmol) was added to a cuvette in each pot. The tops of all mesh‐windowed cores were sealed using gas‐tight rubber septa (SubaSeal) to minimize diffusion of ¹⁴CO₂ into the cores. ¹⁴CO₂ gas was liberated from the NaH¹⁴CO₃ by addition of 10% lactic acid, generating a 1.0175 MBq pulse of ¹⁴CO₂. Samples of 1 ml above‐ground gas and 1 ml below‐ground gas (via the glass wool‐filled core) were taken 1 hr after release of ¹⁴CO₂ and every 4 hr thereafter to monitor the drawdown, respiration and flux of ¹⁴C through the plant–AMF network. Gas samples were injected into gas‐evacuated scintillation vials containing 10 ml Carbosorb^® (Perkin Elmer), a carbon‐trapping compound. To this, 10 ml Permafluor scintillation cocktail (Perkin Elmer) was added, and ¹⁴C content of each sample was quantified by liquid scintillation counting (Tricarb 3100TR scintillation counter; Perkin Elmer).
Pots were maintained under cabinet conditions until detection of maximum below‐ground ¹⁴C flux (20–22 hr after ¹⁴CO₂ liberation) at which point 3 ml 2 M KOH was added to cuvettes within each microcosm to capture remaining gaseous ¹⁴CO₂.

Photosynthetic carbon fixation rates were estimated using ¹⁴C incorporation^{44 (link)}. Water samples were analyzed in triplicates with dark and zero-time controls. Samples (50 mL) collected at each time point were added to polycarbonate bottles (Nalgene) containing 5 μCi of NaH¹⁴CO₃ (Perkin Elmer, specific activity 56 mCi mmol⁻¹) and incubated for 4 h under ambient natural illumination and temperature. To determine the quantity of the added radioactivity, 50 μl of each sample was immediately mixed with 50 μl of ethanolamine and stored for analysis. The incubations were terminated by filtering the spiked seawater through GF/F filters (~0.7 μm nominal pore size) at low pressure (~50 mmHg). The filters were incubated overnight in 5-mL scintillation vials containing 50 μl of 32% HCl in order to remove excess ¹⁴C-bicarbonate. After adding 5 mL of scintillation cocktail (Ultima-Gold) to each vial, the radioactivity was measured using a TRI-CARB 2100 TR (Packard) liquid scintillation counter.