| Summary: | The multifactorial experiment, established in 2003, consists of a control and three treatments: +4 degrees C warming; wetting; and +4 degrees C warming × wetting. The warming treatment was based on temperature models to mimic ~2050. Irrigation treatments increase growing season precipitation by approximately 50% relative to 1971-2000. Measurements were conducted from the end of May to the end of August. NEE was measured with automated clear chambers coupled to a Picarro G1301 analyser. GPP was calculated by the difference between NEE and Reco. Ecosystem respiration and soil pore space CO2 concentrations were measured at least three times a week with infrared gas analysers. Ecosystem respiration was measured using opaque chambers, and soil CO2 concentrations were measured using stainless-steel gas wells inserted throughout the active layer. Gas samples for 14C analysis were collected monthly. Ecosystem respiration, CO2 in ambient air and root-respired CO2 were captured on molecular sieve traps. Roots were manually extracted, rinsed and incubated in CO2-free air for 24 h. Soil gas was collected in pre-evacuated canisters using flow-restricting capillaries. Concentrations of organic C along the soil profile were measured by elemental analysis. For 14C analysis, CO2 was released from molecular sieve traps by heating at 650 degrees C for 45 min or extracted from canisters on a vacuum line, purified cryogenically and reduced to graphite through zinc reduction and analysed at UC Irvine's W. M. Keck Carbon Cycle Accelerator Mass Spectrometer facility28. Soil samples were first combusted to CO2 in pre-combusted, evacuated quartz tubes with cupric oxide for 2 h at 900 degrees C. We used a two-pool mixing model29 to estimate the contributions from older, below-ground permafrost soil C versus young, surface soil C to Reco. data for paper and figures in DOI: 10.1038/NCLIMATE2058
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