Greenhouse gas fluxes in an open air humidity manipulation experiment

Air moisture is one of the main factors causing the greenhouse effect, but humidity has not been thoroughly studied at the ecosystem level. In 2006 a free-air humidity manipulation (FAHM) facility was established in Estonia to investigate the effect of humidity on the performance of two tree species—silver birch (Betula pendula Roth.) and hybrid aspen (Populus tremula L. × P. tremuloides Michx.). The trial is located on an Endogleyic Planosol of former arable land and offers the opportunity to change relative air humidity through controlled artificial misting and drying. We measured the CO₂, CH₄ and N₂O emission from the FAHM system using closed chamber and gas-chromatograph techniques from July 2009 to October 2010. Measurements were carried out once a month in three humidification (h) plots and in three control (c) plots. The vegetation period was rainy in 2009, but the next summer was relatively dry. The dry summer interacted better with the humidification. The CO₂ flux decreased when the air moisture was higher than in the control plots. The soil acted as a sink for methane. Less CH₄ was oxidized in the soil with increased humidity. Emission of N₂O did not correlate with air humidity, although one could observe a clear tendency that there was a smaller N₂O flux during the period with increased humidity. Expectedly, CO₂ emission and CH₄ consumption demonstrated strong positive correlations with soil temperature.