The Effect of Contrasting Moistening Regimes on CO2 Emission from the Gray Forest Soil under a Grass Vegetation and Bare Fallow

The effect of contrasting moisture regimes on the CO2 emission from the gray forest soils (Haplic Luvisols (Loamic, Cutanic, Humic)) under a grass vegetation and bare fallow was studied in a field simulation experiment in June–September, 2015 (Moscow region). Two short soil droughts (53 and 34 days) and a long one (94 days) were simulated on plots isolated from precipitation. A variant with regular irrigation, where the soil moisture was maintained 60–70% of their water holding capacity, was used as a control. Over the whole observation period, the CO2 emissions from the soils studied decreased by a factor of 1.8 compared to the control only in the variant with the grass vegetation under prolonged drought. During the first hours after irrigation of the dry plots, the soil respiration intensified due to the “Birch effect”. The magnitude of this effect was 84–104% in the soils under the grass vegetation and 114–133% in the fallow areas. Owing to this phenomenon, the total CO2 emission from the soils subjected to two short droughts was equal to the CO2 flux under regular moistening for the grass plots and exceeded it by almost 1.3 times for the fallow plots as compared to the control. However, the share of extra CO2 flux induced by moistening of the dry soils did not exceed 8–10% of the total CO2 emission over the whole observation period.