Soil CO2 emission response to the extreme changes in limiting factors: results of field experiments in Central Siberia

Abstract:

In the boreal region as already known, there are two limiting factors control the biogeochemical processes – amount of precipitation and nutrients supply. Soil CO₂ emission as one of the biggest fluxes in the ecosystem also modifies by them. Due to the different origin of two factors, the mechanisms of adaptation to their influence depend on the amount of water or nutrients, ecosystem type, exposure period (snow or snow-free period of the year).

In this study, we conducted two field experiments with adding different amounts of water and nitrogen (NH₄NO₃ - ammonium nitrate solution) and fixation the reaction of soil emission to this impact. For the water experiment was chosen five replicates – 0, 25, 50, 100% water addition from each rain event. In the nitrogen experiment the treatments were: 0, 2, 4, 8, 15, 20, 25, 50, 100, 150 kg N per ha per year. Such a large concentration range allows finding the exact border of nitrogen limitation for the emission process.

From two field experiments was observed the response soil CO₂ emission to impacts of natural (water addition) and anthropogenic (nitrogenous fertilizers) origin factors, which will allow us to conclude the functional role (sink or carbon dioxide source) of our region. A peculiarity of the pine forests of Central Siberia is their high sensitivity to the moistening of the territory during the growing season. A change in their amount may inhibit (at significant amounts) or intensify (at optimal moistening conditions) the rate of CO₂ fluxes from the soil. The meteorological characteristics of the soil during the season participate differently in the formation of soil CO₂ emissions. SWC is equal to 30% is the threshold moisture conditions in this area under which the CO₂ flux was the highest, i.e. there was no restriction on water availability. At extremely low soil moisture, the following trend was observed: the relationship between the CO₂ flux and soil temperature was very high and statistically significant. Such dependence is supposedly a manifestation of a stress reaction to conditions of complete drought. Nitrogen has a similar mechanism of action on the soil emission: it was estimated the amount of nitrogen for the max emission rates.