Modelling of the impact of climate change on the transformation of nitrogen forms in the soil and N2O emissions from the agroecosystems of Eastern Ukraine

The moisture-temperature regime influences the nitrogen status of the soil and the microbiological processes of the transformation of nitrogen forms. Therefore, we assumed that ongoing climate change may affect the emission of nitrous oxide N2O, which is formed in the course of the transformation of nitrogen forms in the soil and is one of the most aggressive greenhouse gases that determine the global warming potential. To test this hypothesis and make a quantitative assessment of the impact of climate change on the transformation of nitrogen forms in the soil and N2O emissions from the agroecosystem soils, we used a complex model of greenhouse gas emissions from an agroecosystem soil that we developed and the RCP4.5 climate change scenario. The research was performed for the chernozem soils of Eastern Ukraine, with winter wheat being the cultivated crop. ‘Dry’ and ‘wet’ years typical for the period of 2021–2050 were selected among climate conditions in accordance with the climate change scenario. A ‘dry’ year was considered to be a year with a precipitation of 60% or less of the long-term average, a ‘wet’ year was a year with a precipitation of 130% or more of the long-term average for the studied period. The level of ammonification during the growing season for both ‘dry’ and partially ‘wet’ years was mainly controlled by temperature. At the beginning of the period, at negative temperatures below –3 °C ammonification and nitrification almost stopped, and at temperatures above –2 °C they slightly increased. Indices for the intensity of these processes increased with a rise in temperature from 0 °C to 10–11 °C. In the conditions of a ‘dry’ year an increment in temperature to the level of 22.4–27.8 °C caused a sharp increase in the rate of ammonification and caused a high level of the rate of nitrification. For the conditions of a ‘wet’ year with a sufficiently low (compared to a ‘dry’ year) temperature regime, a relatively significant rise in temperature with still quite good humidification increased the intensity of the ammonification process. The dependence of the intensity of the ammonification and nitrification processes on the moisture reserves in the soil is traced. The high level of moisture reserves in the arable layer at the beginning of the growing season in both ‘dry’ and ‘wet’ years formed anaerobic conditions in the upper layer of the soil. Against the background of rising temperatures, this called forth the denitrification process. N2O emission was 0.03–0.29 g N-N2O/ha•day due to the denitrification process in a ‘dry’ year, and 0.7–5.2 g N-N2O/ha•day in a ‘wet’ year. The highest level of N2O emission due to nitrification was observed in a ‘dry’ year in the middle of the vegetation period at high temperatures (22.7–27.8 °С) and amounted to 8.2–11.2 g N-N2O/ha•day. A decrease in soil moisture reserves during the second half of the growing season reduced the level of N2O emissions. Nitrification was the main process producing N2O. The emissions of N2O with moisture reserves greater than 55 mm in the arable layer occurred due to denitrification. A rise in temperature increases the level of N2O emissions. The peculiarities of the influence of moisture and temperature conditions of ‘dry’ and ‘wet’ years on the processes of ammonification, nitrification, denitrification, and N2O emissions from chernozem soils according to the RCP4.5 climate change scenario in Eastern Ukraine were assessed.