Effects of acetylene at low concentrations on nitrification, mineralization and microbial biomass nitrogen concentrations in forest soils

Temperate forest surface soils at the varying distances from main trunks (e.g., Pinus koraiensis and Quercus mongolica) were used to study the effects of acetylene (C₂H₂) at low concentrations on nitrification, mineralization and microbial biomass N concentrations of the soils, and to assess the contribution of heterotrophic nitrification to nitrous oxide (N₂O) emissions from soils. The use of acetylene at partial pressures within a range from 10 to 100 Pa C₂H₂ in headspace gas gave a significant decrease in N₂O emission at soil moisture of c. 45% water-filled porosity space, and the decrease was almost the same in each soil after exposure of C₂H₂ at low concentrations. Heterotrophic nitrification could account for 21%–48% of total N₂O emission from each soil; the contribution would increase with increasing distances from the Pinus koraiensis trunks rather than from the Quercus mongolica trunks. Under the experimental conditions, the use of C₂H₂ at low concentrations showed no significant influence on soil microbial biomass N, net N mineralization and microbial respiration. However, 100 Pa C₂H₂ in headspace gas could reduce carbon dioxide (CO₂) emissions from soils. According to the rapid consumption of 10 Pa C₂H₂ by forest soils and convenience for laboratory incubations, 50 Pa C₂H₂ in headspace gas can be used to study the origin of N₂O emissions from forest soils under aerobic conditions and the key associated driving mechanisms. The N₂O and CO₂ emissions from the soils at the same distances from the Quercus mongolica trunks were larger than those from the Pinus koraiensis trunks, and both emissions decreased as the distances from trunks increased. The stepwise regression analysis showed that 95% of the variability in soil CO₂ emissions could be accounted for by the concentrations of soil total C and water soluble organic C and soil pH, and that 72% of the variability in soil N₂O emissions could be accounted for by the concentrations of soil total N, exchangeable NH₄ ⁺-N and microbial biomass N and 25% of the variability in heterotrophic nitrification by the soil microbial biomass N concentration. The emissions of N₂O and CO₂ from forest soils after exposure of C₂H₂ at low concentrations were positively related to the net nitrification of the soils. Supported jointly by National Natural Science Foundation of China (Grant Nos. 20777071 and 20477044) and Hundred Talents Project of the Chinese Academy of Sciences