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 influ- ence 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 con- ditions 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 NH+4-N and microbial biomass N and 25% of
Soil acidification via acid precipitation is recognized to have detrimental impacts on forest ecosystems, which is in part associated with the function of ethylene released from the soil. However, the impacts of acidification on the cycling of ethylene in forest soils have not been fully taken into consideration in global change studies. Forest topsoils （0-5 cm） under four temperate forest stands were sampled to study the effects of a pH change on the emissions of ethylene and carbon dioxide from the soils and concentrations of dissolved organic carbon （DOC） released into the soils. Increasing acidification or alkalinization of forest soils could increase concentrations of DOC released into the soils under anoxic and oxic conditions. The ethylene emission from these forest topsoils could significantly increase with a decreasing pH, when the soils were acidified experimentally to a pH〈4.0, and it increased with an increasing concentration of DOC released into the soils, which was different from the carbon dioxide emission from the soils. Hence, the short-term stimulating responses of ethylene emission to a decreasing pH in such forest soils resulted from the increase in the DOC concentration due to acidification rather than carbon mineralization. The results would promote one to study the effects of soil acidification on the cycling of ethylene under different forest stands, particularly under degraded forest stands with heavy acid depositions.
Change in temperature affects the activity of soil microorganisms.However,there is limited knowledge about temperature effects on ethylene(C2H4) and methane(CH4) production from forest soils.Topsoil samples(0―5 cm) collected from different temperate forest stands(e.g.,Pinus sylvestris L.,Cryptomeria japonica,and Quercus serrata) were used to compare C2H4 and CH4 production from soils at temperature from 5 to 35℃ under oxic and anoxic conditions.The rates of C2H4 and CH4 production from soils under oxic conditions were measured by using inhibition of acetylene(C2H2) and carbon monoxide(CO).The consumption of C2H2 by soils at an initial concentration of c.250 Pa C2H2 was negligible at 5 and 15℃,but it was significantly increased at 25 and 35℃.The presence of 2 kPa CO in the headspace gases tended to decrease the consumption of C2H2 by soils at high temperature.The Q10 values for the soil C2H2 consumption ranged from 2.3 to 3.8,and there were no significant differences in Q10 values between these topsoil samples.The rate of CH4 production from each sample under oxic conditions was larger than the soil C2H4 production at 5―35℃,particularly at low temperature,and presented a smaller Q10 value.Ethylene production from soil after 1 week of anoxic incubation at 5―35℃ was larger than the soil CH4 production,and presented a larger Q10 value.However,CH4 produc-tion from Quercus serrata forest soil and its response to temperature increased significantly with in-cubation time.Long-term anoxic conditions of in situ upland forest soils are normally not prevalent,so it can be reasonably concluded that there is a larger C2H4 production rather than CH4 production under temperate forest stands due to heavy rainfall in summer.
An increase in atmospheric nitrogen （N） deposition can promote soil acidification, which may increase the release of ethylene （C2H4） under forest floors. Unfortunately, knowledge of whether increasing N deposition and C2H4 releases have synergistic effects on soil methane （CH4） uptake is limited and certainly deserves to be examined. We conducted some field measurements and laboratory experiments to examine this issue. The addition of （NH4）2SO4 or NH4Cl at a rate of 45 kg N ha-1 yr-1 reduced the soil CH4 uptake under a temperate old-growth forest in northeast China, and there were synergistic effects of N amendments in the presence of C2H4 concentrations equal to atmospheric CH4 concentration on the soil CH4 uptake, particularly in the NH4Cl-treated plots. Effective concentrations of added C2H4 on the soil CH4 uptake were smaller in NH＋4 -treated plots than in KNO3-treated plots. The concentration of ca 0.3 μl C2H4 L-1 in the headspace gases reduced by 20% soil atmospheric CH4 uptake in the NH4Cl-treated plots, and this concentration was easily produced in temperate forest topsoils under short-term anoxic conditions. Together with short-term stimulating effects of N amendments and soil acidification on C2H4 production from forest soils, our observations suggest that knowledge of synergistic effects of NH＋4 , rather than NO3- , amendments and C2H4 on the in situ soil CH4 uptake is critical for understanding the role of atmospheric N deposition and cycling of C2H4 under forest floors in reducing global atmospheric CH4 uptake by forests. Synergistic functions of NH4＋ -N deposition and C2H4 release due to soil acidification in reducing atmospheric CH4 uptake by forests are discussed.
