Effects Of Simulated Acid Rain On Greenhouse Gas (CO₂, CH₄, N₂O) Emission From Different Soil-Crop Systems

Global warming and acid rain have become two mostly concerned environmental problems of worldwide importance. Anthropogenic increase of greenhouse gases is the main reason for Global warming. Meanwhile, SO2 and NOX emission from fossil fuel combustion is the main reason for acid rain.Agroecosystem plays an important status in the global change, and it is manipulated intensively by human. Acid rainwater increases the input of sulfate ion, nitrate ion, hydrogen ion to soil, and then has effects on the chemical properties of soil, the physiological characteristics and growth of crops, the community structure and activities of soil microorganisms, then induces the change to greenhouse gas (CO2, CH4, N2O) product and emission in agroecosystem. The quantitative investigation on the greenhouse gas emission from farmland treated with simulated acid rainwater will provide the method to evaluate the ecological effects of acid rain objectively. Furthermore, the results of this study will provide the theoretical basis to estimate the regional farmland greenhouse gas emission and its long-term trend in acid rain contaminated.The goal of this research is to quantitatively investigate the greenhouse gas (CO2, CH4, N2O) emission from soil-crop systems treated with different rainwater, which with higher ionic concentrations or lower pH values. The influence way of acid rain on greenhouse gas (CO2, CH4, N2O) emission from soil-crop systems was discussed.In this research, an outdoor pot experiment was used to study the effects of acid rain, and three incubation tests were conducted to discuss the influence way of acid rain. The outdoor pot experiment was conducted with paddy soils of pH 5.48 (S1), pH 6.70 (S2) and pH 8.18 (S3) during two years of rice-wheat growing season. Soil-crop systems were exposed to acid rainwater by spraying the crop foliage and irrigating the soil with simulated rainwater of T1 (pH 6.0), T2 (pH 6.0, ionic concentration was twice as rainwater T1), and T3 (pH 4.4, ionic concentration was twice as rainwater T1), respectively. The quantity of spray rainwater was calculated according to the monthly mean precipitation during 1970-2000. The static opaque chamber-gas chromatograph method was used to measure the flux of CO2, CH4, and N2O from soil-crop systems.Soil incubation test I, a 40-day incubation test, was conducted with the same paddy soils as the pot test. The soils were amended with 0 and 15 g-kg-1 of rice straw, adjusted to the moisture content of 400g-kg-1 air dried soil by using simulated rain of pH 6.0 (TC1), 4.5 (TC2), and 3.0 (TC3), and incubated at 20℃. Soil incubation test II, a 51-day incubation test, was conducted with the soil S2. The soil was amended with 0 and 15 g-kg-1 of rice straw, adjusted to the moisture content of 400g-kg-1 air dried soil or flooded, by using simulated rain of CR1 (pH 5.6, ionic concentration was twice as rainwater T2), CR2 (pH 5.6, ionic concentration was twice as rainwater CR1), and CR3 (pH 3.5, ionic concentration was the same as rainwater CR1), and incubated at room-temperature. Seeds culture test was conducted with rice seeds treated with the same simulated rainwater as in pot test, and incubated at 25℃in dark.Main results of this study are presented as follows:1. During the wheat-growing season, acid rain significantly promoted the average respiration rate and N2O flux from the alkaline soil-wheat systems. During 2005-2006, after the alkaline soil-wheat systems were treated with rainwater T3, the average respiration rate was 23.6% and 27.6% higher than that of alkaline soil-crop systems treated with rainwater T1 and T2, respectively. The N2O average emission rate in treatments S3T3 was 25.6% higher than that in treatments S3T2. Acid rain had significant effects on the dark respiration rate and N2O flux from neutral soil-wheat systems or acidic soil-wheat systems during some specific growth stage (seedling stage, jointing stage, and filling stage), while the average respiration rates and average N2O flux of these two soil-crop systems were not significantly influenced by acid rain.2. During the rice-growing season, the effects of acid rainwater on greenhouse gas emission from soil-rice systems were different between 2006 and 2007.2.1 Rainwater with lower pH value had no significant effects on the average respiration rates in three soil-rice systems. In acidic soil-rice systems, the average respiration rate was significantly promoted by the rainwater with higher ionic concentrations (T2) in 2007. Rainwater, with higher ionic concentrations and lower pH value, reduced the average respiration rate in neutral soil-rice systems during 2006 and that rate in alkaline soil-rice systems during 2007, while it increased the average respiration rate in neutral soil-rice systems during 2007. After the acidic soil-rice systems were treated with rainwater T2, the average respiration rate was 8.1%(p<0.05) higher than that of acidic soil-rice systems treated with rainwater T1. After soil-rice systems treated by rainwater T3, the average respiration rate was 21.4%(p<0.05) lower in neutral soil-rice systems during 2006,7.3%(p<0.05) lower in alkaline soil-rice systems during 2007, and 7.9%(p<0.05) higher in neutral soil-rice systems during 2007 than that rate in soil-rice systems treated with rainwater T1 during the same period respectively.2.2 Rainwater with higher ionic concentrations enhanced the CH4 emission, while rainwater with lower pH value reduced the CH4 emission from soil-rice systems. Higher ionic concentration rainwater could increase the CH4 cumulative emission from acidic soil-rice systems and from neutral soil-rice systems, but these effects did not reach significant level. Compared to the CH4 cumulative emission in rainwater T2 treatments, the CH4 emission decreased in acidic soil-rice systems and in neutral soil-rice systems treated with rainwater T3. After soil-rice systems treated with rainwater T2, the CH4 cumulative emission was 85.6% and 19.2% higher than that in acidic soil-rice systems and in neutral soil-rice systems treated with rainwater T1, respectively. After soil-rice systems treated with rainwater T3, the CH4 cumulative emission was 51.5% and 31.4% lower than that in acidic soil-rice systems and in neutral soil-rice systems treated with rainwater T2, respectively. However, only the effect reached significant level in neutral soil-rice systems.2.3 Rainwater with lower pH value had no significant effects on N2O cumulative emission from the three soil-rice systems. Higher ionic concentrations rainwater (T2) reduced the N2O cumulative emission from acidic soil-rice systems and from neutral soil-rice systems. Rainwater (T3), with higher ionic concentrations and lower pH value, could decrease the N2O cumulative emission from the neutral soil-rice systems. After soil-rice systems treated with rainwater T2, the N2O cumulative emission was 16.6% (p=0.039) (2006) and 56.4% (p=0.019) (2007) lower than that in acidic soil-rice systems and neutral soil-rice systems treated with rainwater T1, respectively. After neutral soil-rice systems treated with rainwater T3, the N2O cumulative emission was 71.9%(p=0.047) lower than that from neutral soil-rice systems treated with rainwater T1.3. It was demonstrated that acid rain affected the greenhouse gas emission from soil by disturbing the decomposition rate of straw. The moisture of soil played some important roles in the influence of acid rain.3.1 Lower pH value rainwater had no significant effect on the soil organic carbon decomposition while it significantly influenced the N2O emission and the straw decomposition. So it significantly influenced the greenhouse gas emission from soil systems. Compared with pH 6.0 rainwater treatment, acid rainwater of pH 3.0 treatment promoted the straw decomposition rates in both acidic paddy soil and alkaline paddy soil to 8% increase during 40 days, while it reduced by 15% in neutral paddy soil. Compared with pH 6.0 rainwater treatments, acid rainwater of pH 3.0 treatments promoted N2O cumulative emission from acidic paddy soil without straw incorporated by 51.3% higher (p <0.05) during 40 days. Acid rain changed the inhibitory effect of straw on the N2O emission, so the difference became smaller among N2O cumulative emission from the three soils with straw incorporated (AS).3.2 Rainwater, with higher ionic concentrations or with lower pH values, had no significant effects on the cumulative emission of CH4, N2O during 51 days. The CO2 cumulative emission was decreased in soils without straw by higher ionic concentration rainwater under submerged condition. After paddy soil treated with higher ionic concentrations rainwater (CR2), the CO2 cumulative emission was 26.1%(p<0.05) lower than that from paddy soil treated with rainwater CR1 under submerged condition. There was significant difference of straw decomposition rates in soils treated with different rainwater. Under submerged condition, rainwater with higher ionic concentrations or lower pH values could enhance the straw decomposition in neutral soil. The straw decomposition in CR2F groups and CR3F groups was 16.1%(p<0.01),2.6%(p<0.01) higher than that in CR1F groups, respectively. Under non-flooded condition, rainwater with higher ionic concentrations or lower pH values could inhibit the straw decomposition in neutral soil. The straw decomposition rates in CR2D groups and CR3D groups were 5.3%(p<0.05), 8.1%(p=0.111) lower than that in CR1D groups, respectively.In summary, the seasonal dynamic of greenhouse gas emission had not been changed by acid rain. During different growth stages, acid rain had different effects on the greenhouse gas product and emission from soil-crop systems. Acid rain had not changed the greenhouse gas emission from soil-rice systems after harvest. During wheat season, alkaline soil-wheat system was more sensitive than the other two soil-wheat systems. During rice season, CH4 and N2O emissions were more easily influenced by acid rain in acidic soil-rice systems and neutral soil-rice systems than that in alkaline soil-rice systems. Acid rain changed the basic physical and chemical properties of crop straw, straw decomposition rate and crop growth cost in soil-crop systems. All these led to the change of green house gas emission from soil-crop systems. Meanwhile, soil pH value and moisture played some important roles on the effects of acid rain on greenhouse gas emission.