| In some regions of mid-, high-latitude and high-altitude area, freeze-thaw event more frequently along with global warming. Freeze-thaw cycle results in changes the soil physical properties, shifts the microbial community composition in original ecological system, then has strong effects on the process of C and N turnover. Meanwhile, with more intensive human activities, more anthropogenic reactive N released to atmosphere, then returned to terrestrial aquatic ecosystems. N deposition the increasing of N deposition can alters the productivity and the biomass accumulation in forest ecosystem, and thus deeply affect carbon storage and nitrogen cycle. At present, the influence of nitrogen deposition on carbon and nitrogen mineralization process had studied by many researchers, they paid more attentions to temperate, subtropical and tropical forest, but little focus on northern coniferous forest which at high latitude and soil heat is more sensitive to the effects of changing environment, such as human activities and climate. Atmospheric N deposition tends to change soil C and N dynamics could interact with freeze-thaw, because N deposition will impacts the supply of C substrate of soil, shifts the microbial community composition and so on. Thus these may change the effects on soil C and N mineralization of freeze-thaw. Coniferous forest in China mainly located in the Daxing’ anling region, which is subjecting to winter warming and atmospheric N deposition clearly, but the combined effects of winter warming and N input on SOM decomposition of this forest type remain unexplored. Using soils of cold-temperate coniferous forest from a 3-year simulated N deposition experiment platform in the Daxing’anling region, we conducted a series of incubation experiments to clarify the effect of freezing-thawing cycle and global N deposition on soil carbon and nitrogen mineralization, the main objectives are to provide theoretical bases for the field studies of soil carbon and nitrogen turnover in non-growing season. The main conclusions are as following:1. Soils of cold-temperate coniferous forest in the Daxing’anling region with different N addition levels were selected to conduct an experiment with 48h incubation and continuous measurement. Two treats of temperature (-1-8℃ FTC and constant 8℃) were set to research the dynamic of soil C mineralization with temperature change and the effect of different N deposition levels on soil C mineralization. The results shown during the measuring time, soil C mineralization rate correlated with the temperature. In freeze-thaw cycle, soil C mineralization rate significantly reduced by decreasing temperature from 8 to -1℃ and strongly enhanced by increasing temperature from -1 to 8℃, but the rate we measured at constant temperature relatively stable. In freezing and thawing cycle of -1℃ constant stage and 8℃ constant stage, soil C mineralization rate was significantly affected by N deposition level, and was the highest in high N addition. Simultaneously, soil C mineralization rate also differed significantly among three levels of NH4NO3 fertilizer (P=0.000) in 8℃ constant temperature without freeze-thaw, while was highest in unfertilized soil, followed by HN and LN. Furthermore, we used exponential function to calculate the Q10 of SOM decomposition during cooling and heating process with different N deposition level treatments and founded, soil temperature change (P=0.000) and N levels (P=0.000) had significant effect on Q10 values. The Q10 values in heating process are higher than the values in cooling process. And in heating process LN is more sensitive to soil temperature change than two other N levels treatments. Soil freezing reduced soil carbon mineralization rate, then affected the amount of CO2 that released from soils, While exogenous nitrogen input into soil would diminish this reducing effect that caused by freezing-thawing. So with increasing deposition of reactive nitrogen in the future, soils in low temperature will release more carbon into the atmosphere.2. Soils of cold-temperate coniferous forest in the Daxing’ anling region were selected to conduct a 30d laboratory incubation experiment with two treats of control (constant 8℃) and freeze-thaw cycles (-5℃ in 24h and then 8℃ in 24h). Soil carbon (C) mineralization, nitrogen (N) mineralization, and the relationship between accumulated soil carbon and nitrogen mineralization were studied. Our results showed that incubation temperature and time had significant effect on soil C mineralization rate and C mineralization accumulated. After the 1st and 5th freeze-thaw cycles the soil C mineralization rate was higher than that in the control. The C mineralization accumulated in the -5-8℃ treatment was significantly lower than that in the control after the 7th and 15th freeze-thaw cycles. Incubation temperature and time had no effect on soil N mineralization rate, while the N mineralization accumulated was significantly affected by incubation temperature and time. After the 5th,7th, and 15th freeze-thaw cycles net N mineralization accumulated was higher in the control than in -5-8℃ treatment. After 30d incubation the C and N mineralization accumulated were higher 1.42 and 2.21 times respectively in the control as against the freeze-thaw treatment. Net N mineralization accumulated was positively correlated with C mineralization accumulated in both control and freeze-thaw treatments. Freeze-thaw cycles of -5-8℃ can decrease C emission and inorganic N accumulation, which might improve soil C storage and decrease nutrient loss.3. Soils of cold-temperate coniferous forest in the Daxing’ anling region with low dose and high dose of NH4NO3 input were selected to conduct a 30d laboratory incubation experiment. Compare the effect of freeze-thaw on soil C and N mineralization between different N deposition levels (including CK soil), and our results showed:N deposition levels (P=0.000) had significant effect on soil C mineralization rate. Compare with CK and HN soils, low N input improve the soil C mineralization rate. The C mineralization rate also had remarkable difference between different temperature treats (-5-8℃ and 8℃), it were higher after frozen than in constant temperature. Soil N mineralization rate significance at P<0.05 of treatments of temperature, incubation time and levels of N input. The effect on soil C mineralization of freeze-thaw with different levels of N input significantly different (P=0.002). Freeze-thaw decreased the C mineralization accumulated of CK and LN soils, and the decrease in CK soil induced by freeze-thaw enlarged along with incubation time, while it diminished in LN soils. In soils with high NH4NO3 addition, the C mineralization accumulated increased by frozen at the beginning of the culture stage. However, the carbon dioxide emissions after 15 times of freeze-thaw cycles lower than the emissions of the soil in constant temperature. The effect on N mineralization accumulated of freezing-thawing with three levels of N input were, in order, CK>0>HN>LN. Freeze-thaw decreased the N mineralization accumulated of original soil (CK), but increased inorganic N accumulation of the soils with nitrogen addition conversely, and the performance in LN is more obvious. |
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