Dynamics Of Soil Carbon And Nitrogen And Responses To Climate Change From Forest And Meadow Ecosystems In Qinling Mountain, China

Study has shown that the Qinling Mountains of China in the transitional climate zone are considered to be susceptible to disturbance and climate changes.In this research,we studied the emissions of the major greenhouse gases in response to different temperatures and the relationship between these gases in major ecosystems?pine forest,oak forest and meadow?in this region.In view of the low temperature conditions was very important for the mountain ecosystem,the soil carbon emissions and the related soil carbon and nitrogen dynamics,as well as the soil nitrogen transformation were conducted in different forests and meadows.The seasonal and temporal variations of dissolved organic carbon and dissolved inorganic nitrogen,and the soil characteristics in the field were analyzed.Using the open top OTC?Open-top chambers?simulated warming and decreasing precipitation simulated throughfall reduction.Four treatments including warming,throughfall reduction,warming and throughfall reduction,and control were set to explore the dynamic of soil carbon and nitrogen in different seasons and the soil depth.Moreover,the variation of soil microbial biomass carbon in different forests and meadow was studied under warming and control conditions.The main results and conclusions of this paper are as follows:?1?An incubation experiment was conducted at three temperature levels?8,18 and 28°C?to quantify the response of soil carbon dioxide?CO₂?and nitrous oxide?N₂O?emissions to temperature in three ecosystems.The soil CO₂ emission rates increased with temperature and decreased with soil depth;they were the highest in the oak forest?broadleaf forest?and were lower in the pine forest?coniferous forest?and the meadow ecosystem.However,there was no significant difference in the soil N₂ O emission rates among the three ecosystems.The temperature sensitivity of CO₂ and N₂ O was higher in the forest than in the meadow ecosystem.The Q10 values?temperature sensitivity coefficient?for CO₂ and N₂ O were 1.07-2.25 and 0.82-1.22,respectively,for the three ecosystems.There was also evidence that the CO₂ and N₂ O emission rates were positively correlated?P<0.01?.The soil characteristics exhibited different effects on CO₂ and N₂ O emissions among different ecosystems at the three temperature levels.Moreover,the soil dissolved organic carbon?DOC?,specific ultraviolet absorbance?SUVA?and nitrate?NO3-?were important factors for CO₂ emissions,whereas the soil ammonium?NH₄+?and pH were the major controllers of N₂ O emissions.Unexpectedly,our results indicated that CO₂ emissions are more sensitive to increasing temperature than N₂ O,noting the different feedback of CO₂ and N₂ O emissions to global warming in this region.The different responses of greenhouse gas emissions in different forest types and a meadow ecosystem suggest that it is critical to conduct a comprehensive investigation of the complex mountain forest and meadow ecosystem in the transitional climate zone under global warming.Our research results provide new insight and advanced understanding of the variations in major greenhouse gas emissions?CO₂ and N₂O?and soil characteristics in response to warming.?2?Temperature has a profound influence on soil carbon fluxes and nitrogen transformation.Little is known about soil carbon release and nitrogen transformation in complex mountain forest-meadow ecosystems at low temperature in a climate transition zone.To elucidate this effect,soil carbon fluxes and nitrogen mineralization dynamics in a pine?coniferous?forest,oak?broadleaf?forest and meadow were investigated under laboratory incubation.The dissolved organic carbon and aromatic compound content of the soil varied with carbon mineralization in different ecosystems.Both forests and meadow exhibited methane?CH₄?uptake,indicating that these ecosystems in the transition zone may act as weak sinks at low temperature.There was no significant difference among different ecosystems in CH₄ uptake,whereas the forests had greater CO₂ emissions than the meadow,with higher CO₂ emissions in the oak forest than in the pine forest.A strong positive correlation was found between cumulative soil CH₄ and CO₂ fluxes in an aerobic environment for each ecosystem?P<0.01?.The soil mineralization and nitrification rates decreased with incubation times for all three ecosystems.This study demonstrated that forests may have a greater effect on soil nitrogen dynamics than meadow and that the pine forest had the highest soil nitrogen transformation rate.There were weak correlations between the soil carbon fluxes?CH₄ and CO₂?and DOC,ammonium nitrogen and nitrate nitrogen concentrations.These results indicate that the soil inorganic nitrogen in different ecosystems was slightly stimulatory or suppressive of soil CH₄ and CO₂ dynamics.This study advanced the understanding of soil carbon and nitrogen cycling in a fragile mountain complex ecosystem.?3?Vegetation plays an important part in soil carbon and nitrogen cycling in terrestrial ecosystems.We determined the effects of forest type?pine and oak forests?and meadow on soil dissolved organic carbon and dissolved inorganic nitrogen in a mountain ecosystem.Results showed that the DOC and dissolved inorganic nitrogen concentrations exhibited an obvious seasonal pattern and decreased with increasing soil layer.There were differences in DOC concentrations between the two soil layers for each ecosystem?P<0.05?.The soil DOC and dissolved inorganic nitrogen concentrations were higher in the meadow than in the forests,moreover,they were higher in the pine forest than in the oak forest.Besides,for soil organic carbon?SOC?and total nitrogen?TN?contents,as well as soil pH value the pine forest had higher value than in the meadow and oak forest.Whereas the meadow had lowest C/N ratio and the highest value appeared in the pine forest among the three ecosystems.Soil surface temperature and soil water content were positively correlated with soil DOC and dissolved inorganic nitrogen concentrations?P<0.01?.It was demonstrated that the major controlling factors on soil DOC and dissolved inorganic nitrogen concentrations were soil SOC,TN and soil C/N ratio.In addition,there were significant negative correlations between soil DOC concentrations and pH value?P<0.01?.Moreover,soil DOC were positively correlated with soil dissolved inorganic nitrogen concentrations?P<0.01?.?4?The effects of warming and throughfall reduction on soil DOC and dissolved inorganic nitrogen?DIN?contents varied with the seasons.The highest values of DOC and DIN in soil appeared in the meadow,and the lowest value appeared in the oak forest.The soil DOC contents in the 0-10 cm soil layer were in the range of 471.53-731.48 mg/kg,while the soil DOC contents in 10-20 cm soil layer were in the range of 435.92-694.94 mg/kg in different forest and meadow ecosystems.For soil inorganic nitrogen,the range of the contents in the 0-10 cm and 10-20 cm soil layers was 22.79-55.30 mg/kg,13.29-34.71 mg/kg,respectively in different vegetation types.The effects of warming and throughfall reduction on soil SOC and TN varied with seasons and soil profiles in different vegetation types.The correlation between soil DOC and DIN,soil water content,surface temperature and TN was significant?P<0.05?,and the correlation between soil DOC and SOC and pH was not significant.?5?The soil microbial biomass carbon?MBC?contents in the three different vegetation types varied with seasons,and showed a decreasing trend with the increasing soil depth.The seasonal variation of soil MBC contents was the highest in summer and lowest in autumn under the warming condition.Whereas soil MBC contents in the oak forest showed different seasonal variation in the trend of spring > autumn > summer > winter.Under the control condition,the seasonal variation of soil MBC contents in the two forest types was the same as that in the oak forest under warming.The change of soil MBC contents in different seasons in the meadow was in the order of spring > summer > autumn both under the warming and control treatments.There were significant differences in soil MBC contents in the cases of different vegetation types and soil depth under each treatment?P<0.05?.The soil microbial biomass carbon in different forest and meadow was the highest in the meadow,followed by the pine and oak forest.Correlation analysis showed that soil MBC content was positively correlated with soil DOC,SOC,DIN,TN and soil temperature and water content respectively,while it was negatively correlated with soil C/N ratio and pH value.It suggested that soil characteristics were important in the changes of soil microbial biomass carbon.