Soil Respiration And Its Environmental Responses Under Different Soil Water Conditions Of Typical Plantations In North China Plain

Soil respiration is the main way of CO₂ exchange from soil to atmosphere,and is an important part of the carbon cycle in forest ecosystems.As an important part of forests,the plantation forest area has expanded worldwide and currently plays an important role in climate change mitigation and ecological and environmental improvement.However,there is a lack of knowledge about the spatial heterogeneity of soil respiration(RS)in different tree species plantations and changes in the responses and related adjustment processes of typical plantation forest soil respiration to seasonal rainfall events and long-term differences in soil moisture under climate change.This study took the common afforestation tree species Populus×canadensis(poplar),Robinia pseudoacacia(black locust),Pinus tabuliformis,andmixed poplar and black locust forest as research objects,and conducted measurements with survey and long-term automatic chambers at Beijing Gongqing Forest Farm.We measured soil respiration in different tree species,and estimated the autotrophic and heterotrophic components of soil respiration with the trenches method in poplar plantations.We thus explored the spatial and temporal variations of soil respiration among different species and the response of soil respiration to rainfall events and long-term soil moisture changes in a poplar plantations.We furthermore focused on the mechanisms of soil respiration responding to biophysical factors at different spatial and temporal scales.The main results are as follows:(1)The daily dynamic changes of RS in different tree species plantations all showed a single-peak curve pattern.At 11 o'clock in the morning,RS measurements of the Chinese pine forest represent the daily average well,with the deviation and the root mean square error 0.15 and 0.22 g CO₂m^-2day^-1,respectively.In the three broad-leaved forests,two RS measurements per day(one at 7 am and one at 7 pm)are needed to adequately represent the daily average,with the root mean square error being reduced by 69.2%compared with one measurement(Poplar),67.1%(Robinia)and 63.6%(mixed).On a seasonal scale,the RS of different forest stands showed the highest changes in summer and the lowest in winter.However,regardless of monoculture forest or mixed forest,the RS of the three broad-leaved forests did not differ significantly,but they were all significantly higher than those of the young Pinus tabuliformis forest.(2)At the daily scale,when the soil moisture content is high,the relationship between soil respiration and soil temperature of different tree species plantations has an exponential relationship.When the soil moisture content is low,the soil respiration is decoupled from the soil temperature in the different forest stands.In poplar plantations,the time lag between soil respiration and soil temperature may occur in any season,and the time lag has a significant negative correlation with soil moisture content.At the seasonal scale,soil respiration of different forest stands is significantly related to soil temperature and moisture content,with soil temperature being the main controlling factor of the seasonal variation of soil respiration.Fine root biomass is the main driving factor affecting the spatial variability of soil respiration among different forest stands.In addition,there are seasonal differences in the contribution of soil heterotrophic respiration to soil respiration in poplar plantations.The average contribution of heterotrophic respiration to total respiration is twice that of autotrophic respiration,which is dominant in soil respiration.Soil autotrophic and heterotrophic respiration respond differently to environmental factors.While heterotrophic respiration is more responsive to changes in soil water content,autotrophic respiration is more sensitive to changes in soil temperature.(3)The response of soil respiration to rainfall in poplar plantations varies seasonally.Soil respiration is increased by rainfall in spring,but in summer and autumn,with increases of rainfall,soil respiration is inhibited.Integrating all rainfall events in the three seasons,when rainfall event exceeds moderate(10-25 mm)rainfall intensity,the response relationship of soil respiration to soil temperature is changed,and the temperature sensitivity of soil respiration(Q10)is significantly reduced.(4)The three long-term significant differences in soil moisture environment(high:H,medium:M and low:L)have a significant impact on the spatial variability of soil respiration in poplar plantations.Although the soil respiration in the high soil moisture environment is significantly(P<0.05)lower than that of the middle and low moisture forest stands,there is no significant difference in soil respiration between the middle and low forest stands.However,when comparing different measurement years,when M and L forests have similarly low soil moisture content,the soil respiration of M forests is significantly inhibited.Long-term differences in water environment have triggered different sensitivities of soil respiration to changes in soil moisture content in forests of the same tree species.Soil organic carbon,organic nitrogen and soil water content have a significant negative correlation with the spatial variability of soil respiration in forests with different moisture environments.In summary,this study explored the temporal and spatial variation of RS in different tree species plantations,and clarified that the RS of different tree species plantations has similar temporal variation characteristics.Among them,there is no significant difference in RS between broad-leaved pure forest and mixed forest,but both are significantly higher than Pinus.In mature monoculture coniferous forests,fine root biomass is the main factor affecting the spatial variability of soil respiration in different forest stands.Additionally,from the perspective of long-term water differences caused by rainfall events that are strongly affected by climate change and the special topography of the riparian zone,the response characteristics and regulation mechanisms of typical European and American poplar plantation RS to long-term and short-term water environment changes are discussed in depth.The results of this study are helpful to more accurately estimate the soil carbon emissions of typical plantation ecosystems with special topography in northern China under climate change,and provide a scientific basis for formulating the best strategies for plantation management.