Effects Of Agroforestry Use And Vegetation Degradation On Soil Respiration And Its Components In Reclaimed Wetlands At Dongtan Of Chongming Island

The natural coastal wetland is an ecosystem with important carbon sink functions in the estuary coastal area.With the continuous acceleration of the urbanization process in the estuary and coastal areas,a large number of coastal wetlands have been reclaimed by artificial dams,and the reclaimed wetlands have been used in various ways for agriculture and forestry.At the same time,the unused wetlands are also in the reclaimed environment,facing the problem of vegetation degradation.These all have a profound impact on the carbon cycle of the ecosystem.Soil respiration is an important part of the global carbon cycle,and its small changes will also have a great impact on atmospheric CO₂.Soil respiration is mainly composed of the autotrophic respiration of roots and the heterotrophic respiration of soil microorganisms and animals.Different components of soil respiration involve different biological and ecological processes,and their responses to environmental changes such as changes in land use patterns and vegetation degradation may have significant differences.Therefore,it is very important to study the seasonal changes of different components of respiration and their response to environmental factors.In order to understand the impact of agricultural and forestry utilization and vegetation degradation on soil respiration and its components after reclamation of coastal wetlands,this paper takes the Chongming Dongtan reclamation area of the Yangtze River estuary as the study area,and selects different agricultural and forestry utilization types under the same reclamation period（reclaimed by dykes in 1998）:Reed wetland（VW）,Young forest（YF）,Middle-age forest（MF）,Imperata wetland（IW）,Artificial pond（AP）,Cropland（CP）,and reed wetlands with different levels of vegetation degradation:No significant degradation wetlands（VW）,Low degraded wetlands（LW）,Moderately degraded wetlands（MW）,and Highly degraded wetlands（IW）were set up as sample plots,Combining the box method（LI-8100 soil carbon flux measurement system）and the layered root removal method,continuously measured different types of agricultural and forestry use（July 2018 to July 2019）and different levels of vegetation degradation in reed wetlands（June 2019 to November 2019）total soil respiration,soil autotrophic respiration and soil heterotrophic respiration.And simultaneously monitor the 0～5cm surface soil temperature,volumetric water content and electrical conductivity changes.It is hoped to explore:（1）The effects of different agricultural and forestry utilization methods on reclaimed wetland soil respiration and its components;（2）The effects of different vegetation degradation levels on reclaimed wetland soil respiration and its components.The main results obtained in this study are as follows:（1）The order of total soil respiration rate of different agricultural and forestry use types is:VW(10.41±0.96μmol·m^-2·s^-1)>IW(8.15±0.72μmol·m^-2·s^-1)>MF(7.87±0.51μmol·m^-2·s^-1)>YF(7.38±0.46μmol·m^-2·s^-1)>CP(6.08±0.39μmol·m^-2·s^-1)>AP(0.60±0.13μmol·m^-2·s^-1).The order of autotrophic respiration rate is:VW(8.26±0.98μmol·m^-2·s^-1)>IW(5.18±0.91μmol·m^-2·s^-1)>YF(2.12±0.42μmol·m^-2·s^-1)>LP(1.56±0.25μmol·m^-2·s^-1)>MF(0.59±0.16μmol·m^-2·s^-1).The order of respiration rate is:MF(7.28±0.45μmol·m^-2·s^-1)>YF(5.26±0.26μmol·m^-2·s^-1)>CP(4.52±0.42μmol·m^-2·s^-1)>IW(2.97±0.45μmol·m^-2·s^-1)>VW(2.15±0.12μmol·m^-2·s^-1).（2）Compared with the remaining wetlands in this area,the use of agriculture and forestry significantly reduced the total soil respiration rate of the wetland,with a reduction in AP of 94%,and the rest of the use of agriculture and forestry reduced the total soil respiration by 22%to 42%.The contribution of heterotrophic respiration to total respiration of different types of agricultural and forestry use is as follows:MF（93%）>CP（74%）>YF（71%）>IW（36%）>VW（21%）.The contribution of total respiration is in order:VW（79%）>IW（64%）>YF（29%）>CP（26%）>MF（7%）.Under different types of agricultural and forestry use,soil respiration,autotrophic respiration and heterotrophic respiration all showed a single-peak curve trend during the observation period.The total soil respiration rate mostly reaches a low peak from December to February,and peaks from July to August.Agricultural and forestry use patterns significantly affect the peak and change trends of soil respiration and its component respiration rates in wetlands.（3）Soil respiration and its components have a significant exponential relationship with 0～5cm soil temperature,and a weak correlation with water content and electrical conductivity.0～5cm soil temperature explained 33%～84%of total soil respiration seasonal changes under different agricultural and forestry use patterns,explained32%～78%of heterotrophic respiration,and explained 7%～65%of autotrophic respiration.The results of multiple linear regression showed that,in addition to CP,0～5cm soil temperature,electrical conductivity,and water content can explain 71%～88%of total soil respiration changes in different agricultural and forestry use methods;it can explain IW,YF,MF 66%～83%of heterotrophic breathing changes.（4）The order of total soil respiration in wetlands with different vegetation degradation levels is:MW(13.39±0.84μmol·m^-2·s^-1)>IW(10.96±0.89μmol·m^-2·s^-1)>LW(10.89±1.63μmol·m^-2·s^-1)>VW(9.30±1.28μmol·m^-2·s^-1).The annual autotrophic respiration rate of each degraded wetland is in order:MW(9.18±0.83μmol·m^-2·s^-1)>IW(6.30±1.04μmol·m^-2·s^-1)>VW(5.96±1.43μmol·m^-2·s^-1)>LW(4.67±1.54μmol·m^-2·s^-1).The annual average heterotrophic respiration rate of each degraded wetland is in order:LW(6.22±0.43μmol·m^-2·s^-1)>IW(4.66±0.52μmol·m^-2·s^-1)>MW(4.22±0.34μmol·m^-2·s^-1)>VW(3.34±0.39μmol·m^-2·s^-1).（5）Compared with VW,vegetation degradation caused a significant increase in the total soil respiration rate of the reed wetland,with an increase of 17%～44%.The components of autotrophic respiration and heterotrophic respiration increased.Under different vegetation degradation levels,the contribution of heterotrophic respiration to total soil respiration is:LW（57%）>IW（42%）>VW（36%）>MW（31%）;autotrophic respiration contributes to total soil respiration.The contribution of autotrophic respiration to total soil respiration is:MW（69%）>VW（64%）>IW（58%）>LW（43%）.（6）For reed wetlands with different levels of vegetation degradation,soil respiration generally peaks in the growing season from July to August,and the soil respiration rate gradually decreases at the end of the growing season.The level of vegetation degradation significantly changed the change trend and component contribution of soil respiration and its component respiration.There is a significant correlation between soil temperature,humidity,electrical conductivity and the respiration of some degraded wetlands（p<0.05）,but the overall correlation is weak.The results of this study can provide a scientific basis for low-carbon sustainable management of land in the coastal reclamation area of the Yangtze River Delta under the background of climate change.