| With the global warming,the extreme climate events have shown the trend of increasing and strengthening.The typhoon has become one of the main driving forces of coastal canopy gaps forming in China,and the canopy gaps have also showed an increasing trend.Under the background,the study on the effect of canopy gaps on soil respiration is the forefront and hotspot of forest soil carbon cycle and global climate change.The Jianfengling tropical montane rainforest in Hainan Island is a relatively well-preserved original tropical forest region in China at present.It is also a typical area of tropical montane rainforest,which is of great research value.The impact of canopy gaps on annual soil respiration was assessed quantitatively,which could provide scientific basis and and theoretical reference for accurate estimating soil carbon emissions and effective management of soil organic carbon(SOC)accumulation in the tropical montane forest ecosystem.In this study,the 60 hm2 dynamic observation plots and the natural canopy gaps in the tropical montane rainforest observation sites of the Jianfengling National Natural Reserve on Hainan Island,south of China,were selected as the research objects.Meteorological factors(e.g.air temperature,radiation and precipitation),soil environmental parameters,soil respiration and other indicators were observed and investigated regularly.Based on those long-term observation data,daily and seasonal patterns and differences of soil respiration between the inside and outside of canopy gaps(which was formatted by typhoon interference)were analyzed.Then,the effect of canopy gaps on soil respiration and its mechanism was illustrated.In addition,the effects of litter on soil respiration were analyzed through removing/keeping the litter,and the characters of soil respiration at both inside and outside of the canopy gaps was stimulated with the Forest-DNDC.The main research results are as follows:1.The activities of soil invertase,urease,catalase and acid phosphatase in the early rainy season in the Jianfengling tropical montane rainforest were 15.36?60.94 mg·g-1·d-1.0.30-0.68 mg·g-1·d-1,1.59?3.30 ml·g-1·20 min-1,0.61?1.13 mg·h-1g-1 respectively.The results of redundancy analysis of soil enzyme activity and environmental factors showed that SOC and total nitrogen were the most important factors for soil enzyme activity during the rainy season,and they accounted for 36.9%and 26.6%of the soil enzyme activity variations,respectively.2.The air temperature,air relativve humidity,soil temperature and soil water content inside and outside the canopy gaps,showed obvious diurnal and monthly variations,and both diurnal and monthly variation showed a unimodal change.On the time-varying scale.the results of paired T-test showed that the air temperature.relative air humidity,soil temperature inside and outside the canopy gaps were significantly different(P<0.01),the daily average of air temperature,relative air humidity,soil temperature and soil water content inside the canopy gaps were 17.40?,99%,20.48?,and 29%respectively;the air temperature,relative air humidity,and soil temperature and soil water content outside the canopy gaps were 19.43?,95%,19.99 C,and 34%.The results of paired T test showed that the air temperature,relative air humidity,soil temperature and soil water content inside the canopy gaps were significantly different from those outside the canopy gaps(P<0.01),the monthly average of air temperature and relative air humidity,soil temperature and soil water content were 18.90?,98%,20.71?,and 31%,respectively;the air temperature,relative air humidity,and soil temperature outside the canopy gaps were 19.01?,95%,20.03?,and 34%.There was a significant difference between relative air humidity,soil temperature and soil water content(P<0.01).There was no significant difference between the air temperature of inside and outside the canopy gaps(P>0.05).The contents of SOC,total nitrogen,total phosphorus,ammonium nitrogen,nitrate nitrogen,invertase,urease and catalase inside the canopy gaps were 35.84 g·kg-1 and 2.33 g·kg-1,0.14 g·kg-1,8.80 mg·kg-1,11.52 mg·kg-1,33.58 mg·g-1·d-1,0.33 mg·g-1·d-1,3.38 ml·g-1·20 min-1;the content inside the forest were 36.04 g·kg-1,2.37 g·kg-1,0.16 g·kg-1,10.79 mg·kg-1,10.24 mg·kg-1,43.98 0.37 mg·g-1·d-1,3.57 ml·g-1·20 min-1.The results of paired T test showed that there were significant differences between inside and outside the canopy gaps,which included phosphorus,sucrase,urease and catalase and forest(P<0.05);while there were no significant differences(P>0.05)for SOC,total nitrogen,ammonium nitrogen and nitrate nitrogen between the inside and outside of canopy gaps.3.The annual average soil respiration rate were 2.68,2.17,3.10,2.69 and 3.29?mol·m-2·s-1 at the inside canopy gaps in 2014-2018;the annual average soil respiration rate at the outside canopy gaps were 2.54,2.11,2.81,2.49 and 2.38 ?mol·m-2·s-1.The results of analysis of variance showed that there was no significant difference in soil respiration between the inside and outside canopy gaps in 2014-2017(P>0.05).There was significant difference in soil respiration between inside and outside the forest gap in 2018(P<0.05).Soil respiration was significantly greater than soil respiration in the dry season(P<0.05).The contribution rates of litter on soil respiration inside the forest gap and outside the forest gap were 31.96%and 22.38%,respectively.At the monthly and annual scales,the discharge rate of soil respiration showed a multi-peak curve at the both inside and outside forest gaps and the variation trend was almost the same.The overall of soil respiration at rain season was greater than that at the dry season.4.The results of redundancy analysis between the soil respiration and environmental factors showed that soil temperature(at 5 cm)and SOC were the most important environmental factors affecting soil respiration,which explained the variations of 15.7%and 6.7%,respectively(P<0.05).Soil temperature(5 cm)and soil urease were the most important environmental factors affecting soil respiration inside the forest gap,accounting for 26.2%and 3.5%of variations(P<0.05).Soil temperature(5 cm)and organic carbon were the most important factors affecting soil respiration outside the forest gap,explaining the variations of 10.7%and 15.8%,respectively(P<0.05).5.The Forest-DNDC model can effectively simulate the monthly discharge characteristics and annual emissions of soil respiration in the tropical mountain rainforest of Jianfengling.The soil respiration simulated by Forest-DNDC model in 2014 and 2015 were 37.88 t·hm-2·a-1 and 40.79 t·hm-2·a-1,respectively.The results of the sensitivity analysis showed that SOC at the depth of 5 cm was the most sensitive factors of soil respiration at the inside of canopy gaps. |
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