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Soil Respiration In Response To Thinning And Simulated Nitrogen Deposition In Pinus Tabuliformis Forest In Taiyue Mountain

Posted on:2020-07-06Degree:DoctorType:Dissertation
Country:ChinaCandidate:B ZhaoFull Text:PDF
GTID:1360330575991515Subject:Ecology
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Soil is the largest carbon pool in terrestrial ecosystems,which is about twice the carbon storage than atmospheric carbon pool.Artificial activities,such as forest management(thinning and the fertilizer),the use of fossil energy(increased atmospheric nitrogen deposition),inevitably affect the underground carbon cycle in forest ecosystems.Soil respiration accounts for 30%to 80%of total forest ecosystem respiration.Understanding the underlying mechanisms of soil respiration in a context of global changes will help to better mitigate global warming,especially in the scenarios of global warming.The study site is located at the Taishan Mountain Forest Ecosystem Station of State Forestry Administration(Lingkong Mountain)in Shanxi province.This study was in Pinus tabuliformis forests.Based on long-term field data,soil respiration component partitioning(autotrophic respiration and heterotrophic respiration)and simulated nitrogen deposition,the objects of the study were to explore(I)soil respiration and its components of plantation in response to different thinning treatments;(2)the response of soil respiration and its components to simulated nitrogen deposition in natural and planted forests;(3)soil respiration components(root respiration,the decomposition of litter,decomposing of soil organic matter),and their responses to nitrogen additions.The study may help to understan the key mechanisms of soil carbon cycle and provide theoretical support for mitigating climate change.1.Understanding the responses of soil respiration(Rs)to thinning is essential to evaluate the effects of management practices on carbon cycling in plantation forest ecosystems.However,how Rs and its components(autotrophic,Ra and heterotrophic respiration,Rh)vary with thinning intensity and the underlying mechanisms are not well understood.In the present study we monitored Rs,Ra and Rh over five growing seasons using a trenching method in a Pinus tabuliformis plantation subjected to four thinning treatments(no thinning,CK;light thinning,LT;moderate thinning,MT and heavy thinning,HT).On average,LT and MT significantly increased Rs by 13%and 17%,respectively,compared with the CK.These increments of Rs were ascribed to the enhanced Ra in LT and MT plots,because light and moderate thinning promoted root growth and productivity(higher fine root biomass).However,HT did not result in a further increase in Ra,suggesting that increases in the activity of remaining trees and understory plants did not compensate for the reduced photosynthesis and the amount of respiring tree roots by extensive tree-cut.In contrast to Ra.variation in Rh was unrelated to thinning,partly due to the stable forest tloormass(non-living organic materials such as litter and fine woody debris)and microbial biomass carbon content(MBC)between thinned and control plots.The temperature sensitivity(Q10)of Ra and Rh ranged from 1.40-3.07 and 2.34-3.42.respectively.The highest Q10 of Ra was observed in MT while that of Rh occurred in LT.Soil moisture was significantly correlated with Rh but a poor predictor for Ra.Our findings demonstrated that Ra and Rh responded to thinning intensity independently of each other.The intensity of management and plant-mediated biological processes are of particular importance in evaluating the impacts of forest management on C sequestration potential in plantation forests.2.Increased atmospheric N deposition is known to have significant effects on soil respiration(Rs)in natural and planted forest ecosystems.Responses of Rs to such N enrichment has been widely investigated in forest ecosystems,both in natural and planted forests.As natural and planted forests differ in many aspects(e.g.species composition,forest management and succession stage,soil properties etc.),the responses of Rs in natural and planted forests to N addition may be different.However,so far,few studies have made a direct comparison between natural versus planted forests.To fill this gap,we have examined how autotrophic(Ra),heterotrophic(Rh)and total Rs respond to experimental N addition in a natural and a planted Pinus tabulaeformis forests in northern China.Three levels of N addition(CK=0,LN=50 and HN=100 kg N ha-1 yr-1)were applied monthly over five growing seasons.Soil respiration and its components,soil properties,microbial biomass(MBC),enzymes activity and fine root biomass were measured.After 5-years of N addition,our results showed that:(1)for all three treatments,mean annual Rs of the planted forest were significantly higher than that of the natural forest;(2)in the natural forest,mean annual Rh was reduced by 16.8%and 28.3%under LN and HN treatments,respectively,whereas in the planted forest,mean annual Rh was reduced by 14.4%and 18.3%under LN and HN treatments,respectively.(3)mean annual Ra was increased by 47.6%and 59.5%under LN and HN,respectively,in the natural forest.In contrast,in the planted forest,only LN significantly enhanced Ra compared with CK.In both natural and planted forests,the inhibition of Rh was largely associated with the decreased microbial biomass C(MBC)and reduced activity of the cellulose degrading enzyme.However,inconsistent patterns of Ra to N addition of different intensities in the natural vs.planted forests might be due to root ammonium toxicity under high N-availability scenarios.We demonstrated that the natural and planted forests may differ in their Rs responses to N addition,depending on different responses of Ra.Our results may have potential implications for forest management and predicting forest ecosystem carbon balance under future N scenarios.3.Increasing atmospheric Nitrogen(N)deposition and N fertilizer addition have profound effects on carbon(C)cycling in planted forest ecosystems.As an important part of belowground C dynamics,soil respiration is potentially affected by changing N availability.However,the responses of total soil respiration(RSr)and its three components,soil respiration derived from plant roots(RSR),root-free soil(RSs)and litter layer(RSL)to such N enrichment remain poorly understood.To assess the effects of N enrichment on soil respiration components,three levels of N additions ranging from low(LN,50 kg N ha-1 yr-1),medium(MN,100kg N ha-1 yr-1)and high(HN.150 kg N ha-1 yr-1)were conducted over five growing seasons from 2011 to 20 15 in a temperate Pinus tabulaeformis forest in northern China.A control plot without N addition(CK)was also established.The five-year mean annual rates of RST was 2.18±0.43 ?mol m-2 s-1,and the contribution of RSR,RSS and RS1.was 8.8 ± 3.1%.82.2 ± 4.5%and 9.0 ± 5.5%,respectively.Compared with CK,RST was significantly increased by 16.5%in the HN plots,but not in the LN or MN treatments.RSs was significantly decreased by 18.1%.26.6%and 18.4%in the LN.MN and HN plots.respectively,due to the reduction of both microbial biomass carbon(MBC)and enzyme activity.In contrast,RSR was increased by more than twice under the MN treatment,which promoted root growth and activity(higher fine root biomass and N concentration).A significant increment of RSL was only detected in the HN plots,where the increased litter input enhanced litter decomposition and hence RSL.Our findings clearly demonstrated that N addition of different intensities had different effects on soil components.In particular,the above-and belowground components of heterotrophic respiration,RSL and RSR showed contrasting responses to the high level of N addition.Thus,we highlight that the responses of soil respiration components to N addition should be examined individually.Our results may contribute to a better understanding of soil respiration dynamics under future N scenarios and have important implication for forest management.
Keywords/Search Tags:Soil respiration, Autotrophic respiration, Heterotrophic respiration, Canopy tree thinning, Temperature sensitivity, Pinus tabuliformis forest, Cellulose degrading enzyme, Microbial biomass carbon, Nitrogen addition
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