Effects Of Forest Fires On Soil Organic Carbon And Its Components Along With Carbon Pool Stability In Secondary Forest Of Pinus Massoniana

Soil organic carbon pools in forest ecosystems play an essential role in the global carbon cycle and balance.As a discontinuous disturbance factor,forest fires are the drives of the biogeochemical cycle,significantly changing the structure and function of ecosystem,nutrient cycling,and energy transfer,causing the changes in forest carbon pools and carbon distribution patterns,which in turn affect the forest succession process and carbon sequestration capacity.Forest fires affect vegetation carbon pools and nutrient turnover by directly changing vegetation coverage,and hence affect vegetation carbon fixation and carbon transfer from vegetation to soil,resulting in the redistribution of different carbon pools.Studying the impact of forest fires on soil organic carbon in forest ecosystems helps understand the processes of soil carbon sequestration and the carbon cycle in forest ecosystems.It is of great significance to formulating clear and reasonable forest fire management strategies aimed at mitigating global climate change.However,the effects and mechanism of forest fires on soil organic carbon and its components,as well as carbon pool stability,remain unclear.Therefore,taking the secondary forest of Pinus massoniana in Heshan City,Guangdong Province as the research object,by means of the adjacent sample plots comparison and space-for-time substitution,along with the field investigation sampling and indoor test analysis,the effects of forest fires on soil physicochemical properties,organic carbon density,labile organic carbon components,and fine root biomass were quantified,and influencing mechanism of forest fires on the sequestration and stability of soil organic carbon was elaborated.This study can provide scientific support for the evaluation of the impact of forest fires on the stability of the soil carbon pool.Based on the research results,a scientific and reasonable forest fire management strategy is proposed.The main findings are as follows:1)The influence of forest fires on soil physicochemical propertiesForest fires have an impact on the soil physicochemical properties in secondary forest Pinus massoniana at different ages.They increased soil bulk density,pH value,and total phosphorus,but they decreased soil moisture content,total nitrogen,and organic carbon content in the secondary forest of Pinus massoniana at different forest ages.In addition,forest fires increased soil C/N ratio,while reduced C/P ratio and N/P ratio.Soil moisture content,total nitrogen,total phosphorus,and organic carbon content increased with forest age,while soil bulk density and pH decreased with forest age.Soil moisture content,total nitrogen,total phosphorus,and organic carbon content in the secondary forest of Pinus massoniana decreased with the depth of soil layer.In contrast,soil pH value and bulk density increased with the depth of the soil layer.Correlation analysis showed that soil fine root biomass was negatively correlated with soil bulk density and pH value,but positively correlated with soil total nitrogen and total phosphorus along with water content(P<0.01),in the sampling that has been burnt.The correlation coefficients between soil organic carbon content and total nitrogen were relatively high at different forest ages.Nested analysis of variance showed that the causes of variation in soil physicochemical properties in the secondary forest of Pinus massoniana were different forest ages,different soil depths,and forest fires.Forest ages and soil depths accounted for 10.80%-69.80%of the variation of soil physicochemical properties,while forest fires accounted for 2.50%-11.20%of the variation of soil physicochemical properties.2)The reduces of soil organic carbon density by forest firesForest fires reduced the soil organic carbon density in the secondary forest of Pinus massoniana at different forest ages.In the horizontal direction,the decrease in soil organic carbon density reduced with forest age.In the vertical direction,the decrease in soil organic carbon density reduced with soil depth and forest age.Compared with the control,the soil organic carbon density of the young-aged forest,middle-aged forest,and mature-aged forest after forest fires were 35.12,40.80,and 52.34 t·ha-1,respectively,which decreased by 10.93%,8.52%,and 7.56%respectively.Compared with the control,soil organic carbon density in the soil profile(0-60cm)of the young-aged,middle-aged and mature-aged forest was 5.04-7.76,5.26-10.27 and 6.33-13.58t·ha-1,respectively,which decreased by 2.51%-16.83%,1.31%-11.85%,and 1.09%-12.50%respectively;Forest fires significantly reduced soil organic carbon density of 0-3 0cm in the young-aged and middle-aged forest(P<0.05),and 0-20cm in the mature-aged forest(P<0.05).There was a significant correlation between soil organic carbon density and soil physicochemical properties(P<0.05)in the secondary forest of Pinus massoniana.Path analysis showed that the direct effect of soil total nitrogen on soil organic carbon density was the largest in both the control plot and the burnt plot.The direct effect of soil fine root biomass on soil organic carbon density was relatively smaller,but it indirectly affected soil organic carbon density by influencing soil total nitrogen.Nested analysis of variance showed that soil depth accounted for 70.60%of the variation of soil organic carbon density variation,forest age explained 25.35%of the variation,and forest fire reflected 2.34%of the variation.3)Decrease of soil labile organic carbon components by forest firesForest fire reduced the contents of soil labile organic carbon components at the different forest ages of the secondary forest of Pinus massoniana.Soil easily-oxidized organic carbon(EOC)decreased with forest age,while soil particulate organic carbon(POC)decreased with forest age,with the increase of forest age.The decrease of soil microbial biomass carbon(MBC)and dissolved organic carbon(DOC)showed a "U" pattern with forest age.Compared with the control,the soil MBC of the young-aged forest,middle-aged forest,and mature-aged forest were 65.36,107.48,and 142.48 mg·kg-1,respectively,which decreased by 26.03%,18.14%,and 18.63%respectively.Soil DOC of the young-aged forest,middle-aged forest,and mature-aged forest were 23.15,52.86,and 68.49 mg·kg-1,respectively,which decreased by 22.28%,17.81%,and 19.41%.Soil EOC of the young-aged forest,middle-aged forest,and mature-aged forest were 1308.82,1824.19,and 2571.58 mg·kg-1,respectively,decreasing by 23.98%,18.99%,and 13.50%.Soil POC of the young-aged forest,middle-aged forest,and mature-aged forest were 1013.02,1374.75,and 2206.09 mg·kg-1,decreasing by 18.50%,22.96%,and 18.32%respectively,The distribution proportion of soil labile organic carbon in the secondary forest of Pinus massoniana increased first and then decreased with forest age.The distribution proportion of soil EOC to the total labile organic carbon was the largest,and EOC distribution proportion in the young-aged forest,middle-aged forest,and mature-aged forest soil were 29.96%,33.29%,and 33.35%,respectively.In the vertical direction,the proportion of soil MBC and EOC decreased with soil depth.Soil DOC of the mature-aged forest increased with soil depth,but the change of soil POC in the secondary forest of Pinus massoniana was irregular.There was a significant correlation between soil labile organic carbon components and physicochemical properties of soil as well as its soil stoichiometry in the secondary forest of Pinus massoniana(P<0.05).Path analysis showed that the C/P ratio direct affected soil MBC in the burnt area,and while the N/P ratio impacted MBC through soil organic nitrogen content.The largest direct effect on soil DOC was soil total nitrogen,while the effect of soil fine root biomass content on soil DOC was through soil total nitrogen.The direct effect on soil EOC was soil total nitrogen.Soil N/P ratio had an indirect effect on EOC through soil total nitrogen.Soil POC was directly affected by soil organic carbon content.Soil fine root biomass indirectly affected soil POC by influencing total nitrogen.Nested analysis of variance showed that soil depth explained 70.73%,28.34%,79.83%,and 73.35%of soil MBC,DOC,EOC,and POC variation,respectively.Forest age explained 20.09%,62.27%,13.92%,and 18.13%of soil MBC,DOC,EOC,and POC variation,respectively.Forest fires explained 4.69%,5.39%,2.61%,and 3.42%of soil MBC,DOC,EOC,and POC variations,respectively.4)The critical impact of forest fires on the stability of soil organic carbon poolsThe response of soil carbon pool management index to forest fires varied with forest ages in the secondary forest of Pinus massoniana.In the horizontal direction,the stability of soil organic carbon pool increased with forest age.The carbon pool management indexes of the young-aged forest,middle-age,and mature-aged forests were 26.15,30.57,and 32.09,respectively,showing an increasing trend with forest age.Moreover,the variation of carbon pool activity in the secondary forest of Pinus massoniana decreased with the increase of forest age.In the vertical direction,the carbon pool activity in burnt plots of the secondary forest of Pinus massoniana decreased with soil depth.In other words,the stability of soil organic carbon pool increased with soil depth.Correlation analysis showed a significant correlation between soil carbon pool management indexes and soil labile organic carbon components(P<0.01),and they also significantly correlated with soil physicochemical properties along with fine root biomass.Path analysis showed that the largest direct effect of soil total nitrogen on soil carbon pool management indexes,with a path coefficient of 1.786.Soil total phosphorus and fine root biomass have a relatively small effect on soil carbon pool management indexes,with path coefficients of-1.021 and 0.981,respectively.Soil pH value indirectly impacted the soil carbon pool management index.In conclusion,forest fires have essential effects on soil organic carbon conversion and carbon cycle in the secondary forest of Pinus massoniana.In the horizontal direction,the stability of organic carbon pool increased with forest age.In the vertical direction,it increased with soil depth.The research results can provide a reference for the impact of soil carbon sequestration and carbon cycling on global climate change after forest fires,and also scientific support for forestry and its carbon sink management,as well as the formulation of forest carbon sink ecological compensation policies.