Effects Of Broadleaf Transformation Of Masson Pine Plantations On The Storage Of Organic Carbon Components In Soil Aggregate

Soil agglomerates are complex microstructures,formed by soil particles as well as organic and inorganic adhesives.They play an important role in organic carbon storage and can improve the stability of organic carbon by protecting it from microbial degradation.This study focuses on six different species of coniferous and broad-leaved mixed forests and artificial pure forests in the state-owned Longli Forest Farm of Guizhou Province,including Pinus massoniana artificial pure forest(CL),Pinus massoniana mixed forest with Camellia oleifera(CL+YC),Pinus massoniana mixed forest with Bretschneidera sinensis(CL+BL),Pinus massoniana mixed forest with Cercidiphyllum japonicum(CL+LX),Pinus massoniana mixed forest with Manglietia conifera(CL+ML),and Pinus massoniana mixed forest with Michelia maudiae(CL+SSHX).By taking 0-100 cm soil samples from Pinus massoniana plantations and mixed forests after the introduction of broad-leaved tree species,this study aims to explore the changes in soil aggregate stability,organic carbon and component content,and different forms of Fe and Al oxides after the introduction of different broad-leaved tree species.The following scientific questions are to be addressed: 1)What is the impact of broad-leaved transformation on soil aggregate stability? 2)What is the impact of broadleaf transformation on the organic carbon and component storage of soil aggregates? 3)How does the broadleaf transformation affect the content of Fe and Al oxides in aggregates? 4)What are the effects of Fe and Al oxides on the stability of aggregates and the content of organic carbon? The main research findings are as follows:(1)The MWD of water-stable agglomerates of each grain size in the0-100 cm soil layer of the six stands in the study area ranged from0.29-2.76 mm and the GMD ranged from 0.08-0.91 mm,both of which decreased with increasing soil depth and decreasing grain size of the agglomerates.Although the MWD and GMD values varied slightly among the stands at different soil depths,overall CL+YC,CL+LX,and CL+SSHX were significantly higher than CL,CL+ML was slightly higher than CL,and CL+BL was lower than CL,indicating that the introduction of YC,LX,SSHX,and ML improved the stability of agglomerates,while the introduction of BL reduced the stability of agglomerates.(2)The total soil SOC content CL+SSHX in 0-100 cm of six stands in the study area tended to decrease and then increase with soil layer deepening,while the other five stands decreased with soil layer deepening.The total soil SOC content of the six stands ranged from1.2-26.54 g/kg,in which CL+YC and CL+SSHX were significantly higher than CL in all soil layers(The SOC content of >1 mm agglomerates ranged from 2.72-40.74 g/kg,1-0.5 mm agglomerates ranged from 2.61-34.8 g/kg,and 0.5-0.25 mm agglomerates ranged from1.72-40.74 g/kg in the 0-100 cm soil layer of the six stands.The SOC content of agglomerates ranged from 1.07-25.58 g/kg,0.25-0.053 mm agglomerate SOC content ranged from 0.87-12.98 g/kg,and 1.3-19.47g/kg for powdered sticky grains(<0.053mm).The SOC content of the agglomerates of CL+SSHX was always higher than that of CL in the deeper soil layers,indicating that the SOC storage capacity was significantly increased after the introduction of SSHX.(3)The introduction of broadleaf species had a significant effect on the organic carbon fractions obtained by the agglomerate density grouping method,and there were significant differences(P<0.05)in the degree of change in organic carbon fractions and their contribution to soil organic carbon.The introduction of SSHX had the most significant elevation in the concentration of each organic carbon fraction,followed by LX and YC.There were significant differences(P<0.05)in the content of each organic carbon fraction among the different grain classes among the stands,and LF accounted for the least(0.09%-3.4%)of the agglomerate fractions in all grain classes,and c POC accounted for the highest(18.99-61.9% and 23.86-63.66%)in both >1 mm and 1-0.5 mm agglomerates The contribution of different fractions to soil SOC was ranked from high to low as c POC>c MOC>f POC>i-POC>i-MOC>LF.LF,i-POC and i-MOC fractions were all significantly and positively correlated with soil SOC(P< 0.05),and c POC,c MOC and f POC showed highly significant positive correlations(P<0.01)with soil SOC with correlation coefficients reaching 0.955,0.809 and 0.916,respectively,indicating that c POC,c MOC and f POC were the most sensitive indicators of soil SOC changes.The comprehensive integrated evaluation among different stands according to the percentage share and contribution of different organic carbon components was CL+SSHX>CL+LX>CL+YC>CL+ML>CL>CL+BL.(4)The content of different forms of oxides in the six stands showed free(Fed,Ald)> amorphous(Feo,Alo)> complexed(Fep,Alp),and the highest content of all forms of Fe Al oxides in different soil layers of all stands was Fed.There was a highly significant positive correlation between Fed and MWD and GMD of water-stable agglomerates and SOC of agglomerates at each grain level(P<0.01),Ald was not significantly correlated with MWD,GMD and SOC of water-stable agglomerates(P>0.05),Fep was not significantly correlated with MWD and GMD of water-stable agglomerates(P>0.05),but was highly significantly negatively correlated with SOC of >1mm and 0.25-0.053 mm agglomerates(P<0.01).Alp had highly significant negative correlation with MWD,GMD and agglomerate SOC of water-stable agglomerates(P<0.01),Feo had significant positive correlation with MWD,GMD and large agglomerate SOC of water-stable agglomerates(P<0.05),Feo had significant positive correlation with MWD,GMD and large agglomerate SOC of water-stable agglomerates(P<0.05),and Feo had significant positive correlation with MWD,GMD and large agglomerate SOC of water-stable agglomerates(P<0.01).(Alo had no significant correlation with MWD,GMD and agglomerate SOC of water-stable agglomerates(P>0.05).It can be shown that Fed is more favorable to the preservation of organic carbon in hydrostable agglomerates relative to other forms of oxides.The integrated analysis of the variability of Fe Al oxides in each stand shows that Fed content CL+SSHX>CL+YC>CL+LX>CL+ML>CL>CL+BL,so the SOC storage potential is significantly increased after the introduction of CL in SSHX,YC,LX and ML.(5)The coordinate comprehensive method was used to evaluate the soil aggregate stability index and the organic carbon and component related index under the six forests in the study area.The comprehensive evaluation order was:CL+SSHX>CL+LX>CL+YC>CL+ML>CL>CL+BL,indicating that not all broad-leaved species can play a positive role in soil aggregate stability and carbon storage after introduction,This is closely related to the characteristics of its tree species and the content of iron and aluminum oxides.In summary,the selection of broad-leaved tree species for the transformation of pure Pinus massoniana forests can appropriately introduce four broad-leaved tree species: Michelia maudiae,Camellia oleifera,Cercidiphyllum japonicum and Manglietia conifera.