Dynamic Changes Of Soil Chemical Properties In Subtropical Evergreen Broad-Leaved Forest And Cunninghamia Lanceolata Plantation

As the main component of the terrestrial ecosystem,forest is an indispensable material basis and important resource for the development of human society.It not only provides materials such as wood and forest by-products for human beings,but also provides important ecological services.The content and availability of soil nutrients are important factors affecting forest service functions.In subtropical areas,a large number of zonal vegetation,evergreen broad-leaved forest,has been replaced by plantation including Cunninghamia lanceolata,and a large area of plantation has degraded and reduced service functions.By studying the temporal dynamic changes of soil nutrients in evergreen broad-leaved forest and C.lanceolata plantation,the effects of forest type changes on soil nutrients can be compared,so as to provide scientific basis for forest management and soil nutrient management.Therefore,based on the long-term fixed monitoring plots of evergreen broad-leaved forest and C.lanceolata plantation in Huitong Forest Ecosystem National Field Scientific Observation and Research Station in Hunan Province,this study discussed the changes of soil carbon,nitrogen,phosphorus,calcium,magnesium,sodium,manganese,zinc and other nutrient elements contents and related stoichiometric ratios over time and soil layer,and adopted principal component analysis to convert multiple indicators into comprehensive indicators.By comparing soil nutrient content and dynamic changes of different forests,the study provides theoretical guidance for sustainable management and long-term productivity of subtropical forests in China.The main results of this study are as follows:1.With the growth and development of trees,the changes of soil C and N contents in evergreen broad-leaved forest and C.lanceolata plantation were inconsistent with time,while the soil P content decreased first and then increased.The soil C content of evergreen broad-leaved forest and C.lanceolata plantation was the highest at 32 a,17.51g/kg and 23.67 g/kg,respectively,but the lowest soil C content appeared at 22 a and 27a,12.5 g/kg and 13.12 g/kg,respectively.The soil N contents of the two forest types were the highest in 32 a and 22 a,which were 1.6 g/kg and 1.52 g/kg,respectively.The lowest values of soil N content appeared in 22 a and 27 a,which were 1.41 g/kg and 1.12 g/kg,respectively.The soil P content of the two forest types was the highest at 37 a,which was0.23 g/kg and 0.26 g/kg,respectively,and the lowest value appeared at 32 a and 27 a,both of which were 0.13 g/kg.The soil C:N,N:P and C:P of evergreen broad-leaved forest and C.lanceolata plantation increased first and then decreased with the growth and development of trees.The soil C:N of evergreen broad-leaved forest and C.lanceolata plantation was the highest at 27 a,which was 10.22 and 11.52 respectively,and the lowest value appeared at 22 a,which was 8.66 and 9.48 respectively.The soil N:P of the two forest types was the highest at 32 a and 27 a,respectively,12.08 and 8.27,respectively,and the lowest values appeared at 37 a,respectively,6.22 and 5.17.The soil C:P and soil N:P of the two forest types showed the same change rule.2.With the deepening of soil layer,the contents of soil C,N and P in evergreen broad-leaved forest and C.lanceolata plantation decreased gradually.Among them,the soil C content of C.lanceolata plantation was significantly higher than that of evergreen broad-leaved forest by 43.46%at 20-40 cm,but there was no difference in the upper soil.The soil N content of C.lanceolata plantation was significantly lower than that of evergreen broad-leaved forest by 33.33%in 0-10 cm,but the difference disappeared with the deepening of soil layer.The soil C:N,N:P and C:P of the two forest types also showed a gradual downward trend with the deepening of the soil layer.Among them,the soil N:P and soil C:P of the C.lanceolata plantation were significantly lower than that of the evergreen broad-leaved forest in the 0-10 cm soil layer.30.97%and 28.04%,respectively,but with the deepening of the soil layer,the difference gradually disappeared.3.With the growth and development of trees,the contents of other soil nutrients（Mn,Zn,Cu,Fe,Ca,Mg,K,Na）in evergreen broad-leaved forest and C.lanceolata plantation mostly showed an increasing trend,but the soil Mn content of C.lanceolata plantation decreased significantly by 65.22%.Among them,the soil Na content of C.lanceolata plantation was significantly higher than that of evergreen broad-leaved forest by 20%at22 a,but significantly lower than that of evergreen broad-leaved forest by 23.53%at 32 a;the contents of Zn and Mg in soil of C.lanceolata plantation were significantly lower than those of evergreen broad-leaved forest.4.With the increase of soil depth,the soil Ca content of evergreen broad-leaved forest and C.lanceolata plantation showed a gradual downward trend,and their soil Zn,Cu and Fe contents were higher at 20-40 cm than at 0-10 cm,but there was no significant difference.The contents of Zn,Cu,Fe,Mg,K and Na in soil of C.lanceolata plantation were lower than those of evergreen broad-leaved forest,but the contents of Mn and Ca in soil were higher than those of evergreen broad-leaved forest.5.The content of soil exchangeable cations in evergreen broad-leaved forest and C. lanceolata plantation showed a similar trend.Their highest values appeared in 22 a and27 a,which were 166.52 mmol/kg and 112.7 mmol/kg,respectively.The lowest values appeared in 32 a,which were 140.36 mmol/kg and 95.34 mmol/kg,respectively.On the whole,the content of soil exchangeable cations(Ca²⁺,Mg²⁺,K⁺,Na⁺,Al³⁺,H⁺)in C.lanceolata plantation has been significantly lower than that in evergreen broad-leaved forest.The content of soil exchangeable cations in C.lanceolata plantation was 39.59%lower than that in evergreen broad-leaved forest at 22 a,and the difference between the two was 28.72%at 37 a.However,the soil exchangeable acid content of C.lanceolata plantation was also significantly lower than that of evergreen broad-leaved forest,which was 47.69%lower than that of evergreen broad-leaved forest at 22 a and 31.15%lower at37 a.6.Principal component analysis was used to compare the soil nutrients of the two forest types at different times,results are as follows:22 a evergreen broad-leaved forest>27 a evergreen broad-leaved forest>32 a evergreen broad-leaved forest>22 a C.lanceolata plantation>37 a evergreen broad-leaved forest>32 a C.lanceolata plantation>37 a C.lanceolata plantation>27 a C.lanceolata plantation.In summary,the soil nutrient status of evergreen broad-leaved forest has always been better than that of C.lanceolata plantation.This is because the amount of litter in evergreen broad-leaved forest is more than that of C.lanceolata plantation,which has more significant significance for protecting soil fertility.However,compared with C.lanceolata plantation,the content of exchangeable Al³⁺and exchangeable acid in evergreen broad-leaved forest soil is higher,and attention should be paid to the possible negative effect of aluminum toxicity caused by soil acidification.In the future management of C.lanceolata plantation,it is necessary to pay attention to the significant reduction of soil Mn content.Secondly,the development of C.lanceolata plantation to maturity（27 a）is limited by P.In order to ensure the normal development of trees,P fertilizer can be applied in the fast-growing stage of C.lanceolata to promote a virtuous cycle of nutrients.