Decomposition Characteristics Of Refractory Substances In Mixed Litter Of Pinus Massoniana And Broad-leaved Tree Species

Pinus massoniana plantation is the main type of plantation in southern China.It not only plays an important role in the global carbon cycle system,but also plays an important role in the construction of local ecological civilization.Continuous planting of Pinus massoniana plantation may cause problems such as soil fertility decline in artificial pure forest sites,continued decline in productivity,and unsustainable management.The transformation of Pinus massoniana plantation into a mixed forest with high self-fertilization effect is an important way to transform Pinus massoniana plantation into near-natural forest.Litter decomposition is the main way to return nutrients to the soil in forest ecosystems.Litters include branches,leaves and fruits that fall from plants,and leaves are the main part of litter.Decomposition of litter includes leaching of water-soluble compounds and biodegradation of water-insoluble compounds.The mixed planting of Pinus massoniana and broad-leaved tree species can increase the diversity of ecosystems and improve the quality of litter.The nutrient return of litter can effectively improve the physical and chemical properties of soil and improve the soil fertility of forest land.The combination of Pinus massoniana with different broad-leaved species can produce different ecological and physiological effects.However,at present,there is a big difference in the choice of broad-leaved tree species mixed with Pinus massoniana,and the proportion of coniferous and broad-leaf mixed that can achieve better nutrient returning effect is not clear.Therefore,this study studied the litter of Pinus massoniana,Sassafras trumu,Cinnamomum camphora and Toona sinensis.The litter decomposition bag method was used to study the decomposition characteristics of condensed tannins,total phenols,lignin and cellulose during the decomposition of mixed litter and pure Pinus massoniana leaves mixed with three broad-leaved tree species and Pinus massoniana.Seven kinds of mixed treatments such as P+S,P+C,P+T,P+S+C,P+S+T,P+C+T and P+S+C+T were set up,and the proportion of litter of Pinus massoniana was divided into three gradients of60%,70% and 80%.The results show:(1)After 720 d decomposition,the highest concentration of condensed tannin in pure P was 0.025g/kg,and the concentration of condensed tannin in P+T 6:4 combination was at least 0.009g/kg.The condensed tannin degradation rate of pure litter of P was significantly lower than other mixed litter,which was 99.91%.The condensed tannin degradation rate of P+C combination was significantly higher than other combinations,and the highest concentration was P+C 6:4 reached 99.98%.The concentration of condensed tannin in allcombinations decreased rapidly during the first 30 days of decomposition.(2)The total phenolic concentration of the P+S+C 8:1:1 combination was highest 3.66g/kg.The total phenolic concentration of the P+C+T 6:1:3 combination reached a minimum of 2.11 g/kg.The total phenol degradation rate of pure P litter was significantly lower than that of other combinations of mixed litter,which was 79.54%.The total phenol degradation rate of P+C combination was significantly higher than other combinations,and the highest level of P+C 6:4 reached 88.91%.The total phenolic concentration of all combinations in the first 30 days of decomposition decreased rapidly,and then after 30 days,the rate of decline in total phenolic concentration gradually slowed down.The combined phenol degradation rate of P+S+C and P+S+T showed a rapid increase,then decreased and then increased rapidly.(3)The lignin concentration of P+C+T 7:2:1 combination was significantly lower than that of other combinations of mixed litter,which was 201.98g/kg.The lignin concentration of pure P was 286.31g/kg.The lignin degradation rate of pure P litter was the lowest with respect to the mixed litter of other combinations,which was 40.38%.The lignin degradation rate of P+C+T was significantly higher than other combinations,of which P+C+T 6:3:1 reached a maximum of 47.91%.Except for pure P,the lignin degradation rate of mixed litter of P+S+C 8:1:1 was the lowest among all blending treatments,which was42.91%.The degradation rate of mixed litter lignin in all combinations showed a rapid decline to a negative value and then slowly increased during 180d-270 d,and the final degradation rate increased to a positive value at 270 d.(4)The cellulose concentration of P+C+T 6:1:3 combination was significantly lower than other combinations,which was 10.06 g/kg,followed by P+C 6:4 combination reaching 20.19 g/kg.The highest concentration of cellulose in the mixed litter of P+S+C was 53.49g/kg.P+C+T 6:1:3 combination cellulose degradation rate was the highest,reaching 82.3%.The lowest cellulose degradation rate was the P+S combination,which was 71.13%.Except for all combinations of P+S+C,the cellulose degradation rate of other combinations decreased to a negative value in the first 30 days of decomposition,and the degradation rate of cellulose was positive until decomposition to 90 days.The cellulose degradation rate of the combination of three species decreased significantly during the period of 450d-540 d,and then increased rapidly.(5)Nitrogen significantly affected the degradation rate of condensed tannins,total phenols and cellulose in mixed litter,which showed a positive effect.Carbon and phosphorus elements do not have a significant effect on the degradation rate of condensedtannins,total phenols and cellulose.Regression analysis found that only the C/N ratio was significantly correlated with the degraded tannin,total phenol and cellulose degradation rate of mixed litter,showing a negative effect.Nitrogen and phosphorus did not have a significant effect on the rate of lignin degradation.Carbon element significantly affected the degradation rate of lignin in mixed litter,which showed a negative effect.There was no significant relationship between carbon to nitrogen ratio,carbon to phosphorus ratio,nitrogen to phosphorus ratio and lignin degradation rate.In summary,the condensed tannins and total phenols in the mixed litter are rapidly degraded in the early stage of decomposition,and finally show a constant trend at the end of decomposition.In the degradation process,the degradation of cellulose and the degradation characteristics of lignin are the same,and the degradation rate first decreases and then continues to rise.The degradation rates of condensed tannin,total phenol,lignin and cellulose were significantly correlated with the combination of tree species.The degradation rate of total phenol and cellulose was also significantly correlated with the mixing ratio of litter,and the other three except condensed tannin.The refractory substances are significantly correlated with the interaction of tree species and mixing ratios.The decomposition of refractory substances in mixed litter has a significant relationship with the initial matrix quality in litter.In order to make the Pinus massoniana mixed forest have self-fertilization effect,the sustainable management of plantation forest is achieved.When selecting the tree species to construct mixed forest of Pinus massoniana,Cinnamomum camphora and Toona sinensis were better than Sassafras tsumu;the ratio of Pinus massoniana leaves to 60% of the mixed litter was the best,and it was most beneficial to decomposition of refractory substances in the mixed litter.In this study,the proportion of litter mixture which is beneficial to the decomposition of refractory substances was selected for the transformation of Pinus massoniana plantation to near-natural forest,which provided a theoretical basis for the construction of mixed forest.The actual planting ratio of Pinus massoniana and broad-leaved tree species needs to be calculated by the annual litter amount of the mixed tree species.