Litter Decomposition And Nutrient Cycling In Pinus Massoniana Plantation

Pinus massoniana is a fast-growing afforestation tree species which is unique to subtropical regions in China.It is highly adaptable and widely distributed.This study was carried out in Pinus massoniana plantation and Mixed forest of Pinus massoniana and Castanopsis fissa in Dinghu Mountain.By comparing the difference of 0-60 cm soil nutrient content,community structure and ground cover litter between the two forest types,the relationship between soil nutrient,stand factors and ground cover litter were analyzed.Litter decomposition bags with different proportions(mass ratio of Pinus massoniana to Castanopsis fissa is 1:0,1:1,3:2 and 7:3)were set up to carry out a one-year litter decomposition experiment in the field,and the change of decomposition rate and nutrient release characteristics were analyzed.The results of this study are as follows:(1)Forest types had significant effects on soil organic matter,total nitrogen and sulfate content(P<0.05),total phosphorus,exchangeable K⁺,Ca²⁺and Mg²⁺content(P<0.01).Soil nutrient content(except nitrate nitrogen content and exchangeable H⁺content)of Pinus massoniana mixed forest was higher than that of pure Pinus massoniana.There were significant differences in soil nutrient content among different soil layers of the same forest type(P<0.01).Soil organic matter and total nitrogen content decreased with the soil depth,and mainly concentrated in 0-20 cm soil layer;soil exchangeable K⁺,Ca²⁺,exchangeable acid(H⁺and Al³⁺)content decreased with the soil depth on the whole;and soil sulfate content increased with the increase of soil depth on the whole.(2)The average DBH and tree height of pure forest were significantly higher than those of mixed forest(P<0.05),the biomass of tree layer was higher than that of mixed forest but not significant(P>0.05),and the average density and canopy density were significantly lower than that of mixed forest(P<0.05).Margalef species richness index and Shannon diversity index in each layer of the two forest types were herbaceous layer>shrub layer>tree layer.(3)The existing amount of three litter layers(26.96-31.57 t·hm^-2)in mixed forest was significantly higher than that in pure forest(20.44-26.63 t·hm^-2)(P<0.05).There was a significant difference in litter stock among different decomposition layers in the same forest type(P<0.01).RDA analysis of stand factors and litter stock showed that total litter stock,fresh litter and fragmented litter layer stock were extremely significant negative correlated with average DBH and tree height(P<0.01)and extremely significant positive correlated with stand density(P<0.01).(4)Total phosphorus and total calcium contents in litter of pure forest and mixed forest land were significantly(P<0.05)or extremely significant(P<0.01),but organic carbon,total nitrogen,total potassium and total magnesium contents were not significantly different(P>0.05).The organic carbon,total nitrogen,total phosphorus,total calcium and total magnesium contents in litter of pure forest were higher than those of mixed forest,increasing by 4.67%,2.45%,12.89%,27.08%and 20.28%,respectively.The contents of organic carbon,total calcium and total magnesium in litter of different decomposition layers of the same forest type were significantly different(P<0.05),which showed as fresh litter layer>fragmented litter layer>humified and amorphous litter layer.RDA analysis of soil nutrients and litter mass showed that there was a significant negative correlation between soil nutrients and organic carbon in humified and amorphous litter layer(P<0.01)and C/N in humified and amorphous litter layer(P<0.05)in 0-10 cm soil layer,while there was a significant negative correlation between soil nutrients and organic carbon in humified and amorphous litter layer(P<0.05)in 10-20 cm soil layer.(5)Forest types had a significant effect on the loss rate of litter quality(P<0.05)After360 days of natural decomposition,the loss rate of litter quality in each proportion was mixed forest>pure forest.In mixed forest,the loss rate of litter quality was 1:1>7:3>1:0.In pure forest,the loss rate of litter leaf mass was 3:2>7:3>1:1>1:0,and the loss rate of conifer leaf mass in proportion of 3:2,7:3 and 1:1 was significantly higher than 1:0(P<0.05).(6)During the decomposition of litter in different proportions of the two forest types,the release patterns of organic carbon,total phosphorus,total potassium and total magnesium in the litter were in the form of enrichment-release or sustained release,while the release patterns of total nitrogen were in the form of enrichment-release or sustained release.After360 days of decomposition,the release rates of organic carbon,total nitrogen,total phosphorus,total calcium and total magnesium in the mixed forest were faster than those in the pure forest.The correlation analysis between nutrient release rate and mass loss rate showed that the release rates of organic carbon,total nitrogen,total calcium and total magnesium were significantly positively correlated with mass loss rate(P<0.01)and the mass loss rate of all proportion of litter was also significantly positively correlated with total phosphorus release rate(P<0.01)except that the proportion of litter was 7:3.Pinus massoniana,as the main tree species of plantation in China,has abundant resources but poor forest quality.How to restore Pinus massoniana plantation has become an important issue.Studying the mixed decomposition process of different proportions of coniferous and broad-leaved litter can better understand the interaction mechanism of mixed litter and the nutrient cycle of Pinus massoniana plantation.The results provide a scientific theoretical basis for the establishment of Pinus massoniana forests with different mixing ratios and the maintenance of long-term productivity of Pinus massoniana forests.