Effects Of Broadleaf Tree On Soil Phosphorus Fractions And The Accumulation Of Phosphorus In A Reforested Cunninghamia Lanceolata Woodland

In subtropical China,the“homogeneity and conifer problem”of plantations is very serious,so it is encouraged to plant broad-leaved trees instead of conifers for afforestation.Understanding the effect of tree species transition from conifer to broadleaf trees on the ecosystem phosphorus pool and soil phosphorus availability are of great significance for the sustainable development of subtropical forest ecosystems.Therefore,this study was conducted to compare the difference of soil physical and chemical,phosphorus fraction and availability across 0-100 cm soil depth and aboveground phosphorus pool between Mytilaria laosensis and Cunninghamia lanceolata plantations,which were initially reforested from C.lanceolata plantation in the spring of 1993.The results showed that:（1）The soil organic phosphorus content in both plantations decreased significantly with soil depth and Na OH-Po is the main component of organic phosphorus.Compared to C.lanceolate,the M.laosensis plantation significantly increased soil available P content by35.7%and 86.2%in the 0-10 and 10-20 cm,respectively.The soil labile P and moderately labile P content decreased significantly with the soil depth in the two plantations.Compared with C.lanceolata,the content of labile P and moderately labile P were significantly higher in the surface soil.Non-liable P is dominated by the Residual P and evidently decreased under M.laosensis plantation.in the 80-100 cm（2）Soil microbial biomass in the two forests decreased significantly with soil depth.In the M.laosensis,the soil microbial biomass carbon（MBC）in the 0-10 and 20-40 cm and soil microbial biomass nitrogen（MBN）and phosphorus（MBP）in the 0-20 cm were significantly higher than those in C.lanceolata.The q MBP and MBC/MBP ratios in the 0-20 cm and MBN/MBP ratio in the 10-20 cm were significantly lower in the M.laosensis plantation.The q MBC,q MBN,and MBC/MBN ratios were no significantly difference between the two forests.（3）Soil moisture content、SOC、TN and DOC in both plantations decreased significantly with soil depth.Tree species transition from C.lanceolata to M.laosensis could significantly change soil Iron-aluminum oxides in the soil profile.Free iron contents at 20-80 cm and Free aluminum contents at 20-60 cm under M.laosensis were significantly decreased.Dissolved organic carbon and free iron were important factors affecting changes in P fractions of the soil.（4）The plant phosphorus pools storage of M.laosensis and C.lanceolata were 148.91kg·hm^-2and 21.45 kg·hm^-2,which occupied for an average of 2.8%and 0.4%of the total phosphorus storage in forest ecosystem respectively.The content of phosphorus in the leaves and roots of M.laosensis was significantly higher than C.lanceolata,and the leaf was the largest phosphorus pool in M.laosensis,while,the root in C.lanceolata.There were no significant difference in soil phosphorus pools between the two forests.In the 0-10 cm depth,soil P content under M.laosensis were 11.22%higher than that under C.lanceolata.In the 60-80 and 80-100 cm depth,soil P content under M.laosensis were17.4%and 17.9%lower than C.lanceolata plantation.In the 60-80 and 80-100 cm depth,soil P pool under M.laosensis were 18.4%and 17.0%lower than C.lanceolata plantation.It is suggested that compared with C.lanceolata plantation,M.laosensis plantation were more conducive to supply phosphorus and beneficial to improve the availability of soil phosphorus..