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Soil microbe is the most important decomposer in forest ecosystems,and plays a key role in material recycling and energy conversion.In this study,the spatial patterns of soil microbial biomass in typical Karst peak-cluster depressions were analyzed through both geo-statistical and traditional statistical methods.The results showed that large variation existed in soil microbial biomass.Soil microbial biomass carbon(Cmic),soil microbial biomass nitrogen(Nmic),and soil microbial biomass phosphorus(Pmic) varied from 44.29—5209.63,20.91—1894.37,0.34—77.06 mg/kg,respectively.The Cmic was significantly positively correlated with Nmic.The value of Cmic/Nmic is 4.78,which obviously lower than that in other ecosystems.Semivariance analysis revealed that the Gaussian model fitted best for Cmic and Nmic,the Spherical model fitted best for Pmic,and the Exponential model fitted best for Cmic/Nmic.The SH%(C/(C+C0)×100%) of soil microbial biomass exhibited moderate spatial autocorrelation,ranging from 25% to 75%.Therefore,the spatial patterns were affected by both random and structure factors.The autocorrelation distance of Cmic and Nmic were about 50 m.Moran′s I decreased gradually with the increase of separation distance,and showed a statistically negative correlation.Moran′s I of Pmic increased when the separation distance was larger than 70 m,which indicated positive correlation.Moran′s I of Cmic/Nmic was lower than others,ranging from-0.2 to 0.2 m.The spatial distribution of Cmic and Nmic were similar,and showed a convex-type distribution in which the contents were high in the middle and low on the two sides.In contrast,The Pmic was distributed with a pattern of high in the upper slope and depressions.The content of Cmic/Nmic showed a concave-type distribution with many small patches.We conclude that soil microbes exist in a certain spatial pattern,and that its contents sharply reduced after disturbing.As a result,the protection of Karst primary ecosystems against human disturbing should be strengthened.

Karst ecosystem has been shown to be extremely vulnerable under severe water and soil erosion due to improper land uses and human activities.The karst area of Southwestern China covers a land of 550000 km2 but 105000 km2 of that is suffering from serious rocky desertification.Recently,national and provincial afforestation programs have been developed to protect and/or restore the degraded karst lands.Litter decomposition is the major pathway to provide organic and inorganic elements for the nutrient cycling processes and control nutrient return to the forest ecosystem.So,leaf litter decomposition and nutrient release characteristics play an important and leading role in the forest ecosystem restoration and reconstruction in the fragile karst region.In view of litter fall as the important component of forest ecosystem and the shortage study about karst area in southwest China,confronting the restoration programs.Litter bag decomposition experiments were set up at six sites to compare the litter decomposition differentiation under different karst forest ecosystems from Dec.2008 until Nov.2009.In this study,three typical primary forest communities(Dye-tree(Platycarya longipes Wu),Eurycorymbus(Eurycorymbus cavaleriei Hand.,Wing-hackberry(Pteroceltis tatarinowii Maxim)) and three typical secondary forest ones(Round-leaved Tallow-tree(Sapium rotundifolium Hemsl),Chinese Alangium(Alangium chinense(Lour.)Harms),Negundo Chaste-tree(Vitex negundo Linn)) were chosen to elucidate the decomposition characteristics of leaf litter,release pattern of nutrient elements over the course of leaf litter decomposition,decomposition rate and related impact factors in Karst areas of northwest Guangxi of China.The result showed that the decay rates of leaf litter at primary forests were slightly faster than that of secondary forest.Release rates of carbon(C),nitrogen(N) and Potassium(K) were generally higher within initial 180 days and then tended to become stable.The content of total phosphorus(TP) in leaf litter in secondary forest communities displayed a status of net accumulation at the initial stage of decomposition,and subsequently turned to be replaced by a net release.However,the TP content of leaf litter in primary forest communities was remained in net accumulation status till 360 days after litter presence.Correlation analysis showed that decomposition rate of leaf litter was negatively correlated with total initial N content,lignin content and lignin to nitrogen ratio in leaf litter,whereas was positively correlated with leaf C/ N ratio.Comprehensive and comparative analysis suggested that leaf litter decomposition rate and nutrient element release rate within the round-leaved tallow tree(Sapium rotundifolium Hemsl) community were totally greater in secondary forests than those of primary forest communities.With regard to species choices for the re-establishment of nutrient cycling in disturbed forest ecosystems,species with high nutrient recycling capacity(e.g.species with fast litter decomposition rate) may be favored.In this study,it can be argued that the tallow tree could act as a potential constructive and dominant species in the restoration of degraded Karst land and vegetation.So our study results can provide some support and refer for national and provincial afforestation programs in Karst areas of northwest Guangxi of China.

Forests form the main body of terrestrial ecosystems and play an important role in mitigating global climate change through carbon dioxide sequestration in forest biomass. Biomass and productivity are essential characteristics of forestecosystem structure and function,and knowledge of both are essential for the study of carbon sink function and carbon sequestration. Cunninghamia lanceolata,Pinus massoniana,and Eucalyptus urophylla × Eucalyptus grandis are the primary plantation trees used in southern China,accounting for 90% of the plantation area in ten southern provinces. However,few studies have analyzed the dynamics of forest biomass and productivity of these forest types. Forty-five 20 m × 50 m research plots were established and analyzed using three dominant plantation species( C lanceolata,P. massoniana,E. urophylla ×E. grandis) in southern China with the goal of improving our understanding the vertical structure of each community,the distribution of biomass to the various organs,and gaining a better understanding of forest biomass and productivity of differently aged stands. Each plantation type was studied using five age classes: young,middle-aged,near-mature,mature,and over-mature forest,with three replicates of each. Regression models relating tree biomass to diameter at breast height( DBH 1.3 m) were developed to estimate tree biomass. Other biomass components were sampled in quadrats within the same forty-five research plots: shrubs( three 4 m2quadrats),herbs( three 1 m2quadrats),and litterfall( three 1 m2quadrats). This method was used for estimating biomass,productivity,and biomass allocation rates to various organs in the three main plantations types; we also analyzed the relationships between stand age and biomass productivity to determine the biomass allocation rates to various organs. Results showed that a significant relationship exists between stand biomass and stand productivity of the three main plantation types as they relate to stand age,with a high degree of fitting in growth models. The total biomass of C. lanceolata,P. massoniana,and E. urophylla × E.grandis plantations increased with stand age and the biomass of mature forest was 192.30,191.53,105.77 Mg /ha,respectively. Living biomass made up 95.76%—98.39%,75.01%—99.14%,85.60%—97.61% of these three plantation types,respectively. Most of the biomass of the three main plantation types was concentrated in the tree layer and biomass increased with stand age for all three types. The biomass of other layers accounted for only a small percentage of total biomass,and generally litterfall herb shrub biomass. The trunk biomass occupied the greatest proportion of the tree layer; for C. lanceolata,P. massoniana,and E.urophylla × E. grandis truck biomass made up 54. 89%—75. 97%,49. 93%—83. 10%,and 51. 07%—98. 48% of total biomass,respectively,and trunk biomass increased with stand age. Root biomass made up the second largest percent of tree biomass and the percentages of both branch and leaf biomass were lower than root biomass and decreased with stand age.Shrub branches made up the majority of shrub biomass; both the above-and belowground biomass allocation of herbs did not exhibit an obvious trend or pattern of change. Compared with other forest types,the total biomass of C. lanceolata plantations and P. massoniana plantations were both greater than most and the total biomass of E. urophylla × E. grandis was lower than most,but the level of productivity of all three plantation types was high,or 12.37,8.98,21.10 Mg hm-2a-1,respectively. All three are fast growing forest tree species with higher photosynthetic efficiency and potential carbon sequestration than other forest plantation species in South China.