Studies On Soil Carbon Flux In The Short-Rotation Plantation Of Eucalyptus Grandis

Eucalyptus with its inherent characteristics of fast growth accounts for 25%of the world's tropical and subtropical plantations covering more than 10 million hm².First introduced to China in 1890,Eucalyptus,with its rapid growth potential in lean soils,has evolved in last 25 years to be the most widely cultivated species in subtropical China. China now ranks as the world's third largest grower of Eucalyptus.Eucalyptus has the highest net productivity among all the forest ecosystems;given this,there is a growing increase in carbon sequestration in a Eucalyptus ecosystem.This study was conducted in the framework of an integrated program on carbon fluxes and sequestration in perennial tropical plantations.Although there is considerable research data about Eucalyptus plantations in the areas of gene,species,ecosystem and landscape,it has poor knowledge of the carbon effiux or influx in the fast growing and short rotation forest ecosystem of Eucalyptus.Specifically,our objective was first to quantify the annual soil carbon effiux/influx in an age series of Eucalyptus plantation in Southwest China using the Space-for-Time approach.This research primarily examined:(1) Monthly dynamics of soil respiration and its relationship with soil temperature at 5 cm depth(using LCi Portable Photosynthesis System(SRS-1000)),(2) Seasonal variations of fine root(＜2 mm) biomass and their carbon reserves in 0～50 cm depth(soil core method),(3) Seasonal variations of litterfall(collecting the samples in 0.5 m×0.5 m area),and(4) Carbon reserves and soil carbon in the different soil depth in an age series(1～6 year old) of Eucalyptus grandis.The results were as follows:1.The reserves of litter were significantly different(P＜0.05) at different ages of the E. grandis forest.In a descending sequence order they were:5 a(12.648±2.888),6a (12.156±5.584),2 a(11.154±2.575),4 a(9.556±1.891),1 a(8.211±1.757) and 3 a(7.189±1.720 t·hm^-2).The reserves of litter at different periods of the survey were:Jul.-Oct., 2006(5.581)＞Jan.-Apr.,2007(1.619)＞Oct.,2006-Jul.,2007(0.152 t·hm^-2).2.The water holding capacity of litter varies between seasons,as well as the age of E. grandis forest.Basically,water absorption was most rapid in the first 4 hours of rainfall and after 8 hours the water holding capacity was significantly diminished or nonexistent. The relationship between water absorption and the related immersion time was significant at the level of 0.01(R²=0.9479～0.9969).A seasonal discrepancy was obvious for the first 4 hours water absorption;investigative data showed that:autumn＞winter＞summer＞spring(P=0.1407,not significant at the level of 0.05).In forests of different ages,water absorption was lowest for 4 years old forests and better for 1 year old forests.3.Fine root biomass in 0～50 cm depth(soil core method) at different seasons showed: autumn＞summer＞winter＞spring(P＜0.01).Their results for different aged forests showed that 6 a(0.9063)＞5 a(0.7798)＞1 a(0.6277)＞2 a(0.6198)＞4 a(0.5505)＞3 a (0.5377 t·hm^-2)(not significant at the level of 0.05).There were significantly positive correlations between fine root biomass and water holding capacity of litter at forest ages from 1 to 3 years and less significant correlations with increasing forest age.The increase of saturated,capillary and field water-holding supported growth of fine root,as well as the decrease of soil bulk density.4.Monthly soil respiration rate(SRR) ranged from 0.35 to 1.71μmol·m^-2·s^-1 and the seasonal values were different as following descending sequence:summer(1.55),autumn (1.16),spring(0.62),winter(0.41μmol·m^-2·s^-1)(P＜0.01).Soil respiration rate(SRR) varied by forest age in the following descending order:6 year-old(1.10)＞5(0.99)＞2 (0.93)＞1(0.82)＞3(0.80)＞4(0.67μmol·m^-2·s^-1)(P＜0.01).The relationship between early monthly dynamics of SRR and soil temperature at 5 cm depth was significant(R²= 0.7063～0.9657,P＜0.05).The exponential equation of Rs=a.e^b·T and Q₁₀=e^10b were applied to compare the relationships of monthly soil temperature at 5 cm depth and SRR and the temperature sensitivity(Q₁₀)were from 1.73 to 2.29.The binary linear regression between soil temperature at 5 cm depth,fine root(＜2 mm) biomass in 0～50 cm depth and SRR is significant(P＜0.01).5.There was no significant difference between samples of different seasons(t-Test,df =6,P＞0.05) for soil carbon in E.grandis.The investigation data indicated that the 6-year old E.grandis had the largest soil carbon reserve and the soil carbon reserve in a 1-year old forest was smaller than in a 3 or 5-year old forest.