Research On Function Of Nutrient Returns From Litter Layers And Carbon Balance Acacia Plantations

Acacia spp.is an important fast-growing tree and has been introduced all over the world.It plays vital roles on the forests construction of short-period industrial raw material,sustainable forest management and germplasm resources of forest trees in South China.The nutrient return from litter layers is the initial process of organics production in the forest.Nutrient utilization,recycling and flowing was the hub between the organic matter and the soil system interior.The circulation of nutrient elements significantly maintains the ecosystem pattern and forms the considerable productivity and biomass outputs.However,the study on functions of nutrient returns from litter layers and carbon balance has been rarely reported.The location of research was chose at Dongxi work area of Tianma national forest station in Zhangzhou,Fujian,from August 2006 for one year.Three tree species including Acacia concinnatai,Acacia melanoxylon and Acacia mangiun were chose as the treatments and the subtropical species Schima Superba as the control.Our field work covers litter collection,litter decomposition experiment and CO₂ release from soil surface,the plants and soil nutrient were analyzed as well.Our purposes were studying the allocation patterns of biomass of Acacia spp.,exploring the rules of nutrient recycling including C,N,P,K returns from litter layers and assessing carbon source/sink status in Acacia plantations.This research provides the scientific foundation for guiding scientific management of Acacia plantations and sustainable management on Minnan mountainous region and basic database for dealing with future climate negotiations of our country.The conclusions were as follows:(1)The highest individual plant biomass was found in Acacia concinnatai which was 1.34 and 1.59 times as heavy as Acacia melanoxylon and Acacia mangiun,respectively.The order of organ proportion for each plant was stems＞root＞the bark＞branch＞leaf.Significantly higher stand biomass was found in Acacia concinnatai than in Acacia melanoxylon and Acacia mangiun species.In the composition of stand biomass,higher proportion was found in tree layers(87.06%～91.11%)as compared litter layers(6.48%～6.97%).(2)The annual litter-fall of Acacia concinnatai,Acacia melanoxylon and Acacia mangiun were 6171.49, 7338.28 and 6102.16 kg·hm^-2,respectively.Of these,the percentages of leaf litter of Acacia concinnatai,Acacia melanoxylon and Acacia mangiun were 65.32%,83.71%and 68.10%,respectively.The monthly dynamics of total litter-fall showed the double-peak modes.(3)The annual nitrogen and phosphorus returns from litter layers of Acacia concinnatai and Acacia mangiun were similar,but they were significantly(p＜0.05)lower than that of Acacia melanoxylon.The order of annual potassium returns from litter layers were Acacia mangiun(22.28 kg·hm^-2a^-1)＞Acacia concinnata i(19.96 kg·hm^-2a^-1)＞Acacia melanoxylon(17.76 kg·hm^-2a^-1).The order of annual nutrient returns from litter layers were nitrogen(N)＞potassium(K)＞phosphorus(P).The highest carbon returns was found in Acacia melanoxylon than in Acacia concinnatai(1.27 times)and Acacia mangiun(1.23times).The seasonal changes of carbon returns from litter layers in Acacia concinnatai were autumn＞summer＞winter＞spring;Similar higher carbon returns were found in Acacia melanoxylon between spring,summer and autumn than in winter.Higher carbon returns in Acacia mangiun were found on summer and the autumn than that in spring and winter.(4)The residue rates of dry weight of leaf litter and litter branches were Acacia mangiun＞Acacia melanoxylon＞Acacia concinnatai＞Schima Superba by decomposing for one year.The decomposition process of litter branches was more stable than that of leaf litter,but decomposition dynamics were similar to leaf litter.(5)The concentration of carbon(C)and potassium(K)from leaf litter decreased with the litter decomposed, whereas phosphorus concentration increased.The nitrogenous concentration represented ruleless fluctuation with indistinct amplitude.The change of other nutrient concentrations(such as nitrogen,phosphorus and potassium) were complexity except that carbon concentration decreased during the decomposition process owing to discrepancy of plant organs.(6)Obviously net release amount of carbon and nitrogen were found from leaf litter and litter branch during decomposition process.However,net absorption of the potassium release was showed in Acacia mangiun with higher in leaf litter and lower in litter branches.There was not significant(p＜0.05)relativity between release amount of phosphorus and decomposition time for Schima superba and Acacia concinnatai,even represented net sequestration.Except for Acacia mangiun's litter branch,the order of nutrient release rate was k_K＞k_C＞k_N.(7)The carbon release of Acacia melanoxylon was higher than that of Acacia concinnatai and Acacia mangiun.The highest nitrogen release amount was found in Acacia melanoxylon,but lowest was found in Acacia concinnatai.The order of potassium release amount was Acacia mangiun＞Acacia concinnatai＞Acacia melanoxylon.The annual phosphorous absorption in Acacia concinnatai was about 0.45 kg·hm^-2,while net phosphorous release of Acacia melanoxylon and Acacia mangiun were 2.27 kg·hm^-2and 1.65 kg·hm^-2,respectively.(8)There was negative correlation between initial nutrient of litter and the soil nutrient,but positive correlation was found between soil organic carbon and initial nutrient of leaf fitter.The initial nitrogen content of leaf litter was significantly positive correlation with soil total nitrogen content,soil hydrolysable nitrogen and soil available potassium.The initial potassium content of leaf litter was significantly positively related to soil total potassium.Thus,the soil nutrient level has influenced on the initial nutrient level of defoliation.The initial N and P content in litter branches was significantly(p＜0.05)positively related to those in the surface soil.However,there was negative correlation between K content of branches,total K content in soil and available K possibly because the different physiological functions of plants induced the lower K content in branches.(9)Fourteen quality indices of litter and habitats were chose to analyze the decomposition rate using PCA classification.The results showed that the principal component was the initial N content that was followed by soil organic carbon,soil hydrolysis N,total soil N content,the ratio of C/N,the initial P content and soil available K. The second mian component was the initial K content which was followed by total K content and P content in soil, the ratio of C/N in soil,the available P in soil and the initial C content from litters.(10)The percentage of carbon content of each organ was about 45%～50%for the three Acacia plants.The order of carbon content of the Acacia concinnatai and the Acacia melanoxylon were the trunk＞the root＞the bark＞the branch＞the leave and carbon percentage in Acacia mangiun was the trunk＞the tree root＞the bark＞the leaf＞the branch.The average carbon contents in understory forest of the Acacia concinnatai,the Acacia melanoxylon and the Acacia mangiun were 42.29%,42.78%and 41.26%,respectively,which was obviously lower than those in tree layer.The carbon storage of litter in the Acacia concinnatai,the Acacia melanoxylon and the Acacia mangiun were 2.27t·hm^-2,1.61t·hm^-2and 1.63t·hm^-2,respectively.The carbon storage of the Acacia concinnatai,the Acacia melanoxylon and the Acacia mangiun in ecosystem was 93.63 t·hm^-2,109.50 t·hm^-2and 77.37 t·hm^-2;respectively.(11)The seasonal changes of soil respiration,non-root soil respiration and root respiration were consistent with the soil temperature with a single peak.However,the seasonal changes of litter respiration were complex, which higher in summer and lower in winter.(12)The order of a value of soil respiration was Acacia melanoxylon,Acacia melanoxylon,Schima superba and Acacia mangiun.The order of Q₁₀value of soil respiration was Acacia mangiun,Acacia melanoxylon, Schima superba and Acacia melanoxylon.The lowest residuals were found in the Schima superba forest and Acacia trees in winter,but negative residual was found in spring and positive in autumn.For Schima superba and Acacia melanoxylon,the temperature sensitive coefficient Q₁₀was higher in root respiration than that of other components.For the Acacia melanoxylon and Acacia mangiun,the temperature sensitive coefficient Q₁₀was lower in root respiration than that of other components.(13)The soil respiration velocity in the Acacia melanoxylon,Acacia melanoxylon,and Acacia mangiun was 47.91%,36.57%and 29.69%,respectively,using the relevant analysis between litters and soil respiration.The contribution rate of rainfall was less than 20%to the soil respiration using the relevant analysis.(14)The annual release amount of soil respiration was lowest in the schima superba,which was 68.66%, 88.63%and 73.44%to Acacia melanoxylon,Acacia melanoxylon,and Acacia mangiun,respectively.The annual soil respiration of non-root in Acacia melanoxylon,Acacia melanoxylon,and Acacia mangiun was 5.21,5.05 and 5.24tC·ha^-1·a^-1,respectively.The annual release amount of root respiration was highest in the schima superba, which was 1.47,1.51 and 1.23 times to Acacia melanoxylon,Acacia melanoxylon and Acacia mangiun, respectively.Higher litter respiration was found in Acacia melanoxylon than the other stands.(15)The annual net storage amount of carbon in the Acacia melanoxylon,Acacia melanoxylon and Acacia mangiun was 14.01,10.31 and 11.57t·hm^-2·a^-1,respectively,Of these,the C storage amount in the tree layer was 4.98,4.06 and 3.03 t·hm^-2·a^-2,respectively,and the annual C returns amount in litters was 8.63,6.08 and 7.48 t·hm^-2·a^-1,respectively.The annual C accumulation was about 40%and C return was about 60%.The net biomass productions of Acacia melanoxylon,Acacia melanoxylon,and Acacia mangiun in ecosystem were 4.46,3.62 and 3.59 t·hm^-2·a^-1,which was called "carbon sequestration functions".It means equal to fix CO₂ 16.36,13.27 and 13.15 t·hm^-2.a^-1every year for Acacia melanoxylon,Acacia melanoxylon and Acacia mangiun,respectively.