Effects Of Litter Treatment And Nitrogen Addition On Forest Soil Carbon And Nitrogen Fractions And Microbial Processes

Changes in soil carbon and nitrogen input sources due to climate change have important effects on carbon and nitrogen dynamics of forest ecosystems.The processes of soil organic matter transformation are largely dependent of the quantity and quality of its inputs;litter sources of different properties and inputs of different levels of exogenous nitrogen affect and regulate the accumulation and decomposition of soil organic carbon.Understanding the responses of physical and chemical fractionation of soil carbon and nitrogen to above-and below-ground carbon and nitrogen inputs is of great significance for further elucidating and predicting the dynamics of soil carbon and nitrogen under climate change.By using an established long-term above-ground litter treatment and nitrogen addition experiment in a temperate Pinus tabuliformis Carr.and Quercus wutaishansea Mary mixed forest,and a soil displacement and controlled above-and below-ground carbon inputs experiment in a Quercus aliena var.acuteserrata Maxim.ex Wen forest,this thesis studies the relationships of soil carbon and nitrogen fractions with microbial community structure and functions.The specific empahses were to address the questions of:(1)If and how would different organic carbon sources and nitrogen addition significantly change forest soil organics carbon and nitrogen fractions?(2)How would soil microbial community structures and function respond to changes in carbon and nitrogen fractionation?(3)What types of interrelationships would be there between forest soil organic carbon and nitrogen factions and the microbial community structures and function,and what would be the key regulatory factors or constraints?In the above-ground litter treatment and nitrogen addition experiment,the contents of soil total organic carbon(TOC),total nitrogen(TN),dissolved carbon or nitrogen(DOC/N),labile and recalcitrant carbon or nitrogen(LOC,ROC),macro aggregates(250?2000 m)and micro aggregates(53?250 m)were significantly increased in different soil layers(0-5cm,5-10 cm,and 10-20 cm)by the inputs of aboveground leaf litter and mixed litter(containsing twigs,cones and leaves).The contents of TOC,TN,DON,LOC and intermediate and recalcitrant carbon or nitrogen(IOC,ROC/N)pools were significantly increased by nitrogen addition.Above-ground litter manipulations and nitrogen addition only showed significant interactive effects on DOC/N(%),LOC,ROC/N and the Macro-aggregation carbon in the 10-20 cm soil layer,with the double inputs of leaf litter and mixed litter significantly increased their contents under treatments of nitrogen additon.Carbon and nitrogen fractions in the clay and silt aggregates(2 ? 53 ?m and is < 2 ?m)and C:N ratio in all the three aggregates did not differ among the treatments;whereas the double inputs of leaf litter and mixed litter significantly increased the ratios of soil labile and recalcitrant carbon and nitrogen fractions,and enhanced the soil aggregate stability and the stability of carbon and nitrogen fractions in the 0-5 cm soil layer.No significant interaction was found between the litter and nitrogen treatments in affecting soil microbial communication structure(PLFAs)and metabolism function indexes;there were significant interactions of the treatments with time and soil depth,such that microbial PLFAs,?-glycosidase enzymes(BG)and N-acetyl-glycosidase enzymes(NAG)activities significantly decreased,and the microbial respiration(MR),and microbial carbon and nitrogen limitations significantly increased with treatment time,and MR,microbial carbon substrate utilization efficiency(CUEC:N),carbon and nitrogen limitations,and the activities of BG and NAG enzymes significantly decreased with soil depth.The double inputs of leaf litter and mixed litter significantly increased the abundance of the gram-negative bacteria(G-),saprophytic fungi(SAP)and am fungi(AMF),fungi(F),bacteria(B),and the total microbial PLFAs.No significant response was found in the microbial biomass carbon and nitrogen ratios,F:B and gram positive(G+)and G-ratios to various treatments.The double inputs of leaf litter and mixed litter increased the microbial respiration,but the carbon utilization maintained relatively steady across the treatments.In the soil displacement and controlled above-and below-ground carbon inputs experiment,at the early stage of soil carbon accumulation,roots contributed less to different carbon fractions than the aboveground litter,but the content of micro-aggregate carbon was significantly increased by the inputs from roots with time of treatement.In the 0-10 cm soil layer,the above-ground litter contributed more to TOC.In the 10-30 cm soil layer,in addition to the contribution of the leaching of above-ground litter,roots also significantly increased the TOC.There was no significant difference between the microbial nutrient availability and TOC during treatments,and the soil mainly contained labile organic carbon.The input of above-and belowground carbon did not show significant interaction in affecting PLFAs in different microbial communities.In the second year of treatments,the inputs from roots played a dominant role in PLFAs and microbial metabolism;in the fourth year,the inputs from above-ground litter played a dominant role.The microbial biomass carbon,activity,carbon substrate utilization efficiency and BG activity increased significantly with time of treatments.Overall,these results indicate that N addition promoted the turnover of soil carbon and nitrogen fractions.The effects of above-and belowground carbon inputs on the soil carbon and nitrogen fractions depending on the content of soil carbon and nitrogen substrates,which is an important factor affecting microbial community structure and metabolism functions.