贵州山区土壤微生物生物量的碳同位素组成与有机碳同位素效应

选择贵州岩溶区3个不同海拔高度上的旱地作为实验点,进行了绿肥(紫花光叶苕,C3豆科植物)的输入实验,目的是了解通过输入绿肥之后土壤微生物生物量碳(SMBC)的响应,以及微生物生物量中碳同位素比值的变化,来判断植物分解作用期间有机碳的同位素效应,即微生物分解对13C的分馏作用和微生物对有机质的选择性利用.实验结果表明,在实验地里加入绿肥之后,年平均气温较高的实验地的SMBC响应较快,并且其值高于其余两个年平均气温较低的实验地,说明SMBC主要来源于植物残留物,其值的大小受控于气温.在加入绿肥之后,并未在各自的实验地和对照地之间出现δ¹³C值的差异,而是在植物残留物的分解达到一定程度后实验地的δ¹³C值才高于对照地,但在年平均气温较低的实验点上始终未出现这种差异,说明植物残留物的分解程度和质量将影响SMBC的δ¹³C值,这种现象也进一步说明土壤有机碳的同位素效应是微生物对13C的分馏和对有机质的选择性利用并存的过程.3个实验点上的土壤有机碳的δ¹³C值大小的顺序与SMBC的δ¹³C值大小的顺序相一致,表明SMBC的δ¹³C值能够反映相应样品中的土壤有机质δ¹³C值的总的变化趋势,并可能是控制土壤有机质的δ¹³C值大小的主要的影响因素.

CARBON ISOTOPE COMPOSITION IN SOIL MICROBIAL BIOMASS AND ORGANIC CARBON ISOTOPE EFFECT

We tested whether the shift in isotopic composition of plant litter during microbial decomposition can be induced by isotope effect: microbial against 13 C discrimination or selective utilization of organic compounds. The decomposition experiment with green manure was conducted in three soils, located different elevation in southwest China, which lasted from 5 January in 2001 to 25 February, 2002. The concentrations and δ¹³C values of soil microbial biomass carbon (SMBC) were measured in both control and treatment plots with green manure ( Vicia villosa var.). The objective of this study was to investigate the factors affecting on the shift of carbon isotope composition in microbial biomass during decomposition. The manure induced increase in SMBC was higher and emerged earlier in the low elevation with high average annual temperature than that in high elevation, indicating that SMBC came mainly from plant litter and controlled by air temperature during decomposition. Because the δ¹³C value of green manure and that of crop and weeds were similar at each experiment site, the difference between treatment and control plots was only quantity and quality of plant litter incorporated into soils. By mass balance, if the shift of carbon isotope composition in microbial biomass only induced by microbial 13 C discrimination, or only by selective utilization of organic compounds, the δ¹³C values of SMBC in the treated and control plots at each experiment site would be equal. The experimental results showed that there were no differences of δ¹³C values of SMBC bet ween treated and control plots when corresponding increasing of carbon concentrations in microbial biomass. However, those differences occurred following sampling date at both Xiaohai and Bakai sites with relatively higher annual average temperature, and the δ¹³C value of SMBC in treated plot was higher than that in control plot, indicating that the degree of microbial decomposition and quality of plant litter will effect on the shift of δ¹³C value of SMBC, and that isotope effect of microbial decomposition was complex, selective utilization of organic compounds was accompanied by micro bial 13 C discrimination. The sequence of magnitude of δ¹³C value of SMBC was consistent with that of soil organic carbon (SOC) among three experiment sites, indicating that the δ¹³C value of SMBC reflects gross changes in the δ¹³C value of SOC in the corresponding samples, and may be the main factor to control magnitude of the δ¹³C values of SOC.