不同形态有机碳的有效性在两种重金属污染水平下水稻土壤中的差异

Availability of different organic carbon fractions of paddy soils under two heavy metal contamination levels

调查分析了两块不同重金属污染水平下的水稻田土壤有机碳活性组分的含量，以评价重金属胁迫条件下有机碳摹质的微生物有效性。结果显示，重污染水稻土的总有机碳（TOC）、0．5mol·-L^-1 K3SO4溶解态有机碳（K2SO4-C）、微生物生物量碳（MBC）、1～7d（CO2-C1-7d）和8．28d矿化的有机碳（CO2-C8-28d）的含量均显著低于轻污染土壤（P〈0．001）。碳有效性指标中，微生物商（MBC／TOC比）和8-28d的有机碳矿化率（CO2-C8-28d·TOC^-1 d^-1））在轻污染土壤中较高，而K2SO4-C／TOC和代谢商（qCO2）在重污染土壤较高（P〈0．001）。1—7d的有机碳矿化率（CO2-C1-7d·TOC^-1d^-1）在两种土壤中差异不显著。两种土壤培养初期的有机碳矿化率远高于培养后期（P〈0．001）。逐步回归分析显示，CO2-C1-7d与活性组分K2SO4-C和MBC显著相关（r^2=0．83，P〈0．001），而CO2-C8-28d只与TOC显著相关（r^2=0．70，P〈0．001）。研究表明，尽管如SO4溶解态有机碳在供试的重金属污染的水稻土中是较易被分解的活性碳基质，但重金属污染抑制了微生物数量及其呼吸活性，降低了活性有机碳库的矿化率，这是导致土壤中K2SO4溶解态碳积累的原因。溶解性有机碳叉能提高重金属的有效性，进一步抑制微生物对有机碳的矿化。因此，评价重金属污染的水稻土有机碳的有效性，应当综合考虑小同有机碳库的活性、微生物的矿化能力和碳基质在不同阶段的供应潜力。

Biologically active or labile fractions of soil organic carbon are vital in understanding decomposition potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure. Proportions of dissolved organic carbon, microbial biomass carbon and mineralized carbon in total organic carbon are served as sensitive indices of soil organic carbon availability. However, most relevant work over the past decades has focused on forests and grasslands, with little emphasis on quantifying active carbon pool fluxes in human-dominated agricultural systems, especially on heavy metal contaminated subtropicalrice field soils.We investigated different organic carbon fractions in paddy soils under two heavy metal contamination levels in GuangdongProvince, South China, in order to compare and assess the availability of soil organic carbon substrates under long-term heavy metal stress. Our results showed that the contents of total organic carbon （TOC）, 0.5mol· L^-1 K2 SO4 extractahle organic carbon （K2SO4-C）, microbial biomass carbon （MBC）, mineralized organic carbon during 1 - 7 d （CO2-C1-7d） and 8 - 28 d （CO2- C8- 28d ） of laboratory incubation were consistently lower in highly contaminated paddy soils than those in slightly contaminated soils （p 〈 0.001 in all cases）. In contrast, availability indices of organic carbon presented different trends. Compared with slightly contaminated soil, highly contaminated soil were revealed greater K2 SO4-C/TOC ratio and microbial metabolic quetient （CO2-C/ MBC）, whereas, lower MBC/TOC and mineralization rate of organic carbon during 8 - 28 d incubation （CO2-C8-28d· TOC^-1 d^- 1 ）. Differences in mineralization rate of organic carbon during 1 - 7 d incubation （CO2-C1 - 7d·TOC ^- 1 d^- 1 ） of two soils were not remarked. Mineralization rate of organic carbon during initial 7 d of incubation was pronouncely higher than that during subsequent 21 d of incubation. Mutiple stepwise regression showed that CO2-C1- 7d was significantly correlated with K2 SO4-C and MBC （ r2 = 0.83, p 〈 0.001）, while CO2-C8-28d was remarkedly associated with TOC alone （ r2 = 0.70, p 〈 0.001 ） . Our study indicates that K2SO4-C is readily decomposed active C pool.However, long-term heavy metal contamination decreased micriobial population size and respiratory activity, inhibitedmicrobial mineralizaiton efficiency of labile organic carbon. The accumulation of K2S04-C could be attributed to the decreased mineralization rate because more labile organic carbons in paddy soils may directly derived from exudate of intensive fine rice roots rather than decompositon of raw litters. Moreover, elevated K2 S04-C in soil can enhance the solubility, mobility and biological uptake of metals, and thereby increase the biological availiablity of heavy metals. It may further restrained mineralization rate of active carbon pool.On basis of the suppressed turnover process, relatively greater proportion of K2 S04 extractable organic carbon in highly contaminated paddy soil is not doomed to high availability of active carbon substrate supply. Thus, in order to assess the availablity of organic carbon of paddy soils polluted by heavy metals, the combined functioning of active organc carbon pools,mineralization intensity of microbes and supply potential of carbon substrates during different turnover stages should be taken into consideration.