三种南方典型水稻土长期试验下有机碳积累机制研究 Ⅱ．团聚体内有机碳的化学结合机制

SOC ENHANCEMENT IN THREE MAJOR TYPES OF PADDY SOILS IN A LONG-TERM AGRO-ECOSYSTEM EXPERIMENT IN SOUTH CHINA Ⅱ.CHEMICAL BINDING AND PROTECTION IN MICRO-AGGREGATE SIZE FRACTIONS

在对我国南方3种典型的水稻土有机碳团聚体分布研究的基础上,对用低能量超声波分散法分离得到的团聚体颗粒组进一步进行有机碳键合形态分析,考察不同键合态有机碳在不同粒径团聚体的分布,由此探讨这些水稻土中有机碳积累中物理作用基础上的化学机制。结果表明:钙键合态有机碳(Ca-SOC)多分布于2 000～200μm的粗团聚体颗粒组中,且在紫色水稻土中含量最高(达到9 g kg-1左右),而红壤性水稻土中最低(不到3 g kg-1);相反,铁铝键合态有机碳(Fe(Al)-SOC)趋向于在细团聚体颗粒组集中,是<2μm颗粒组中有机碳的优势组分,且以红壤性水稻土中最高(介于20～30 g kg-1之间),紫色水稻土中最低(仅为10 g kg-1左右),这与该团聚体颗粒组中铁铝氧化物的含量分布相吻合。统计分析表明,易氧化态碳(LOC)占有机碳(SOC)的比值与Ca-SOC占SOC的比值表现为正相关关系,而与Fe(Al)-SOC占SOC的比值表现为反相关关系。直径为2 000～200μm的粗团聚体中的Ca-SOC和Fe(Al)-SOC对长期不同耕作和施肥的响应最为敏感,良好的耕作施肥下水稻土有机碳的积累主要表现为Fe(Al)-SOC的增加。氧化铁铝的含量与SOC和LOC的含量之间存在一定的依变关系,说明这些土壤发生的无机组成分在有机碳的保护与稳定中发挥着重要作用,并且以红壤性水稻土粗团聚体中的氧化铁铝对有机碳的保护作用最强。由此看来,团聚体更新中物理保护的有机碳在细团聚体形成中进一步与氧化铁铝的键合可能是这些水稻土中有机碳稳定的重要机制。

As shown in a previous study, soil carbon sequestration of the three typical paddy soils, i.e. Cabhaplic, Acihaplic and Typfec Stagnic-hydroagric Anthrosols in South China was related to physical protection in micro-aggregate size fractions, though varying with the soil types and management practices. Herewith the authors report a study on analysis of SOC binding forms in size fractions of micro-aggregates obtained by low-energy ultrasonic dispersion procedure from the studied types of paddy soils. Calcium bound SOC （Ca-SOC） was predominant in the coarse fraction of 2 000 - 200 μm, with the highest and the lowest content in purple paddy soil and in red paddy soil, respectively. On contrast, iron/ aluminum oxyhydrates-bound SOC （Fe（Al）-SOC） dominated the size fraction of 〈 2 μm, with the highest and the lowest content in red paddy soil and in purple paddy soil, separately, which was in coincidence with the content distribution of Fe/Al oxyhydrates in various particle size fractions. Statistic analysis indicated significant positive correlation of ratio of LOC to SOC with that of Ca-SOC to SOC, and significant negative correlation of the ratio of LOC to SOC with that of Fe （A1）-SOC to SOC. Ca-SOC and Fe（ Al）-SOC in coarse aggregates of 2 000 - 200 μm showed the most sensitive response to long-term tillage and fertilization practices, and SOC accumulation in paddy soils under good management practices was represented mostly as increase in Fe（Al）-SOC. Fe/Al oxyhydrates in coarse aggregates from the red paddy soil among the three showed the most intensive chemical protection of organic carbon. Thus, chemical protection of SOC physically protected in coarse fractions by binding with Fe/Al oxyhydrates during micro-aggregate turnover was supposed to be a prevailing mechanism of SOC sequestration in these paddy soils.