水耕人为土时间序列铁氧化物与磁化率演变特征

Evolution Characteristics of Iron Oxides and Magnetic Susceptibility in Stagnic Anthrosols along Chronosequences

以我国亚热带地区不同母质起源的水耕人为土时间序列为研究对象,分析不同形态铁氧化物和磁化率随成土时间的动态演变特征及其影响因素。结果表明,石灰性母质起源的水耕人为土0~120 cm土体中全铁(Fet)、游离铁(Fed)和游离度(Fed/Fet)随时间序列演变均逐渐增加,0~50 a内Fet、Fed和Fed/Fet增加速率分别为3.2 t hm^(-2) a^(-1)、1.2 t hm^(-2) a^(-1)和0.04%a^(-1),50~1 000 a内Fet、Fed和Fed/Fet增加速率分别为0.1 t hm^(-2) a^(-1)、0.15 t hm^(-2) a^(-1)和0.01%a^(-1);而酸性母质起源的水耕人为土0~120 cm土体中Fet、Fed和Fed/Fet随时间序列演变均逐渐下降,0~60 a内Fet、Fed和Fed/Fet下降速率分别为0.2 t hm^(-2) a^(-1)、0.5 t hm^(-2) a^(-1)和0.03%a^(-1),60~300 a内Fet、Fed和Fed/Fet下降速率分别为0.9 t hm^(-2) a^(-1)、1.2 t hm^(-2) a^(-1)和0.06%a^(-1)。土壤pH、Eh、以及外源铁输入与土体内铁淋失的相对强度是控制不同母质水耕人为土中铁氧化物转化速率与途径的主要因素。石灰性母质起源的水耕人为土中不同磁学指标随时间演变分为三个阶段:0~50 a内表现为质量磁化率(MS)、饱和等温剩磁(SIRM)和软剩磁(IRMs)的急剧降低;50~300 a内表现为MS、SIRM和IRMs的持续、缓慢降低以及硬剩磁(IRMh)的相对稳定发展;300~1 000 a内表现为MS、SIRM和IRMs的持续、缓慢降低以及IRMh的快速下降。酸性母质起源的水耕人为土0~20 cm和20~120 cm土壤中磁学指标演变呈现截然不同的两个阶段:0~60 a 0~20 cm内MS,SIRM和IRMs的急剧降低,IRMh具有明显增加;而20~120 cm内MS、SIRM和IRMs缓慢下降,IRMh明显降低。60~300 a 0~20 cm内不同磁学指标变化幅度均很小,而20~120 cm内IRMh相对比较稳定,MS、SIRM和IRMs在种稻150 a后快速下降。淹水还原条件下亚铁磁性矿物的破坏是不同母质水耕人为土演变过程中磁性衰减的主要机制。

【Objective】The study is oriented to explore characteristics of the evolution of iron oxides and magnetic susceptibility in Stagnic Anthrosols, different in parent material, in subtropical China along chronosequence and their influencing factors. 【Method】Soil samples were collected for extraction of total Fe （Fet）, free Fe oxides （Fed）, amorphous Fe oxides （Feo） and organic-bound Fe （Fep） by the lithium metaborate fusion method, the dithionite-citrate-bicarbonate （DCB） method, acid ammonium oxalate （AAO） at pH=3 in the dark and Na-pyrophosphate at pH=10, separately. Magnetic susceptibility （MS） of the soils was measured at both low （0.47 kHz） and high frequencies （4.7 kHz） using a Bartington MS-2B meter with dual frequency sensor.【Result】Results show that along the chronosequence, weighted mean total Fe （Fet）, free Fe oxides （Fed） and Fed/Fetin the 0~120 cm soil layer increased in the calcareous Stagnic Anthrosols, at a rate of 3.2 t hm-2a-1, 1.2 t hm-2a-1 and 0.04% a-1, respectively, during the first 50 years and at a rate of 0.1 t hm-2a-1, 0.15 t hm-2a-1 and 0.01% a-1, respectively, during the following 950 years （50 ~ 1000 years）, but decreased in the acid Stagnic Anthrosols at a rate of 0.2 t hm-2a-1, 0.5 t hm-2a-1 and 0.03% a-1, respectively, during the first 60 years and at a rate of 0.9 t hm-2a-1, 1.2 t hm-2a-1 and 0.06% a-1 during the following 240 years （60 ~ 300 years）, while weighted mean amorphous Fe oxides （Feo） and Feo/Fed in the 0~120 cm soil layer tended to decrease in the calcareous Stagnic Anthrosols, but did reversely in the acid Stagnic Anthrosols. Soil pH, Eh, and the balance between input of external Fe with irrigation and loss of internal Fe with leaching are major factors controlling the rates and pathways of Fe oxides transformation during the evolution of Stagnic Anthrosols. Along the chronosequence, the Stagnic Anthrosols derived from calcareous parent material underwent chronosequence demonstrated three phases of magnetic changes, i.e. the initial phase lasting a few decades dominated by rapid decreases in MS （magnetic susceptibility）, SIRM （saturation isothermal remanent magnetization） and IRMs （soft isothermal remanent magnetization）; the second phase lasting a bit more than two centuries （50~300 years） demonstrating constant IRMh （hard isothermal remanent magnetization） and slow decline in MS, SIRM, and IRMs; and the third phase （300 ~ 1000 years）, witnessing minimal changes in MS, SIRM and IRMs but drastic decline in IRMh, while the Stagnic Anthrosols derived from acid parent material did two phases, which in the 0~20 cm soil layer were completely different from those in the 20~120 cm soil layer： In the first phase （0~60 years）, MS, SIRM and IRMs declined but IRMh increased rapidly in the 0~20 cm soil layer, while all the magnetic properties declined in the 20~120 cm soil layer; However, in the second phase （60~300 years）, all the magnetic properties did not vary much in the 0~20 cm soil layer, while MS, SIRM, and IRMs declined rapidly in the 20~120 cm soil layer after 150 years of paddy cultivation.【Conclusion】The overall magnetic depletion in the Stagnic Anthrosols, though different in parent material was attributed to the reductive dissolution of ferromagnetic minerals under artificial submergence.