中亚热带山区土壤不同形态铁铝氧化物对团聚体稳定性的影响

Effects of different forms of Fe and Al oxides on soil aggregate stability in midsubtropical mountainous area of southern China

以福建省三明市本底条件一致的三片林分的土壤作为研究对象,采用湿筛法测定了水稳定性团聚体粒径分布,分析了不同形态铁铝氧化物含量与>0.25 mm大团聚体数量及团聚体平均重量直径(MWD)的关系。结果表明:(1)不同形态铁铝氧化物含量呈现出米槠次生林>米槠人工林>杉木人工林,游离结晶态(Fed、Ald)>无定形(Feo、Alo)>络合态(Fes、Als)。(2)>0.25 mm大团聚体含量呈现米槠次生林>米槠人促林>杉木人工林,林分之间差异显著,MWD值的趋势与之相似。(3)线性回归分析表明:不同形态铁铝氧化物均与>0.25 mm水稳定性大团聚体数量及MWD值达到显著甚至极显著相关,但通过分析相关系数R和显著性P说明氧化铝可能比氧化铁更有助于大团聚体的形成与稳定,无定形及络合态铁铝氧化物比游离态铁铝氧化物更能促进大团聚体的形成与稳定。

Soil aggregation is an important soil physical property that controls water storage, aeration, fertility, plant growth, and biological activity. Soil aggregates are formed as a result of rearrangement, floceulation, and cementation of particles. They are controlled by soil organic carbon, biota, and mineral particles （clay and oxides）. Fe and Al oxides, as important inorganic binding agents for aggregates, have attracted increasing attention in tropic and sub-tropic areas where soils are rich in oxides. Oxides can contribute to aggregation in three ways： Oxides as a flocculant in soil solution; organic materials adsorbed on oxide surfaces; and a coat of oxides formed on the surface of minerals that creates bridges between primary and secondary particles. However, owing to the difference in degree of crystallinity, particle size, and distribution of oxides, the debate on the effect of oxides on aggregate formation and stability has continued. The subtropical mountainous area of southern China is a major timber plantation region, where large areas of native broad-leaved forests have been converted to tree plantations. Such forest conversion involved intensive anthropogenic disturbance, which has significantly altered soil structures. Because of the fragile system and complex soil processes and conditions, investigating changes in soil structure （especially soil aggregate formation and stability ） associated with forest conversion are very important for understanding soil carbon dynamics. In this study, soils （at a depth of 0-10 cm） were collected from three forests： naturally regenerated Castanopsis carlesii forest （NR） （CF） in Sanming city, Fujian Province. These forests Castanopsis carlesii plantation （CC）, and Chinese fir plantation spread over the same parent rock type and have the same topography. Wet sieving method was used to determine the size distribution of water-stable aggregates. Different forms of Fe and Al oxides in mineral soil were extracted selectively and their contents were measured. The relationships between Fe and AI oxide contents and the amount of water-stable macroaggregates 〉0.25 mm in diameter as well as the aggregate mean weight diameter （MWD） were analyzed. The amount of different forms of Fe oxides ranged from 0.74 to 5.82 g/kg. Both types of oxides were most oxides ranged from 0.83 to 12.41 g/kg, and that of Al abundant in NR followed by CC and CF. The dominant forms of these oxides were present in the following order： dithionite-citrate-bicarbonate-extractable oxides （Fe0, Al0） 〉〉 acid ammonium oxalate-extractable oxides （Fe0, Al0） of water-stable macroaggregates 〉0.25 mm in diameter 〉sodium pyrophosphate-extractable oxides （ Fe0, Al0 ）. The amount ranged from 69.71 to 82.41% and were present in the order of NR 〉 CC 〉CF, following the same trend of MWD. These results may be due to the increasing artificial disturbance caused by forest management and the decrease of binding agents. Linear regression analysis indicated a significant positive correlation between Fe, Al oxides, and the amount of water-stable macroaggregates 〉0.25 mm in diameter and the MWD, irrespective of oxides form. Although the contents of Feo, Al0, Fe0, and Al0 were far below those of Fed and Al0, the higher correlation coefficient （R） and significant P value indicated that they were more important factors contributing to the formation and stability of macroaggregates. Compared to Fe oxides, Al oxides had higher R and P, implying that they may play a relatively more important role in promoting aggregates formation and stability. It is concluded that Feo, Al0. Fe0, and Al0 may benefit more aggregate formation and stability than Fe0 and Al0.